Direct investigation of halogen bonds by solid-state multinuclear magnetic resonance spectroscopy and molecular orbital analysis

Article

pubs.acs.org/JACS

Direct Investigation of Halogen Bonds by Solid-State Multinuclear

Magnetic Resonance Spectroscopy and Molecular Orbital Analysis

Jasmine Viger-Gravel, Sophie Leclerc, Ilia Korobkov, and David L. Bryce*

Department of Chemistry and Center for Catalysis Research and Innovation, University of Ottawa, 10 Marie Curie Private, Ottawa,

Ontario, Canada K1N 6N5

S

* Supporting Information

ABSTRACT: Noncovalent interactions play a ubiquitous role in the

structure, stability, and reactivity of a wide range of molecular and ionic

cocrystals, pharmaceuticals, materials, and biomolecules. The halogen

bond continues to be the focus of much attention, due in part to its

strength and unique directionality. Here, we report a multifaceted

experimental and computational study of halogen bonds in the solid state.

A series of cocrystals of three diﬀerent diiodobenzene molecules and

various onium halide (Cl⁻or Br⁻) salts, designed to exhibit moderately

strong halogen bonds (C−I···X⁻) in the absence of competing hydrogen bonds, has been prepared and characterized by single-

crystal X-ray diﬀraction. Interestingly, a wide range of geometries about the halide anion are observed. ^35/37Cl and ^79/81Br solid-

state NMR spectroscopy is applied to characterize the nuclear quadrupolar coupling constants (C_Q) and asymmetry parameters

(η_Q) for the halogen-bonded anions at the center of bonding environments ranging from approximately linear to distorted square

planar to octahedral. The relationship between the halogen bond environment and the quadrupolar parameters is elucidated

through a natural localized molecular orbital (NLMO) analysis in the framework of density functional theory (DFT). These

calculations reveal that the lone pair type orbitals on the halogen-bonded anion govern the magnitude and orientation of the

quadrupolar tensor as the geometry about the anion is systematically altered. In −C−I···X⁻···I−C− environments, the value of

η_Qis well-correlated to the I···X⁻···I angle. ¹³C NMR and DFT calculations show a correlation between chemical shifts and

halogen bond strength (through the C−I distance) in o-diiodotetraﬂuorobenzene cocrystals. Overall, this work provides a

chemically intuitive understanding of the connection between the geometry and electronic structure of halogen bonds and

various NMR parameters with the aid of NLMO analysis.

(R), which can be an organic or inorganic moiety or another

halogen atom (acting as an electron withdrawing group), and a

halogen bond acceptor, YZ, a nucleophilic region of another

molecule, such as a Lewis base, halide, or π-electrons.²⁶The

electron-withdrawing group, R, will help generate a positive

electrostatic potential along the far end of the RX bond,

known as the σ-hole, which is surrounded by negative

electrostatic potential.^27,28

Over the past decade in particular, halogen bonds have been

extensively characterized with X-ray diﬀraction in the solid

state.^29,30Simultaneously, Legon and co-workers have made

signiﬁcant contributions to understanding halogen bonds in the

gas phase.³¹They have examined experimentally how the

structures of halogen-bonded complexes depend on the nature

of the halogen and the Lewis base with the use of rotational

spectroscopy. Among the parameters which can be extracted

from such experiments are the halogen nuclear quadrupolar

coupling constants, C_Q, which give quantitative information

about the electric ﬁeld redistribution associated with the

halogen bonding process.^32,33

INTRODUCTION

■

Halogen bonds (XBs) are currently attracting much attention

in the literature due to their multiple applications in diverse

research ﬁelds, including crystal engineering^1,2and biochemis-

try.^3,4This class of noncovalent interaction continues to ﬁnd

applications in these varied areas of research, as halogen bonds

can be comparable to or stronger (ranging from 1.2 kcal/mol

−

for Cl···Cl interactions in chlorocarbons to 43 kcal/mol in I₃···

I₂)⁵than hydrogen bonds (typically 3−7 kcal/mol)⁶and other

noncovalent interactions such as anion−π,^6,7cation−π,⁸and

Lewis acid−Lewis base interactions.⁶Additionally, halogen

bonds are highly directional and can align their components

with speciﬁc orientations which are appealing in the

architecture of supramolecular⁹or functional materials, such

as anion−organic frameworks^10,11and crystalline assem-

blies.^12,13As for the biochemical relevance of halogen bonds,

they have been observed in molecular recognition and folding

processes and in ligand binding.¹⁴⁻¹⁶This has sparked interest

in the development of new drugs,^17,18where halogen bonds

play a key role,¹⁹as well as in anion binding²⁰⁻²⁴and

catalysis.²⁵

The halogen bond (RX···YZ) is deﬁned as an

electrostatic interaction between a halogen bond donor, an

electrophilic region of a halogen atom (X), part of a molecule

Received: October 3, 2013

A

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The IUPAC deﬁnition of XBs states that the X···Y halogen

bond usually leads to characteristic changes in the nuclear

magnetic resonance signals of RX and YZ.²⁶In solution

¹H NMR studies, solvent shifts of haloformic protons have

been monitored when halogen bonds are formed with various

electron-rich solvents.³⁴¹³C NMR has been used to detect, in a

series of iodoalkynes, the eﬀects of XB interactions on the

δ_iso(¹³Cα) value, which may vary by up to 15 ppm depending

on the nature of the solvent.³⁵Solution ¹⁹F NMR has also been

successfully applied to the determination of association

constants of halogen bonding anion receptors, where the

binding aﬃnity of the receptor is a good parameter for

describing selectivity, which is important in molecular

parameters by allowing one to identify contributions of

individual occupied molecular orbitals (MOs) to the NMR

interaction tensors.⁵⁵This work therefore combines exper-

imental and theoretical methods to provide insight into the

relationship between the geometry and electronic structure of

halogen bonds and their corresponding spectroscopic observ-

ables.

EXPERIMENTAL SECTION

■

Synthesis. Starting materials were purchased from Sigma-Aldrich

and were used without further puriﬁcation. At room temperature, an

equimolar amount of p-diiodotetraﬂuorobenzene (p-DITFB, 1), o-

diiodotetraﬂuorobenzene (o-DITFB, 2), or p-diiodobenzene (p-DIB,

3) was dissolved in dichloromethane with an ammonium or

phosphonium salt (n-Bu₄NCl (A), n-Bu₄PCl (B), n-Bu₄PBr (C), n-

Bu₄NBr (D), Ph₄PCl (E), EtPh₃PBr (G)), yielding the following

halogen-bonded cocrystals: [(n-Bu₄NCl)(p-DITFB)] (1A),⁷⁷[(n-

Bu₄PCl)(p-DITFB)] (1B), [(n-Bu₄PCl)(o-DITFB)] (2B), [(Ph₄PCl)-

(p-DITFB)] (1E), [(Ph₄PCl)(o-DITFB)]·2CH₂Cl₂(2E), [(n-

Bu₄PBr)(p-DITFB)] (1C), [(n-Bu₄NBr)(p-DITFB)] (1D), [(n-

Bu₄PBr)(o-DITFB)] (2C), [(EtPh₃PBr)(p-DITFB)] (1G), and

[(EtPh₃PBr)₂(p-DIB)] (3G). The preparation and crystal structures

of some of the halogen-bonded compounds containing p-DITFB were

previously reported.^39,77The numbering used for the compounds is

summarized in Table S1 of the Supporting Information.

recognition.^20,21Erdel

́

yi has used NMR to elegantly demon-

strate that symmetric halogen bonding is preferred in solution

in certain [N−X−N]⁺systems.³⁶In the solid state, Weingarth

and co-workers have determined N···I distances from ¹⁵N−¹²⁷I

dipolar coupling interactions through rotary resonance

recoupling ¹⁵N solid-state NMR (SSNMR) experiments.³⁷

Bouchmella et al. observed imidazolyl-containing haloalkenes

and haloalkynes involved in halogen bonds by ¹H, ¹⁵N, and ¹³

C

SSNMR.³⁸The ¹³C NMR data were, however, inconclusive

with respect to establishing a link between the halogen bond

and the spectral data, as they were not able to observe the

resonance for the carbon directly involved in the interaction.

Our group has observed, in a series of XB compounds

containing iodobenzenes, signiﬁcant changes in the chemical

shift (CS) tensors of the nuclei involved in the halogen bond in

comparison to the starting material, which is devoid of the

noncovalent interaction with multinuclear solid-state magnetic

resonance.³⁹⁻⁴¹Attrell and co-workers⁴²studied a series of

solid haloanilium halides where they used halogen SSNMR

Crystals of compounds 2B and 1E were obtained by slow

evaporation from a minimum amount of dichloromethane. Crystals

of compounds 2C,E were obtained using a slow diﬀusion method. The

compounds were dissolved in a minimum amount of dichloromethane

and placed in a vial which was then placed in a jar containing hexane

and sealed.

X-ray Crystallography. Data collection results for compounds 1E,

2B,C,E, 3G, and o-DITFB represent the best data sets obtained in

several trials for each sample. The crystals were mounted on thin glass

ﬁbers using paraﬃn oil. Prior to data collection, crystals were cooled to

200.15 K. Data were collected on a Bruker AXS SMART single-crystal

diﬀractometer equipped with a sealed Mo tube source (wavelength

0.71073 Å) APEX II CCD detector. Raw data collection and

processing were performed with the APEX II software package from

Bruker AXS.⁵⁶Diﬀraction data for 1E, 2B,C, 3G, and o-DITFB were

collected with a sequence of 0.5° ω scans at 0, 120, and 240° in φ. In

order to ensure adequate data redundancy, diﬀraction data for 2E were

collected with a sequence of 0.5° ω scans at 0, 90, 180, and 270° in φ

due to their lower symmetry. Initial unit cell parameters were

determined from 60 data frames with a 0.3° ω scan, each collected for

diﬀerent sections of the Ewald sphere. Semiempirical absorption

corrections based on equivalent reﬂections were applied.⁵⁷Systematic

absences in the diﬀraction data set and unit cell parameters were

(

^35/37Cl, ^79/81Br, and ¹²⁷I)⁴³⁻⁵⁰as a probe to study the halogen

bond acceptor. Due to competing hydrogen-bonding inter-

actions in those compounds and the fact that the halogen

bonds were weak, it was diﬃcult to unambiguously correlate

the NMR parameters to the halogen bonding environment.

One of the open questions in characterizing halogen bonds

concerns the understanding of how the NMR parameters of the

nuclei involved in halogen bonds, particularly the halogens

themselves, may be more rigorously related to (i) the local XB

geometry and (ii) the local electronic structure; more

speciﬁcally, the molecular orbitals (MOs) involved in the

halogen bond.

In the present study, novel complexes have been synthesized

to speciﬁcally have moderately strong halogen bonds, with R_XB

values ranging between 0.79 and 0.91. The normalized distance

parameter, R_XB= d_I···X¯/∑d_VdW, is the ratio of the distance

between the halogen and the electron donor to the sum of the

consistent with triclinic P1 (No. 2) for compound 2E, monoclinic

̅

P2₁/c (No. 14) for compounds 2B,C, monoclinic P2₁/n (No. 14) for

compound 3G and o-DITFB, and monoclinic C2/c (No. 15) for

compound 1E. Solutions in centrosymmetric space groups for all

compounds yielded chemically reasonable and computationally stable

results of reﬁnement. The structures were solved by direct methods,

completed with diﬀerence Fourier synthesis, and reﬁned with full-

matrix least-squares procedures based on F².

For compound 2E, thermal motion parameters for atoms of two

partially occupied CH₂Cl₂solvent molecules suggested a positional

disorder not related to the symmetry elements. The disorder was

successfully modeled; however, a set of geometrical (SADI) and

thermal motion (SIMU, DELU) restraints was applied to achieve

acceptable molecular geometries and thermal motion values.

Disordered fragment occupancies were reﬁned with satisfactory results

at 25%:25% for both CH₂Cl₂solvent molecules.

van der Waals radii of the atoms or ions involved, ∑d_VdW.

⁵¹In

addition, these cocrystals were designed to be devoid of

competing interactions such as hydrogen bonding. The

compounds used in this study contain iodoperﬂuorobenzenes,

“now considered to be the ‘iconic’ halogen bond donors”,⁵²

cocrystallized with diﬀerent ammonium or phosphonium halide

XB acceptors. In addition to characterization by single-crystal

X-ray diﬀraction, comprehensive multinuclear solid-state

magnetic resonance spectroscopic studies are reported. The

origins of the observed NMR parameters, such as the

quadrupolar coupling constant, C_Q, and asymmetry parameter,

η_Q, are further examined with natural localized molecular

orbital (NLMO) analyses.^53,54NLMO analyses provide direct

insight into the relationship between structure and spectral

In the structural models of 2B,C,E and o-DITFB, all molecular

fragments of the structures are located in general positions (i.e., not on

symmetry elements). In the structure of 1E, two molecules of p-C₆I₂F₄

are located on two diﬀerent inversion centers, whereas all of the other

structural fragments are situated in general positions. Similarly, one

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molecule of p-C₆I₂H₄in the structure of 3G also occupies an inversion

center, with the rest of the moieties located in general positions.

Further details on the reﬁnements of the structural models for

compounds 1E and 2B are given in the Supporting Information. In

structural models for all compounds, hydrogen atoms were located

from the diﬀerences in Fourier maps. However, after initial

positioning, all hydrogen atoms were constrained to suitable

geometries and subsequently treated as idealized contributions during

the reﬁnement. All scattering factors are contained in several versions

of the SHELXTL program library, with the latest version being used

(6.12).⁵⁸

data sets was omitted due to probe ringing. No apodization was used

when processing these spectra. For the VOCS method, each piece was

processed identically and coadded together to yield the ﬁnal spectrum.

Simulations were performed with WSolids1⁶⁴and DMFIT (v.2011).⁶⁵

Simulations of ¹³C NMR spectra included chemical shifts, line

broadening, and the appropriate number of crystallographic sites

determined by X-ray crystallography. Stack plots of the experimental

and simulated spectra were prepared using DMFIT. Further

information is given in the Supporting Information.

Computational Details. Models were generated from X-ray

crystal structure atomic coordinates for each halogen-bonded

compound using one halide ion and the nearest interacting p- or o-

DITFB or p-DIB molecule(s) (see Scheme 1 and Figure 1). Only for

Solid-State NMR Spectroscopy. Carbon-13 and chlorine-35/37

SSNMR experiments were carried out on a 400 MHz (B₀= 9.4 T)

Bruker Avance III wide-bore spectrometer equipped with a triple-

resonance 4 mm MAS probe and a 5 mm solenoid probe, respectively.

¹³C, ^35/37Cl, and ^79/81Br SSNMR experiments were also carried out at

the National Ultrahigh-ﬁeld NMR Facility for Solids in Ottawa using a

21.1 T Bruker Avance II spectrometer equipped with either a Bruker 4

mm double-resonance MAS probe for ¹³C or a home-built 5 mm

solenoid probe for ^35/37Cl and ^79/81Br. Samples were powdered and

packed in 4 mm o.d. zirconium oxide rotors for ¹³C experiments and in

5 mm glass tubes for static ^35/37Cl and ^79/81Br experiments.

Scheme 1. General Halogen-Bonding Motif (X = Cl, Br) for

1A−D and 2B,C

¹³C SSNMR. Ramped amplitude ¹³C cross-polarization (CP) MAS

60

NMR⁵⁹spectra were collected with SPINAL-64 H decoupling for

all samples containing protons at two ﬁelds, 9.4 and 21.1 T (¹³C

Larmor frequencies of 100.613 and 226.338 MHz, respectively).

Chemical shifts were referenced to solid glycine (δ_iso(¹³CO) 176.4

ppm with respect to TMS). The π/2 pulse length and contact times

were 3.5 μs and 3 or 5 ms at 9.4 T and 2.5 μs and 3 ms at 21.1 T.

Recycle delays ranged from 5 to 15 s. A single-channel rotor-

synchronized Hahn-echo experiment (i.e., π/2−τ₁−π−τ₂−acq) was

used for the acquisition of the spectrum of o-DITFB. Further details

are provided in the Supporting Information.

1

Cl2 (site A) of 1E were the diiodotetraﬂuorobenzenes replaced by

iodotriﬂuoromethane molecules to ensure convergence. The positions

of the ﬂuorine atoms in that model were optimized in Gaussian09⁶⁶

(B3LYP/3-21G). The NLMOs, electric ﬁeld gradient (EFG) tensors,

and magnetic shielding (MS) tensors were calculated with the

Amsterdam Density Functional (ADF) program (version 2009.01).

Scalar and spin−orbit relativistic eﬀects were included using the

zeroth-order regular approximation (ZORA). The revised Perdew,

Burke, and Ernzerhof generalized gradient approximation exchange-

correlation functional (GGA revPBE) of Zhang and Yang was used for

all calculations.⁶⁷The basis sets used for all atoms were Slater-type

triple-ζ with polarization functions. A diﬀuse function was included in

the basis set for the halide ion (i.e., AUG/ATZP),⁶⁸whereas the basis

set used for all other atoms was relativistically optimized (i.e., ZORA/

TZP).⁶⁹

The EFG and shielding tensors contained in the output ﬁles were

analyzed using a modiﬁed version of the EFGShield program (version

1.1).⁷⁰Magnetic shielding constants were converted to chemical shifts

according to the formula δ_ij= (σ_ref− σ_ij)/(1 − σ_ref), where σ_refis the

absolute shielding constant of a reference compound (184.1 ppm for

¹³C in TMS;⁷¹974 ppm for ^35/37Cl in inﬁnitely dilute Cl⁻(aq)).⁷²To

properly compare the calculated and experimental δ_iso(^35/37Cl) values,

we also accounted for the concentration of 0.1 M NaCl in D₂O⁷³and

the signiﬁcant isotope shift of ∼5 ppm caused by D₂O.⁷⁴

^35/37Cl SSNMR. Chlorine chemical shifts and pulse widths were

calibrated using the ^35/37Cl NMR signal of KCl powder (δ_iso(KCl(s))

8.54 ppm with respect to 0.1 M NaCl in D₂O). The CT-selective pulse

widths were scaled by 1/(I + 1/2). Spectra were collected under

stationary conditions at two ﬁelds (v_L(³⁵Cl) = 39.204 MHz at 9.4 T

and 88.194 MHz at 21.1 T; v_L(³⁷Cl) = 32.634 MHz at 9.4 T and

73.412 MHz at 21.1 T). At 9.4 T, WURST-QCPMG⁶¹experiments

combined with proton continuous wave (CW) decoupling to acquire

^35/37Cl NMR spectra in combination with the variable oﬀset

cumulative spectrum (VOCS) method (100 kHz steps) were

advantageous, as the nuclei of interest are dilute in the halogen-

bonded complexes (^35/37Cl ranges from 6.29 to 33.1 mg/cm³) and the

spectra are broad.⁶²A 50 μs WURST pulse with a 500 kHz bandwidth

sweep from high to low frequency was used with pulse powers

optimized experimentally (see the Supporting Information). The

spikelet separation was set to 5 kHz by setting the echo duration to

106 μs with pulse ring-down times of 20 μs; 128 echoes were acquired

in every scan. The recycle delays used for these compounds varied

between 2 and 4 s. At 21.1 T, ^35/37Cl SSNMR signals were acquired

using an echo (i.e., π/2−τ₁−π/2−τ₂−acq)⁶³pulse sequence combined

with the VOCS method and CW proton decoupling. Typical

acquisition parameters were CT-selective π/2 pulse lengths of 4 μs,

spectral windows of 500 kHz, τ₁values of 45 μs, pulse delays of 1 or 2

s, and 14k to 32k scans per piece.

RESULTS AND DISCUSSION

■

X-ray Crystal Structures and Halogen Bond Geome-

try. Summarized in Table 1 are the crystallographic data for the

new XB compounds, and in Table 2 are given the relevant

halogen bonding distances (d_I···X¯) and angles (θ_{C−I···X¯}) as well

as the I···X⁻···I angles (θ_{I···X¯···I}) for the compounds studied

presently (see Scheme 1). Also presented in Table 2 are the

carbon−iodine bond lengths, which have been demonstrated to

increase upon XB formation.²⁶The crystal structures for XB

compounds 1A−D,G show a range of Cl⁻···I and Br⁻···I

halogen bond environments and have been discussed

previously.³⁹We therefore focus the discussion below on

crystallographic aspects of the new XB compounds. One

measure of the presence of a halogen bond is when the distance

between the halogen (i.e., I) and the electron donor (i.e., Cl⁻or

Br⁻) is shorter than the sum of their van der Waals radii (1.98

Å for covalent I and 1.81 and 1.96 Å for Cl⁻and Br⁻,

^79/81Br SSNMR. The experimental setup, pulse calibration, and

referencing were done using solid KBr (δ_iso(KBr(s)) 54.51 ppm with

respect to 0.03 M NaBr in D₂O). The Larmor frequencies for bromine

were v_L(⁷⁹Br) = 225.518 MHz and v_L(⁸¹Br) 243.094 MHz at 21.1 T.

The CT-selective ^79/81Br pulses were 1.5 μs for the Solomon echo

experiment used to acquire the SSNMR spectra. A recycle delay of

0.2 s, spectral window of 2 MHz, and transmitter oﬀsets of 500 kHz

were used. τ₁varied between 18 and 175 μs, and each piece was

collected with 14k to 24k scans.

Spectral Simulation and Processing. All NMR data were

processed with Bruker TopSpin 3.0 software. Echoes were left-shifted

where required. The WURST-QCPMG spectra were processed in

magnitude mode. The ﬁrst echo of each of the ³⁷Cl WURST-QCPMG

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ranging from 167.6 to 179.4° for θ_{C−I···Cl¯}, and 170.7 to 177.9°

for θ_{C−I···Br¯}

.

3G exhibits the same dianionic motif ([Br···I−C₆H₄I···

Br]²⁻) as the previously described 1G³⁹(see Figure 1), as well

as the same space group and crystal system (P2₁/n and

monoclinic). 3G is the only reported compound in this study

where the halogen donor molecule is p-diiodobenzene, and it

exhibits the weakest XB interaction with an R_XBvalue of 0.91.

This is attributed to the fact that protons are less electron

withdrawing than ﬂuorine; this leads to the formation of a

smaller σ hole to interact with the halogen bond acceptor

(Br⁻).²⁸The unit cell may be viewed in the Supporting

Information in Figure S1, where it is observed that the

dianionic species form discrete entities in alternating rows with

PPh₃Et⁺cations which are associated two by two into an

inversion-centered phenyl embrace motif. This motif has been

observed for bulky phosphonium salt (i.e., Ph₃PR) derivatives⁷⁶

and was also observed in compound 1G.³⁹

Halogen-bonded compounds 2B,C form one-dimensional

architectures (see Figure S5 in the Supporting Information).

Each halogen bond acceptor (Cl⁻or Br⁻) interacts with two

iodine atoms, forming polymeric anionic zigzag chains as

described for compounds 1A−D³⁹and by Abate et al.⁷⁷

Interestingly, for 2B there are three crystallographically distinct

chloride ion sites; the environment surrounding each halide is

quite diﬀerent, with I···Cl⁻···I angles of 112.0, 144.5, and 123.4°

for Cl1, Cl2, and Cl3, respectively. One bromide site is present

in 2C, and the I···Br⁻···I angle is rather acute at 80.4°. Such

acute angles have been observed previously by Grebe et al. in

similar compounds.⁷⁸2B,C both pack in the monoclinic crystal

system with the P2₁/c space group (see the Supporting

Information for packing diagrams).

Compound 2E packs in the triclinic crystal system and P1

space group. Two molecules of o-DITFB and two chlorides

interact together, as shown in Figure 1, where two halogen

bond donors (I) interact with one halogen bond acceptor

(Cl⁻), forming a I···Cl⁻···I angle of 120.1°. In the unit cell along

the c axis, there are two halogen-bonded complexes forming a

row. These rows are separated by Ph₄P⁺cations associated two

by two into an inversion-centered phenyl embrace motif

(Figure S6, Supporting Information), as observed for 3G and

1G.^39,76Dichloromethane molecules crystallize in the unit cell.

Finally, in 1E ([(Ph₄PCl)(p-DITFB)]) there are two unique

chloride sites in the crystal structure with two very diﬀerent

halogen-bonding environments and crystal networks. The ﬁrst

chloride site is at the center of a distorted-square-planar motif,

where four diﬀerent XB donor molecules interact with a single

chloride anion (see Figure 1). These form inﬁnite one-

dimensional networks along the c axis (see the Supporting

Information). The second chloride site sits at the center of a

distorted octahedron by interacting with six iodine atoms

(Figure 1) and forms a secondary intrinsic two-dimensional

network (see the Supporting Information). In 1E, when both

chloride sites are observed, they alternate rows along the b axis

(Supporting Information). Looking at the overall structure

along the c axis, the Ph₄P⁺cation forms columns with the

halide, which alternates rows with p-DITFB molecules.

̅

Figure 1. Local halogen-bonding geometries for some of the

compounds studied in this work, from X-ray diﬀraction (left), and

corresponding molecular structure schemes (right). Each atom is color

coded: iodine is violet, chlorine is turquoise, bromine is orange,

ﬂuorine is green, carbon is black, and hydrogen is gray. Relative atom

sizes are based on their relative van der Waals radii. See the angles and

bond distances in Table 2. 1E has two crystallographically distinct

chloride sites, where the symmetries about sites A and B are almost

perfectly octahedral and square planar, respectively. 3G exhibits

dianionic species of the form [Br···I−C₆H₄−I···Br]²⁻. Cations are not

shown.

respectively).⁵¹Also shown in Table 2 are the normalized

distance parameters, R_XB, which range from 0.79 (1A) to 0.91

(3G). These values indicate the presence of moderately strong

(1A) to weak (3G) halogen bonds. Very strong halogen bonds

can have R_XBvalues as low as 0.69.⁷⁵Generally, the RX···Y

angle tends to be linear in halogen bonds²⁶(angles varying

between 160 and 180°), as the halogen aligns with the n lone

pair of electrons of the XB acceptor. All of the reported

compounds exhibit near-linear halogen bonds, with angles

The C−H hydrogen atoms in the onium counterions are

well-removed from the coordination spheres of the halide ions

in all compounds. Aliphatic CH−chloride contacts typically

cluster around 2.39 Å,⁷⁹whereas most of the shortest H−anion

distances in the compounds studied here are around 3 Å.

D

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Table 1. Crystallographic Data and Selected Data Collection Parameters

a

2B

2E

2C

1E

3G

empirical formula

formula wt

cryst size, mm

cryst syst

space group

Z

C₆₆H₁₀₈Cl₃F₁₂I₆P₃

2090.18

C₃₁H₂₂Cl₃F₄I₂P

861.60

C₂₂H₃₆BrF₄I₂P

741.19

C₄₂H₂₀ClF₁₂I₆P

1580.40

C₄₆H₄₄Br₂I₂P₂

1072.37

0.27 × 0.23 × 0.11

monoclinic

P2₁/c

0.21 × 0.14 × 0.14

triclinic

0.24 × 0.18 × 0.10

monoclinic

P2₁/c

0.21 × 0.17 × 0.15

monoclinic

C2/c

0.24 × 0.20 × 0.17

monoclinic

P2₁/n

P1

̅

2

4

8

2

a, Å

13.4256(3)

25.3718(6)

25.3449(6)

90.00

11.8135(2)

12.8379(3)

13.4744(3)

92.2880(10)

114.0890(10)

104.8300(10)

1779.12(7)

0.0390

13.1132 (4)

15.1457(5)

14.4157(5)

90.00

18.5756(2)

19.3258(2)

26.4008(3)

90.00

9.24230(10)

12.5396(2)

18.5824(3)

90.00

b, Å

c, Å

α, deg

β, deg

98.915(2)

90.00

98.7470(10)

90.00

99.1840(10)

90.00

98.3230(10)

90.00

γ, deg

V, Å³

8529.0(3)

0.1716

2829.78(16)

0.0289

9356.08(18)

0.0366

2130.92(5)

0.0228

R₁(ref)

wR₂(ref)

0.1541

0.1208

0.0604

0.0763

0.0520

a

2E is a solvate.

Table 2. Selected Intermolecular Contact Distances, Angles, and Halogen Bonding Environment Surrounding the Halides in

a

Halogen-Bonded Compounds

b

c

compd

#I

d_I−C/Å

d_I···X¯/Å

R_XB

θ

_{C−I···X¯}/deg

θ_{I···X¯···I}/deg

I_x···X⁻···I_y

I₁···Cl⁻···I₂

d

1A

1B

2B

1

2

1

2

2.1071

2.0963

2.0946

2.1022

2.988

3.104

3.038

2.976

0.79

0.82

0.80

0.79

178.1

170.2

175.0

177.0

109.1

d

155.7

I₁···Cl⁻···I₂

Cl1

Cl2

Cl3

1

2

3

4

5

6

2.1152

2.0965

2.1173

2.115

3.157

3.088

3.115

3.150

3.052

3.062

0.84

0.82

0.83

0.81

175.8

175.3

178.0

174.9

169.6

176.8

112.0

144.5

123.4

I₃···Cl⁻···I₂

I₁···Cl⁻···I₆

I₅···Cl⁻···I₄

2.0948

2.0874

1E

Cl1

Cl2

1

2

3

4

5

6

1

2

1

2

1

2

1

2

1

2.0931

2.0952

2.0653

2.0872

2.0944

2.0925

2.1058

2.1065

2.1121

2.0963

2.1046

2.0995

2.1085

2.1047

2.1081

2.1125

3.204

3.240

3.332

3.191

3.204

3.132

3.152

3.166

3.236

3.189

3.196

3.347

3.269

3.147

3.472

0.85

0.86

0.88

0.85

0.83

0.84

0.83

0.87

0.85

0.82

174.4

167.6

174.6

179.4

176.1

174.5

177.4

176.1

175.3

177.9

176.8

177.7

174.6

174.1

175.5

170.7

91.6

75.7

I₁···Cl⁻···I₂

I₂···Cl⁻···I₃

I₁···Cl⁻···I₃

I₄···Cl⁻···I₅

100.4

176.3

e

2E

120.1

139.2

140.9

80.4

I₁···Cl⁻···I₂

I₁···Br⁻···I₂

d

1D

d

1C

2C

d

1G

3G

0.91

a

b

Refer to Scheme 1 for information on the reported angles and distances. #I refers to crystallographically diﬀerent iodine atoms in each of the

c

structures. R_XBis the normalized distance parameter, R_XB= d_I···X¯/∑d_VdW, where d_I···X¯is the shortest contact distance between the halogen and the

halide and d_VdWis the sum of their van der Waals radii (1.98 Å for I and 1.81, 1.96 Å for Cl⁻and Br⁻, respectively).⁵¹Crystal structure reported in

d

e

ref 39. Disordered solvate (DCM).

Solid-State NMR Spectroscopy: Correlation of NMR

Data with Local Structure. ¹³C NMR. The ¹³C CP/MAS

NMR spectra of 1E, 2B,C, and 3G corresponding to the region

of the carbons covalently bonded to iodine are shown in Figure

2. Analogous spectra of the pure non-halogen-bonded aromatic

compounds (o-DITFB, p-DITFB, p-DIB) are also shown. The

chemical shifts, which were obtained by simultaneously ﬁtting

the spectra at magnetic ﬁelds of 9.4 and 21.1 T, are summarized

in Table 3. The full ¹³C CP/MAS NMR spectra and chemical

shift assignments may be found in the Supporting Information.

As previously described,³⁹such spectra are challenging to

acquire due to possible relaxation caused by the directly bonded

E

dx.doi.org/10.1021/ja5013239 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

Figure 2. Selected regions of the experimental (solid lines) and simulated (dashed lines) ¹³C CP/MAS SSNMR spectra of (a, d) o-DITFB, (b, e) 2C,

and (c, f) 2B recorded at 21.1 T (left, a−c) and 9.4 T (middle, d−f) and of (g) p-DIB, (h) 3G, (i) p-DITFB (this spectrum was also shown in ref

39), and (j) 1E, recorded at 21.1 T (right). In the case of 2B, the intensities of the lines in the simulations were adjusted to match the experimental

data. This accounts in an ad hoc manner for partial overlap of unresolved resonances and possible small diﬀerences in site intensities due to

diﬀerential cross-polarization eﬃciencies.

these values shows that a fully ¹²⁷I coupled ¹³C multiplet line

shape should span about 12 ppm at 21.1 T, suggesting that the

¹³C line shapes obtained presently are at least partially ¹²⁷I self-

decoupled.

Table 3. Experimental ¹³C Isotropic Chemical Shifts of

Carbons Covalently Bonded to Iodine

a

compd

δ_iso(¹³C)/ppm

Higher ¹³C chemical shifts are observed to generally correlate

with longer C−I bonds in the halogen-bonded o-DITFB

compounds. The values of δ_iso(¹³C) for the C−I carbons in 2C

(98.6(0.4) and 101.4(0.4) ppm) and 2B (102.5(0.3),

101.4(0.3), 100.2(0.5), and 97.9(0.2) ppm) are all signiﬁcantly

larger than that for pure o-DITFB, where no XB is present

(92.6(1.3) ppm). This is consistent with the trend noted for

halogen-bonded p-DITFB compounds.³⁹The substantially

lower chemical shift observed for the carbon bonded to iodine

in comparison to the typical chemical shift of carbons in

aromatic rings is due to a relativistic spin-orbit-induced heavy

atom substituent eﬀect; the magnitude of the relativistic shift is

largely due to the magnitude of the spin−orbit splitting of the

heavy atom and tends to increase with the valence s orbital

character of the observed nucleus.⁸⁰However, a precise

correlation between d_C−Iand δ_iso(¹³C) cannot be established

for the halogen-bonded o-DITFB compounds, since for two out

of the four compounds studied here, not all of the isotropic

peaks of the diﬀerent crystallographically distinct carbons are

resolved. In the case of 2B, six nonequivalent carbons are

expected from the crystal structure and only four resonances

are resolved in the NMR spectra at both ﬁelds (Figure 2c,f).

The ¹³C resonance of the halogen-bonded complex 3G is

more deshielded (δ_iso(¹³C) 99.5(0.5) ppm) in comparison to

pure p-DIB (Figure 2g) (97.1(2.6) ppm). In this example, the

increase in chemical shift upon halogen bonding may be clearly

o-C₆F₄I₂

p-C₆H₄I₂

92.6(1.3)

97.1(2.6)

98.6(0.4)

101.4(0.4)

102.5(0.3)

101.4(0.3)

100.2(0.5)

97.9(0.2)

99.5(0.5)

80.2(1.5)

2C

[(n-Bu₄PBr)(o-C₆F₄I₂)]

b

2B

[(n-Bu₄PCl)(o-C₆F₄I₂)]

3G

[(EtPh₃PBr)₂(p-C₆H₄I₂)]

[(Ph₄PCl)(p-C₆F₄I₂)]

b

1E

a

Experimental ¹³C isotropic chemical shift of the carbon covalently

bonded to iodine. Errors are given in parentheses, which are equal to

b

the line width at half-height. For 2B and 1E, six distinct carbon sites

are expected for each, from X-ray crystallography.

iodine, nearby ﬂuorines causing dipolar broadening, and distant

protons rendering CP ineﬃcient. The use of a 21.1 T

spectrometer was essential to resolve some of the chemical

shifts observed for the carbons involved in the weak XB

interaction. The residual dipolar coupling (RDC) between a

spin ¹/₂nucleus such as ¹³C and a quadrupolar nucleus such as

¹²⁷I is reduced at higher magnetic ﬁelds, as is apparent when the

line widths observed at 9.4 T are compared to those measured

at 21.1 T (∼125 vs ∼90 Hz). No splitting or asymmetric

broadening attributable to RDC is observed, and spectra at

both ﬁelds were ﬁt with consistent chemical shift values. A

calculation using typical parameters for the ¹²⁷I−¹³C dipolar

coupling constant (657 Hz) and the ¹²⁷I quadrupolar coupling

constant (1000 MHz) gives a residual dipolar coupling constant

of about −1.1 kHz at 21.1 T. A spectral simulation based on

correlated to an increased carbon−iodine distance (d_C−I

=

2.09635 and 2.1125 Å for p-DIB and 3G, respectively) found

from X-ray data, since only one distinct carbon site is present

for both the XB compound and the starting material.

F

dx.doi.org/10.1021/ja5013239 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

Figure 3. ⁸¹Br solid-state NMR spectra of stationary powdered halogen-bonded compounds acquired at 21.1 T. Experimental spectra are shown in

(a) 1D, (c) 2C, (e) 1C, (g) 3G, and (i) 1G, and their respective simulated spectra are given in (b), (d), (f), (h), and (j). Asterisks in (i) mark an

impurity.

a

Table 4. Experimental ⁸¹Br EFG and CS Tensor Parameters for Halogen-Bonded Compounds

b

c

|C_Q(⁸¹Br)|/MHz

η_Q

δ_iso/ppm

Ω/ppm

κ

α/deg

β/deg

γ/deg

1D

1C

2C

1G

3G

58.0(0.2)

57.0(0.2)

30.8(0.1)

40.7(0.1)

21.5(0.1)

0.22(0.01)

0.19(0.01)

0.27(0.01)

0.26(0.01)

250(6)

310(10)

256(4)

255(20)

110(10)

192(10)

320(20)

115(15)

1.0(0.2)

0.9(0.3)

0.0(0.1)

0.9(0.1)

−0.9(0.2)

0(0)

20(10)

82(2)

5(1)

90(20)

90(0)

12(2)

90(40)

30(5)

34(4)

0(20)

90(2)

210(10)

140(10)

0.32(0.01)

b

0(5)

a

Error bounds are given in parentheses. The EFG tensor, V, can be diagonalized to provide three principal components deﬁned as |V₃₃| ≥ |V₂₂| ≥

|V₁₁|. The quadrupolar coupling constant, C_Q, is equal to eQV₃₃/h; the asymmetry parameter, η_Q, is equal to (V₁₁− V₂₂)/V₃₃. While C_Qmay take any

c

real value, only |C_Q| can be measured from conventional SSNMR experiments. The isotropic chemical shift, δ_iso, is equal to (δ₁₁+ δ₂₂+ δ₃₃)/3, the

span, Ω, is equal to δ₁₁− δ₃₃, and the skew, κ, is equal to 3(δ₂₂− δ_iso)/Ω, where δ₁₁≥ δ₂₂≥ δ_33.

Compound 1E should have a total of six distinct carbon sites

which are involved in halogen bonds. In spectra recorded at

ﬁelds of 9.4 and 21.1 T, only one very broad peak is observed;

hence, none of the chemical shifts can be resolved (Figure 2j).

However, it is clear that the carbon isotropic chemical shift of

the halogen-bonded complex is deshielded with respect to p-

DITFB, as was previously observed for 1A−D,G.³⁹This follows

the overall trend, even though a site-speciﬁc correlation with

the six diﬀerent C−I distances cannot be established.

with larger C_Qvalues. They are in the same range (C_Q(⁸¹Br) =

12.3−45.3 MHz) as the values reported by Attrell et al. for

halogen-bonded haloanilium halides.⁴²The ⁸¹Br isotropic

chemical shifts are within the known range for bromides,⁴⁴

ranging from 140(10) to 310(10) ppm, and the CS tensor

spans range from less than 110(10) ppm to 320(20) ppm.

Compounds 1D,C are isostructural and feature polymeric

anionic chains with θ_{I···Br¯···I}angles of 139.2 and 140.9°,

respectively. Two polymorphs are known for 1D: that of

Metrangolo et al.⁷⁷crystallizes in the space group C2/c,

whereas our group reported the space group Pccn.³⁹Compound

1C also packs in the space group C2/c. These halogen-bonded

compounds are characterized by similar ⁸¹Br EFG tensor

parameters (Table 4): |C_Q(⁸¹Br)| values of 58.0(0.2) and

57.0(0.2) MHz and η_Qvalues of 0.22(0.01) and 0.19(0.01) for

1D,C, respectively. At higher magnetic ﬁeld, bromine chemical

shift anisotropy (CSA) contributes signiﬁcantly to the NMR

line shapes (Figure 3a,e; the span, Ω, is 255(20) ppm for 1D

and 110(10) ppm for 1C; see also Figure S14 in the Supporting

Information). The halogen-bonded compound 2C also forms

polymeric anionic zigzag chains, but with acute I···Br⁻···I angles

(80.4°) and packs in the space group P2₁/c. Qualitatively, this

compound gives a much narrower ⁸¹Br NMR line shape, which

is quantiﬁed by a |C_Q(⁸¹Br)| value of 30.8(0.1) MHz (Figure

3g). This value may be related to the local halogen bonding

environment (vide infra). Bromine CSA parameters for 2C are

between the values reported for 1C,D: e.g., a span, Ω, of

^79/81Br NMR. Compounds with a bromide anion as halogen

bond acceptor have been characterized with ^79/81Br SSNMR at

21.1 T (see Figure 3 and the Supporting Information). The

bromine NMR parameters summarized in Table 4 were

obtained by simultaneously simulating the NMR spectra of

both bromine isotopes when possible. These spectra span

several MHz due to second-order quadrupolar broadening.

Attempts to acquire these spectra at moderate magnetic ﬁelds

of 9.4 and 11.75 T were impractical due to the combined eﬀect

of the large breadth of the spectra and the bromide

concentration being quite low for ^79/81Br⁻, ranging from 23

mg/cm³(2C) to 64 mg/cm³(3G).

The ⁸¹Br quadrupolar coupling constants for this series of

halogen-bonded compounds vary from 21.5(0.1) MHz for 3G

to 58.0(0.2) MHz for 1D. These values are an order of

magnitude greater than the values for the pure bromide starting

materials: i.e., ammonium and phosphonium bromide salts.^45,81

Generally, less symmetric electronic environments will correlate

G

dx.doi.org/10.1021/ja5013239 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

Figure 4. ³⁵Cl solid-state NMR spectra of static powdered halogen-bonded compounds acquired at 21.1 T. Experimental spectra are shown in (a)

1E, (c) 2E, (e) 1B, (g) 2B, and (i) 1A, and their simulated spectra are in (b), (d), (f), (h), and (j), respectively. The residual Ph₄PCl in (c) is marked

by a red asterisk. There are two crystallographically distinct sites in compound 1E (a) and three crystallographically distinct sites in 2B (g)

(simulations shown with dashed lines). Shown in the insets are the experimental and simulated ³⁷Cl NMR spectra recorded at 21.1 T for (a) 1E and

(i) 1A.

a

Table 5. Experimental ³⁵Cl EFG and CS Tensor Parameters for Halogen-Bonded Compounds

|C_Q(³⁵Cl)|/MHz

η_Q

δ_iso/ppm

Ω/ppm

κ

α/deg

β/deg

γ/deg

1A

1B

2B

5.43(0.05)

1.00(0.01)

0.07(0.01)

117(3)

132(4)

94(4)

−0.3(0.2)

0.85(0.10)

0(5)

0(8)

2(2)

106(4)

45(20)

10.42(0.04)

180(15)

35(30)

b

site 1

site 2

site 3

4.57(0.20)

8.22(0.30)

7.15(0.05)

1.00(0.04)

0.12(0.04)

0.70(0.04)

112(10)

148(20)

135(20)

<60

c

1E

2E

site 1

site 2

3.66(0.10)

6.85(0.10)

6.77(0.20)

0.38(0.04)

0.12(0.02)

0.44(0.04)

177(8)

98(4)

60(15)

96(10)

−0.4(0.2)

0.0(0.2)

120(20)

0(10)

106(16)

90(10)

5(4)

35(10)

40(10)

94(10)

60(20)

1.00(0.04)

a

b

Error bounds are given in parentheses. See also footnotes b and c of Table 4. Three crystallographically distinct chlorine sites were determined

from the X-ray diﬀraction structure. The asymmetry parameter is characteristic of the halogen-bonding environment, and each site may be assigned

to the corresponding chloride. Details of the assignment are given in the main text. No CSA is incorporated in these ﬁts to avoid overinterpretation

c

of the NMR spectrum. Two crystallographically distinct chlorine sites are present in the crystal structure of 1E. The chloride in the pseudo-O_h

geometry was assigned as Cl1, as the C_Qvalue is the smaller value that is expected for a quadrupolar nucleus in a symmetric environment. The

second site has a larger C_Qvalue, in agreement with its square-planar geometry.

192(10) ppm. To improve the precision of the reported

quadrupolar and CS tensor parameters, ⁷⁹Br NMR spectra of

compounds 1D and 2C were also acquired and are shown in

the Supporting Information.

the nearby iodine, and its local electronic environment more

closely approximates that of an isolated Br⁻anion, for which

the value of C_Qis 0. The asymmetry parameters are similar,

with values of 0.26(0.01) and 0.32(0.01) for 1G and 3G,

respectively. In addition, the more weakly halogen bonded

complex 3G has a smaller bromine chemical shift (140(10)

ppm) and span (115(15) ppm) in comparison to 1G (δ_iso

210(10) ppm and Ω = 320(20) ppm), corroborating the trend

reported by Attrell et al.⁴²

Two dianionic bromide-containing species, 3G and 1G, are

isostructural and pack in the same space group, P2₁/n. Their

⁸¹Br NMR spectra are shown in Figure 3g,i. As stated above,

compound 3G exhibits a much weaker halogen bond in

comparsion to 1G. This diﬀerence is reﬂected in their

respective ⁸¹Br EFG and CSA parameters. For example, the

value of |C_Q(⁸¹Br)| for 3G (21.5(0.1) MHz) is half the value

observed for 1G (40.7(0.1) MHz). The smaller value of C_Qfor

3G may be qualitatively rationalized by considering that the

bromide anion in this compound is much further removed from

^35/37Cl SSNMR. Presented in Figure 4 are the ³⁵Cl NMR

spectra (21.1 T) of the halogen-bonded complexes in which the

halogen bond acceptor is a chloride anion. The spectra acquired

at 9.4 T for ³⁵Cl and ³⁷Cl are shown in Figures S15 and S16 in

the Supporting Information, respectively. The ³⁵Cl EFG and

H

dx.doi.org/10.1021/ja5013239 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society

Article

Figure 5. (a) Simpliﬁed molecular orbital diagram for a halogen bond between a C−I moiety and a chloride ion (after Pinter et al.⁸⁵). (b, c) NLMO

contributions to the largest principal component of the ³⁵Cl EFG tensor in a model halogen-bonded system (F₃C−I···Cl⁻···I−CF₃). In (b) the lone

pair orbital for chloride is represented for I···Cl⁻···I angles less than or equal to 110°, and the inset gives the representation of the less important core

−

orbital NLMO. In (c) is the chlorine lone pair orbital for θ_{I···Cl ···I}≥ 115°, and in the inset the core orbital is presented. The lone pair orbitals

dominate the resulting EFG.

CS tensor parameters are presented in Table 5 and were

obtained by simultaneously modeling the NMR spectra of both

chlorine isotopes at two magnetic ﬁelds.

environments where one chloride anion interacts with two

iodine atoms, thereby forming inﬁnite linear polymeric anionic

chains. 1A,B have very similar structures and only diﬀer by their

cation, n-Bu₄N⁺or n-Bu₄P⁺, respectively; they additionally pack

in the same space group, P2₁/c. This change in cation is

accompanied by a change in the angle surrounding the halide:

θ_{I···Cl¯···I}= 109.1° for 1A and 155.7° for 1B. This interesting

feature in their structures might explain the very diﬀerent line

shapes observed for these halogen-bonded compounds (see

Figure 4e,i), as will be demonstrated through an NLMO

analysis (vide infra). The |C_Q(³⁵Cl)| value for 1A (5.43(0.05)

MHz) is about half that for 1B (10.42(0.04) MHz). Even more

intriguing, the asymmetry parameters take on two limiting

values where in 1A two spectral discontinuities (“horns”)

collapse with an η_Qvalue equal to 1 (1.00(0.01)) and for 1B we

distinctly see the two horns where the asymmetry parameter

approaches 0 (0.07(0.01)).

Compound 2B exhibits three crystallographically distinct

chloride sites in the unit cell. They all have similar halogen

bonding environments, where one chloride anion interacts with

two halogen bond donors, forming a one-dimensional network

(Figure S5 in the Supporting Information). However, from the

³⁵Cl NMR spectrum (see Figure 4g), it is not directly obvious

how to assign the three powder patterns to the three

crystallographic sites (vide infra).

Discussion: NLMO Analysis of the Halide Quadrupolar

Coupling Tensors in the Context of the Experimental

NMR and X-ray Data. The origins of the quadrupolar

interaction at the halide nuclei as a result of the formation of a

halogen bond may be intuitively grasped by considering a

standard molecular orbital diagram (Figure 5a, after Pinter et

al.⁸⁵). Interaction of a halide lone pair (represented here by a p

orbital) with the σ* orbital of the carbon−iodine bond results

in a halogen bond. It is the polarization of this same p orbital

which gives rise to the quadrupolar interaction at the halide

nucleus. While this qualitative picture is useful, ZORA DFT

computations were carried out to provide a more theoretically

rigorous insight into the relationship between the local

environment of the halogen bond acceptor and the observed

NMR parameters. The calculations are used to analyze the

contributions of key molecular orbitals to the halide EFG

The ³⁵Cl quadrupolar coupling constants range from

3.66(0.10) MHz for 1E to 10.42(0.04) MHz for 1B. The

largest of these values is slightly greater than that observed

previously⁴²for more weakly halogen bonded compounds. The

chloride compounds in this study are moderately strongly

halogen bonded, with R_XBvalues ranging from 0.79 to 0.88.

The asymmetry parameters vary across the entire range of

possible values, from 0.07(0.01) for 1B to 1.00(0.01) for 1A.

Spectra of the halogen-bonded complexes were simulated with

CSA, where the eﬀect is more prominent at 21.1 T; however,

due to the small magnitudes of the anisotropies and small

overall range in their values, no clear correlation with any

structural element is noted. The chlorine isotropic chemical

shifts range from 98(4) to 177(8) ppm and the spans range

from 60(15) ppm for 1E to 180(15) ppm for 1B, which are

within the known range of values for chloride ions in

nonsymmetric environments.^43,49,82,83

The chlorine EFG tensor parameters may be related to the

chloride anion halogen-bonding environment and to the local

electronic structure (vide infra). First, consider the spectrum of

1E, where two crystallographically nonequivalent chlorine sites

are present, shown in Figure 4a. From the NMR line shape, the

two diﬀerent sites are clearly diﬀerentiated, as shown by the

spectral deconvolution. It is possible to resolve multiple sites

from a spectrum of quadrupolar nuclei, as the quadrupolar

coupling constant is dependent on the coordination environ-

ment. When the coordination environment surrounding the

nuclide is spherically symmetric, the C_Qvalue will be null and

any divergence from that symmetry will cause an increase in

C_Q.⁸⁴In 1E, chlorine site 1 (Cl1) is in a nearly octahedral

geometry according to the X-ray crystal structure. This site may

be assigned to the narrower ³⁵Cl NMR powder pattern with the

smaller |C_Q(³⁵Cl)| value of 3.66(0.10) MHz. The second

chlorine site is in a pseudo-square-planar geometry and may be

assigned to the broader ³⁵Cl line shape characterized by a

quadrupolar coupling constant of 6.85(0.10) MHz (this

assignment is supported by DFT calculations; vide infra).

Compounds 2B and 1A,B have similar halogen bonding

I

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tensors, through an NLMO analysis.⁵³⁻⁵⁵The theoretical basis

of this analysis has been described and applied by Autschbach

et al.⁵³One of this method’s advantages is that it provides an

interpretation of the NMR parameters in terms of orbitals

which are familiar to chemists (e.g., lone pairs, bonding orbitals,

and core functions). Here, a model composed of two molecules

of iodotriﬂuoromethane halogen-bonded with a chloride anion,

where the I···Cl⁻···I angle was varied between 85 and 180° in

5° increments, was employed. The calculated value of V₃₃was

chosen for more in-depth analysis, as it determines the value of

the chlorine quadrupolar coupling constant. The major NLMO

contributions to V₃₃(³⁵Cl) as core and lone-pair orbitals are

presented in the Supporting Information.

that the contributions from the sum of the chloride core

orbitals are minor (black line) and do not vary signiﬁcantly

between 80 and 180°. The same can be observed qualitatively

from a plot of these core orbitals (Figure 5). Conversely, the

contributions to V₃₃from the sum of the chloride ion lone pair

NLMOs are more important: the same change in sign is

observed between the angles 110 and 115° as for V₃₃(black

squares). Additional details as well as each contributing lone

pair orbital have been tabulated and plotted in the Supporting

Information. Examination of the lone pair NLMO which largely

determines the value of V₃₃in Figure 5b,c reveals that the main

contributing orbital changes at ∼115°. At 115° and for larger

angles, the principal axis of the orbital is aligned with the iodine

atom; this corresponds to the localized orbital which is

responsible for the halogen bond. To put this in the context

of more familiar orbital types, consider again the MO diagram

in Figure 5a, which depicts explicitly the chloride p orbital

involved in the halogen bond. This p orbital may be readily

identiﬁed with the p-type NLMO resulting from our DFT

calculations and shown in Figure 5c, thus providing an intuitive

understanding of the connection among the simple MO

diagram, the NLMO analysis, and the resulting NMR

parameters.

Plotted in Figure 6a are the calculated ³⁵Cl principal

components of the EFG tensor as a function of the I···Cl⁻···I

Additionally, when the I···Cl⁻···I angle approaches linearity

there is an increase in the contribution from the sum of the

chloride lone pair orbitals to V₃₃, since there is a greater degree

of halogen bonding which further distorts this “p” orbital.

Hence, we propose that the main contribution from the

chloride lone pair orbital after 115°, which dictates the value of

the chlorine quadrupolar coupling constant, may also be

visualized as participating in electron donation to the σ-hole

characteristic of the halogen bond. In the limit of an idealized

model system, this interpretation of the value of C_Qcan thus be

taken as a measure of the halogen bonding interaction.

To validate the above hypothesis, the same NLMO analysis

of V₃₃was conducted for each of the halogen-bonded

complexes we studied experimentally (1C,D and 2C for

bromide and 1A,B and 2B,E for chloride; see Table 6).

When the halogen-bonded environment is composed of two

XB donors (iodine) interacting with a halide (I···X⁻···I motif),

the same contributions are observed from the core and lone

pair orbitals for the halide as were observed for the model

system discussed above. Examination of the results plotted in

Figure 6b shows that the calculated contributions for the

halogen-bonded compounds follow the same trend for chloride

(red) and bromide (black outline; the values are larger for this

nuclide) and also follow the general trend established for the

iodotriﬂuoromethane model. A change in sign is observed

around 110−115° from the contributing lone pair orbitals

(squares) to V₃₃, and a linear trend is observed for the

contribution of the core orbital (diamonds) to V_33.The two

dianionic species 1G and 3G have additional minor

contributions from iodine lone pair orbitals to V₃₃. Compound

1E has a diﬀerent geometry. Its main contribution to V₃₃is

from the chloride core and lone pair orbitals. The octahedral

site in 1E also has a very minor contribution from the iodine

lone pair.

Figure 6. (a) Plot of the calculated EFG tensor components (where

V₃₃are the red triangles, V₂₂are the green squares, and V₁₁are the blue

diamonds, with |V₃₃| ≥ |V₂₂| ≥ |V₁₁|) for ³⁵Cl in a F₃C−I···Cl⁻···I−CF₃

model system where the I···Cl⁻···I angle is varied. (b) Plot of the

NLMO contributions to the largest principal component V₃₃as a

function of θ_{I···Cl¯···I}, where the solid black squares represent the

contributions from the chlorine lone pair orbitals and the black line

represents the contributions from the chlorine core orbitals (see the

Supporting Information). Overlaid on this plot are the values of the

calculated contributions for the experimental halogen-bonded

complexes (values from Table 6). In red are data for the chloride

compounds (squares, lone pair NLMOs; diamonds, core NLMOs),

showing excellent agreement with the data for the model system. The

corresponding data for the bromide compounds are shown as empty

squares and diamonds; as expected, the values are larger than those for

chloride due to diﬀerences in geometries, unit cell volumes, and

Sternheimer antishielding factors.

angle. A change in sign of the principal components V₁₁and V₃₃

is evident for angles exceeding the tetrahedral angle of about

110°. From the calculations it is possible to determine the

orientation of the EFG tensor, which has been plotted in Figure

S18 of the Supporting Information for three I···Cl⁻···I angles of

interest: 110, 115, and 180°. At θ_{I···Cl¯···I}= 110°, the eigenvector

corresponding to V₃₃lies in the I···Cl⁻···I plane and bisects the

θ_{I···Cl¯···I}angle. Around 115°, the eigenvector reorients to lie

close to the I···Cl⁻internuclear vector, and ﬁnally at 180° it is

completely aligned with the I···Cl⁻···I axis. When the NLMO

contributions to V₃₃are examined, two main types of MOs

centered on the chloride anion dominate: core orbitals and lone

pair orbitals. From the plot shown in Figure 6b, one can see

Finally, similar plots were prepared for the experimental

values of V₃₃as a function of the I···X⁻···I angle determined

from X-ray crystallography for the halogen-bonded complexes

in this study (Figure 7b). Satisfyingly, when the I···X⁻···I angle

increases, the largest principal component of the halide EFG

tensor also increases. This is in agreement with the computa-

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are equal due to symmetry, resulting again in an asymmetry

parameter of 0. For all intermediate angles, the values of V₁₁

and V₂₂will not be equal due to the lower symmetry of the

molecule, and the resulting asymmetry parameter will be

nonzero. The asymmetry parameter reaches a maximum value

of unity when the I···X⁻···I angle is near the tetrahedral angle

and the EFG tensor component V₂₂lies close to one of the

iodine−chloride internuclear vectors (Figure S20).

Once this dependence on the local structure surrounding the

halide was understood, it was possible to successfully simulate

the ³⁵Cl NMR spectrum for [(n-Bu₄PCl)(o-DITFB)] (2B),

where three distinct chloride ion sites are present. The I···Cl⁻···

I angles allowed for a good prediction of the asymmetry

Table 6. Analysis of Calculated Chlorine and Bromine EFG

Tensor NLMO Contributions to the Largest Principal

Component (V₃₃) in Halogen-Bonded Compounds

core

lone pair

(X⁻)

lone pair

a

b

compd

(X⁻)

(I)

sum

V₃₃

1D

1C

2C

1G

3G

1A

1B

2E

1E

0.271

0.277

2.383

2.381

2.654

2.658

2.456

2.452

−0.165

0.247

−1.567

2.127

−1.732

2.268

−1.581

2.165

−0.106

−0.074

0.116

0.876

0.918

0.840

−0.070

0.128

−0.809

1.301

−0.879

1.429

−0.831

1.337

0.067

0.757

−0.035

−0.042

0.789

0.764

parameters for the three sites. For chloride site 1 θ_{I···Cl¯···I}

=

112.0° and η_Q= 0.94(0.02), whereas the second chloride site

has a much larger angle (θ_{I···Cl¯···I}= 144.5°) and the value of η_Q

is 0.14(0.04). The third chloride site has an I···Cl⁻···I angle

site 1

0.029

0.262

0.249

0.245

site 2 −0.098

−0.999

−1.097

−0.968

2B

between those for the two other sites (123.4°,) and η_Q

=

site 1 −0.068

−0.832

1.215

−0.900

1.323

−0.848

1.246

0.72(0.10). This is a satisfying example of how the halogen-

bonding environment surrounding the halide is manifested in

the NMR parameters and highlights the advantages of a

combined experimental−theoretical approach to evaluate the

NMR parameters when there are multiple nonequivalent sites

in the sample of interest, where site assignment may be

ambiguous. In addition to the fundamental insights provided by

this analysis, value also lies in the extension of the spectroscopic

and computational methods employed herein to characterize

halogen-bonded complexes where diﬀraction methods may not

be amenable.

Given the clear-cut dependence of the halide quadrupolar

asymmetry parameter on the I···Cl⁻···I angle in halogen-

bonded systems, it is of interest to compare this result to similar

ﬁndings available for hydrogen bonds. The ²H asymmetry

parameters for deuterons in hydrogen bonds are known to

deviate somewhat from 0 for nonlinear geometries,⁸⁶but the

dependence on structure can be complicated.⁸⁷However,

deuterons are electron acceptors in hydrogen bonds, whereas

halide ions in halogen bonds are electron donors. A more

apposite discussion is then of ¹⁷O quadrupolar asymmetry

parameters in hydrogen bonds of the form −N−H···OC,

where the oxygen acts as the electron donor. Wu and co-

workers have demonstrated that η_Q(¹⁷O) decreases as r_N···O

increases in hydrogen-bonded amides.⁸⁸They have also shown

that η_Q(¹⁷O) correlates with the hydrogen bond strength in a

series of nucleic acid bases.⁸⁹The full bell-shaped dependence

noted presently for the halide quadrupolar asymmetry

parameter is not seen for typical hydrogen bonds, likely simply

site 2

site 3

0.108

0.079

0.882

0.961

0.898

a

Blank lines indicate that contributions are less than 5% of the sum.

Sum of “core” and “sum(LP)”. Values obtained in this way diﬀer from

b

the total calculated V₃₃(ﬁnal column) because not all of the

contributions are printed.

tional study discussed above. The calculated asymmetry

parameters are plotted as a function of the I···X⁻···I angle

extracted from the NLMO EFG tensor study in Figure 7a. The

plot forms a bell shape over the range 80−180°. Overlaid on

the same plot are the experimental values of the asymmetry

parameters of the halogen-bonded complexes as a function of

their experimentally determined I···X⁻···I angles. Data for the

chloride and bromide compounds both follow the predicted

trend. From this we conclude that the experimentally observed

asymmetry parameters are largely determined by the I···X⁻···I

angles, as suggested by ZORA DFT. A simple picture

explaining the reasons behind this may be gleaned from a

reinspection of the data in Figure 6 and Figure S20 (Supporting

Information). When the I···X⁻···I angle is 0°, the asymmetry

parameter is 0 due to the cylindrical I···X⁻···I symmetry, with

the largest, unique EFG tensor component (V₃₃) oriented along

the symmetry axis of the two halogen bonds. The chloride p

orbital is oriented along this same axis and contributes to the

electric ﬁeld gradient in that direction. Similarly, when the I···

X⁻···I angle is 90° (the other limiting case), the V₃₃component

lies perpendicular to the plane containing those three atoms,

and the remaining components lying in the plane (V₁₁and V₂₂)

Figure 7. (a) Plot of quadrupolar asymmetry parameter versus I···Cl⁻···I angle. The black diamonds represent the calculated values for ³⁵Cl in a

F₃C−I···Cl⁻···I−CF₃model system. The red triangles and blue circles represent the experimental values for ³⁵Cl and ⁸¹Br, respectively. (b) Plot of

experimentally determined values of the largest component of the EFG tensor for ³⁵Cl (red triangles) and ⁸¹Br (blue circles) versus the value of the

I···X⁻···I angle for compounds where two iodines interact with one halide anion. Solid lines show the best linear ﬁt; V₃₃(⁸¹Br) = 0.0073(θ_{I···Br¯···I}) −

0.0084 and V₃₃(³⁵Cl) = 0.0056(θ_{I···Cl¯···I}) − 0.3383.

K

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due to constraints on possible hydrogen bond geometries

relative to what is seen for halogen bonds. Interestingly,

however, Smith and co-workers reported⁹⁰that computed ¹⁷O

EFG tensor orientations change with the strength of the

hydrogen bond, analogously to what we have noted for halogen

bonds (Figure S20, Supporting Information).

Additional computations resulted in the NMR parameters for

¹³C, ^35/37Cl, and ^79/81Br for the compounds studied

experimentally (see the Supporting Information for values).

The models used consist of the simple halogen-bonding

environment surrounding the halide (see Figure 1). The ¹³C

chemical shifts are overestimated by the calculations, consistent

with our previous report on p-diiodotetraﬂuorobenzene

complexes.³⁹Nevertheless, in cases where distinct carbon

resonances were observed experimentally, assignments to

particular crystallographic sites were possible on the basis of

diﬀerences in the carbon−iodine bond lengths. The calculations

similarly overestimate the halide quadrupolar coupling

parameters and show correlations of varying quality (Figure

8). Multiple reasons may explain this inability to reproduce the

for the present work many of the unit cells are too large for us

to carry out such computations. Furthermore, we were

interested in the NLMO analysis oﬀered with the ZORA

DFT method. The inclusion of relativistic eﬀects via ZORA is

important for computing the ¹³C magnetic shielding tensors for

carbon atoms bonded to iodine. Our calculations are carried

out at 0 K, while the X-ray and NMR data are acquired at

higher temperatures. This may also contribute to the

discrepancies between the experimental and calculated values

shown in Figure 8.

Since there is no single generally available computational

approach at present which will simultaneously properly account

for all of the aforementioned issues, compromises were made in

choosing the best approach to address the problem at hand.

The priorities for this study ((i) the ability to carry out an

NLMO analysis to gain insight into the relationship between

the halogen bond and the NMR observables and (ii) a proper

consideration of relativistic eﬀects on the NMR parameters)

meant that the ZORA DFT method oﬀered the best

compromise.

CONCLUSIONS

■

A multifaceted study of halogen bonds has been presented.

New halogen-bonded compounds without competing hydrogen

bonds were prepared and characterized with X-ray crystallog-

raphy and multinuclear magnetic resonance spectroscopy (¹³C,

^35/37Cl, ^79/81Br). These iodobenzene-onium halide cocrystals

exhibit C−I···X⁻distances and angles which allow them to be

classiﬁed as moderately strongly halogen bonded compounds,

as quantiﬁed by their reduced distance parameters, R_XB,of

0.79−0.91. The main conclusions of this study are as follows.

(1) From ¹³C CP/MAS SSNMR, we observe an increase in

the chemical shift of the ipso C−I carbon for the halogen o-

diiodotetraﬂuorobenzene-bonded complexes in comparison to

the starting materials; ¹³C chemical shifts are therefore

diagnostic of the strength of the halogen bond.

(2) From halogen SSNMR studies of a larger series of

cocrystals, the NMR parameters of chloride and bromide

anions were correlated to the local halogen-bonding environ-

ment. For example, octahedral and square-planar chloride ion

coordination environments are clearly diﬀerentiated on the

basis of their ³⁵Cl nuclear quadrupolar coupling constants. For

C−I···X⁻···I−C systems, the value of the halogen quadrupolar

coupling constant correlates with the strength of the halogen

bond.

(3) A natural localized molecular orbital DFT study aﬀorded

further chemical insight into the relationship among the

quadrupolar NMR parameters of the halide ions, their local

halogen bond environment, and an intuitive molecular orbital

picture. The value of the quadrupolar coupling constant and the

quadrupolar asymmetry parameter were found to be well-

correlated with the I···X⁻···I angle. Furthermore, this depend-

ence could be explained, via both an NLMO analysis and a

simpler MO picture, as arising largely from the contribution of

a lone pair type orbital centered on the halide ion. The

experimental asymmetry parameters follow the DFT trend and

are highly correlated to the I···X⁻···I angle.

Figure 8. Plots of calculated versus experimental EFG tensors for

bromine (a, b) and chlorine (c). The principal components V₃₃, V₂₂,

and V₁₁are represented by the red triangles, green squares, and blue

diamonds, respectively. Solid lines represent the best linear ﬁt: (a)

C_Q(calcd) = 2.4446[C_Q(exptl)] + 13.423, R²= 0.8933; (b) V₁₁(calcd)

= 1.4926[V₁₁(exptl)] − 0.1952, R²= 2.2722, V₂₂(calcd) = 2.413-

[V₂₂(exptl)] − 0.0548, R²= 0.9765, V₃₃(calcd) = 2.4446[V₃₃(exptl)] −

0.2185, R²= 0.8933; (c) V₁₁(calcd) = 2.1957[V₁₁(exptl)] − 0.0235, R²

= 0.8826, V₂₂(calcd) = 2.7712[V₂₂(exptl)]− 0.041, R²= 0.8826,

V₃₃(calcd) = 2.5907[V₃₃(exptl)] − 0.0148, R²= 0.7080.

experimental values precisely. For one, DFT calculations are

performed in the gas phase and use a simple cluster model,

whereas the compounds of interest crystallize in the solid state.

A full treatment of the eﬀect of the ions in the crystal lattice on

the computed NMR parameters is possible with the gauge-

including projector-augmented wave (GIPAW) DFT method⁹¹

for calculations using periodic boundary conditions; however,

(4) The correlations noted between the halide quadrupolar

coupling parameters and the local halogen bond geometry are

in qualitative agreement with the limited corresponding data

available for the comparable electron donors in hydrogen bonds

(i.e., ¹⁷O quadrupolar parameters for oxygen); however, the

data for halogen bonds cover a broader range of geometries.

L

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Journal of the American Chemical Society

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Overall, this work contributes to a chemically intuitive

understanding of the connection between the geometry and

electronic structure of halogen bonds and various NMR

parameters with the aid of NLMO analysis. The experimental

and computational methods employed here, as well as the

insights gained, set the stage and hold promise for studying

more complex halogen bond environments, such as those

found in biomolecules,⁹²gels,⁹receptors,²¹MOF-like frame-

work materials,¹¹and other systems where diﬀraction methods

may provide an incomplete picture.

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M. J. Am. Chem. Soc. 2013,

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Stihle, M.; Gsell, B.; Thoma, R.; Diez, J.; Benz, J.; Plancher, J.−M.;

Hartmann, G.; Banner, D. W.; Haap, W.; Diederich, F. Angew. Chem.,

Int. Ed. 2011, 50, 314−318.

ASSOCIATED CONTENT

* Supporting Information

■

S

Text, ﬁgures, tables, and CIF ﬁles giving additional

experimental details, crystallographic data for 1E, 2B,C,E, 3G,

and o-DITFB, powder X-ray diﬀractograms, elemental analyses,

images of the packing diagrams for the halogen-bonded

compounds, full ¹³C SSNMR spectra and assignments,

NLMO analysis of XB compounds and ³⁵Cl EFG tensor

orientations, additional halogen NMR spectra, and calculated

NMR tensors. This material is available free of charge via the

Internet at http://pubs.acs.org.

(18) Lu, Y.; Shi, T.; Wang, Y.; Yang, H.; Yan, X.; Luo, X.; Jiang, H.;

Zhu, W. J. Med. Chem. 2009, 52, 2854−2862.

(19) (a) Wilcken, R.; Zimmermann, M. O.; Lange, A.; Joerger, A. C.;

Boeckler, F. M. J. Med. Chem. 2013, 56, 1363−1398. (b) Wilcken, R.;

Liu, X.; Zimmermann, M. O.; Rutherford, T. J.; Fersht, A. R.; Joerger,

A. C.; Boeckler, F. M. J. Am. Chem. Soc. 2012, 134, 6810−6818.

(c) Lemke, C. T.; Goudreau, N.; Zhao, S.; Hucke, O.; Thibeault, D.;

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Llinas-Brunet, M.; White, P. W. J. Biol. Chem. 2011, 286, 11434−

AUTHOR INFORMATION

11443.

■

(20) Kilah, N. L.; Wise, M. D.; Serpell, C. J.; Thompson, A. L.;

White, N. G.; Christensen, K. E.; Beer, P. D. J. Am. Chem. Soc. 2010,

132, 11893−11895.

Corresponding Author

*D.L.B.: tel, +1 613 562 5800 ext. 2018; fax, +1 613 562 5170;

e-mail, dbryce@uottawa.ca.

(21) Chudzinski, M. G.; McClary, C. A.; Taylor, M. S. J. Am. Chem.

Soc. 2011, 133, 10559−10567.

Notes

The authors declare no competing ﬁnancial interest.

(22) Zapata, F.; Caballero, A.; White, N. G.; Claridge, T. D. W.;

Costa, P. J.; Fel

11541.

(23) Gilday, L. C.; Lang, T.; Caballero, A.; Costa, P. J.; Fel

Beer, P. D. Angew. Chem., Int. Ed. 2013, 52, 4356−4360.

(24) Jentzsch, A. V.; Emery, D.; Mareda, J.; Nayak, S. K.; Metrangolo,

P.; Resnati, G.; Sakai, N.; Matile, S. Nat. Commun. 2012, 3:905, 1−8.

(25) Walter, S. M.; Kniep, F.; Herdtweck, E.; Huber, S. M. Angew.

Chem., Int. Ed. 2011, 50, 7187−7191.

(26) Desiraju, G. R.; Ho, P. S.; Kloo, L.; Legon, A. C.; Marquardt, R.;

Metrangolo, P.; Politzer, P. A.; Resnati, G.; Rissanen, K. Pure Appl.

Chem. 2013, 85, 1711−1713.

́

ix, V.; Beer, P. D. J. Am. Chem. Soc. 2012, 134, 11533−

ACKNOWLEDGMENTS

■

́

ix, V.;

D.L.B. thanks the Natural Sciences and Engineering Research

Council (NSERC) of Canada for funding and J.V.-G. thanks

the Fonds de Recherche du Quebec-Nature et Technologies

́

(FRQNT) for a scholarship. Dr. Victor Terskikh, Dr. Eric Ye,

and Dr. Glenn Facey are thanked for technical support. Access

to the 900 MHz NMR spectrometer was provided by the

National Ultrahigh-Field NMR Facility for Solids (Ottawa,

Canada), a national research facility funded by the Canada

Foundation for Innovation, the Ontario Innovation Trust,

(27) Politzer, P.; Murray, J. S. ChemPhysChem. 2013, 14, 278−294.

(28) Clark, T.; Hennemann, M.; Murray, J. S.; Politzer, P. J. Mol.

Model. 2007, 13, 291−296.

́

Recherche Quebec, the National Research Council Canada, and

Bruker BioSpin and is managed by the University of Ottawa

(www.nmr900.ca). NSERC is acknowledged for a Major

Resources Support grant.

(29) Fourmigue,

45.

́

M. Curr. Opin. Solid State Mater. Sci. 2009, 13, 36−

(30) Mooibroek, T. J.; Gamez, P. CrystEngComm 2013, 15, 4565−

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Direct investigation of halogen bonds by solid-state multinuclear magnetic resonance spectroscopy and molecular orbital analysis

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