Water Docking Bias in [4]Helicene

Article abstract of DOI:10.1002/anie.201902889

We report on the one- and two-water clusters of [4]helicene, the smallest polycyclic aromatic hydrocarbon with a helical sense, which were captured in the gas phase using high-resolution rotational spectroscopy. The structures of the complexes are unambiguously revealed using microwave spectra of isotopically enriched species. In the one-water cluster, the apparent splitting pattern is consistent with a tunneling motion that encompasses an exchange of strongly and weakly bonded water hydrogens. This motion is “locked” in the two-water cluster. The relevant intermolecular contacts, symmetry, and aromaticity effects are unveiled for the microsolvated chiral topologies. These observations entail the first glance at the structures and internal dynamics of the water binding motifs of a chiral polycyclic aromatic hydrocarbon.

Full text of DOI:10.1002/anie.201902889

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Accepted Article
Title: Water Docking Bias in [4]Helicene
Authors: Sérgio Rosa Domingos, Kévin Martin, Narcis Avarvari, and
Melanie Schnell
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902889
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http://dx.doi.org/10.1002/ange.201902889

Angewandte Chemie International Edition
10.1002/anie.201902889
Water Docking Bias in [4]Helicene
1∗
2
2
Dr. Sérgio R. Domingos , Dr. Kévin Martin , Dr. Narcis Avarvari ,
and Prof. Dr. Melanie Schnell1
,3∗
May 13, 2019
1
2
3
Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, 22607 Hamburg, Germany
MOLTECH-Anjou, UMR 6200, CNRS, UNIV Angers, 2 bd Lavoisier, 49045 ANGERS Cedex, France
Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Max-Eyth-Str.1, 24118
Kiel, Germany
*
corresponding author: sergio.domingos@desy.de, melanie.schnell@desy.de
Abstract
We report on the one- and two-water clusters of [4]helicene, the smallest polycyclic aromatic hydro-
carbon with a helical sense, captured in the gas phase using high-resolution rotational spectroscopy.
The structures of the complexes are unambiguously revealed using microwave spectra of isotopically-
enriched species. In the one-water cluster, the apparent splitting pattern is consistent with a tunnelling
motion that encompasses an exchange of strongly- and weakly-bonded water hydrogens. This motion
is “locked" in the two-water cluster. The pertaining intermolecular contacts, symmetry, and aromaticity
effects are unveiled for the micro-solvated chiral topologies. These observations entail the first glance
at the structures and internal dynamics of water binding-motifs comprising a chiral polycyclic aromatic
hydrocarbon.
1
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Angewandte Chemie International Edition
10.1002/anie.201902889
The interstellar medium’s carbon resources are thought to be loaded with a plethora of polycyclic
aromatic hydrocarbons (PAHs), given the prominent and abundant mid-IR emission bands attributed
to them.[1, 2] Among dense clouds, however, and because of suppressed IR emission, these carbon-
based aromatic molecules are believed to also exist as part of cold grains, forming interstellar ices.
Cosmic water ices containing PAHs rise thus as likely catalytic sites for chemical reactions driven by
ultraviolet radiation. These complex molecular systems have started to be explored over the last decade
by employing advanced laboratory astrophysics experiments aiming to reveal the photochemistry of
PAH-containing ices.[3, 4]
Prototypical clusters of small PAHs with a few water molecules provide simplified models to deter-
mine the initial steps of ice grain formation and infer chemical reactivity from the local environment.[3–5]
Studies performed on non-centrosymmetric substrates are, however, yet poorly explored. Chiral PAHs
such as helicenes are good model systems to investigate the initial steps of ice grain evolution taking place
in a medium intrinsically lacking a center of symmetry. These scenarios are rich probes for symmetry-
breaking events leading to handedness biasing in space and homochirality.[6] In addition, given the
proposed relationship between adiabatic ionisation potentials of PAH oligomers and aromaticity[7], he-
licenes and their aromatically-twisted topologies emerge as good case studies within an astrochemical
framework.
Here we report on the structure, steric effects and internal dynamics of water aggregated with
a chiral aromatic substrate, mimicked here as [4]helicene, the smallest PAH with a helical sense.[8]
By combining chirped-pulse Fourier-transform microwave (CP-FTMW) spectroscopy[9] and the cold
conditions of a supersonic jet, we obtained the rotational spectrum of [4]helicene complexed with
one and two water molecules, capturing the first steps in the solvation of a chiral PAH. Rotational
spectroscopy demonstrates here once again an undeniable ability to provide a definite assignment of
complex molecular geometries.[10–18] Because of the direct correspondence between unique moments
of inertia and molecular structure, geometries and cluster topologies can be differentiated from their
rotational spectrum and directly compared with theoretical structure predictions. Furthermore, isotopic
substitution allows to precisely solve the atomic make-up of a molecular complex from the local changes
in the atomic mass of the cluster, resolving conformational ambiguities on the theoretical potential energy
surface.
Lacking a sizeable permanent dipole moment, [4]helicene is by itself undetectable using microwave
spectroscopy. However, the very first addition of one water molecule onto the surface of [4]helicene
generates a permanent dipole moment. In Fig. 1 we show theoretical predictions for the three energet-
ically more stable molecular complexes of [4]helicene with one water molecule. In all complexes, the
permanent dipole moment is > 2 Debye. Unlike other observations of water aggregated with achiral
PAHs of comparable size,[19] the asymmetry of the aromatic template generates distinct, inequivalent
binding motifs. In the complexes [4]h−AB and [4]h−BC, the water engages on a dual aromatic em-
bedding via two OH· · π interactions with rings (A and B) and (B and C) of [4]helicene, respectively.
2
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[4]h-AB
[4]h-BC
[4]h-CD
2
.749 Å
3
.015 Å
3.605 Å
2
.597 Å
2.553 Å
2
.581 Å
|!|=2.4 D
|!|=2.4 D
|!|=2.3 D
A
A
A
B
B
C
B
C
C
D
D
D
ΔE=+0.45 kJ/mol
ΔE=+0.56 kJ/mol
ΔE=0
Figure 1: Molecular geometries of the [4]helicene−H2O complex calculated at the B3LYP-D3(BJ)/def2-
TZVP level of theory using Gaussian09.[20] In the global minimum geometry, [4]h−CD, the water
molecule docks onto the substrate via two OH· · π interactions to rings C and D, and a secondary CH··O
contact via the inner hydrogen of ring A. The species [4]h−AB and [4]h−BC (which are slightly higher in
energy with +0.45 kJ/mol and +0.56 kJ/mol, respectively, considering zero-point corrected energies) are
shown where water aggregation to the template occurs via two contacts to rings (A and B) and (B and C),
respectively. Bond lengths between the water hydrogens and the relevant ring centroids are depicted.
In the [4]h−CD topology, the water molecule is anchored onto the [4]helicene via three contact points:
two OH· · π interactions to rings D and C, and one CH··O interaction where the inner hydrogen from
ring A participates and locks the position of the water. The zero-point corrected energies of [4]h−AB
and [4]h−BC are predicted to be only +0.45 kJ/mol and +0.56 kJ/mol higher in energy with respect to
[
4]h−CD. This is no longer the case if one considers the Gibbs energies: [4]h−AB is more stable than
4]h−CD by 0.82 kJ/mol. Similar differences in the predicted relative energies for the three complexes
[
prevail using other levels of theory (see table S4 of the supporting information).
Two spectral portions of the microwave spectrum of [4]helicene with water (H216O:H218O, 1:1) are
shown in Fig. 2. The experimental spectrum, shown as the upper trace (in black), has been recorded
using the Hamburg COMPACT[21, 22] spectrometer. Experimental details are given in the supporting
information. The coloured spectral lines represent a simulation obtained from the fitted spectroscopic
0
parameters given in Table 1. Rotational transitions JK K ← J
0
0
are denoted as J, Ka and Kc, with J
a
c
K K
a
c
being the rotational angular momentum quantum number and Ka and Kc being the projections of J onto
the principal axes at the prolate and oblate symmetric top limits, respectively. Panels C and D show
quartet progressions of [bccb] transitions for the [4]H:H218O and [4]H:H216O isotopologs, respectively.
The observed splitting of each K−doublet into a quartet can be ascribed to the tunnelling motion en-
compassing interchange of water hydrogens (1) and (2) (Fig. 2). For further reading on water tunnelling
dynamics the reader is referred to previous work and references therein.[23–26] A total of 609 rotational
transitions were assigned for [4]H:H216O and [4]H:H218O isotopologs. The primary rotational constants
(A, B, C) and quartic centrifugal distortion constants were determined through a recurrent fit using the
3
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Angewandte Chemie International Edition
10.1002/anie.201902889
C
2
axis
1
2
c
A
B
a
b
b
a
c
r
s
C
D
5
EXP
0
c
c
c
c
c
[4]H:H
2
18O
c
c
[4]H:H
0— / 0+
2
16O
c
b
0—
/ 0+
b
b
b
-5
SIM
b
b
b
b
6
.745
6.75
Frequency (GHz)
6.885
6.89
Figure 2: [4]helicene−H2O topology: panel A and panel B show two views of an overlay between
the calculated structure of the [4]h−CD complex (B3LYP-D3(BJ)/def2-TZVP) and the experimentally
determined position of the water oxygen retrieved via the rs method. The principal axis frame is shown
with relative intensities of the dipole moment components to scale. Splitting of rotational transitions
1
8
in the [4]H:H2O complex: panels C and D show rotational patterns assigned to the [4]H:H2 O and
1
6
[
4]H:H2 O isotopologs, respectively. The split transitions arise from the tunnelling states of the water
−
+
moiety, commonly represented as 0 and 0 . See main text for further information.
A-reduced semirigid rotor Hamiltonian as implemented in PGOPHER.[27] Rotational frequencies and
quantum numbers of all assigned transitions are given in the supporting information.
In Table 1 we compare the experimentally obtained results with quantum chemistry calculations.
−
+
The 0 and 0 states for both isotopologs are unambiguously resolved despite the small difference in
rotational constants. The splitting is on the order of a few tens of kHz. Moreover, the predicted rotational
constants for the [4]h−CD complex are in very good agreement with the experimental parameters:
dispersion corrected functionals seem to capture accurately the topology of the complex.[28] Calculations
at other levels of theory, including the rotational constants for the other complexes, are given in the
supporting information. A definitive position for the water oxygen atom within the principal axis
frame is determined using the Kraitchman equations [29–31] including the rotational parameters for
both isotopologs (rs method). Again, the agreement with theoretical predictions is very good, as can be
visually assessed in Fig. 2 (panels A and B). A comparison of experimental and predicted coordinates for
the water oxygen is given in the supporting information. With a validated model for the global minimum
geometry, 4[h]−CD, and in agreement with the zero-point energy predictions, we will now further
analyse its structural details with a focus on the hierarchy of the steric and electronic intermolecular
interactions at play.
4
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Table 1: Rotational constants, dipole moments, and centrifugal distortion constants for [4]H:H2O and
4]H:(H2O)2. The errors for the measured values are standard errors. N is the number of lines included
[
in the fit, and σ is the standard deviation of the fit. Values between brackets were kept fixed during the
1
fit. The experimental frequency accuracy is 25 kHz. [4]h−CD complex at the B3LYP-D3(BJ)/def2-TZVP
2
18
level of theory. predicted/scaled constants for ([4]h−CD) [4]H:H2 O using the CORSCL program.[32]
3
y=observed; n=not observed;
1
6
[4]H:H218O
[4]H:(H216O)2
[
4]H:H2
O
−
+
theory1
−
+
theory2
theory1
0
0
0
0
expt.
A/MHz
610.01581(13) 610.06705(30) 616.98 595.796279(80) 595.847894(99) 595.12
457.28296(26)
303.03780(13)
205.20434(12)
479.67
302.79
209.95
B/MHz 318.301928(94) 318.29312(15) 324.29 315.191437(62) 315.181188(93) 315.38
C/MHz 236.969422(79) 236.97448(12) 238.41 235.237870(44) 235.243434(48) 235.30
DJ/kHz
DJK/kHz
DK/kHz
dJ/kHz
0.01552(43)
0.0662(18)
0.0474(16)
0.00285(20)
-0.0295(31)
0.01542(71)
0.0735(36)
0.0446(33)
0.00256(32)
-
-
-
-
-
0.2
1.8
1.3
-
[0.01552(43)]
[0.0662(18)]
[0.0474(16)]
[0.00285(20)]
[0.01542(71)]
[0.0735(36)]
[0.0446(33)]
[0.00256(32)]
-
-
-
-
-
-
-
-
-
-
0.00669(24)
0.0349(33)
0.0345(64)
-
-
-
-
-
0.0
1.4
0.6
-
-
-
n
y
y
dK/kHz
-0.0173(57)
[-0.0295(31)]
[-0.0173(57)]
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
3
µ ꢀ/D
y
y
y
202
5.1
n
y
y
150
6.6
n
y
y
157
6.3
n
y
y
100
5.3
a
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
µ ꢀ/D
b
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
µ ꢀ/D
c
N
140
6.5
σ/kHz
-
-
To guide our interpretation of the intermolecular binding contacts, we performed a non-covalent
interaction (NCI) analysis using NCIPLOT[33]. This methodology has been shown to assist in the
interpretation of molecular assemblies and to quantify the type of intermolecular interactions present
in a large variety of complex topologies.[34, 35] In Fig. 3 we show the results of the NCI study for the
[
4]h−CD complex. Intramolecular interactions are dominated by steric hindrance, located in the fjord
◦
region, i.e., the inner part of the helix, with a twist angle of 29 (angle between the planes defined by
rings A and D). The intermolecular contacts arise primarily via two OH· · π interactions with markedly
different strengths, featuring a dual-aromatic embedding of the water hydrogen atoms onto the helicene
substrate (relevant interaction points are depicted in Fig. 1 and Fig. 3). We note that, because of its
relative orientation, the weaker OH· · π contact (3.605 Å to ring C centroid) is prone to being initially
disregarded as an intermolecular interaction and considered a free water hydrogen. The NCI analysis
provides evidence otherwise, and an actual aromatic embedding is predicted between the water and
both rings C and D (see Fig. 3).
Finally, an interaction is established via a CH··O bond between the water oxygen lone pair and the
inner hydrogen of ring A. This interaction seems to tip the scale and provide that additional stability
that results in the small preference for this topology rather than the [4]h−AB and [4]h−BC complexes.
This intuitive explanation is consistent with the results from symmetry adapted perturbation theory
(
[
SAPT)[36] calculations, given in table S5 of the supporting information. We find that whilst in the
4]h−AB and [4]h−BC complexes electrostatic and dispersion contributions are similar, in the [4]h−CD
cluster electrostatic energy has a noticeable dominance over dispersion. Furthermore, the existence of a
secondary CH··O weak hydrogen bond hampers the large amplitude motion of the water molecule along
its C2 axis as observed in its almost free internal rotation on corannulene, the bowl-shaped PAH.[37]
The asymmetry of the helical substrate thus results in a binding configuration where the water acts as
5
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Angewandte Chemie International Edition
10.1002/anie.201902889
CH · · O
(2.764 Å)
OH · · ⇡
steric hindrance
(
!=29.0º)
-
0
8.436 NICS(0)
.854 HOMA
A
-6.362
0
.605
CH · · O
B
C
(
2.764 Å)
-
0
6.436
.605
OH · · ⇡
(
2.581 Å)
D
OH · · ⇡
-
8.240
0
(
3.605 Å)
.852
Figure 3: Non-covalent interaction analysis, HOMA and NICS(0) values for the [4]h−CD complex. The
isovalue for the NCI plot is 0.5. The red dots at the center of each ring indicate strong repulsion forces.
α is the angle between the planes defined by rings A and D. The NICS(0) values were calculated at the
same level of theory (B3LYP/6-311++G**) for each ring centroid. The HOMA values were determined
for each ring independently. Calculated bond lengths between the water hydrogens and the relevant
ring centroids are depicted.
a hydrogen donor to rings C and D on the bottom-end of the helix, and as an acceptor to the top-end
of the helix (ring A). Consequently, the symmetry of the system drops the C2 point group to become C1
upon hydration.
To tentatively quantify the water-induced effects on the aromaticity of [4]helicene we utilised the
harmonic oscillator model of aromaticity (HOMA)[38]. This simple model delivers a normalised sum
of squared deviations of bond lengths with respect to a fully aromatic system, such as benzene, and is
P
2
57.7
n
i
2
defined as: HOMA= 1 −
(dopt − di) , where n is the number of C=C bonds, dopt is the aromatic-
n
optimised bond length and di is the predicted bond length (Å). In addition, we also compare the
HOMA values with the nucleus-independent chemical shift (NICS)[39] values, that deliver the absolute
magnetic shielding at the center of each ring. Relevant values are depicted in Fig. 3. Whilst the HOMA
index provides rather inconclusive values, the NICS values suggest that the water molecule alters the
aromaticity of the bound ring "D" with respect to its antipodal equivalent ring "A", resulting in electronic
asymmetry along the helical plane. Moreover, a time-dependent DFT calculation on the [4]h−CD cluster
anticipates a shift of the first electronic excitation by ∼ 13 nm (0.22 eV) to the red, when compared with
the unbound [4]helicene.
To ascertain if the biased docking observed in the one-water complex progresses over larger order
clusters, we extended our spectral search to track rotational signatures of the two-water complex.
Interestingly, all the remaining lines in our spectrum can in fact be assigned to a species heavier than
6
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Angewandte Chemie International Edition
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NICS(0)
HOMA
-8.428
0.837
(
2.861 Å)
OH · · O
A
O—O (r ) 2.889(5) Å
s
-
0
6.302
.593
B
C
r
s
CH · · O
r
s
(
3.306 Å)
(
2.342 Å)
2.416 Å)
OH · · ⇡
(
-
0
6.298
.593
steric hindrance
(
!=27.9º)
-8.363
.852
0
Figure 4: Overlay of calculated (B3LYP-D3(BJ)/def2-TZVP) and rs structure for [4]H:(H2O)2. NCI analysis
(
isovalue 0.25), HOMA and NICS(0) values for the complex depicting additional OH ·· O interactions
between water molecules. Similarly to [4]H:(H2O), dual OH · · π interactions with rings C and D are
present. Steric hindrance at the fjord region is characterised by the angle α between the planes defined by
◦
rings A and D, ∼ 1 smaller approaching planarity when compared with [4]H:(H2O). The NICS(0) values
were calculated at the same level of theory for each ring centroid. The HOMA values were determined
for each ring independently. Bond lengths between the water hydrogens and the relevant ring centroids
are depicted. Other relevant intermolecular contacts are also shown.
that of the one-water complex. In Table 1 we show the fitted and calculated rotational parameters for
the complex of [4]helicene with the water dimer. The assignment is readily conclusive based on the
quantum chemistry calculations. Nonetheless, using both singly-substituted 18O isotopologs (see the
ESI) we were able to experimentally lock the positions of the water oxygens and further confirm our
assignment of the observed molecular geometry of [4]H:(H2O)2. Remarkably, we find that the previous
bias (docking to rings C and D) observed for the one-water cluster prevails for the two-water aggregate.
In Fig. 4 we show the overlay of the experimental oxygen atom positions (rs method) and the calculated
geometry, as well as the NCI analysis for the cluster. The second water molecule acts as a bridge between
the first water and the inner hydrogen of ring A, thereby maintaining the CD docking onto the aromatic
substrate. The HOMA and NICS indices reflect again a break of symmetry in aromaticity along the
helical plane and inspire further studies to build a model for ice grain evolution including more water
molecules aggregated onto a chiral PAH substrate.
In summary, we uncovered the structure and prevailing intermolecular interactions, symmetry prop-
erties, aromaticity and tunnelling dynamics of the one- and two-water complexes with [4]helicene, the
smallest chiral PAH. Using high resolution rotational spectroscopy of isotopically-enriched species we
determined the structures of the complexes and studied its intermolecular contacts aided by theoretical
methods. The observed splitting pattern in the rotational spectrum of the one-water complex is in line
with an interchange of strong- and weakly-bonded water hydrogens to the aromatic substrate. Quan-
tum chemistry computations and an NCI analysis paired with SAPT calculations provide us with a solid
hierarchy of the intermolecular interactions at play for all observed species. These results lay the ground
work for the pursue of higher-order clusters of water with chiral aromatic templates aiming at revealing
the principles of asymmetrical ice grain evolution in an astrochemical framework.
7
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1
Acknowledgments
This work has been funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foun-
dation) - Projektnummer 328961117 - SFB 1319 ELCH. S.R.D. acknowledges a Postdoctoral Research
Fellowship from the Alexander von Humboldt Foundation. We acknowledge the use of the GWDG
computer cluster. Financial support in France by the CNRS, the University of Angers and the Région
Pays de la Loire through the RFI LUMOMAT (grant to K.M.) is acknowledged.
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