Chloromethyl-oxirane and chloromethyl-thiirane in liquid phase: A joint experimental and quantum chemical study

Chemical Physics 473 (2016) 24–31

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Chloromethyl-oxirane and chloromethyl-thiirane in liquid phase: A joint experimental and quantum chemical study M. Campetella a, L. Bencivenni a, R. Caminiti a, C. Zazza b,1, S. Di Trapani a, A. Martino a, L. Gontrani a,⇑ a b

Dipartimento di Chimica, Università di Roma La Sapienza, P.le A. Moro 5, 00185 Roma, Italy Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy

a r t i c l e

i n f o

Article history: Received 18 January 2016 In final form 31 March 2016 Available online 9 April 2016 Keywords: Chloromethyl-oxirane Chloromethyl-thiirane ADXD Conformational stability DFT Molecular dynamics

a b s t r a c t The X-ray diffraction spectra of liquid chloromethyl-oxirane (ClMO) and chloromethyl-thiirane (ClMT) have been recorded for the first time. The interpretation of X-ray measurements was based on ab initio molecular dynamics simulations at finite temperature conditions. Both liquids show conformational equilibrium, which is discussed in terms of Gauche-2, Gauche-1 and Cis structures. The occurrence of the various forms estimated from X-ray and AIMD data has been compared with spectroscopy data from the literature, with the FTIR spectra of the liquids newly recorded in this work, and with theoretical in vacuo calculations. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction Chloromethyl-oxirane (ClMO) and chloromethyl-thiirane (ClMT) are, among monohalomethyl oxiranes, the molecules which have been more extensively studied by microwave [1–4] and vibrational spectroscopy [5–8], in most cases supported by computational methods [6–8]. These monohalomethyl oxiranes are known to exist, as a mixture of the three conformers Gauche-1, Gauche-2 and Cis in vapor, as well as in liquid state, while the solid state of chloromethyl-oxirane would only consist of the Gauche-1 form [5–8]. The vapor and liquid state composition of these species has been long matter of debate, as skillfully depicted by Durig [6,7]. The amount of the three conformers in the different phases seems actually to be influenced by the conformer’s polarity, as asserted by Durig and confirmed by means of vibrational spectroscopy studies [6,7]. In particular, Gauche-1 and Gauche-2 forms are expected to be the considerable components of the liquid phase of both the molecules, being the Gauche-1 conformer the only one present in solid state [5,9,10] of chloromethyl-oxirane, whereas the annealed solid of chloromethyl-thiirane would only consist of the Gauche-2 conformer [7].

⇑ Corresponding author. E-mail address: [email protected] (L. Gontrani). Present address: ASSCO, Università di Roma ‘‘La Sapienza”, P.le A. Moro 5, 00185 Roma (Italy). 1

http://dx.doi.org/10.1016/j.chemphys.2016.03.027 0301-0104/Ó 2016 Elsevier B.V. All rights reserved.

It would be particularly interesting to re-examine the whole matter by combining together theoretical and experimental approaches to determine the amount of the three conformers in liquid and vapor state. For this reason, we have undertaken a joint experimental and computational study of ClMO and ClMT liquids combining X-ray Diffraction, Density Functional-level studies of isolated molecules and molecular dynamics of simulation cells representative of the bulk liquid. Several studies regarding the interpretation of X-ray patterns with simulation techniques have been published so far [11]. The results of previous IR spectroscopy studies of liquid oxiranes have been reconsidered on the basis of more refined calculations to reach further conclusions on the abundances of the conformers in gas-phase and in the liquids.

2. Computational details Calculations were carried out employing Gaussian-09 program [12] for geometry optimizations, vibrational frequencies and conformational stability studies. B3LYP [13] and PBE1PBE [14] functionals were chosen for basic calculations (geometries and molecular vibrations) with the 6-311++G(3df,3pd) basis set [15]. The same calculations were accomplished at the MP2(full)/6-311 ++G(3df,3pd) level [16]. Stability studies were based on singlepoint MP4(SDTQ)/6-311++G(3df,3pd) energies [17] and on G2MP2 and G4MP2 compound methodologies [18]. To investigate the structure of ClMO and ClMT through dynamics methods, we studied a bulk system composed of 120 molecules. Pre-equilibra-

M. Campetella et al. / Chemical Physics 473 (2016) 24–31

tion of the molecules in their respective boxes was performed employing classical molecular dynamics within periodic boundary conditions, using the AMBER program [19] with Gaff force field [20,21]. 1 ns of physical time was simulated for the system, setting the simulation temperature at 300 K, the experimental one. The final configurations obtained from this procedure were used as starting structures for ab initio molecular dynamics simulations accomplished by means of the program package CP2k [22], employing the Quickstep module [23] and orbital transformation [24] for faster convergence. The electronic structure was calculated utilizing the PBE [25] functional, with the explicit Van der Waals terms including the empirical dispersion correction (D3) from Grimme [26]. MOLOPT-DZVP-SR-GTH basis sets and GTH pseudopotentials [27,28] were chosen. The time step was 0.5 fs and the temperature was set at 300 K by a Nose–Hoover chain thermostat [29]. After 5 ps of QM-equilibration, a 32 ps NVT trajectory was acquired. 3. Experimental details

25

in this formula xi, are the numerical concentrations of the species, fi their Q-dependent X-ray scattering factors and q0 is the bulk number density. The structure functions, both the one derived from experimental diffraction patterns and that obtained from theoretical radial distribution functions were multiplied by a modification function M(Q), a sharpening factor, necessary to improve the curve resolution at high Q, and then Fourier-transformed in the distance domain, according to the relation

DiffðrÞ ¼

2r

p

Z

1

QIðQ Þ sinðrQ ÞMðQ ÞdQ :

ð3Þ

0

The Differential Correlation function Diff(r) function contains only the structural contribution to the distribution function. For a comprehensive report of all the formulas, see [30,31]. Summarizing, the comparison between X-ray experimental data and simulations is performed through the analysis of both reciprocal space (I(Q)) and distance space (Diff(r)) functions. This methodology has been successfully applied to the study of molecular [32,33] and ionic [34,35] liquids, as well as solutions [36,37].

3.1. FTIR spectra The spectra of the liquids were recorded in the frequency range 4000–200 cm
1 by means of the Bruker IFS-113 FTIR spectrophotometer. Liquids were spread on the surface of a CsI optical window. The samples (CAS numbers: 106-89-8 (chloromethyloxirane) and 3221-15-6 (chloromethyl-thiirane), both having certified purity > 97%) were purchased by Manchester Chemicals Ltd (UK). The samples were purified by distillation and stored in dark glass sample tubes. 3.2. X-ray About 0.5 ml of liquid were introduced in amorphous quartz capillary (2 mm radius), which was afterward sealed with a Teflon band and kept in dry atmosphere, just before the measurements. The X-ray experiments (WAXS) was carried out on a Bruker D8 Advance with DaVinci design diffractometer (angle dispersive) equipped with a Mo KaX-Ray tube (k = 0.7107 Å), whose radiation was focused onto the sample with Göbel mirrors. The 2# angle range available was 2.75–142° with a step of 0.25° within the Bragg–Brentano para-focusing geometry. The scattered intensity was gathered with the Lynxeye XE Energy-Dispersive 1-D detector. Primary data were processed performing the necessary corrections for the background and sample absorption and subtracting the independent atomic scattering, to obtain the ‘‘total (static) structure function” I(Q), which is equal to:

IEXP ðQ ÞE:U ¼

N X

2

xi f i þ IðQ Þ

ð1Þ

i¼1

The first term represents the independent atomic scattering from the atoms in a stoichiometric unit, while I(Q) is the ‘‘total structure function” and constitutes the structurally sensitive part of the scattering intensity, being due to the interference contributions from different atoms. The variable Q is the magnitude of the transferred momentum, and depends on the scattering angle (2#), according to the relation Q = 4p (sin #/k). The function I(Q) is related to the radial distribution functions descriptive of the structure and obtainable from the simulations, according to the formula

IðQ Þ ¼

Z N X N X xi xj f i f j 4pq0 i¼1 j¼1

o

qmax

r 2 ðg ij ðrÞ
1Þ

sinðQrÞ dr ðQrÞ

ð2Þ

4. Results and discussion 4.1. DFT and vibrational spectroscopy study As preliminary step of the work, we considered worthwhile summarizing the main results acquired for the three conformers of the molecules from theoretical calculations. For this reason, it is helpful to show the structures of the three stable molecular conformations of ClMO/ClMT system that can be obtained by rotating around the bond linking the oxygen ring and the chlorine atom, namely Gauche-2, Gauche-1 and Cis, and of the three transitions (TS) states connecting them; the structure are shown in the attached chart (Fig. 1). The most significant geometrical parameters determined the conformers of the two molecules at the DFT level are summarized in Table 1. The complete energy profile, determined for the two molecules at the B3LYP/6-311++G(3df,3pd) level and shown in Fig. 2 provides a more complete insight concerning the stability features of the two systems. Each profile was obtained as a sequence of optimized geometries for a fixed value of the Cl–C– C–C–X torsional angle value. For a deeper inspection of the matter, the set of energy differences evaluated at different levels of theory can be better displayed considering the results reported in Table 2. The main results acquired from the calculations can be summarized as follows: (I) The energy differences calculated for the less stable gasphase Gauche-1 and Cis conformers of ClMO are lower (nearly the half, being 2 and 5 kJ mol
1) with respect to those determined for ClMT (against 5 and 10 kJ mol
1). (II) The height of the conformational barrier separating the Gauche-1 conformer from the lowest energy one Gauche-2 is higher for ClMT (13 kJ mol
1) than for ClMO (9 kJ mol
1). At the same way, the height of the conformational barrier separating the Cis form from Gauche-2 is higher for ClMT (8 kJ mol
1) than for ClMO (5 kJ mol
1). At 298 K, the DH° values related to the conformational gasphase equilibria Gauche-2 M Gauche-1 and Gauche-1 M Cis are quite low for ClMO (both 2 kJ mol
1) if compared to the values found for ClMT (both 5 kJ mol
1). An additional remarkable result is that G4MP2 calculations performed in the same temperature range reported by Durig et al. [6,7], taking into account the xenon matrix environ-

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M. Campetella et al. / Chemical Physics 473 (2016) 24–31

Fig. 1. Minimum energy conformers and transition state structures.

Table 1 Most significant molecular geometry parameters

a

of the conformers of ClMO and ClMT.

ClMO

C–Cl C–O Cl  X C–X–C Cl–C–C–C Cl–C–C–X a

ClMT

Gauche-2

Gauche-1

Cis

Gauche-2

Gauche-1

Cis

1.805 1.428/1.433 4.020 61.4° 94.2° 162.1°

1.807 1.424/1.427 3.426 61.9°
150.9°
82.5°

1.797 1.419/1.434 3.216 61.6°
19.6° 52.0°

1.818 1.827/1.832 4.421 47.6° 85.8° 161.8°

1.816 1.834/1.835 3.747 47.7°
158.6°
81.6°

1.796 1.821/1.827 3.491 47.8°
31.7° 49.2°

X = O for ClMO and X = S for ClMT. Distances are in Å and angles in degrees.

ment through the PCM method, provide for ClMO average DH° values 2.2 and 2.8 kJ mol
1 (in the temperature range 168 < T/K < 233) for the conformational gas-phase equilibria Gauche-2 M Gauche-1 and Gauche-1 M Cis, while 4.9 and 5.0 kJ mol
1 are calculated for the analogous average DH° values in the case of ClMT within 123–218 K range. At the same reference temperature, the calculated gas-phase conformational population indicates the Gauche2 conformer as the most abundant species, that is 68% for ClMO and even higher for ClMT (88%). The gas-phase abundance expected for the Gauche-1 conformer is 25% for ClMO but quite less for ClMT (11%). At last, the Cis rotamer is confirmed to be a minor species for both the molecules, barely 5% for ClMO and even less for ClMT (1%). The agreement between our in vacuo theoretical results and gas-phase electron diffraction data (see Table 3) is satisfactory for both molecules, 4.2. DFT and vibrational spectroscopy study As already asserted in the introduction, several studies are available for the gas, low-temperature matrix, liquid and solid

state of ClMO and ClMT and those reported by Durig [6,7], Wurrey [6,8,41] and us [8] are of great interest for the purpose of the present investigation aimed to the conformational study in the liquid state of the two molecules. The infrared spectra of liquid ClMO and ClMT were measured in the course of the present work and compared with those of the gaseous, matrix and solid phase reported in literature [6–8,41]. Further, the comparison between the theoretical infrared and Raman spectra of the molecules with those of the gaseous, liquid and solid states assumes valuable importance when the presence of the three conformers has to be verified. The whole comparison was performed between the set of all the experimentally available and theoretical results based on DFT (B3LYP, PBE1PBE) and MP2 (full) calculations for free (in vacuo) molecules and ab initio molecular dynamics calculations for liquid phase. The results of these studies are reported in Table S1 of the Supplementary Material. The vibrational spectra calculated at the B3LYP, PBE1PBE and MP2 levels with the 6-311++G(3df,3pd) basis set do not show notable differences. In particular, the spectra determined at the B3LYP and PBE1PBE levels are almost identical. Furthermore, the xenon

27

M. Campetella et al. / Chemical Physics 473 (2016) 24–31

Table 2 Gas-phase conformational stability and thermochemistry (T = 298.15 K, all values in kJ/mol). (G2 and G1 stand for Gauche-2 and Gauche-1 conformers). ClMO (DE values relative to the lowest energy conformer, including thermal corrections). Method

G2

G1

Cis

B3LYP/6-311++G(3df,3pd) PBE1PBE/6-311++G(3df,3pd) MP2(full)/6-311++G(3df,3pd) MP4(SDTQ)/6-311++G(3df,3pd) G2MP2 G4MP2

0 0 0 0 0 0

1.6 1.3 2.1 2.6 2.4 2.2

6.5 5.0 4.4 5.2 5.4 5.0

Transition statesa

DE (A,B) = EA
EB

TS1,G2

TS2,G1

TS3,G2

TS1,G1

TS2,Cis

TS3,Cis

B3LYP PBE1PBE MP4

9.7 9.7 11.3

19.2 18.9 18.1

10.4 10.2 10.7

8.1 8.4 8.8

14.4 15.3 15.1

4.0 5.2 5.5

Vapour phase conformational population at 298.15 K

B3LYP/6-311++G(3df,3pd) PBE1PBE/6-311++G(3df,3pd) MP4(SDTQ)/6-311++G(3df,3pd) G2MP2 G4MP2

Fig. 2. Relative energy profiles calculated for ClMO and ClMT; the zero is the lowest energy G2 conformer.

matrix effect on the vibrational spectra, simulated at the PCM/ B3LYP level, was indeed found to be negligible. Owing to these premises and considerations, the results summarized in Table 4 for free ClMO and ClMT molecules are restricted to those obtained from B3LYP/6-311++G(3df,3pd) calculation. A further and likely much more interesting frequency calculation was accomplished from molecular dynamics simulations of the vibrational spectra of the liquids. Incidentally, one has to observe that DFT calculations provide vibrational spectra according to a static protocol based on energy Hessian matrix, while molecular dynamics vibrational spectra are determined starting from dynamic quantities such as velocity autocorrelation function [38]. With respect to static calculations, the autocorrelation function provides a distribution of frequencies having a finite bandwidth rather than a delta function. These simulations, however, are not able to extract very low vibrational frequencies because these are merged into internal translational modes. Another limitation of these dynamic techniques is that they do not allow to distinguish the normal modes of conformers because their vibrational frequencies are quite often too close to each other. There are no particular comments about the vibrational assignments for the FTIR spectra of the two molecules as our calculations confirm the previous ones [6–8], suggesting that the gas phase of ClMO consists of the three conformers, while that of ClMT only of the two Gauche forms. There is just a point to be considered, that is the spectroscopic evidence of the Cis conformers in liquid ClMO and ClMT. Relying on the calculations, the theoretical IR spectra of the three conformers would often display bands lying within a narrow wavenumber range (see Table S1) and suggest that the existence of the Cis conformer can be proved by the presence of a fundamental mode around 500 cm
1 having, at least in gas phase, suitable IR band intensity. There is no doubt that the FTIR spectrum of liquid ClMO shows bands due to the Cis conformer, including a band at 518 cm
1, unquestionably assigned to the t21 mode. The FTIR spectrum of liquid ClMT shows an extremely weak band at 492 cm
1, which might reveal the presence of the Cis form of the molecule. However, provided that this band is a fundamental band associated to the t21 mode of the Cis conformer and not a combination band or overtone, one might conclude that if the Cis form were actually present in liquid ClMT its amount would be thoroughly negligible.

G2 (%)

G1 (%)

Cis (%)

65.8 56.9 66.5 69.7 67.6

30.5 37.8 25.9 24.3 25.6

3.7 5.3 7.6 6.0 6.8

Thermochemistry of the conformational gas-phase equilibria at 298.15 K (DH° values)

B3LYP/6-311++G(3df,3pd) PBE1PBE/6-311++G(3df,3pd) MP4(SDTQ)/6-311+G(3df,3pd) G2MP2 G4MP2 b

G2 M G1

G1 M Cis

G2 M Cis

1.6 1.3 2.1 2.4 2.2 0.8

4.9 3.7 2.3 3.0 2.8 5.1

6.5 5.0 4.4 5.4 5.0 5.9

ClMT (DE values relative to the lowest energy conformer, including thermal corrections) Method

G2

G1

Cis

B3LYP/6-311++G(3df,3pd) PBE1PBE/6-311++G(3df,3pd) MP2(full)/6-311++G(3df,3pd) MP4(SDTQ)/6-311++G(3df,3pd) G2MP2 G4MP2

0 0 0 0 0 0

6.0 5.4 5.0 4.8 5.2 4.9

12.8 10.9 9.5 9.7 10.3 9.9

Transition statesa DE (A,B) = EA
EB

TS1,G2

TS2,G1

TS3,G2

TS1,G1

TS2,Cis

TS3,Cis

B3LYP PBE1PBE MP4

16.7 17.0 17.7

21.1 20.9 20.1

18.7 18.3 17.9

10.7 11.6 12.6

14.3 15.5 16.1

5.8 7.5 8.1

Vapour phase conformational population at 298.15 K G2 (%)

G1 (%)

Cis (%)

B3LYP/6-311++G(3df,3pd) 92.0 7.6 0.4 PBE1PBE/6-311++G(3df,3pd) 89.6 9.5 0.9 MP4(SDTQ)/6-311++G(3df,3pd) 87.7 11.0 1.3 G2MP2 88.8 10.1 1.1 G4MP2 87.3 11.4 1.3 Thermochemistry of the conformational gas-phase equilibria at 298.15 K (DH° values)

B3LYP/6-311++G(3df,3pd) PBE1PBE/6-311++G(3df,3pd) MP4(SDTQ)/6-311++G(3df,3pd) G2MP2 G4MP2 b a b

G2 M G1

G1 M Cis

G2 M Cis

6.0 5.4 5.0 5.2 4.9 5.4

6.8 5.4 4.5 5.1 5.0 6.9

12.8 10.9 9.5 10.3 9.9 12.3

G2 and G1 stand for Gauche-2 and Gauche-1, respectively. G4MP2 values from PCM calculations at 298.15 K in Xe.

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M. Campetella et al. / Chemical Physics 473 (2016) 24–31

Table 3 Experimental abundances for chloromethyl-oxirane and chloromethyl-thiirane. ClMO

References Gauche-2 Gauche-1 Cis

GED (340 K)

VCD

Dipole moment

Vibrational spectroscopy (liquid, 300 K)

[46] 67% 33%

[42] 35.7% 54.6% 9.7%

[44] 54%

[5] 21% 70% 9%

ClMT

References Gauche-2 Gauche-1 Cis

GED (308 K)

Xe-matrix (123 K/218 K)

Dipole moment

Vibrational spectroscopy (liquid, 300 K)

[45] 82.2% 17.8%

[7] 62% 38%

[42] 83.5% 16.5%

[43] 59% 28% 13%

5. Results and discussion 5.1. Theoretical results The optimized geometries of ClMO and ClMT in Gauche-2 form are superimposed in Fig. 3. It can be seen that the two molecules share the almost invariant carbon atom skeleton, while the oxygen–sulfur substitution has a large effect on the ring structure. The different electronic structure leads to a 0.3 Å increase of the carbon–heteroatom bond and to a contraction of the C–X–C angle (see Table 3). The different spatial arrangement of the C–X–C moiety causes a relevant modification of the overall intramolecular distance distribution that can be appreciated in a diffraction experiment. In particular, considering that the most important contributions to the scattering pattern come from distances between heavy atoms, an important role is played by, the X–Cl distance of Gauche2 conformer, that changes from 3.98 to 4.37 Å. Passing to AIMD simulations, the structural parameters of the two systems were obtained from the analysis of the radial distribution functions, calculated for the two trajectories. The most interesting features regard the relative spatial arrangement of the chlorine atom with respect to the oxirane ring; based on this issue, each molecule can be found in one of the three main conformations (Gauche-1, Gauche-2 and Cis) already defined. A suitable parameter to quantify such behavior is the distribution of the intramolecular distance ClX calculated along the trajectory, where X = O, S. Such radial distribution (Fig. 4) shows two peaks centered at 3.40 and 4.04 Å. A thorough inspection of the curves shows that, while the latter peak has a single contribution, the former appears to have two components, ascribable to Cis conformer (the shorter distance contact, around 3.20 Å) and to Gauche-1 (the shoulder at 3.5–3.6 Å), at the right side of the peak. An equivalent pattern is found for thiirane, where the peak values are 3.48 and 4.41 Å, while the shoulder falls at 3.74 Å. The areas’ ratio of the two peaks, that can be related to the (Gauche-1 and Cis)-Gauche-2 triad of conformers, are about 60:40 and 65:35 for ClMO and ClMT, respectively. These distance values comply satisfactorily with those obtained from in vacuo geometry optimiza-

Fig. 3. Superposition between the two molecules skeleton (from ab initio calculations in vacuo). Blue: ClMO; Red: ClMT. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. Cl-X(O,S) intramolecular radial distribution functions. Black: ClMO; Red: ClMT. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

tion (see Tables 1 and 4), which yield 3.98/3.16/3.40 (ClMO) and 4.37/3.68/3.42 (ClMT) for Gauche-2, Gauche-1 and Cis structures, respectively. The discrimination of the population within the G1– Cis couple (first peak) is hardly feasible using g(r) data only; a much clearer way to obtain the population ratio exploits the Dihedral Distribution Functions (DDF) of the Cl–C–C–X (O,S) dihedral angle for both molecules, shown in Fig. 5. Now three distinct peaks are evident, and the ratio of the peak integral is 44.7:15.6:39.7 (ClMO) and 21.2:14.6:64.2 (ClMT). A summary of such analysis is reported in the following chart.

Table 4 Dihedral angle distribution. Conformer

G1 G2 Cis

ClMO AIMD

ClMO opt in vacuo

ClMT AIMD

ClMT opt in vacuo

Angle

Population%

Angle

Population%

Angle

Population%

Angle

Population%


79 (31) 164 (36) 46 (32)

44.7 39.7 15.6


82.5 162.1 50.0

25.2 67.6 6.8


81 (40) 162 (43) 43 (37)

21.2 64.2 14.6


81.6 161.8 49.2

11.4 87.3 1.3

M. Campetella et al. / Chemical Physics 473 (2016) 24–31

29

The peak maxima fall at the angular values reported in Table 4, where they are compared to the same angles observed in the structures optimized in vacuo (see Table 1). In the same table, an analogous comparison between the population ratios is reported. From the table we can notice that the maximum values are very similar, while the relative populations differ notably in the two calculations. The first noteworthy issue is a more homogeneous percentage distribution observed in the liquid, as expected for a condensed phase at room temperature; moreover, while the occupancy order calculated in vacuo is retained for ClMT, a Gauche-1–Gauche-2 rank reversal is observed for ClMO. These results comply with the calculated order of dipole moments (Gauche-1 > Cis > Gauche-2, namely 3.28–2.66–0.61 D for ClMO and 3.00–2.62–0.52 D for ClMT), considering that in the liquid phase, the stability of polar forms (maximally Gauche-1 in ClMO) is generally enhanced by the surrounding medium. The effect of the neighbor molecules is further confirmed by the AIMD simulations of the isolated molecules at 300 K. As a matter of fact, in the dihedral distribution functions calculated from the trajectories, which are reported in the Supplementary Material, only the Gauche-2 conformer is largely prevalent. Turning back to the liquid simulation, the analysis of full width at medium height values (FWMH), extracted from the Gaussian fit of the peaks (in parentheses), indicates that the ClMT molecule has a slightly higher conformational mobility. An evident stumble of our AIMD simulation is the too large population predicted for ClMT Cis structure, that is not in agreement with the infrared spectra. While the 32 ps trajectory is sufficient to determine a proper local sampling of the constant temperature ensemble, and therefore the finite temperature local structure of the system, it is worth remarking that this time might be too short to achieve a very accurate determination of the relative abundance of the three conformers. The error in the estimation of the free energy of this structure with respect to the experimental one, that can be obtained from the exponential relation between probability density and the free energy for a given conformation C Fig. 5. Dihedral Distribution Functions (DDF) of the Cl–C–C–X(O,S). Top: ClMO; Bottom: ClMT.

Fig. 6. Comparison between experimental (dotted/dashed) and MD diffraction patterns (line). Green: CLMO; Blue: CLMT. Panel A: Diff(r), Panel B: Diff(r) of the two MD patterns superimposed, Panel C: structure functions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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M. Campetella et al. / Chemical Physics 473 (2016) 24–31

  P MD ðCÞ
ðF MD
F EXP Þ ¼ exp PEXP ðCÞ KBT

ð4Þ

is actually smaller than the thermal energy at room temperature (KBT), i.e. within the typical accuracy of MD simulations. Improving this accuracy would require either a very long simulation time, such that the system can visit all the metastable states a large number of times, or the use of special techniques for rare events (e.g. Metadynamics [39] or Temperature Accelerated MD [40] or MC [41]). Not being the accurate quantitative estimation of the relative abundance of the conformers the objective of this work, we preferred to draw qualitative, and locally quantitative, conclusions from present simulations. Another possible source of inaccuracy is the treatment of dispersion forces, which we include via the D3 Grimme approach (see Section 2). In general, the effectiveness of these methods is system dependent. Summarizing, we can state that the distortion of the epoxy ring (Fig. 3) brought about by the oxygen–sulfur substitution has little influence on the conformations of the chain but has a much more relevant effect on the energy and the resulting population ratio. 5.2. Comparison with experimental X-ray patterns In Fig. 6, the experimental diffraction patterns of the two investigated systems are compared to the AIMD models. As concerns the structure function (panel C), we can notice that both curves are poorly structured in the intermediate Q range (4–10 Å
1), indicating the absence of strong intermolecular interactions associated to polar groups, though ClMT shows taller peaks, owing to the sulfur higher scattering factor. The AIMD model is able to reproduce the experimental pattern very accurately in the whole Q range. Regarding the radial pattern (Diff(r), panel A), optimally reproduced by our simulations, after the first two peaks that are related to 1,2 (first neighbor) and 1,3 (second neighbor) intramolecular contacts, in the range 3–6 Å the superposition of both intra and intermolecular contributions can be observed. In particular, in the sulfur containing system (ClMT), that has the larger X-ray contrast, the presence of the two peaks at around 3.7 and 4.5 Å that emerge clearly from the intermolecular bundle (see Fig. 6, panel B), nicely mirrors the Gauche-2:Gauche-1 population ratio predicted by the AIMD study. This issue is less evident in ClMO, where two different effects cooperate: the reduced X-ray contrast of oxygen and the more homogeneous conformer distribution. Nevertheless, a peak at 4 Å and a shoulder around 3.5 Å are visible in Diff(r) spectrum (green), suggesting the coexistence of both forms in comparable amounts, as the AIMD results suggest for both liquids. The presence of the Cis conformer cannot be assessed from the diffraction patterns, since its weak contribution is buried into the first intermolecular peak, even for ClMT. Moving rightwards on the spectrum, a large peak centered at about 5.1 Å can be found in both ClMO and ClMT. From the analysis of all partial g(r)’s, it results that this feature is originated by all intermolecular contacts and is not due to a specific interaction. Two other faint peaks due to long range structural correlation are visible at 13 Å (ClMO) and 15 Å (ClMT) (see Fig. S1, additional material).

present in the liquid where the percentage of the two most polar forms, Gauche-1 and Cis, increases with respect to the vapor phase. Furthermore, the two Gauche conformers reach a nearly identical amount. ClMT, on the contrary, shows a quite different conformational population, since the amount of the Cis conformer is extremely negligible or absent both in vapor and liquid phase. The vapor consists of the two Gauche conformers, Gauche-2, the less polar having calculated in vacuo dipole moment 0.52 D, and the most polar one Gauche-1 having calculated in vacuo dipole moment 3.00 D. The Cis form cannot be pointed out clearly in liquid X-ray measurements for both molecules, since its weak scattering peak would be buried in the first-shell intermolecular envelope. Our ab initio molecular dynamics simulations overestimate the population of the Cis conformers of both molecules, although the amount of the two Gauche conformers is perfectly in line with the expectations based on conformer’s polarity. Conflict of interest No conflict of interest. Acknowledgments We would like to kindly thank Simone Meloni for the fruitful and helpful discussions about AIMD simulations, and Prof. Ruggero Caminiti (Chemistry Department, Sapienza University of Rome) for providing free access to Narten computing cluster. Computational support from PRACE (grant no. 2013091962) is also acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemphys.2016. 03.027. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

[12]

6. Conclusions In this study we report a detailed survey of several experimental and theoretical investigations on the conformational propensities of 2-(chloromethyl)- oxirane and thiirane. Our experimental and theoretical studies provide grounded evidence that ClMO exists in its three conformations both in vapor and liquid state. Its vapor phase mainly consists of the less polar Gauche-2 and the most polar Gauche-1 conformers and of a not entirely negligible amount of the other polar Cis form. The same three conformers are

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