Structural and Conformational Analysis of 1-Butyl-3-methylimidazolium Nitrate

Article abstract of DOI:10.1007/s10870-021-00899-w

The crystal structures of two polymorphic forms of 1-butyl-3-methylimidazolium nitrate are reported. The form observed at 100 K and 200 K (Ia and Ib) crystallizes in the P1 ˉ space group and contains two independent imidazolium cations and nitrate anions. At 100 K the butyl chain of one cation adopts a TT (trans–trans) conformation and the other cation adopts a G′G′ (gauche–gauche) conformation. At 200 K, the G′G′ chain is disordered with about twelve percent of the GT conformation present. A different polymorph (Form II, also crystallizing in the P1 ˉ space group with two independent ion pairs) is present at 273 K displaying significant disorder in the butyl chains with a mixture of G′T, G′G′, and GT conformers. Raman spectra were collected on samples between 100 and 350 K and show changes in band frequencies and intensities consistent with conversion between different butyl chain conformations. Hydrogen bond interactions are present between cation C–H’s and oxygen atoms of the nitrate ions, with significant lengthening observed for three of the six close contacts (and formation of one new contact) upon conversion to the higher-temperature form. The structural details revealed in this study shed light on the intermolecular forces and the conformational changes that accompany phase changes in 1-butyl-3-methylimidazolium nitrate. Graphic Abstract: X-ray diffraction and Raman spectroscopy of the ionic liquid 1-butyl-3-methylimidazolium nitrate at temperatures from 100 to 300 K showevidence of two different polymorphs and significant temperature-dependentconformational changes.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10870-021-00899-w

Journal of Chemical Crystallography
https://doi.org/10.1007/s10870-021-00899-w
ORIGINAL PAPER
Structural and Conformational Analysis
of 1‑Butyl‑3‑methylimidazolium Nitrate
Dean H. Johnston1 · Mallory Gasbarre1 · Carolanne E. Norris1
Received: 29 April 2021 / Accepted: 3 August 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
The crystal structures of two polymorphic forms of 1-butyl-3-methylimidazolium nitrate are reported. The form observed at
̄
100 K and 200 K (Ia and Ib) crystallizes in the P1space group and contains two independent imidazolium cations and nitrate
anions. At 100 K the butyl chain of one cation adopts a TT (trans–trans) conformation and the other cation adopts a G′G′
(gauche–gauche) conformation. At 200 K, the G′G′ chain is disordered with about twelve percent of the GT conformation
̄
present. A diferent polymorph (Form II, also crystallizing in the P1space group with two independent ion pairs) is present
at 273 K displaying signifcant disorder in the butyl chains with a mixture of G′T, G′G′, and GT conformers. Raman spectra
were collected on samples between 100 and 350 K and show changes in band frequencies and intensities consistent with
conversion between diferent butyl chain conformations. Hydrogen bond interactions are present between cation C–H’s and
oxygen atoms of the nitrate ions, with signifcant lengthening observed for three of the six close contacts (and formation of
one new contact) upon conversion to the higher-temperature form. The structural details revealed in this study shed light on
the intermolecular forces and the conformational changes that accompany phase changes in 1-butyl-3-methylimidazolium
nitrate.
Graphic Abstract
X-ray difraction and Raman spectroscopy of the ionic liquid 1-butyl-3-methylimidazolium nitrate at temperatures from 100
to 300 K showevidence of two diferent polymorphs and signifcant temperature-dependentconformational changes.
N
N+
–
273 K
T = 273 K
NO3
phase transition
100 K
T = 100K
1000
800
600
400
200
Raman shift (cm-1)
Keywords Ionic liquids · Raman spectroscopy · Conformational isomerism · Crystal polymorphism
Extended author information available on the last page of the article
Vol.:(0123456789)
1 3

Journal of Chemical Crystallography
Raman spectroscopy has been an important tool for the
identifcation of polymorphic forms of [C4mim]+ salts, tak-
ing advantage of distinctive bands observed for the vibra-
tions of the cation. In 2003, Hamaguchi et al. [15, 28]
identifed Raman bands unique to specifc polymorphs and
crystal forms of the chloride and bromide salts assignable
to conformational isomers of the 1-butyl-3-methylimida-
zolium cation. This work was quickly followed by Raman
spectroscopy and ab initio calculations of the [C4mim][PF6]
ionic liquid [34, 35]. Additional studies have examined the
vibrational spectroscopy and conformational isomerism
in the tetrafuoroborate salt [36, 37]. More recent studies
have combined detailed computational work with spectro-
scopic, thermal and X-ray characterization [18, 29, 32]. All
of this work has been summarized in several comprehen-
sive reviews [38–41] and demonstrates how valuable Raman
spectroscopy can be in identifying conformers present in
both crystalline and liquid samples of alkylimidazolium-
based ionic liquids, including a recent analysis of water
confnement in [C4mim][NO3] [42].
Introduction
Ionic liquids have widespread applications in chemical
synthesis, catalysis and electrochemistry due to their high
thermal and electrochemical stability, large liquid range,
low vapor pressure and ability to solubilize a variety of
materials [1–3]. A major challenge in the study and appli-
cation of ionic liquids is understanding their structures
and how intermolecular forces determine their properties.
The wide range of molecular interactions present in ionic
liquids [4], from weak dispersion forces to strong Coulom-
bic interactions and hydrogen bonds, further complicates
this analysis.
Crystallography is a powerful tool for analyzing the
structures of ionic liquids and for determining the pres-
ence and infuence of diferent intermolecular forces [5,
6]. Approaches such as in situ crystallization [7] and
high-pressure studies [8] have helped overcome the chal-
lenges in isolating single-crystal samples of ionic liq-
uids. The structures of at least eighteen metal-free salts
of the 1-butyl-3-methylimidazolium ([C4mim]+) cation
are known, including fuoride [9, 10], chloride [11–14],
bromide [11, 15, 16], hexfuorophosphate [7, 17, 18],
several tetraphenyl or tetraalkylborates [19–23], tosylate
[16], bis(trifuoromethanesulfonyl)imide [24], trifate [7]
and others [25–27]. Both the chloride and hexafuorophos-
phate salts exhibit polymorphism [11, 18, 28, 29] leading
to extensive studies of the origins and the infuence of
intermolecular interactions on the nature of the observed
polymorphic forms.
The torsion angles of interest correspond to
N−Cꢀ−C −Cꢂ and Cꢀ−Cꢁ−Cꢂ −C (see Fig. 1) for the butyl
groups ofꢁeach imidazolium cationꢃ. The lowest-energy con-
formations are trans (T,180◦) and gauche (G, 60◦), with the
added possibility of positive or negative torsion angles for
the gauche confguration, annotated here as G and G′ [18].
These possibilities generate at least nine conformers, some
of which can be distinguished by Raman spectroscopy [41]
and all of which have been extensively modeled via compu-
tational methods [18, 32].
In this study we report the frst single crystal X-ray struc-
tures of two crystal forms of 1-butyl-3-methylimidazolium
nitrate including an analysis of the hydrogen bonding net-
work formed in the solid state. We also present Raman
data collected at temperatures ranging from 100 to 350 K,
spanning both the solid and liquid forms, and correlate the
spectral features with the conformers identifed from the
crystallographic data. Through these data we confrm the
existence of two diferent polymorphs of [C4mim][NO3],
providing insight into the importance of hydrogen bonding
interactions, the relative stability of the two structures, and
the transition from solid to liquid.
While the[C4mim][NO3]ionic liquid has not been struc-
turally characterized by single-crystal X-ray methods, sev-
eral studies have examined the properties of the 1-alkyl-
3-methylimidazolium nitrate ([Cnmim][NO3]) family of
ionic liquids. Molecular dynamics simulations have been
used to model the phase transitions of the [Cnmim][NO3]
(n = 4–12) ionic liquids, identifying the formation of a
metastable state prior to the melting of the ionic liquid
[30]. Strechan and coworkers published a detailed analysis
of the thermal properties of 1-butyl-3-methylimidazolium
nitrate [31], demonstrating the existence of four diferent
crystal forms from the three solid-phase phase transitions
observed between 278 and 310 K. They speculate that the
phase transitions in [C4mim][NO3] result from an order-
disorder transformation of the nitrate ions, but conforma-
tional rearrangements of the butyl sidechain of the cation
likely also play a signifcant role [32]. Abe et al. [33] used
simultaneous diferential scanning calorimetry (DSC) and
powder X-ray difraction measurements to identify two of
the crystal phases that form during the heating of cold-
crystallized neat [C4mim][NO3], but only reported unit cell
parameters and possible space groups.
Experimental
Synthesis and Crystallization
All reagents, 1-methylimidazole (ACROS Organics, 99%),
1-bromobutane (ACROS Organics, 99%) and silver nitrate
(Fisher ACS) were purchased from Fisher Scientifc and
used as received.
1 3

Journal of Chemical Crystallography
The 1-butyl-3-methylimidazolium bromide and 1-butyl-
3-methylimidazolium nitrate were prepared using a modi-
fed version of the procedure described by Gruzdev et al.
[43] and outlined in Scheme 1. A quantity of 10.17 g of
1-methylimidazole was placed in a 150-mL round bottom
fask to which was added 16.69 g of 1-bromobutane. The
fask was capped and the mixture was allowed to stir at room
temperature for 1 week producing a viscous honey-colored
liquid. The product was placed under vacuum to remove any
potentially unreacted 1-bromobutane.
collect data at 283 K indexed to the same unit cell found at
273 K, but did not give data of sufcient quality to produce
a structure solution.
Cell refnement and data integration were performed
using SAINT [45] with a wide-frame algorithm and SAD-
ABS [45] was used for absorption correction.
X‑Ray Difraction: Structure Solution, Refnement,
and Treatment of Disorder
The 1-butyl-3-methylimidazolium bromide obtained
in the frst step was used without further purifcation and
16.67 g was placed in an Erlenmeyer fask along with 30
mL of deionized water. A sample of 12.70 g of silver nitrate
was dissolved in 20 mL of deionized water and added drop-
wise to the 1-butyl-3-methylimidazolium bromide solu-
tion. The solution was stirred in the dark for 1 day after
which the silver bromide precipitate was fltered of and the
water was removed using a rotary evaporator. The 1-butyl-
3-methylimidazolium nitrate product was obtained as a light-
Structures were solved using SHELXT [46] and refned
using SHELXL [47] using the Olex2 GUI [48]. All
hydrogen atoms were located in the diference map and
placed in idealized positions using a riding model, with
Uiso( H) = 1.2Ueq( C) (ring or CH2) orUiso(H) = 1.5Ueq( C)
(CH3). Potential C−H⋯O contacts were identifed using
the HTAB command in Olex2 [48] with a minimum angle
of 120◦ and maximum donor-acceptor distance of 3.34 Å.
Molecular graphics were prepared using CrystalMaker [49].
̄
Structure Ia The structure at 100 K crystallized in the P1
1
yellow viscous oil. H NMR (300 MHz DMSO): ꢀ = 0.84
space group and was fully ordered.
(t, 3H, CH2CH3, JHH = 7.35 Hz), 1.22 (m, 2H, CH2CH3,
JHH = 7.62 Hz), 1.75 (m, 2H, CH2CH2CH3, JHH = 7.26 Hz),
3.85 (s, 3H, NCH3), 4.15 (t, 2H, JHH = 7.26 Hz), 7.62 [s, 1H,
H5 (Im)], 7.70 [s, 1H, H4 (Im)], 9.00 [s, 1H, H2 (Im)]. Stor-
age of the product in an air-tight Schlenk fask for several
months resulted in formation of a solid crystalline product.
Structure Ib The structure at 200 K crystallized in
̄
the P1 space group and showed disorder in the C12, C13
and C14 atoms of the butyl chain of one [C4mim]+ cation
[0.876(4):0.124(4)]. All 1–2 and 1–3 distances in the butyl
chains were restrained to be the same (SADI) for the two
cations using default tolerances. Rigid bond restraints
(RIGU) were applied to all carbon atoms of the disordered
butyl chain. The hydrogen atoms on the methyl group of one
cation (C16) were modeled as rotationally disordered (AFIX
123), resulting in a refned occupancy of [0.61(2):0.39(2)]
for the two positions.
X‑Ray Difraction: Sample Preparation, Data
Collection, and Data Processing
Due to the tendency for samples to melt during inspection
and mounting, crystals were prepared on a cooled micro-
scope slide using a ꢀCHILL [44] apparatus connected to an
Oxford DTC Desktop Cooler.
Structure II The structure at 273 K also crystallized in
̄
the P1space group and showed disorder for the butyl chains
of both imidazolium cations. The disorder of the two chains
was originally modeled independently, but after refnement
showed identical occupancies for the major component of
both chains, a single free variable was assigned to model
both disordered chains, refning to [0.569(6):0.431(6)]. All
1–2 and 1–3 distances in the butyl chains were restrained
to be the same (SADI) for the two cations using default tol-
erances. Rigid bond restraints (RIGU) were applied to all
carbon atoms of the disordered butyl chains.
Diffraction data was collected using a Bruker AXS
SMART APEX II CCD difractometer equipped with Mo
Kꢀ sealed-tube radiation (ꢀ = 0.71073 Å) and an Oxford
Cryostream 700 low temperature device. Data were col-
◦
lected using a wide frame (2.0 ) strategy (APEX3 [45]) for
all structures.
Data sets were collected at three diferent temperatures,
100 K, 200 K and 273 K. Multiple samples were analyzed at
200 K, ensuring that the same unit cell and structural param-
eters were obtained for samples that were fash-cooled to
200 K versus samples that were initially analyzed at higher
temperature and then cooled to 200 K. Several attempts to
Crystal data, data collection, and structure refnement
details are summarized in Table 1. Crystallographic data
has been deposited with the Cambridge Crystallographic
Data Centre as CCDC 2010507 (Ia) CCDC 2010508 (Ib)
+
+
AgNO
25 °C
7 d
3
N
N
N
N
N
N
Br
AgBr
(s)
+
+
D.I. HO
Br–
NO3–
2
Scheme 1 Preparation of 1-butyl-3-methylimidazolium nitrate
1 3

Journal of Chemical Crystallography
Table 1 Summary of
crystallographic data collection
and refnement
(Ia)
(Ib)
(II)
Chemical formula
Temperature (K)
Space group
a (Å)
C8H15N2⋅NO3
C8H15N2⋅NO3
C8H15N2⋅NO3
100
̄
P1
200
̄
P1
273
̄
P1
8.0411 (5)
11.3114 (7)
11.5416 (7)
82.364 (1)
84.123 (1)
86.510 (1)
1033.81 (11)
4
8.1074 (7)
11.4318 (10)
11.5759 (10)
81.866 (2)
84.952 (2)
86.620 (2)
1056.79 (16)
4
7.9872 (7)
11.7615 (10)
11.8119 (10)
88.523 (2)
80.462 (2)
87.778 (2)
1093.25 (16)
4
b (Å)
c (Å)
◦
ꢀ ( )
◦
ꢀ ( )
◦
ꢀ ( )
V (Å )
3
Z
−1
0.10
0.10
0.09
ꢀ (mm )
Crystal size (mm)
Tmin, Tmax
0.46 × 0.35 × 0.23
0.725, 0.746
27633
4756
3917
257
0
0.037
0.56 × 0.52 × 0.50
0.594, 0.745
22995
3746
3035
286
48
0.038
0.56 × 0.52 × 0.50
0.705, 0.745
25189
3869
3028
314
126
0.026
Measured refections
Unique refections
Obs. refections [I > 2ꢀ(I)]
Number of parameters
Number of restraints
Rint
GOF
R[I > 2ꢀ(I)]
1.03
0.035
0.085
0.23, − 0.23
1.07
0.039
0.104
0.20, − 0.19
1.03
0.055
0.166
0.21, − 0.17
wR
Resid. density (e Å−3
)
and CCDC 2010509 (II). These data can be obtained free
of charge via https://www.ccdc.cam.ac.uk/structures/ (or
from the Cambridge Crystallographic Data Centre,12 Union
Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
or deposit@ccdc.ca.ac.uk).
θ = 0
S
-60
60
G'
G
gauche
syn
θ
gauche'
eclipsed'
E'
+
N
N
eclipsed
θ
trans
E
Conformational Analysis and Database Searching
-120
120
T
A detailed analysis of the imidazolium butyl chain confor-
mation within Ia, Ib, II, and related structures was com-
pleted. The torsion angle notation described by Saoane,
et al. [18] is used here (see Fig. 1): S (syn, − 30 to + 30◦),
G (gauche, + 30 to + 90◦), G′ (gauche’, − 30 to − 90◦), E
(eclipsed, + 90 to + 150◦), E′ (eclipsed’, − 90 to − 150◦), and
T (trans, − 150 to + 150◦).
180
Fig. 1 Illustration of the imidazolium butyl chain torsion angles, ꢀ1
and ꢀ2, and the corresponding conformational labels (Color fgure
online)
Graphs were generated using DataGraph, Version 4.7. Copy-
right ©2021 Visual Data Tools, Inc., Chapel Hill, NC, USA.
Available at: https://www.visualdatatools.com/
The Cambridge Structural Database [50] was searched
using a combination of conventional search queries and
SMARTS [51] substructure searches executed using the
CSD Python API. The fexibility of the SMARTS query pro-
vided comprehensive search results since the placement and
representation of the bonds in the imidazolium ring can vary
between entries. A Python script was used to analyze butyl
chain conformations including conformations and occupan-
cies of disordered chains. The script and documentation is
available at https://gitlab.com/djohnston66/bmim-analysis.
Raman Spectroscopy
Dispersive Raman spectra were collected using a Jobin-
Yvon LabRam 800HR at an excitation wavelength of
532.15 nm generated by a frequency-doubled Nd:YAG
laser. Spectra were collected with a 950 grooves/mm
1 3

Journal of Chemical Crystallography
grating and calibrated using a silicon reference. Samples
were loaded into a Linkam BCS196 low temperature stage
and Raman spectra were collected at temperatures ranging
from 100 to 350 K.
Results and Discussion
X‑Ray Crystal Structures and Hydrogen Bonding
This study represents the frst full structural characteriza-
tion of the [C4mim][NO3] ionic liquid. Full datasets were
collected at temperatures of 100 K, 200 K and 273 K. The
Thermal Analysis
̄
space group for all three structures was P1. Displacement
Differential scanning calorimetry (DSC) measurements
were conducted using a TA Instruments 2010 DSC. A
15.800 mg sample of 1-butyl-3-methylimidazoium nitrate
was analyzed with heating rate of 10 ◦C∕min and the
resulting curve is shown in Fig. 2. The curve displays a
broad melting curve between 15 and 40 ◦C that likely rep-
resents multiple phase transitions. [31] A smaller endo-
thermic transition occurs around 5 ◦C, presumably corre-
sponding to the conversion from Form I to Form II. The
results are qualitatively in agreement with previous work
[31, 43] but show differences due to differing techniques
( [31]) or the absence of substantial water content ( [43]).
ellipsoid diagrams showing the atom numbering schemes for
all three structures are shown in Fig. 3. The asymmetric unit
contains two independent copies of the [C4mim][NO3] unit.
The structures at 100 K and 200 K (Ia and Ib, respectively)
were found to be very similar with relatively small changes
in the unit cell parameters (ꢀa∕ꢁ = +111, ꢀb∕ꢁ = +142,
ꢀc∕ꢁ = +40) and disorder in one of the two independent
butyl groups at 200 K not seen at 100 K. The structure at 273
K (II), however, difered signifcantly, with large changes
in the unit cell parameters from Ib to II (ꢀa∕ꢁ = −172,
ꢀb∕ꢁ = +330, ꢀc∕ꢁ = +236) and conformational changes
and additional disorder appearing for both butyl groups.
A primary structural feature found in all three structures
is the orientation of the butyl groups towards each other in
a columnar arrangement along the a axis (see Fig. 4) with
hydrogen bonding between the ions on the “exterior” of
the columns. This type of segregation is a common feature
found in many ionic liquid crystal structures [4] that for
longer chain (n = 12−18) 1-alkyl-3-methylimidazolium-
based ionic liquids results in a bilayer structure. The range
of hydrogen bonding interactions is similar to those found
in the closely-related [C2mim][NO3] [6, 53], but there is no
Computational Analysis
Density functional calculations were run using Spartan'
18 [52] with the B3LYP functional and the 6-311+G(2df,
2p) basis set using the refined crystallographic atomic
coordinates of each unique [C4mim]+ ion conformer as a
starting point. Approximate Raman intensities were cal-
culated assuming an excitation wavelength of 532 cm-1
and a temperature of 298 K.
1.0
Fig. 2 Diferential scanning
calorimetry (DSC) curve dis-
playing phase transitions for a
sample of 1-butyl-3-methylimi-
dazoium nitrate. Heating rate =
freezing
10 ◦C∕min (Color fgure online)
-30.93°C
0.5
-24.28°C
34.98J/g
0.0
5.66°C
3.219J/g
27.61°C
34.99J/g
6.92°C
-0.5
solid-solid
phase transition
melting
Sample: BMIM-NO3
Size: 15.8000 mg
33.45°C
-1.0
-60
-40
-20
0
20
40
60
80
100
Exo Up
Universal V3.9A TA Instruments
Temperature (°C)
1 3

Journal of Chemical Crystallography
O6
N6
O6
O6
N6
C7
O3
O3
O3
C7
C6
C5
C7
O5
C1
O5
C1
C7A
C6
N6
C15A
O5
C1
C6
C5
O2
O2
O2
N5
O1
N5
O1
N5
O1
C15
C15
C5A
C5
C4
O4
C15A
C14
O4
C14
C13
O4
C6A
C15
C12
C14A
C13A
C14
C13
C13A
C4
N1
C2
C4
N1
C2
C9
C9
C9
N1
C2
C12
N4
C13
N4
N4
N2
C12
N2
N2
C8
N3
C10
N3
C10
C16
C16
C16
N3
C8
C8
C10
C3
C11
C11
C3
C3
C11
Ia (T = 100K)
Ib (T = 200K)
II (T = 273K)
Fig. 3 Displacement ellipsoid diagrams of Ia, Ib and II. Disordered carbon atoms drawn as gray spheres without attached hydrogen atoms.
Ellipsoids are drawn at the 50% level (Color fgure online)
Fig. 4 Hydrogen bonding interactions (dashed red lines) in Ia viewed along the a axis (left) and c axis (right). Labels correspond to column one
of Table 2 (Color fgure online)
The hydrogen bonding extends to form chains:
C9−H9⋯O2⋯H11−C11, but only in the a direction. The
interactions observed at 200 K (Ib) are very similar to those
seen at 100 K (Ia) apart from a slight lengthening of the
C−H⋯O interactions (see Table 3 and Fig. 5). As illustrated
in Fig. 5 and listed in Tables 2, 3, and 4, at 273 K (II) three
of the six C−H⋯O contacts are broken or significantly
evidence of the ꢀ⋯ꢀ interactions seen in [C2mim][NO3].
There are interactions between the imidazolium ring and
⋯
the nitrate ions [d(Cg N) = 3.44Å] similar to those found
in [C2mim][NO3] [6].
Hydrogen bonding plays a signifcant role in the struc-
ture and properties of ionic liquids [54–56]. Analysis of the
hydrogen bonding interactions in Ia and Ib (Tables 2 and
3) show a number of weak C−H⋯O interactions. At lower
temperature (100 K and 200 K) there are three C−H⋯O
contacts between each[C4mim]+ cation and the nitrate ions,
as illustrated in Fig. 4. Most of the C−H⋯O contacts are
from the carbon atoms in the 2 and 5 ring positions (C1, C2,
C9, C10, labeled as contacts (a), (b), (d), and (e), respec-
tively), but the alpha carbon (C4) of the butyl group on one
cation also forms a weak interaction (labeled as (c)). Similar
hydrogen-bonding interactions are seen in other ionic liquids
with oxyanions [7, 25].
Table 2 Hydrogen-bond distances and angles for Ia
D−H⋯A
D–H H⋯A D⋯A
D−H⋯A
(a)
(b)
(c)
(d)
(e)
(f)
C1−H1⋯O5
0.95
0.95
0.99
0.95
0.95
0.95
2.32
2.36
2.42
2.33
2.27
2.53
3.1308 (14) 143
C2−H2⋯O3i
C4−H4B⋯O1i
C9−H9⋯O2
3.3027 (14) 170
3.2215 (15) 138
3.2455 (15) 161
3.1637 (15) 157
3.2610 (15) 134
C10−H10⋯O6ii
C11−H11⋯O3ii
i
Symmetry codes: x − 1, y, z; ii −x + 1, −y + 1, −z + 1
1 3

Journal of Chemical Crystallography
Table 3 Hydrogen-bond distances and angles for Ib
whether this refers to a GG, TG, or GT conformation. The
derived unit cell parameters do not agree with the actual
values for structure Ib (200 K), but the cell volume matches
D−H⋯A
D–H H⋯A D⋯A
D−H⋯A
3
(a)
(b)
(c)
(d)
(e)
(f)
C1−H1⋯O5
0.95
0.95
0.99
0.95
0.95
0.95
2.34
2.40
2.42
2.36
2.29
2.55
3.1649 (19) 145
exactly within experimental error (1057 Å ). In their com-
C2−H2⋯O3
3.332 (2)
3.263 (2)
3.265 (2)
3.186 (2)
3.296 (2)
168
143
160
158
135
bined DSC/X-ray analysis, Abe et al. indexed the low-tem-
perature form as monoclinic (ꢀ = 95.35◦) and the second
(higher-temperature) form as orthorhombic and obtain a
value of 8 for Z, none of which is consistent with our data.
And comparison of the unit cell parameters for Ia, Ib and II
to those obtained from powder difraction experiments [33]
also shows a lack of agreement.
C4−H4A⋯O1
C9−H9⋯O2
C10−H10⋯O6i
C11−H11⋯O3ii
Symmetry codes: i −x, −y + 1, −z + 1; ii x − 1, y, z
Another useful comparison can be found between the
molecular dynamics simulations of [Cnmim][NO3][30] and
the three structures determined in this report. From their
simulations, Cao et al. describe the melting process as con-
sisting of a transition to a metastable state where the polar
and non-polar portions do not melt simultaneously. Rather
they observe disorder in the alkyl side chains while the
charged portions remain ordered. This description closely
matches what we observe as the diferences between the
structures at 200 K (Ib) and 273 K (II).
Table 4 Hydrogen-bond distances and angles for II
D−H⋯A
D–H
H⋯A D⋯A
D−H⋯A
C1−H1⋯O5i
(a)
(c)
(d)
(e)
(g)
0.93
0.97
0.93
0.93
0.97
2.34
2.36
2.40
2.49
2.47
3.190 (3)
152
162
156
146
149
C4−H4BD⋯O1
3.298 (3)
3.268 (3)
3.302 (3)
3.337 (6)
C9−H9⋯O2
C10−H10⋯O6ii
C13−H13B⋯O6ii
Symmetry codes: i −x + 1, −y + 1, −z + 1; ii −x, −y + 1, −z + 1
Conformational Analysis
weakened resulting in a loss of the (b) and (f) C−H⋯O con-
tacts (see Fig. 6) and formation of a new C−H⋯O contact
labeled as (g).
The imidazolium butyl chain conformations observed for
all three structures are summarized in Table 5. At 100 K
(Form Ia), one butyl group is in the lowest-energy TT [18,
32] conformation while the other is in the less common
(and higher energy) G′G′ conformation. At a temperature of
200 K (Form Ib), the butyl chain of the frst cation remains
ordered in the TT conformation, while the butyl chain of
the second imidazolium is disordered with 87.6(4)% in the
G′G′ conformation and 12.4(4)% in the lower energy [32]
GT conformation.
It is interesting to compare the structures obtained to
those modeled by Cao, et al. [30] for the molecular dynam-
ics simulations of a series of [Cnmim][NO3] ionic liquids
(n = 4–12). They used the bilayer structure found for other
long-chain [Cnmim]+ salts with alkyl chains in the all-trans
conformation as a starting point. For [C4mim][NO3] they
found that a pair of chains “take the gauche confguration”
after equilibration. It is not clear from the text or fgures
In the structure of Form II, determined at 273 K, both
butyl chains are disordered, with complete loss of the TT
conformation resulting in one chain distributed between G′T
[56.9(6)%] and GT [43.1(6)%], and the second between G′G′
[56.9(6)%] and GT [43.1(6)%]. The two disordered chains
were initially modeled with independent occupancies and
refned to the same value (within error). Inspection of a
spaceflling model showed spatial clashes between the two
butyl chains thus requiring that the conformations and occu-
pancies of the two chains be correlated. The model was then
modifed to include a single free variable for the disorder of
both butyl groups.
A search of the Cambridge Structural Database (CSD) for
1-butyl-3-methylimidazolium salts resulted in 225 hits repre-
senting 192 compounds, with some duplication and several
examples of polymorphism [8, 11, 18, 28]. Of the 415 indi-
vidual[C4mim]+ ions, seventy eight (19%) were modeled as
disordered ions. The reported disorder frequency is likely
an underestimate, however, as the displacement ellipsoids
Fig. 5 Donor-acceptor distances (Å) found in each of the three struc-
tures, plotted versus temperature (K). Distances with large changes
between 200 and 273 K are shown in green (increase) or blue
(decrease). The dashed red line represents the donor-acceptor dis-
tance criterion used for tabulating contacts (3.34 Å) (Color fgure
online)
1 3

Journal of Chemical Crystallography
Fig. 6 Hydrogen bonding interactions (dashed red lines) in II viewed along the a axis (left) and the c axis (right). Labels correspond to column
one of Table 4. Note the loss of C−H⋯O contacts (b) and (f) and the gain of (g) relative to Ia and Ib (Color fgure online)
Table 5 Torsion angles and
Structure
T (K)
Ion Pair
Occupancy
ꢀ1
ꢀ2
Conf. Label
respective conformation
labels for the 1-butyl-3-
methylimidazolium ions in Ia,
Ib, and II. See Fig. 1 for torsion
angle (ꢀ) and conformation
label defnitions
−
Ia
Ia
Ib
Ib
Ib
II
100
100
200
200
200
273
273
273
273
1
2
1
2
2
1
1
2
2
1.000
1.000
1.000
0.876 (4)
0.124 (4)
0.569 (6)
0.431 (6)
0.569 (6)
0.431 (6)
178.05 (9)
171.3 (1)
TT
G′G′
TT
G′G′
GT
G′T
GT
−
−
61.0 (1)
64.5 (2)
−
176.7 (1)
172.4 (2)
−
−
60.7 (3)
67.0 (4)
−
67 (1)
175 (2)
−
−
70.9 (5)
166.6 (7)
II
70.0 (6)
51.6 (9)
167 (1)
73 (1)
II
G′G′
GT
−
−
II
59.0 (8)
161 (1)
of many structures show evidence of unmodeled disorder as
well as numerous entries containing whole-molecule disor-
der or where the cation electron density was modeled using
SQUEEZE [57] (those structures were not included in this
analysis). A histogram showing the relative abundance of ten
common butyl chain conformations for the 225 hits found in
the CSD is shown in Fig. 7. The distribution follows closely
with the relative energy [32] of each conformation, with the
TT, G′T and GT conformations found for nearly seventy
percent of all ions.
A summary of crystallographic data and butyl confor-
mations of [C4mim]+ salts with metal-free anions is shown
in Table 6. This collection demonstrates a greater prefer-
ence for the G′T and GT conformations (>60%) and less
for the TT conformation (22%) than the overall analysis
and energetics would predict. This could be due in part
to the limited space available when the C4mim+ cation
packs with smaller ions. The G′G′ conformation found in
[C4mim][NO3] is quite uncommon (1.3% overall) and has
not been seen previously in any of the small-ion[C4mim]+
structures.
Fig. 7 Relative abundance and relative energy (kJ/mol) of ten com-
mon butyl chain confgurations for 225 structures in the CSD (Color
fgure online)
Raman Spectroscopy
Raman spectra were collected on a sample of [C4mim][NO3]
at a range of temperatures and are shown in Fig. 8. Most
of the major peaks in the 100 K spectrum can be assigned
1 3

Journal of Chemical Crystallography
Table 6 Summary of crystallographic details and conformations for selected metal-free 1-butyl-3-methylimidazolium salt structures
Anion
T (K)
Space group
Z′
Disorder
CSD refcode
Conformation 1
Conformation 2
Citation
−
̄
P1
NO3
NO3
NO3
100
200
273
173
199
173
173
293
150
150
173
293
110
93
173
183
100
193
193
293
263
120
173
173
110
110
200
150
123
100
173
120
200
173
173
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
1
2
1
1
1
1
2
1
1
1
1
2
2
1
1
No
Yes
Yes
No
No
No
No
No
No
No
No
No
No
No
No
No
No
Yes
Yes
Yes
Yes
Yes
No
Yes
Yes
No
Yes
No
No
No
Yes
Yes
No
No
No
TT
TT
G′G′
G′G′ (0.88)/GT
G′G′ (0.57)/GT
This work
This work
This work
[9]
−
̄
P1
−
̄
P1
G′T (0.57)/GT
G′T
G′T
TT
GT
TT
TT
GT
GT
GT
GT
GT
G′T
G′T
GT
GT (0.75)/G′T
TT
TT (0.70)/TE′
G′T (0.92)/GT
G′T
GT
GT (0.82)/G′T
GG (0.73)/GT
G′T
TG′ (0.75)/ET
TT
G′T
GT
TT
TG
G′T
GT
GT
F−·H2O
F−·H2O
Cl−
P212121
P212121
P21/c
Pna21
P21∕n
P21/c
Pna21
Pna21
Pna21
Pna21
P212121
NAZHUS
NAZHUS01
TAJCUD
TAJCUD01
TAJCUD02
TAJCUD03
TAJCUD04
TAJDAK
TAJDAK01
TAJDAK02
KETMAY
MAZXOB
MAZXOB01
MAZXOB02
MAZXOB03
MAZXOB04
MAZXOB05
MAZXOB06
IGUPUW
WITJUE
WITJUE01
IKAQUG
IKARAN
TAHMAR
IKARER
PENREH
EKIBEH
DAZJIA
MIKYEL
LAZRUA
[10]
[11]
[11]
[12]
[13]
[14]
[11]
[15]
[16]
[58]
[17]
[7]
[18]
[18]
[18]
[18]
[18]
[24]
[19]
[20]
[21]
[21]
[22]
[21]
[23]
[27]
[26]
[16]
[7]
Cl−
Cl−
Cl−
Cl−
Br−
Br−
Br−
I−
−
̄
P1
PF6
PF6
−
̄
P1
−
PF6−
Pbca (alpha)
Pbca (alpha)
PF6−
PF6
̄
TG′ (0.68)/TE′
TT (0.70)/TE′
P1 (beta)
−
̄
PF6
PF6
P1 (beta)
−
̄
P1 (gamma)
Tf2N−
P21∕n
P212121
P212121
P21/c
P21/c
P21∕n
G′T (0.70)/GT
−
BPh4
−
BPh4
B(p-tol)−4
B(CF3Ar)−4
BARF−
TT (0.50)/TG
−
B(C6F13Ar)4
−
B(C6F5)4
HS−
P41
P21/c
Pna21
2−
P21/c
̄
P1
G′T (0.92)/GT
G′T (0.60)/GT
G′T
C2O4
−
p-tolSO3
CF3SO3
−
P21∕n
−
̄
P1
CH3SO3
CH3OSO3
VIGLUT
VIGMAA
[25]
[25]
−
P21/c
based on the structure Ia and its TT and G′G′ conformations.
The strongest bands assignable to the TT conformation are
found at 914, 742, 631 and 341 wavenumbers. These cor-
relate well with data presented in previous work [39, 41]
identifying characteristic bands for the TT conformer. All of
these bands except the 631 cm-1 band can be seen to signif-
cantly decrease in intensity as the temperature is increased,
consistent with the crystallographic results. The lack of a
corresponding change in intensity of the 631 cm-1 band is
puzzling, though the TT conformation might be expected
to be present in samples at 300 K and above that are in the
liquid state. A similar decrease in intensity of the peaks at
950, 815 and 615 cm-1 assigned to the GG′ conformation is
observed in this set of data.
Additional peaks assignable to the G′T and GT con-
formations are also marked in Fig. 8. In some cases these
peaks are coincident or nearly coincident with other peaks
and thus may be difcult to distinguish. However, there
is a clear increase in the peak at 330 cm-1 that can be
assigned to the G′T conformation that becomes particu-
larly apparent at about 250 K. The liquid form (at 350 K)
appears to contain signifcant amounts of the TT and G′T
conformers, though this is not a comprehensive analysis
and other conformers are likely to be present.
1 3

Journal of Chemical Crystallography
Fig. 8 Raman spectra collected
on a sample of [C4mim][NO3]
at temperatures ranging from
100 to 350 K. DFT calculated
bands for four conformers
shown on top and highlighting
signifcant peaks. Note: linear
baseline correction applied to
remove fuorescence back-
ground (Color fgure online)
Conclusion
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Authors and Afliations
Dean H. Johnston1 · Mallory Gasbarre1 · Carolanne E. Norris1
1
* Dean H. Johnston
Department of Chemistry, Otterbein University, Westerville,
djohnston@otterbein.edu
OH 43081, USA
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