Finding the Better Fit: The Microwave Spectrum and Sterically

Sep 26, 2018 - The rotational spectrum of the gas-phase bimolecular heterodimer formed between trans-1,2-difluoroethylene and hydrogen chloride is ...
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Finding the Better Fit: The Microwave Spectrum and Sterically Preferred Structure of trans-1,2-Difluoroethylene−Hydrogen Chloride Helen O. Leung* and Mark D. Marshall* Department of Chemistry, Amherst College, P.O. Box 5000, Amherst, Massachusetts 01002-5000, United States

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S Supporting Information *

ABSTRACT: The rotational spectrum of the gas-phase bimolecular heterodimer formed between trans-1,2-difluoroethylene and hydrogen chloride is obtained using Fourier transform microwave spectroscopy from 5.6 to 20.6 GHz. Analysis of the spectrum provides rotational constants and nuclear quadrupole coupling constants that are used to determine the structure of the complex. The HCl molecule forms a hydrogen bond with one of the two electrostatically equivalent fluorine atoms of the difluoroethylene, and this hydrogen bond bends from linearity to allow a secondary interaction between the chlorine atom and the hydrogen atom located cis to the fluorine atom in the hydrogen bond. Because the two hydrogen atoms are likewise electrostatically equivalent, the structure indicates that this is the sterically preferred arrangement in HCl binding to a fluoroethylene rather than the one with the secondary interaction to the geminal hydrogen atom. Detailed comparisons among the geometries of the complexes formed between HCl and HF, on the one hand, and vinyl fluoride, 1,1-difluoroethylene, trans-1,2-difluoroethylene, 1,1,2-trifluoroethylene, and (E)-1-chloro-2-fluoroethylene, on the other, reveal structural trends accompanying increasing fluorination and substitution of chlorine for fluorine. HCCH,13 and that to (E)-1-chloro-2-fluoroethylene is the same for HF15 and HCl,14 with both sets assuming the top binding configuration where the acid binds to the F (in preference to the Cl) and H atoms across the double bond. For other complexes, however, we have observed a dependence of binding motif on acid identity. In the case of vinyl chloride, HF binds in a top binding configuration,16 HCCH adopts a side configuration,17 and HCl shows yet a third motif: nonplanar with tunneling motion.18,19 For (Z)-1-chloro-2-fluoroethylene, HCCH binds to the Cl and H atoms at one end of the double bond,20 the first time in haloethylene−acid complexes when, in the presence of an F atom, a Cl binding configuration was observed, while HCl forms a bifurcated hydrogen bond with both the F and Cl atoms.21 We have been able to rationalize the different structural motifs through a consideration of the operative electrostatic and steric factors, and of the different electronic density distributions about the F and Cl atoms in the ethylene subunit. It is important to complete the work on acid complexes of haloethylenes that contain both nucleophilic and electrophilic sites (i.e., both halogen and hydrogen atoms) to gain a thorough investigation of the nature of intermolecular

1. INTRODUCTION To understand and to unravel the competing factors in intermolecular interactions, we have been employing complexes formed between haloethylenes and three protic acids (HF, HCl, HCCH) as our molecular systems. Thus far, the fluoroethylene complexes studied by our group and by the Legon group that involve vinyl fluoride,1,2,4,5 1,1-difluoroethylene,3,6,7 and 1,1,2-trifluoroethylene8−10 are all planar or effectively planar. However, they do show two different binding motifs. The first, which we call “top” binding, is where the acid binds across the double bond of the ethylene subunit and interacts with the F and H atoms connected to different C atoms, as illustrated in Figure 1 for vinyl fluoride− HCl and 1,1-difluoroethylene−HCl. The second, called “side” binding, has the acid at one end of the double bond, also interacting with a pair of F and H atoms, but this time they are connected to the same C atom, such as in 1,1,2-trifluoroethylene−HCl in Figure 1. In either motif, the primary interaction is a hydrogen bond, which bends to allow a secondary interaction between the nucleophilic portion of the acid and an electropositive atom of the ethylene. Although the structural parameters differ for different acid partners, for a given fluoroethylene subunit, the three acids bind in the same manner. In other words, the binding motif to the fluoroethylene is independent of acid identity. Similarly, the binding mode to 1-chloro-1-fluoroethylene is the same for HF11 and © XXXX American Chemical Society

Received: August 16, 2018 Revised: September 25, 2018 Published: September 26, 2018 A

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Figure 1. Structures of (a) vinyl fluoride−HCl,1,2 (b) 1,1-difluoroethylene−HCl, originally determined in ref 3 and redetermined in ref 6, (c) 1,1,2trifluoroethylene−HCl,9 (d) trans-1,2-difluoroethylene−HF,12 (e) trans-1,2-difluoroethylene−HCl (this work), and (f) (E)-1-chloro-2fluoroethylene−HCl.14 The carbon atoms are dark gray, hydrogen atoms are light gray, the fluorine atoms are light blue, and chlorine atoms are green.

Figure 2. Left: coordinate system used in ab initio calculations to determine the minima in the interaction potential energy surface for trans-1,2difluoroethylene and HCl. The origin is at the center of the CC bond of the ethylene, which defines the x−z plane. The H atom of HCl is located using the spherical polar coordinates R, θ, and ϕ. Right: relaxed potential scan at fixed values of θ while R, ϕ, and the position of Cl in HCl are optimized. Points marked by open squares are obtained without BSSE correction while those marked by filled circles are with BSSE correction. The red symbols denote optimized minima.

interactions in these species. Of the fluoroethylene complexes, there remain two sets to be examined: the complexes of trans1,2-difluoroethylene and cis-1,2-difluoroethylene. We have obtained preliminary results on the complexes of HCl and HCCH with cis-1,2-difluoroethylene and will report them in

the near future. In this paper, we consider trans-1,2difluoroethylene and its complex with HCl. trans-1,2-Difluoroethylene is an ideal molecule to help in teasing apart the interplay between electrostatic and steric factors in intermolecular interactions. It offers both top and B

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The Journal of Physical Chemistry A side binding modes to an acid. Because the two F atoms are electrostatically equivalent, as are the two H atoms, the electrostatic factors in the two different binding configurations are identical. Thus, the observed structure of a trans-1,2difluoroethylene−acid complex reveals the sterically more stable configuration. When the acid is HF, the top binding mode is observed12 (Figure 1d). Coupling this result with the fact that the values of the CF···H angle in all top binding configurations of fluoroethylene−acid complexes are approximately the same (∼120°), we are able to conclude that the top binding configuration, when the acid is HF, is sterically more stable. As a result, the observed top binding mode of vinyl fluoride−HF4 indicates that steric considerations are more important for this complex even though a side binding configuration would allow F in HF to interact with a more electropositive hydrogen, the one geminal to the F atom in vinyl fluoride. Conversely, electrostatics is more important for the side binding 1,1,2-trifluoroethylene−HF8 complex even though it experiences steric strain. Here, we explore the consequences of changing the acid HF to HCl, a weaker acid in the gas phase and also a larger molecule, to examine if the binding mode to trans-1,2-difluoroethylene depends on acid identity, and from which we can establish the sterically more favorable configuration for fluoroethylene−HCl complexes.

Figure 3. Optimized structures corresponding to the (a) local minimum and (b) global minimum in the relaxed potential scan in Figure 2. The geometric parameters without (with) BSSE correction taken into account are labeled on top (bottom) in black (purple).

Table 1. Relative Energies, without and with BSSE Corrections and Rotational Constants for Two Isomers of trans-1,2-Difluoroethylene−HCl (Shown in Figure 3) Obtained from ab Initio Calculations isomer

2. AB INITIO CALCULATIONS Before searching for the rotational spectrum of trans-1,2difluoroethylene−HCl, we perform ab initio calculations at the MP2/6-311++G(2d,2p) level with GAUSSIAN 0922 to determine the possible structures for the complex. Fixing the subunits at their ground state, average structures,23,24 we use a coordinate system with the origin at the middle of the double bond of the ethylene subunit (Figure 2). The H atom of HCl is then described by R, θ, and ϕ. We carry out a relaxed scan by incrementing the values of θ in 5° or smaller intervals while optimizing the values of R, ϕ, and the position of the Cl atom in HCl at each value of θ. Because of the inversion symmetry of the ethylene subunit, it is only necessary to consider the range of θ values from 0° to 90°. The minimum energy path is presented in Figure 2 (labeled by open squares) and shows two minima, which were then optimized (θ = 14.5° and 79.8°) and their energies are included in the same figure. The structures of the two isomers are shown in Figure 3, and their relative energies, rotational constants, and dipole moments components are listed in Table 1. When basis set superposition error (BSSE) correction is taken into account, the two minima are slightly shifted, to θ = 14.3° and 81.9°. To ensure that no minima are missed in the relaxed scan, we repeat it, but this time applying the BSSE correction at every value of θ. This minimum energy path is labeled with filled circles in Figure 2. The corresponding minimum energy structures are shown in Figure 3, with their energies, rotational constants, and dipole moments components included in Table 1. We loosely label the analogous structures obtained with and without BSSE correction, by (a) and (b). In each instance, (a) represents the structure at the local minimum and (b) the global minimum. A list of the atomic positions of each isomer (with and without BSSE correction) is available as Supporting Information. Isomer a is planar with HCl binding at one end of the double bond of the ethylene while isomer b is slightly nonplanar (ϕ = 24.6° and 15.9°, respectively, without and with BSSE correction), with HCl binding across the double bond of

relative energy/ cm−1

a b

95 0

a b

62 0

A/MHz

B/ MHz

C/ MHz

μa/D

Without BSSE Correction 11614 928 859 1.280 5493 1317 1074 1.120 With BSSE Correction 11006 912 842 1.103 5580 1231 1013 0.994

μb/D

μc/D

0.836 0.907

0.000 0.011

0.887 0.916

0.000 0.039

the ethylene. A comparison between the two isomers shows that, regardless of whether BSSE correction is included or not, the hydrogen bond in the side binding structure is slightly longer (by 0.013 Å) and bends more from linearity (by 5.4− 6.7°) while the secondary interaction is also longer (by 0.066− 0.103 Å). Having both a weaker primary and secondary interactions results in a higher energy for isomer a. Additionally, the barrier that separates the two isomers is only 29 cm−1 higher in energy than isomer a when BSSE correction is taken into account (or 14 cm−1 without BSSE correction), which is likely less than the zero point energy of isomer a. Thus, this isomer should relax to isomer b in our experiment. Isomer b is only slightly nonplanar, and can adopt two equivalent configurations with HCl on one side or the other of the ethylene plane. Consequently, it is useful to use theory to investigate the possibility of a tunneling motion connecting them. We thus perform another relaxed scan, this time scanning the values of ϕ while allowing the values of R and θ and the position of Cl in HCl to optimize. The results, without (open squares) and with (filled circles) BSSE correction are plotted in Figure 4. The barrier corresponds to a planar, top binding structure, with BSSE-corrected energy of 2.4 cm−1 (and 15.7 cm−1 without BSSE correction), low enough that we expect the complex to have an average planar structure and an absence of tunneling motion.

3. EXPERIMENT The trans-1,2-difluoroethylene−HCl complex is formed by expanding a gas mixture of 1% each of trans-1,2-difluoroethylene and 1% HCl in argon with a backing pressure of 1−2 atm. The broadband, chirped pulse25−27 rotational spectrum C

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components for the 625−524 transition in the most abundant species.

Figure 4. Relaxed potential scan at fixed values of ϕ while R, θ, and the position of Cl in HCl are optimized. Points marked by open squares are obtained without BSSE correction, while those marked by filled circles are with BSSE correction. The red symbols denote optimized minima that correspond to the two equivalent configurations of isomer b, where HCl lies on different sides of the ethylene plane.

Figure 5. 625−524 transition of the most abundant isotopologue of trans-1,2-difluoroethylene−HCl showing four hyperfine components (appearing as Doppler doublets) due to the 35Cl quadrupolar nucleus.

The spectrum of each isotopologue is analyzed using the Watson A-reduced Hamiltonian29 with the inclusion of nuclear quadrupole coupling interactions due to the chlorine nucleus. Using Pickett’s nonlinear SPFIT program30 for each species, we fit three rotational constants, five quartic centrifugal distortion constants, and for the 37Cl isotopologue, one sextic centrifugal distortion constant. We have also determined diagonal components of the chlorine nuclear quadrupole coupling tensor, and the absolute value of the only nonzero offdiagonal component. (As described in the following, the complex has an average planar structure. Thus, the off-diagonal quadrupole coupling constants χac and χbc are zero.) The spectroscopic constants are listed in Table 2, and tables of observed and calculated transition frequencies with assignments for both isotopologues studied are in the Supporting Information. The rms deviation of each fit is less than 2 kHz, smaller than the resolution element of 2.4 kHz. For the most abundant species, we additionally observe five weak spectral lines that could be assigned to two a-type Ka = 4 transitions (642−541 and 643−542) if the sextic centrifugal distortion constant, ΦKJ, was fitted. Because of the lack of other Ka = 4 transitions, we are not entirely confident of this assignment; thus, we omit these five lines from the fit reported in this paper. These five lines and the fit that includes them can be found in the Supporting Information. The results and conclusions established in this work are the same regardless of which fit is used. B. Structure Determination. The values of the asymmetry parameters for the 35Cl and 37Cl isotopologues of trans-1,2-difluoroethylene−HCl are −0.899 and −0.894, respectively, indicating that the complex is a near prolate asymmetric top. The experimental rotational constants agree closely with those of isomer b, whether determined with or without BSSE correction, establishing that HCl binds to trans1,2-difluoroethylene in a top binding configuration. Furthermore, the values of the inertial defect of the two isotopologues are small and positive, 0.060 and 0.064 u Å2, respectively,

was collected in the 5.6−18.1 GHz region in three segments of 4.0 or 4.5 GHz. After the gas mixture expands through two pulsed valves, each with a 0.8 mm diameter nozzle, the sample is polarized using a chirped microwave polarization pulse of 4 μs duration and 20−25 W of power. The resulting free induction decay (FID) is digitized at 50 Gs s−1 for 10 μs beginning 0.5 μs after the end of the excitation pulse. Ten FIDs are collected during each 800 μs opening of the pulsed valves, which typically operate at 4 Hz, although this is reduced to 0.8 Hz for overnight operation. A total of 540 000−822 000 FIDs are averaged for each segment, and as described previously,26 the average is Fourier transformed to give a frequency domain spectrum with a resolution element of 23.8 kHz and typical line widths (fwhm) of 225 kHz. After transitions are assigned to the most abundant species and the 37Cl containing isotopologue, the chlorine quadrupole hyperfine components of transitions between 5.7 and 20.6 GHz were then better resolved with the narrow band, Balle−Flygare26,28 Fourier transform microwave spectrometer. Using only one pulsed valve in this instrument, the background-corrected time domain signals for the narrowband spectra are digitized for 2048 data points and zero-filled to a 4096-point record length before Fourier transformation to give a frequency domain signal with a resolution element of 2.4 kHz. Because the molecular beam axis is parallel to the resonator axis, the spectral lines are Doppler doubled.

4. RESULTS A. Spectral Analysis. With the Balle−Flygare spectrometer we observe over 70 a and b type transitions, which split into 363−376 chlorine hyperfine components for each of the two chlorine isotopologues of the trans-1,2-difluoroethylene− HCl complex. J was sampled from 0 to at least 9, and Ka from 0 to 3. A typical spectrum is included in Figure 5 showing four Doppler doublets that correspond to the 35Cl hyperfine D

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isotopologues using Schwendeman’s STRFTQ program.32 The rms deviation of the fit is 0.045 u Å2. In terms of chemically relevant parameters, HCl forms a hydrogen bond to one of the F atoms in trans-1,2-difluoroethylene with a length of 2.20030(53) Å. This bond then bends by 30.72° (an angle fixed to reproduce the value of θa) from linearity to allow the Cl atom in HCl to form a secondary interaction, with a length of 3.0626(9) Å, with the H atom cis to the F atom (Figure 1e).

Table 2. Spectroscopic Constants (MHz, unless Otherwise Noted) for Two Isotopologues of trans-1,2Difluoroethylene−HCla,b A B C ΔJ/10−3 ΔJK/10−3 ΔK/10−3 δJ/10−3 δK/10−3 ΦJ/10−6 χaa χbb χcc |χab| no. of rotational transitions no. of a type no. of b type no. of hyperfine components J range Ka range rms/kHz

CHFCHF−H35Cl

CHFCHF−H37Cl

5527.82550(27) 1274.635251(53) 1035.669701(44) 2.92276(38) −22.5500(31) 130.134(54) 0.76422(21) 7.791(19) −0.0349c 1.56143(99) −28.7065(11) 27.14509(88) 38.0403(29) 73 41 32 376 0−9 0−3 1.73

5506.03633(23) 1237.011550(49) 1009.952329(47) 2.76316(73) −21.4669(25) 127.098(47) 0.70750(16) 7.618(16) −0.0349(44) 0.84604(84) −22.24098(85) 21.39494(82) 30.1433(24) 74 40 34 363 0−10 0−3 1.41

5. DISCUSSION Guided by a prediction from ab initio calculations of a slightly nonplanar, top binding global minimum structure and a low barrier between two equivalent configurations of this structure, we are able to assign the rotational spectrum of trans-1,2difluoroethylene−HCl and determine the average structure of this species. The trans-1,2-difluoroethylene−HCl complex has the same binding motif as its HF counterpart,12 indicating that at least for the acids HF and HCl, the top binding configuration is acid-independent for trans-1,2-difluoroethylene. The value of the CFH angle, 125.106(19)o, is similar to those for other haloethylene−acid complexes with a top binding configuration where a hydrogen bond involves an F atom in the ethylene subunit. Our earlier conclusion regarding the HF complex also applies here:12 because the two H atoms in trans-1,2-difluoroethylene are electrostatically equivalent to each other, as are the two F atoms with each other, the complex is sterically more stable when HCl adopts a top binding mode. Consequently, the top binding configuration for vinyl fluoride−HCl is driven by steric factors, while electrostatic factors are more important in the side-binding configuration of 1,1,2-trifluoroethylene−HCl.9 A comparison between trans-1,2-difluoroethylene−HCl (Figure 1e) and its HF counterpart (Figure 1d) shows that the hydrogen bond in the HCl complex is 0.290 Å longer and bends 9.07° more from linearity, indicating that the weaker gas-phase acid HCl forms a weaker hydrogen bond with the ethylene subunit. The length of the secondary interaction in the HCl complex, 3.0626(9) Å, is 0.453 Å longer than in its HF counterpart, which is 1.6 times greater than the difference between the van der Waals radii of Cl and F. Thus, the secondary bond in the HCl complex is also weaker. We have previously discussed the effect of an increase in fluorine substitution on intermolecular interactions for fluoroethylene−HCl complexes9 by comparing the structures of the vinyl fluoride, 1,1-difluoroethylene, and 1,1,2-trifluoroethylene complexes (Figure 1). We have concluded that the hydrogenbonded fluorine atom becomes less nucleophilic as the number of F atoms in the ethylene subunit increases, resulting in a weakened hydrogen bond as indicated by a longer bond length and a greater deviation from linearity. The secondary bond lengths for the 1,1-difluoroethylene and 1,1,2-trifluoroethylene complexes are virtually identical [3.07619(30) Å and 3.0796(5) Å] but that for vinyl fluoride−HCl is slightly longer (3.162 Å), suggesting that the stronger hydrogen bond in the vinyl fluoride complex can accept a slightly weaker secondary interaction to stabilize the complex. Nevertheless, these three secondary interaction lengths are similar enough that a length in the 3.08−3.16 Å range appears to be appropriate for an optimal Cl···H distance and that HCl will bend to achieve this. Using the results of this study, we can make additional observations regarding an ethylene subunit containing two F atoms.

a

1σ standard deviations in the parameters are given in parentheses. The nuclear quadrupole coupling constants of chlorine are fitted as 1.5 χaa and (χbb − χcc)/4, and the Laplace condition is used to calculate the individual hyperfine constants. cFixed at the value appropriate to the 37Cl-containing isotopologue. b

which indicates that the complex has an average planar structure with in-plane vibrational motions making the greater contributor to the inertial defect. [The inertial defect of isomer b, determined in the BSSE corrected calculation is −2.152 u Å2.] Because the complex is planar, only three geometrical parameters are required to describe its structure: a distance between the two subunits and their relative orientations. The orientation of HCl, however, cannot be determined in a structure fit to any combination of the two independent (of the three) moments of inertia for each isotopologue. We therefore turn to the nuclear quadrupole coupling constants of HCl to supply this information. Making the usual assumption that the electric field gradient of the quadrupolar nucleus (Cl in this case) is not perturbed upon complexation, the nuclear quadrupole coupling constant of the H35Cl subunit along the a inertial axis, χaa, is a ⟨P2⟩ projection of that of the free H35Cl monomer, χHCl.31 Expressed mathematically, we have χaa =

3 cos2 θa − 1 χHCl 2

where θa is the angle formed by the H35Cl subunit and the a inertial axis. Using the value of χaa for the most abundant isotopologue of the complex, the value of θa can be approximated as cos−1( cos2 θa ) and is determined to be 55.6769(6)o. (This uncertainty, deriving only from the uncertainty in the two experimentally determined values for χ, severely underestimates the uncertainly in the angle.) Fixing the orientation of HCl to reproduce this value, we fit the two remaining geometric parameters to Ia and Ic of each of the two E

DOI: 10.1021/acs.jpca.8b07992 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Although both trans-1,2-difluoroethylene and 1,1-difluoroethylene contain two F substituents, the hydrogen bonds formed by HCl are of different strengths. Specifically, the hydrogen bond in trans-1,2-difluoroethylene−HCl (Figure 1e) is 0.131 Å shorter and bends 3.50° less from linearity compared to its 1,1-difluoroethylene counterpart (Figure 1b).3,6 The F atoms in trans-1,2-difluoroethylene are therefore more nucleophilic than those in 1,1-difluoroethylene, consistent with our observations for the corresponding HF complexes. In fact, this effect is also seen for the HF complexes: the hydrogen bond in trans-1,2-difluoroethylene− HF12 is 0.078 Å shorter, and bends 8.34° less from linearity compared to its 1,1-difluoroethylene counterpart.6 The greater nucleophilicity of the F atoms likely come from their close proximity to sources of polarizable electron density: each F atom is two bonds away from a hydrogen atom in trans-1,2difluoroethylene, but in the case of 1,1-difluoroethylene, each F is three bonds away from a hydrogen atom. The secondary interaction length for the trans-1,2-difluoroethylene−HCl complex is 3.0626(9) Å, which is similar to those in the 1,1-difluoroethylene and 1,1,2-trifluoroethylene counterparts (Figure 1), supporting our argument regarding the optimal range for the Cl···H distance. The same structural trends discussed earlier concerning an increase in the number of F substituents are seen here, with the hydrogen bond in trans-1,2-difluoroethylene−HCl weaker than that in vinyl fluoride−HCl but stronger than in 1,1,2-trifluoroethylene− HCl. As with the HCl complexes, the secondary interaction lengths in fluoroethylene−HF complexes are also similar, lying in the range 2.73−2.75 Å for vinyl fluoride,4 1,1-difluoroethylene,6 and 1,1,2-trifluoroethylene.8 For trans-1,2-difluoroethylene−HF, however, the secondary bond length is shorter and outside of this range, 2.6095(1) Å.12 One possible explanation is that this is the only case where the H atom involved in the secondary interaction is geminal to an F atom that can effectively withdraw electron density from it, making it more electropositive and enhancing the interaction with F in HF and to a smaller extent, Cl in HCl, compared to the other fluoroethylenes. When the nonbonding F atom in trans-1,2-difluoroethylene is substituted with a Cl atom to give (E)-1-chloro-2fluoroethylene, the primary interaction length with HCl, 2.19481(34) Å (Figure 1f), is only 0.005 Å shorter than that in its trans-1,2-difluoroethylene counterpart;14 thus, these bonds are of similar strength. However, the secondary interaction length in (E)-1-chloro-2-fluoroethylene−HCl, 2.9011(5) Å,14 again lies outside of the 3.06−3.16 Å range observed for four fluoroethylene−HCl complex and is 0.162 Å shorter than that in trans-1,2-difluoroethylene−HCl. Turning to the acid partner HF, while the hydrogen bond in (E)-1chloro-2-fluoroethylene−HF15 is slightly longer (by 0.030 Å) than that in trans-1,2-difluoroethylene−HF,12 once again indicating that they are of similar strength, there is a notable difference in the secondary interaction lengths. This difference is similar to that for the HCl complexes: the secondary bond of (E)-1-chloro-2-fluoroethylene−HF is 2.4510(26) Å, which is 0.159 Å shorter than that of the trans-1,2-difluoroethylene− HF. The relative strengths of these bonds are somewhat puzzling. It appears that while the hydrogen bonds formed by HF to (E)-1-chloro-2-fluoroethylene and trans-1,2-difluoroethylene are of similar strength and likewise for HCl, the secondary interaction for both is stronger in the (E)-1-chloro2-fluoroethylene complex than in the trans-1,2-difluoro-

ethylene complex. Thus, a second halogen atom (F or Cl) located trans to the hydrogen bonded F atom exerts the same effect on it, but when that halogen atom is Cl, it affects the geminal H that is involved in the secondary interaction to a greater extent. In fact, these secondary interaction lengths in (E)-1-chloro-2-fluoroethylene complexes are shorter than the corresponding ones seen in fluoroethylene−acid complexes. It appears that replacing an F atom with a Cl atom results in a more electrophilic geminal H atom. This is contrary to expectations based on the electronegativities of the two halogens and thus must be attributed to a resonance withdrawal of electron density, perhaps via hyperconjugation.

6. CONCLUSIONS The microwave rotational spectrum of trans-1,2-difluoroethylene−HCl has been obtained and analyzed, and the molecular structure of this bimolecular heterodimer has been determined. Similar to the complex formed between trans-1,2difluoroethylene and HF,12 trans-1,2-difluoroethylene−HCl adopts a top-binding geometry where the acid interacts with F and H atoms on the fluoroethylene that are cis to each other. Because the pairs of F atoms and H atoms are electrostatically equivalent in trans-1,2-difluoroethylene, this result reveals that, as for HF, the top binding configuration for HCl is sterically favored over the alternative side binding one (as observed for complexes of HF and HCl with 1,1,2-trifluoroethylene,8,9 where electrostatic considerations determine the geometry). More detailed comparisons of the structural parameters for trans-1,2-difluoroethylene−HF and trans-1,2-difluoroethylene−HCl indicate that the primary, hydrogen-bonding interaction and the secondary interaction are both weaker in the HCl species than the corresponding interactions in the HF complex. However, similar to the analogous HF species,6,12 HCl interacts more strongly with trans-1,2-difluoroethylene than with 1,1-difluoroethylene3 despite there being the same number of fluorine atoms on the two ethylenes. While replacing one of the F atoms in trans-1,2-difluoroethylene with a Cl atom to give (E)-1-chloro-2-fluoroethylene has only a small effect on the primary interaction in both (E)-1-chloro2-fluoroethylene−HF15 and (E)-1-chloro-2-fluoroethylene− HCl,14 there is a significant shortening of the secondary bond length, suggesting that hyperconjugation may play a role in this interaction.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.8b07992.

■

Tables of observed and calculated transition frequencies for two isotopologues of trans-1,2-difluoroethylene−HCl that are reported in this study, the atomic coordinates for the theoretical structures shown in Figure 3 and the experimental structure shown in Figure 1e, the results of a fit that includes five lines with Ka = 4 for transCHFCHF−H35Cl, and the complete citation for Gaussian 09 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*H. O. Leung. Fax: +1-413-542-2735. E-mail: hleung@ amherst.edu. F

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The Journal of Physical Chemistry A *M. D. Marshall. Fax: +1-413-542-2735. E-mail: mdmarshall@ amherst.edu.

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ORCID

Mark D. Marshall: 0000-0002-4052-4840 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based on work supported by the National Science Foundation under Grant No. CHE-1465014. REFERENCES

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DOI: 10.1021/acs.jpca.8b07992 J. Phys. Chem. A XXXX, XXX, XXX−XXX