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The Microwave Spectrum and Molecular Structure of (E)‑1-Chloro-2Fluoroethylene−HF: Revealing the Balance among Electrostatics, Sterics, and Resonance in Intermolecular Interactions Helen O. Leung,* Mark D. Marshall,* and Alex J. Lee† Department of Chemistry, Amherst College, P.O. Box 5000, Amherst, Massachusetts 01002-5000, United States S Supporting Information *

ABSTRACT: The structure of the gas-phase bimolecular complex formed between (E)-1-chloro-2-fluoroethylene and hydrogen fluoride is determined via Fourier transform microwave spectroscopy from 6.9−21.6 GHz. Although the complex adopts a geometry very similar to that of previously studied dihalosubstituted ethylene−HF species, trends observed in the values of structural parameters such as bond lengths, bond angles, and deviations of the primary hydrogenbonding interaction from linearity provide information regarding the balance among electrostatic, steric, and resonance effects in the structures of these complexes. Consideration of the ab initio interaction potential between (E)-1-chloro-2-fluoroethylene and hydrogen fluoride suggests that it is the strength of the intermolecular bond formed between the hydrogen atom of HF and the fluorine atom of the substituted ethylene that plays the significant role in determining the geometry. In addition to determining the complete nuclear quadrupole coupling constant tensor for the (E)-1-chloro-2-fluoroethylene−HF complex, the corresponding tensor for (E)-1chloro-2-fluoroethylene itself was measured with greater precision than previously availabe, including the first reported determination of the single, nonzero off-diagonal element, χab.

1. INTRODUCTION As an extension of our understanding of the nature of intermolecular interactions between fluoroethylenes and protic acids, we have been conducting a systematic investigation of the effects of chlorine substitution in the ethylene subunit. For vinyl fluoride, where only one fluorine atom is present, the protic acids HF,1 HCl,2 and HCCH3 bind to the ethylene subunit in a similar manner: the acid forms a primary hydrogen bond with the F atom, which bends to allow a secondary interaction between the nucleophilic portion of the acid and the H atom located cis to the F atom in vinyl fluoride to give a planar or effectively planar complex. We call this the “top” binding mode. On the one hand, the structural parameters for these complexes provide important details regarding the strength of the interactions. On the other hand, for vinyl chloride, where only one chlorine atom is present, the binding modes for the three acids are all different. HF binds across the CC double bond in a manner similar to its vinyl fluoride counterpart (“top” binding),4 with the notable difference that the angle between the hydrogen bond and the C−halogen bond is only 102.4°, much smaller than the corresponding angle of 121.4° in the vinyl fluoride−HF complex, a reflection of the difference in electron density distribution of the two vinyl halide molecules. Although the complex still adopts a planar geometry, the binding mode of HCCH to vinyl chloride is different from its HF counterpart. HCCH still forms a hydrogen bond with Cl, © 2016 American Chemical Society

but with a second interaction between the acetylenic bond and the H atom geminal to Cl (“side” binding mode).5 HCl shows yet another mode of interaction with vinyl chloride: the complex is nonplanar and exhibits tunneling motion.6,7 Thus, acid identity does matter when it comes to binding with vinyl chloride. When both F and Cl are present in the ethylene subunit such as in 1-chloro-1-fluoroethylene, there are two possibilities for a protic acid to bind across the CC double bond: it can interact with the F and H atoms that are cis to each other or with the similarly situated Cl and H atoms in the ethylene subunit. For both HF and HCCH complexes,8,9 we have observed binding to the former pair, suggesting that the acids preferentially interact with the more nucleophilic F atom. In this work we explore the situation when the F and Cl atoms are located trans to each other by using (E)-1-chloro-2-fluoroethylene, and the acid we choose is HF. Our work with 1chloro-1-fluoroethylene−HF suggests that HF would likely bind to the F atom instead of the Cl atom, but (E)-1-chloro-2fluoroethylene also offers two additional binding modes: “side” binding to either F or Cl. The “side” binding mode involving HF has been observed in 1,1,2-trifluoroethylene−HF.10 Our Received: July 19, 2016 Revised: September 5, 2016 Published: September 17, 2016 7935

DOI: 10.1021/acs.jpca.6b07252 J. Phys. Chem. A 2016, 120, 7935−7946

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The Journal of Physical Chemistry A

Figure 1. Coordinate system used to describe the position of H in HF with respect to the plane of the (E)-1-chloro-2-fluoroethylene molecule. The point of reference is the center of the CC bond. H is placed at a distance of R from the origin and forms a polar angle of θ and azimuthal angle of ϕ. The relaxed potential scan is constructed at fixed values of θ, while R, ϕ, and the position of F in HF are optimized. Four minima are labeled on the minimum-energy path. Atom colors: carbon, dark gray; hydrogen, light gray; fluorine, light blue; chlorine, green.

Figure 2. Optimized structures (a−d) corresponding to the four minima found in the relaxed potential scan for (E)-1-chloro-2-fluoroethylene−HF showing chemically relevant geometric parameters. A fifth minimum with HF bonded with H and Cl across the double bond is shown in (e). Atom colors: carbon, dark gray; hydrogen, light gray; fluorine, light blue; chlorine, green.

2. AB INITIO CALCULATIONS FOR THE (E)-1-CHLORO-2-FLUOROETHYLENE−HF COMPLEX

goal is to examine how the presence of F and Cl trans to each other affects the electron density distribution in ethylene and thus, its ability to interact with HF. In our investigation, we also observe the rotational spectrum of uncomplexed (E)-1-chloro-2-fluoroethylene. This molecule has been previously studied extensively in the 75−800 GHz region by Cazzoli et al.11 We seek here to supplement this work by reporting our results in a lower frequency region and to measure more precisely the chlorine nuclear quadrupole coupling constants.

To guide our search for the spectrum of (E)-1-chloro-2fluoroethylene−HF, we turn to theory to calculate the interaction potential between the two subunits. With the structures of both species fixed at their ground-state, experimental configurations,12,13 we use the coordinate system shown in Figure 1 to locate the H atom of HF. The origin is at the center of the CC bond, and the ethylene defines the x−z plane. The polar angle θ is scanned in 10° steps, while R (the distance between H in HF and the origin), the azimuthal angle 7936


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The Journal of Physical Chemistry A ϕ, and the position of F in HF are optimized at each value of θ using ab initio calculations at the MP2/6-311G++(2d,2p) level performed with Gaussian 09.14 When necessary, finer values of θ are used to show more clearly the minimum-energy path. Four minima are found in this relaxed scan (Figure 1), and the structure at each minimum is then optimized. The chemically relevant geometric parameters for these structures are displayed in Figure 2a−d, and their relative energies and rotational constants are listed in Table 1. These minima do not represent

Cl, giving a long secondary interaction length. Consequently, Structure (a) is 94 cm−1 higher in energy than Structure (e). These two structures, however, are both much higher in energy than Structure (d), even though Structure (d) has a secondary H···F bond longer than the one in Structure (e) and comparable to that in Structure (a), suggesting that the H···F primary hydrogen bond contributes much more significantly to the stabilization of the complex compared to a H···Cl hydrogen bond. Interestingly, Structure (b), where HF interacts only with the Cl atom in the ethylene via an almost linear hydrogen bond (there is only a 5.7° deviation from linearity), is slightly lower in energy than Structure (a), where there are both primary and secondary interactions, indicating that the bend in the hydrogen bond in Structure (a) cannot be compensated by the stabilization arising from the secondary interaction. Putting all the arguments together, the computational results thus show that the stability of Structure (c) arises both from a stronger primary bond combined with a stronger secondary bond. We also corrected the ab initio energies of the five isomers due to basis set superposition error (BSSE), and the results are listed in Table 1. There is no reordering of the energies among the species. Under our molecular beam conditions, we expect to detect only Structure (c). The predicted dipole moment components for this isomer are μa = 1.27 D, μb = 1.85 D, and μc = 0 D.

Table 1. Relative Energies, without and with BSSE Corrections and Rotational Constants for Five Isomers of (E)-1-Chloro-2-fluoroethylene−HF (shown in Figure 2) Obtained from ab Initio Calculations relative energy/cm−1 structure

no BSSE correction

with BSSE correction

A/MHz

B/ MHz

C/ MHz

a b c d e

518 494 0 152 424

453 440 0 117 415

5437 4668 6480 11 232 3598

1449 1508 1428 1076 2093

1144 1189 1170 982 1341

all the local minima on the potential energy interaction surface, and in fact, in our preliminary computational work, we found a fifth minimum, which corresponds to a nonplanar structure where HF interacts with H and Cl located across the double bond in ethylene (Figure 2e and Table 1). The relative energy for this structure is 424 cm−1, much higher than the 259 cm−1 for the structure with the same polar angle (θ = 90.43°) shown in the minimum-energy path (Figure 1). The lower-energy structure is not a stationary point on the potential. The Cartesian coordinates for all structures in Figure 2 are available as Supporting Information. Four of the optimized structures show a hydrogen bond between HF and a halogen atom in (E)-1-chloro-2-fluoroethylene with a secondary interaction involving F in HF with an H atom in the ethylene subunit, but only three [Structures (a), (c), and (d)] are planar. In the two configurations where the hydrogen bond is formed with the F atom in (E)-1-chloro-2fluoroethylene [Structures (c) and (d)], the bond length of each is ∼1.87 Å, and the bend of the bond from linearity is ∼22°. The value of the CF···H angle in the side-binding configuration [Structure (d)] is typical of 1,1,2-trifluoroethylene−protic acid complexes,10,15,16 whereas that in the top-binding configuration [Structure (c)] is typical of protic acid complexes of vinyl fluoride,1−3 trans-1,2-difluoroethylene,17 and 1,1-difluoroethylene.18−20 On the one hand, the F atom in HF can form a favorable interaction with the H atom located cis to F in (E)-1-chloro-2-fluoroethylene, giving a bond length of 2.46 Å [Structure (c)]. On the other hand, the interaction length between F in HF and the H atom geminal to F in (E)-1-chloro-2-fluoroethylene in the side-binding configuration [Structure (d)] is too long (2.91 Å) to contribute much to the stability of the complex. Indeed, Structure (d) is 152 cm−1 higher in energy than Structure (c). In Structures (a) and (e), where the hydrogen bond is formed between HF and Cl in (E)-1-chloro-2-fluoroethylene, the lengths of this bond are both ∼2.3 Å. A 17° bend of the bond in Structure (e) allows F in HF to approach closely to the H atom located cis to Cl (2.70 Å), but even a larger bend in the hydrogen bond (21°) in Structure (a) still places F in HF far (3.01 Å) from the H atom geminal to

3. EXPERIMENT The spectra of six isotopologues of the (E)-1-chloro-2fluoroethylene monomer (CH 35 ClCHF, CH 35 ClCDF, CD35ClCHF, and their 37Cl counterparts) and eight isotopologues of the (E)-1-chloro-2-fluoroethylene−HF complex (formed by the six isotopologues of the ethylene subunit with HF and by CH35ClCHF and CH37ClCHF with DF) are studied using a narrow band cavity, Balle−Flygare Fourier transform microwave spectrometer in the 6.9−21.6 GHz region. The nondeuterated isotopologues are studied in natural abundance, while the deuterated ethylene species is a generous gift from Prof. Norman Craig at Oberlin College, and DF comes from Icon Isotopes. The microwave spectrometer has been described previously, and thus only a few details are presented here.21 An 0.8 mm pulsed nozzle is mounted behind one of the mirrors that form the Fabry−Pérot cavity. Both the (E)-1-chloro-2fluoroethylene monomer and its HF complex are studied in an expansion of 1% of (E)-1-chloro-2-fluoroethylene and 1% of HF (or DF) in argon through the nozzle at a stagnation pressure of ∼2 atm. The free induction decay is detected after down conversion in two steps to a center frequency of 2.5 MHz, digitized at a sampling frequency of 10 MHz, and is background corrected. Typically, signals are digitized for 2048 data points and zero filled to a 4096 record length before Fourier transformation to give a frequency domain signal with a resolution element of 2.4 kHz. The weaker signals, especially those for complexes containing both D and 37Cl in the ethylene subunit and the (E)-CH37ClCHF−DF isotopologues, are digitized for 1024 data points and zero filled to a 2048 record length, resulting in a 4.8 kHz resolution in the frequency domain signal. Since the molecular beam is parallel to the mirror axis, each transition appears as a Doppler doublet, and the rest frequency of the transition is the mean frequency of the two lines in the Doppler doublet. 7937


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4. RESULTS 4.1. Spectral Analysis. 4.1.1. (E)-1-Chloro-2-fluoroethylene. We observed 11−12 rotational transitions for each isotopologue of (E)-1-choro-2-fluoroethylene. These include J levels between 1 and at least 6, and Ka values of 0 and 1. Transitions in all isotopologues studied exhibit splitting due to the chlorine nuclear quadrupole coupling interaction and are further split when a deuterium nucleus is present. Additional splitting due to the fluorine nuclear spin-rotation coupling interaction is observed for some transitions. The rotational spectrum for each isotopologue is analyzed using the Watson A-reduced Hamiltonian in the Ir representation22 with the inclusion of nuclear quadrupolar coupling interactions for the Cl nucleus and, as appropriate, the nuclear quadrupolar coupling interaction for the D nucleus and the fluorine spinrotation coupling interaction. The previous rotational study by Cazzoli et al. of eight isotopologues of (E)-1-chloro-2-fluoroethylene,11 including the six isotopologues considered here and two doubly deuterated species (CD35ClCDF and CD37ClCDF), accessed levels with large values for J and Ka, allowing a more precise determination of the rotational constants and higher-order centrifugal distortion constants than we can achieve. Since our data are not sensitive to three of the quartic centrifugal distortion constants (ΔK, δJ, and δK), we fix them to those determined in the earlier study. Using Pickett’s nonlinear least-squares program,23 we are able to fit, for each isotopologue, the rotational constants and two quartic centrifugal distortion constants. However, the lower values of J and Ka accessed in the present work are affected to a greater extent by hyperfine interactions so that the diagonal components of the chlorine nuclear quadrupole coupling tensor are determined either to greater precision than in the earlier work or for the first time. It was also possible to fit the only nonzero off-diagonal component (χab) of the chlorine quadrupole coupling tensor for each isotopologue, thereby completing the entire chlorine quadrupole coupling tensor, and one component of the spinrotation coupling constant, Maa, due to the F nucleus (Tables 2 and 3). Where appropriate, the diagonal components of the deuterium quadrupole coupling tensor were determined. The rotational constants and the quartic centrifugal distortion constants ΔJ and ΔJK in our fit agree excellently with those determined in the earlier work. The root-mean-square (rms) error for each fit is ∼2 kHz, much smaller than a fraction of the observed line width (which is typically 5−7 kHz). Tables of observed and calculated transition frequencies with assignments for the six isotopologues are available as Supporting Information. 4.1.2. (E)-1-Chloro-2-fluoroethylene−HF. As each isotopologue of the dimer has a greater moment of inertia than the corresponding (E)-1-chloro-2-fluoroethylene monomer, the effects of spin rotation are not detectable in the complex. Once again, all transitions are split by the chlorine nuclear quadrupole coupling interaction. For the two nondeuterated ethylene−HF isotopologues (E)-CH35ClCHF−HF and (E)CH37ClCHF−HF, further splitting due to HF spin−spin coupling interaction is observed. An example is shown in Figure 3 for the most abundant isotopologue. For those isotopologues containing a deuterated ethylene, neither the effects of deuterium quadrupole coupling interaction nor HF spin−spin coupling interaction are observed, likely because both effects are so small that a combination of them is

Table 2. Spectroscopic Constants (in MHz, unless as otherwise noted) for Three 35Cl-Containing Isotopologues of (E)-1-Chloro-2-fluoroethylene, A B C ΔJ/1 × 10−3 ΔJK/1 × 10−3 ΔKc/1 × 10−3 δJc/1 × 10−3 δKc/1 × 10−3 χaa (Cl) χbb (Cl) χcc (Cl) |χab| (Cl)d χaa (D) χbb (D) χcc (D) Maa No. of rotational transitions No. of a type No. of b type No. of hyperfine components J range Ka range rms/kHz

CH35ClCHF

CH35ClCDF

CD35ClCHF

53 655.7302(29) 2476.606 48(17) 2366.410 00(15) 0.3177(28) −10.384(83) 759.992 0.020 720 2.2410 −63.6484(18) 27.8649(21) 35.7836(21) 34.4(11)

0.0518(27) 12

43 786.7307(21) 2466.657 51(15) 2334.139 01(13) 0.3201(31) −5.574(71) 342.026 0.0200 2.39 −63.1476(15) 27.3526(19) 35.7949(19) 36.10(63) −0.0813(26) 0.1859(24) −0.1045(26) 0.0422(26) 11

42 810.7588(12) 2476.127 56(13) 2339.704 16(11) 0.3142(25) −7.118(56) 502.694 0.024 74 2.086 −63.7822(13) 28.0412(16) 35.7410(15) 33.88(56) −0.0800(23) 0.1870(18) −0.1070(22) 0.0421(20) 12

9 3 91

9 2 168

9 3 187

1−8 0−1 2.22

1−6 0−1 2.32

1−6 0−1 1.96

a

1σ standard deviations in the parameters are given in parentheses. The nuclear quadrupole coupling constants of chlorine and deuterium, where appropriate, are fitted as 1.5χaa and (χbb − χcc)/4, and the Laplace condition is used to calculate the individual hyperfine constants. cFixed to corresponding values reported in ref 11. dThe signs of all χab values are determined to be negative. See Section 4.3 for details. b

unresolvable in a spectral line. In fact, according to ab initio calculations at the MP2/6-311G++(2d,2p) level, the magnitudes of the deuterium coupling constants χaa, χbb, and χcc in the dimer are just a fraction of those for the (E)-1-chloro-2fluoroethylene monomer. For the two DF-containing isotopologues, many of the chlorine hyperfine components are seen to be split further by the deuterium quadrupole coupling interaction (which, in accord with ab initio calculations, is of greater importance than that when deuterium is present in the (E)-1-chloro-2-fluoroethylene subunit). Both a- and b-type transitions are observed for each isotopologue of the complex, and we are able to access J levels between 0 and at least 7 and K levels between 0 and at least 2. The transitions for each species are analyzed using the Watson A-reduced Hamiltonian in the Ir representation22 with the inclusion of the chlorine nuclear quadrupole coupling interaction and, where appropriate, the HF spin−spin coupling interaction and the deuterium quadrupole coupling interaction. We are able to fit the rotational constants, four quartic centrifugal distortion constants, and the complete chlorine nuclear quadrupole coupling tensor (again, because of the planarity of the complex, only one off-diagonal component is nonzero). For the two nondeuterated ethylene−HF isotopologues and the two nondeuterated ethylene−DF isotopologues, we also determine the HF spin−spin coupling constants and 7938


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The Journal of Physical Chemistry A Table 3. Spectroscopic Constants (in MHz, unless as otherwise noted) for Three 37Cl-Containing Isotopologues of (E)-1-Chloro-2-fluoroethylene, A B C ΔJ/1 × 10−3 ΔJK/1 × 10−3 ΔKc/1 × 10−3 δJc/1 × 10−3 δKc/1 × 10−3 χaa (Cl) χbb (Cl) χcc (Cl) |χab| (Cl)d χaa (D) χbb (D) χcc (D) Maa No. of rotational transitions No. of a type No. of b type No. of hyperfine components J range Ka range rms/kHz

CH37ClCHF

CH37ClCDF

CD37ClCHF

53 612.9228(32) 2415.965 78(16) 2310.904 36(13) 0.3031(23) −10.315(83) 763.42 0.019 397 2.158 −50.2162(16) 22.0143(20) 28.2020(20) 26.30(93)

0.0521(23) 12

43 763.8288(19) 2405.686 60(14) 2279.407 68(12) 0.3007(28) −5.613(69) 343.05 0.020 37 2.15 −49.8181(14) 21.6102(19) 28.2079(18) 27.45(43) −0.0809(25) 0.1840(22) −0.1031(25) 0.0443(21) 11

42 766.9211(14) 2415.331 55(15) 2285.222 02(12) 0.3004(28) −7.126(62) 505.815 0.024 10 2.002 −50.3364(17) 22.1696(19) 28.1669(18) 26.35(66) −0.0794(29) 0.1856(22) −0.1062(27) 0.0439(22) 12

9 3 82

9 2 141

9 3 143

1−9 0−1 1.89

1−6 0−1 1.96

1−6 0−1 1.91

Figure 3. A portion of the 414−303 transition over a 0.5 MHz range showing two chlorine quadrupole hyperfine components, J + ICl = 7/ 2−5/2 (lower frequency) and 11/2−9/2 (higher frequency), for the most abundant isotopologue (E)-CH35ClCHF−HF. The Doppler doublets of each hyperfine component are connected by a comb. Each comb connects the doublets that are further split by the HF spin−spin coupling interaction. The corresponding doublets are linked by teeth of the same length.

(c); specifically, the values of the A constant differ by 4%, and those for the B and C constants differ by no more than 0.5%. Thus, the observed structure is that of HF binding across the CC double bond to the F and H atoms in the (E)-1-chloro2-fluoroethylene subunit. With four of the isotopologues [(E)-CH37ClCHF−HF, (E)CH35ClCDF−HF, (E)-CD35ClCHF−HF, (E)-CH35ClCHF− DF] containing a single substitution in (E)-CH35ClCHF−HF, we employ a Kraitchman analysis24 to determine the absolute values of the substitution coordinates for Cl and three H atoms (Table 6). By arbitrarily placing the Cl atom in the fourth quadrant, we can easily assign the location of the H atom connected to C-2, H2, to the first quadrant using the known structure of the (E)-1-chloro-2-fluoroethylene monomer. These two atoms establish the structure of the ethylene subunit; thus, even though the Kraitchman a-coordinate for the H atom connected to C-1, H1, is unphysical, indicating that it is close to the b axis, the only reasonable sign for its b coordinate is negative. For the HF subunit, the b coordinate of the H atom (HF) is unphysical, suggesting that the atom lies more or less on the a axis. The a coordinate of this H atom must therefore be opposite to that of the Cl atom in (E)-1-chloro-2fluoroethylene to give a plausible structure. These four substitutions again establish a structure consistent with Structure (c), where HF binds to (E)-1-chloro-2-fluoroethylene in the “top” binding mode. To obtain more precise geometric parameters, we assume that the structures of (E)-CHClCHF and HF remain unchanged upon complexation and perform least-squares fit to two of the three independent moments of inertia Ia and Ic of different combinations of isotopologues of the complex (Ia + Ib + Ic = 0 for a planar species). Only three geometric parameters are necessary to describe the planar complex: a distance between the subunits and the orientations of the subunits. We then translate the geometric parameters to chemically relevant

a

1σ standard deviations in the parameters are given in parentheses. The nuclear quadrupole coupling constants of chlorine and deuterium, where appropriate, are fitted as 1.5χaa and (χbb − χcc)/4, and the Laplace condition is used to calculate the individual hyperfine constants. cFixed to corresponding values reported in ref 11. dThe signs of all χab values are determined to be negative. See Section 4.3 for details. b

the diagonal deuterium quadrupole coupling constants, respectively. The one remaining quartic centrifugal distortion constant, ΔK, is strongly correlated with other parameters. If left unfitted, however, the rms errors for three species (CH35ClCHF−HF, CH37ClCHF−HF, and CH35ClCHF−DF) are 1−2 kHz greater than if this parameter is included. We thus find the best fitted value for ΔK and fix it at this value for each of the three isotopologues before running a final fit. For the other isotopologues, we fix the values of ΔK to that appropriate to one of these three isotopologues as noted in Tables 4 and 5, which contain the spectroscopic constants for all eight isotopologues. The rms error for each fit is between 1.4 and 2.1 kHz, commensurate with our measurement precision. Tables of observed and calculated transition frequencies with assignments for the eight isotopologues are available as Supporting Information. 4.2. Structure Determination for (E)-1-Chloro-2-fluoroethylene−HF. For each isotopologue of (E)-1-chloro-2fluoroethylene−HF, the inertial defect is small and positive (between 0.36 and 0.44 amu Å2), and the asymmetry parameter is between −0.90 and −0.88. These values are consistent with an asymmetric planar complex, where in-plane vibrational motions make the dominant contribution to the inertial defect. The experimentally determined rotational constants for the most abundant species agree excellently with those for structure 7939


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Table 4. Spectroscopic Constants (in MHz, unless as otherwise noted) for Four 35Cl-Containing Isotopologues of the (E)-1Chloro-2-fluoroethylene−HF Complex, A B C ΔJ/1 × 10−3 ΔJK/1 × 10−3 ΔK/1 × 10−3 δJ/1 × 10−3 δK/1 × 10−3 χaa (Cl) χbb (Cl) χcc (Cl) |χab| (Cl)g No. of rotational transitions No. of a type No. of b type No. of hyperfine components J range Ka range rms/kHz

CH35ClCHF−HFc

CH35ClCDF−HF

CD35ClCHF−HF

CH35ClCHF−DFd

6229.739 68(22) 1434.096 10(11) 1164.565 713(98) 1.892 04(73) −13.8073(57) 119.7e 0.52409(32) 6.823(46) −72.3125(10) 36.9660(12) 35.34659(97) 16.851(17) 42 30 12 431 0−8 0−3 1.87

5940.641 33(89) 1432.664 16(18) 1153.308 78(11) 1.869 88(79) −13.096(22) 119.7f 0.52827(86) 6.823f −72.2523(22) 36.8829(33) 35.3694(23) 17.314(23) 20 18 2 79 0−8 0−2 1.64

6128.151 90(67) 1437.951 38(14) 1163.704 797(82) 1.852 07(59) −13.052(12) 119.7f 0.51377(61) 6.823f −72.2850(15) 36.9444(24) 35.3406(20) 16.758(88) 21 19 2 74 0−8 0−2 1.16

6255.812 69(62) 1409.265 40(18) 1149.081 13(17) 1.7590(10) −13.0073(98) 107.6e 0.47754(53) 6.154(66) −72.3385(25) 36.9827(27) 35.3558(22) 16.8982(70) 33 25 8 243 0−8 0−2 2.07

1σ standard deviations in the parameters are given in parentheses. bThe nuclear quadrupole coupling constants of chlorine are fitted as 1.5χaa and (χbb − χcc)/4, and the Laplace condition is used to calculate the individual hyperfine constants. cThe HF nuclear spin−spin coupling constants are fitted as 1.5Daa and (Dbb − Dcc)/4, and the Laplace condition is used to calculate the individual hyperfine constants. The values of Daa, Dbb, and Dcc are 0.1239(39), −0.2383(21), and 0.1144(27) MHz, respectively. dThe nuclear quadrupole coupling constants of deuterium in DF are fitted as 1.5χaa and (χbb − χcc)/4, and the Laplace condition is used to calculate the individual hyperfine constants. The values of χaa, χbb, and χcc are −0.1295(37), 0.2527(29), and −0.1232(29) MHz, respectively. eThis value for ΔK gives the smallest rms deviation for a fit of all parameters. Since ΔK is correlated with A, a final fit is then performed by fixing ΔK to this specific value while allowing all other parameters to vary. fFixed to the value appropriate for CH35ClCHF−HF. gThe signs of all χab values are determined to be positive. See Section 4.3 for details. a

Table 5. Spectroscopic Constants (in MHz, unless as otherwise noted) for Four 37Cl-Containing Isotopologues of the (E)-1Chloro-2-fluoroethylene−HF Complex, A B C ΔJ/1 × 10−3 ΔJK/1 × 10−3 ΔK/1 × 10−3 δJ/1 × 10−3 δK/1 × 10−3 χaa (Cl) χbb (Cl) χcc (Cl) |χab| (Cl)g No. of rotational transitions No. of a type No. of b type No. of hyperfine components J range Ka range rms/kHz

CH37ClCHF−HFc

CH37ClCDF−HF

CD37ClCHF−HF

CH37ClCHF−DFd

6207.997 23(50) 1397.849 87(15) 1139.812 26(14) 1.772 91(88) −13.0099(87) 100.0e 0.483 81(49) 6.393(57) −57.1461(15) 29.2828(17) 27.8633(13) 12.7740(58) 34 25 9 304 0−8 0−2 1.86

5917.995 04(85) 1396.684 67(16) 1129.049 30(12) 1.752 52(68) −12.347(20) 100.0f 0.487 91(84) 6.393f −57.1145(21) 29.2350(38) 27.8795(28) 13.08(14) 18 16 2 71 0−7 0−2 1.38

6108.1311(12) 1401.522 26(19) 1139.030 18(12) 1.73420(88) −12.289(19) 100.0f 0.473 43(84) 6.393f −57.1245(33) 29.2818(62) 27.8427(58) 12.809(78) 17 15 2 63 0−8 0−2 1.64

6233.869 57(85) 1373.457 68(23) 1124.443 96(21) 1.6458(13) −12.248(13) 100.0f 0.439 71(79) 5.833(80) −57.1716(36) 29.3030(31) 27.8686(32) 12.76(11) 26 19 7 150 0−8 0−2 2.10

a 1σ standard deviations in the parameters are given in parentheses. bThe nuclear quadrupole coupling constants of chlorine are fitted as 1.5χaa and (χbb − χcc)/4, and the Laplace condition is used to calculate the individual hyperfine constants. cThe HF nuclear spin−spin coupling constants are fitted as 1.5Daa and (Dbb − Dcc)/4, and the Laplace condition is used to calculate the individual hyperfine constants. The values of Daa, Dbb, and Dcc are 0.1258(57), −0.2456(27), and 0.1198(41) MHz, respectively. dThe nuclear quadrupole coupling constants of deuterium in DF are fitted as 1.5χaa and (χbb − χcc)/4, and the Laplace condition is used to calculate the individual hyperfine constants. The values of χaa, χbb, and χcc are −0.1521(66), 0.2659(43), and −0.1138(42) MHz, respectively. eThis value for ΔK is one that gives the smallest rms deviation for a fit of all parameters. Since ΔK is correlated with A, a final fit is then performed by fixing ΔK to this specific value while allowing all other parameters to vary. fFixed to the value appropriate for CH37ClCHF−HF. gThe signs of all χab values are determined to be positive. See Section 4.3 for details.

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The Journal of Physical Chemistry A Table 6. Coordinates of Atoms with a Single Substitution in (E)-CHClCHF−HF Cl

H1

(i) substitution coordinatesb,c a/Å 2.159 20(69) unphysical b/Å −0.3873(39) −1.1598(13) (ii) from fit to moments of inertia using all isotopologues a/Å 2.1625 −0.3056 b/Å −0.3836 −0.9537 (iii) from fit to moments of inertia using six HF-containing isotopologues a/Å 2.1623 −0.3059 b/Å −0.3834 −0.9533 (iv) from fit to moments of inertia using two HF-nondeuterated ethylene isotopologues a/Å 2.1631 −0.3049 b/Å −0.3833 −0.9540 (v) from fit to moments of inertia using two DF-containing isotopologues a/Å 2.1874 −0.2813 b/Å −0.3846 −0.9525

H2

HF

0.5903(25) 1.99194(75)

−2.498 07(60) unphysical

0.5195 2.0030

−2.4592 −0.2989

0.5195 2.0034

−2.5493 −0.2755

0.5196 2.0030

−2.5516 −0.2740

0.5467 2.0036

−2.4533 −0.2835

a

H1, H2, and HF refer to the H atoms connected to C-1, C-2, and F in HF, respectively. bCostain errors27 in the parameters are given in parentheses. A Kraitchman analysis gives the absolute values of the substitution coordinates. The signs are derived from a consideration of known distances between atoms and, in the case of HF, a reasonable distance between itself and the ethylene subunit. c

upon complexation, the HF nuclear spin−spin coupling constants depend only on the orientation of the subunit in the complex. Thus, the coupling constants can supply information on the average orientation of the HF subunit. Specifically, the spin−spin coupling constant along the g axis in the complex, Dgg, is simply a ⟨P2⟩ projection of the spin−spin coupling constant of free HF, Dmonomer = −286.75(5) kHz.25,26

ones: the angle formed by the hydrogen bond and the C−F bond (∠CF···H), the bond length of the H···F hydrogen bond (rprimary), and the deviation of the hydrogen bond from linearity (δHF). These parameters, together with the calculated value of the distance between F in HF and the H atom located cis to F in (E)-1-chloro-2-fluoroethylene (rsecondary), are listed in Table 7. The first fit utilizes all isotopologues. Recognizing that HF Table 7. Geometric Parameters Obtained from Three Fits to the Moments of Inertia (Ia and Ic) of (E)-1-Chloro-2fluoroethylene−HF rprimary/Å

rsecondary/Å

∠CF···H/deg

using all isotopologues 1.906(17) 2.4563(35) 115.6(18) using six HF-containing isotopologues 1.9399(19) 2.4510(26) 118.200(55) using two HF-nondeuterated ethylene isotopologues 1.93954(14) 2.4541(2) 118.3235(40) using two DF-containing isotopologues 1.90685(13) 2.4575(2) 116.3086(37) a b

δHF/deg

ΔIrmsb/ amu Å2

16.0(56)

0.324

24.75c

0.300

24.85c

0.012

17.79d

0.012

Dgg =

3 cos2 θg − 1 2

Dmonomer

In this equation, θg denotes the angle between the g axis of the complex and the HF axis. Using the values of spin−spin coupling constants for the most abundant isotopologue, the values of θg (or more precisely, cos−1 ⟨cos2 θg⟩ ) are calculated and listed in Table 8. Similarly, average angular Table 8. Values of θg Determined Using Hydrogen Fluoride Nuclear Spin−Spin Coupling Constants and Deuterium Quadrupole Coupling Constants

The uncertainties r epr ese nt 1 σ standard de viations. ΔIrms

θa/deg θb/deg θc/deg

= ∑ (Iobs − Icalc)2 /(No. of observables − No. of parameters ). Fixed at the value to reproduce the angle between HF and the a axis given by the HF spin−spin coupling constant Daa. dFixed at the value to reproduce the angle between DF and the a axis given by the deuterium quadrupole coupling constant χaa.

c

a

HF spin−spin

D quadrupole

77.7(13) 19.62(46) 74.96(73)

72.59(71) 25.92(40) 71.43(52)

1σ standard deviations in the parameters are given in parentheses.

information can be obtained for the DF-containing isotopologues. Making the usual assumption that there is negligible electric field gradient perturbation in the DF subunit upon complexation, an analogous equation to that given above for the spin−spin coupling constants relates the nuclear quadrupole coupling constants of DF in the complex, χgg, to that of the monomer, χmonomer = 354.238(78) kHz.25 The values of θg for (E)-CH35ClCHF−DF so calculated are also listed in Table 8. Indeed, these values differ from those calculated from HF spin−spin coupling constants by a few degrees, suggesting that the HF and DF isotopologues do exhibit slightly different average structures.

and DF could have different zero-point motions in the complex and that the same may be true, albeit to a lesser extent, for the deuterated and nondeuterated ethylenes, we perform three additional fits using all six HF-containing isotopologues, two HF-nondeuterated ethylene isotopologues, and two DFcontaining isotopologues, respectively. The orientations of HF and DF, however, cannot be determined from these latter three fits that involve only a portion of the isotopologues; we thus seek to use hyperfine constants to locate these subunits. Continuing with the assumption that the HF bond length remains unchanged 7941


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Figure 4. Inertial axes of (E)-CH35ClCHF (drawn from structure given in ref 12) and (E)-CH35ClCHF−HF (this work). Atom colors: carbon, dark gray; hydrogen, light gray; fluorine, light blue; chlorine, green.

nuclear quadrupole coupling tensors for both the most abundant isotopologues of the (E)-1-chloro-2-fluoroethylene monomer and of the (E)-1-chloro-2-fluoroethylene−HF complex, we are able to determine the signs consistent with the directions chosen for the positive coordinates in the inertial axis system for the χab components by recognizing the angle required to rotate the inertial axis system of the monomer to that of the complex should be the same as the angle required to rotate the nuclear quadrupole coupling tensor of the monomer to that of the complex. With the arrangement of atoms shown in Figure 4a for (E)-CHClCHF and in Figure 4b for the HF complex, the rotational matrix connecting the two inertial tensors rotates the quadrupole coupling tensor of the monomer to reproduce the values of χaa and χbb for the dimer to within 0.8% and 0.3%, respectively. Using a positive or a negative χab value for the monomer the rotation gives 57.24 and 15.94 MHz, respectively, as the χab values for the rotated quadrupole tensor. Because the experimental magnitude for χab in the dimer is 16.851 MHz, we can definitively assign a negative sign to χab in the monomer, and a positive sign to that in the dimer. The rotated and experimental values of χab of the dimer differ only by 5.7%. The signs of χab for other isotopologues of the monomer and complex are similarly determined. Diagonalizing the chlorine quadrupole coupling tensor for the most abundant (E)-1-chloro-2-fluoroethylene monomer, we obtain the values of the coupling components in the principal electric field gradient axis system, χxx, χyy, and χzz, and they are 39.3536, 35.7836, and −75.1371 MHz, respectively. The y axis is perpendicular to the molecular plane and thus is identical to the c inertial axis. The magnitude of the asymmetry parameter for the coupling constants, |η| = |(χxx − χyy)/χzz| = 0.0475, suggests that there is a small deviation from a cylindrical electronic charge distribution about the z electric field gradient axis. Performing the same calculations for the most abundant (E)-1-chloro-2-fluoroethylene−HF complex, the values of χxx, χyy, and χzz are found to be, respectively, 39.5054, 35.3466, and −74.8520 MHz, giving 0.0556 as the

Fixing the deviation from linearity of the hydrogen bond formed by HF to reproduce the value of θa derived from the a component of the HF spin−spin coupling constant, we perform the two fits to the six HF-containing isotopologues and to two HF-nondeuterated ethylene isotopologues allowing the CF···H angle and rprimary to vary. Similarly, we perform another fit by fixing the position of DF to reproduce the value of θa derived from deuterium quadrupole coupling constants for the two DFcontaining isotopologues. The results are listed in Table 7. The two fits using HF-containing species give almost identical results. Furthermore, it is noteworthy to mention that the values of CF···H angle for these three fits are similar and within 2° [118° for the HF species and 116° for the DF species]. The hyperfine data, however, results in a 7° difference in the values for the angle of deviation of the acid from linearity (25° for HF and 18° for DF), which translates to a difference of 0.033 Å in length for the primary bond for the HF and DF isotopologues [1.940 Å for H···F and 1.907 Å for D···F]. On the other hand, the secondary interaction distance is remarkably constant in all four fits, further indicating that the fluorine atom of the HF (or DF) is well-located with most of the zero-point motion, as expected, occurring in the lighter atom. The Kraitchman coordinates of the four atoms with a single substitution, previously determined, are compared with those obtained in each of the four structure fits in Table 6. The coordinates for Cl determined by the structure fits agree well with the substitution coordinates. The agreement is not quite as good for the coordinates of the hydrogen atoms, which is not surprising given the fact that hydrogen and deuterium do exhibit rather different zero-point motions. Because of the structural difference in the HF- and DF-containing species and because the two fits involving only HF-containing species are similar, we will take the structure determined by the six HF-containing isotopologues to represent (E)-CHClCHF−HF in the following Discussion. 4.3. Chlorine Nuclear Quadrupole Coupling Constants. With the determination of the complete chlorine 7942


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The Journal of Physical Chemistry A absolute value for the asymmetry parameter. The values of these quadrupole constants differ from their (E)-1-chloro-2fluoroethylene counterparts by between 0.4% and 1.2%. These small differences, together with the fact that the intermolecular interaction between HF and the ethylene likely occurs too far away to disturb the electronic environment at Cl, serve to indicate a slight change in zero-point motion when the ethylene is complexed. From the diagonalization, the angle between the z axis of the quadrupole coupling tensor and the a axis of the inertial system is found to be 18.5° in (E)-1-chloro-2-fluoroethylene. From the structure of the molecule, the angle between the a axis and the C−Cl bond is 19.0°. For the dimer, the respective values are 8.6° and 7.5°. From these we can conclude that the z electric field gradient axis lies almost along the C−Cl bond in both (E)1-chloro-2-fluoroethylene and its HF complex. Because the c axis is perpendicular to the plane of the molecule, it has the same orientation for all species considered here. Therefore, the values for χcc of the 35Cl nucleus should be identical in all the isotopologues of both the monomer and the complex containing this nucleus and similarly for 37Clcontaining species. Indeed, the values differ by no more than 0.15% among the corresponding monomer species and similarly when comparing only the dimer species. However, the value in the dimer is consistently smaller than that for the corresponding monomer by 1.2%. This is another indication that there is merely a difference in zero-point motions between (E)-CHClCHF and its complex rather than an electric field gradient perturbation at the Cl nucleus upon complexation. In fact, in the complex, the zero-point motion of the HF subunit is quite large, making an average angle of 15° with the molecular plane (the complement to θc in Table 8).

Figure 5. Structures of (a) (E)-1-chloro-2-fluoroethylene−HF (this work), (b) 1-chloro-1-fluoroethylene−HF (ref 8), (c) trans-1,2difluoroethylene−HF (ref 17), and (d) 1,1-difluoroethylene−HF (ref 18). Atom colors: carbon, dark gray; hydrogen, light gray; fluorine, light blue; chlorine, green.

helpful in providing insights regarding how chlorine affects the nature of intermolecular interactions of the ethylene subunit. For the HF−difluoroethylene complexes, the position of the second F atom does exert a noticeable effect in the manner in which the ethylene interacts with HF. The hydrogen bond is 0.078 Å longer, and the secondary bond is 0.173 Å longer in 1,1-difluoroethylene−HF than the corresponding bonds in trans-1,2-difluoroethylene−HF. Previously, we suggested that the second F atom in the ethylene subunit, when placed only two bonds away from the hydrogen-bonded F atom (as in 1,1difluoroethylene), can reduce the nucleophilicity of the hydrogen-bonded F atom more effectively than when it is placed three bonds away (as in trans-1,2-difluoroethylene).17 Furthermore, the longer secondary bond in 1,1-difluoroethylene−HF indicates that the H atom of the ethylene that participates in the interaction is less electropositive than the corresponding atom in trans-1,2-difluoroethylene. Indeed, these are corroborated by mapping the electrostatic potential of each ethylene subunit onto the electron density surface, calculated at the MP2/6-311G++(2d,2p) level (Figure 6), as was done previously for 1,1-difluoroethylene (Figure 6d) and trans-1,2difluoroethylene (Figure 6c).17 Focusing on the F and H atoms in these two ethylenes that participate in intermolecular interactions, for 1,1-difluoroethylene, the F atom is less negative and the H atom is less positive than their counterparts in trans-1,2-difluoroethylene. Similar trends are observed, albeit to a different extent, when both F and Cl atoms are present in the ethylene subunit and each is located cis to an H atom. For these complexes, as in the case for the difluoroethylene complexes, HF binds to the F and H pair at each end of the double bond. The hydrogen bond in

5. DISCUSSION The experimental average structure of (E)-1-chloro-2-fluoroethylene−HF agrees very well with the theoretical lowestenergy structure [Structure (c)]. In fact, the experimental value of the CF···H angle differs from the theoretical value by 0.3%, and the same deviation is found as well between the experimental and theoretical values for the secondary bond length. In absolute terms, the experimentally determined average values of 118.200(55)o for the CF···H angle and of 2.4510(26) Å for the secondary bond length are smaller than the theoretical equilibrium values by 0.4° and 0.008 Å, respectively. The largest differences between experiment and theory are the primary hydrogen bond length and the extent it deviates from linearity. Specifically, the experimental hydrogen bond length, 1.9399(19) Å, is 0.067 Å (or 3.6%) longer, and the 24.75° deviation from linearity is 2.4° (or 10.5%) larger than the corresponding theoretical values. These differences, however, are expected because the zero-point motion of the subunits, especially the H atom in HF, is likely very significant. The binding mode of HF in (E)-1-chloro-2-fluoroethylene− HF is the same as that for the HF complexes of vinyl fluoride,1 1,1-difluoroethylene,18 trans-1,2-difluoroethylene,17 and 1chloro-1-fluoroethylene,8 namely, HF binds across the CC double bond, and the interactions are with F and H on the two ends of this bond. Of these complexes, vinyl fluoride−HF has the shortest, and hence strongest, hydrogen bond [1.892(14) Å], which is not surprising, because there is no other halogen atom in the ethylene subunit to compete with the F atom for electron density. A comparison among the four complexes formed by disubstituted ethylene (Figure 5) is instructive and 7943


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Figure 6. Electrostatic potential of (a) (E)-1-chloro-2-fluoroethylene, (b) 1-chloro-1-fluoroethylene, (c) trans-1,2-difluoroethylene, and (d) 1,1difluoroethylene. Blue color represents positive electrostatic potential, and red indicates negative electrostatic potential.

(E)-1-chloro-2-fluoroethylene is only very slightly shorter (by 0.008 Å) than that in 1-chloro-1-fluoroethylene−HF, suggesting that the position of Cl (geminal or trans) relative to F affects the nucleophilicity of the F atom to only a small extent: Cl draws slightly more electron density away from F when it is in the geminal position. There is, however, a significant difference in the length of the secondary interaction: it is 0.288 Å shorter in the (E)-1-chloro-2-fluoroethylene, indicating that Cl, when placed two bonds away from the H atom that interacts with HF, is more effective in drawing electron density from H than when it is three bonds away as in 1-chloro-1fluoroethylene. These observations are supported by the electrostatic potential maps of the chlorofluoroethylene (Figure 6). The F atom in (E)-1-chloro-2-fluoroethylene is more negative, while the H atom located in the cis position is more positive than the corresponding atoms in 1-chloro-1-fluoroethylene. When all four complexes are compared, it appears that a second F atom exerts the greatest influence on the hydrogen bond when it is placed geminal to the hydrogen-bonded F atom, followed by, in order of decreasing influence, the presence of a geminal Cl, the presence of Cl in the trans position, and finally the presence of F in the trans position (Figure 5). Specifically, (E)-1-chloro-2-fluoroethylene−HF has a longer hydrogen bond (by 0.030 Å) than trans-1,2-

difluoroethylene. On the one hand, the fact that the hydrogen-bonded F atom is less nucleophilic when the second halogen atom located in the trans position is Cl instead of F suggests that the less electronegative Cl can withdraw electron density from the first F atom through resonance via a contribution from a Lewis structure having a double bond between each halogen and its respective carbon, a carbon− carbon single bond, and a negative (positive) formal charge on chlorine (fluorine). On the other hand, the hydrogen bond of 1-chloro-1-fluoroethylene is 0.040 Å shorter than that in 1,1difluoroethylene, indicating that the hydrogen-bonded F atom is less nucleophilic when the second halogen in the geminal position is F instead of Cl. Thus, the inductive effect of the second F atom withdraws electron density from the first F atom more effectively compared to the combined inductive and resonance effects of a geminal Cl atom. It is interesting to note that the secondary interaction in (E)-1-chloro-2-fluoroethylene−HF is the shortest in the four complexes. The H atom in the interaction should be less positive than that in trans-1,2difluoroethylene−HF because the geminal Cl should withdraw less electron density than the geminal F; thus, the secondary interaction in the former species must confer enough stability for the additional 3° bend in the hydrogen bond observed in the latter complex. It is also noteworthy that although the H atom geminal to F in (E)-1-chloro-2-fluoroethylene is more 7944



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positive than the one in the trans position, the secondary bond is formed by F in HF to the trans hydrogen, supporting the fact that the top binding configuration in this complex is sterically more stable than the side binding configuration.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (H.O.L.) *Phone: (413) 542-2006. Fax: (413) 542-2735. E-mail: [email protected]. (M.D.M.)

6. CONCLUSIONS As it does in forming complexes with previously studied 1,1and trans-1,2-dihalogenethylenes, HF binds across the CC double bond of (E)-1-chloro-2-fluoroethylene, forming a planar species with a primary hydrogen-bonding interaction between the hydrogen atom of HF and a fluorine atom in the ethylene (Figure 5). A secondary interaction to a hydrogen atom cis to the fluorine confers further stability to the interaction and allows the hydrogen bond to depart from linearity. Nevertheless, a detailed examination of the structural parameters of the four complexes HF forms with 1,1-difluoroethylene, 1chloro-1-fluoroethylene, trans-1,2-difluoroethylene, and (E)-1chloro-2-fluoroethylene reveals the effects of both the location and identity of the second halogen in changing the electron density of the ethylene. A comparison of the two difluoro species shows that the inductive effect of a fluorine atom is better at withdrawing electron density from an atom bonded to the same carbon atom (i.e., geminal) than it is for an atom situated in the trans position.17 A similar comparison between the chlorofluoro analogues leads to the same conclusion regarding the (not surprisingly weaker) electron withdrawing ability of chlorine upon atoms at the geminal and cis locations. However, cross comparisons that consider the 1,1- and transdisubstituted pairs indicate that a trans chlorine atom is more effective at electron withdrawal than a similarly situated fluorine atom and suggests that resonance effects come into play. In all four complexes, the primary interaction is determined by electrostatics and is formed at the most nucleophilic site of the ethylene. Both 1,1-difluoroethylene−HF and 1-chloro-1fluoroethylene−HF can only accommodate a secondary interaction in what we term the top configuration, or binding across the CC double bond. The two trans-1,2-dihalogen− HF complexes each offer the option of binding at one end of the CC bond in the so-called side bonding motif. Even though this would involve a more electropositive geminal hydrogen atom in the secondary interaction, unfavorable steric requirements forced upon the hydrogen bond in the sidebinding geometry lead to top binding in these species as well. Consideration of higher-energy structures predicted by ab initio calculations confirm the importance of the H···F hydrogen bond in determining the global minimum energy structure and suggest that the observed experimental structure provides the best balance of a strong primary, hydrogen-bonding interaction with the most favorable secondary interaction compatible with it.

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Article

Present Address †

University of Texas at Austin, 200 E. Dean Keeton St., Stop C0400, Austin, TX, USA 78712. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This material is based on work supported by the National Science Foundation under Grant No. CHE-1465014. We are grateful to Prof. N. Craig at Oberlin College for supplying us with deuterated (E)-1-chloro-2-fluoroethylene.

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REFERENCES

(1) Cole, G. C.; Legon, A. C. A characterisation of the complex vinyl fluoride•••hydrogen fluoride by rotational spectroscopy and ab initio calculations. Chem. Phys. Lett. 2004, 400, 419−424. (2) Kisiel, Z.; Fowler, P. W.; Legon, A. C. Rotational spectrum, structure, and chlorine nuclear quadrupole tensor of the vinyl fluoridehydrogen chloride dimer. J. Chem. Phys. 1990, 93, 3054−3062. (3) Cole, G. C.; Legon, A. C. Non-linearity of weak B ••• H-C hydrogen bonds: an investigation of a complex of vinyl fluoride and ethyne by rotational spectroscopy. Chem. Phys. Lett. 2003, 369, 31−40. (4) Leung, H. O.; Marshall, M. D. The effect of acid identity on the geometry of intermolecular complexes: The microwave spectrum and molecular structure of vinyl chloride-HF. J. Phys. Chem. A 2014, 118, 9783−9790. (5) Leung, H. O.; Marshall, M. D.; Feng, F. The microwave spectrum and molecular structure of vinyl chloride-acetylene, a side-binding complex. J. Phys. Chem. A 2013, 117, 13419−13428. (6) Messinger, J. P.; Leung, H. O.; Marshall, M. D. The effect of protic acid identity on the structures of complexes with vinyl chloride: Fourier transform spectroscopy and molecular structure of the vinyl chloride-hydrogen chloride complex. 69th International Symposium on Molecular Spectroscopy, Urbana-Champaign, IL, 2014; Talk TE07.110.15278/isms.2014.TE07 (7) Leung, H. O.; Marshall, M. D.; Messinger, J. P. Chlorine nuclear quadrupole hyperfine structure in the vinyl chloride-hydrogen chloride complex. 70th International Symposium on Molecular Spectroscopy, Urbana-Champaign, IL, 2015; Talk WJ06.110.15278/isms.2015.WJ06 (8) Leung, H. O.; Marshall, M. D.; Bozzi, A. T.; Cohen, P. M.; Lam, M. Microwave spectrum and molecular structure of the 1-chloro-1fluoroethylene-hydrogen fluoride complex. J. Mol. Spectrosc. 2011, 267, 43−49. (9) Leung, H. O.; Marshall, M. D.; Grimes, D. D. Rotational spectroscopy and molecular structure of the 1-chloro-1-fluoroethyleneacetylene complex. J. Chem. Phys. 2011, 134, 034303. (10) Leung, H. O.; Marshall, M. D. Rotational spectroscopy and molecular structure of 1,1,2-trifluoroethylene and the 1,1,2-trifluoroethylene-hydrogen fluoride complex. J. Chem. Phys. 2007, 126, 114310. (11) Cazzoli, G.; Puzzarini, C.; Gambi, A.; Gauss, J. Rotational spectra of 1-chloro-2-fluoroethylene. I. Main isotopologues and deuterated species of the trans isomer. J. Chem. Phys. 2006, 125, 054313. (12) Puzzarini, C.; Cazzoli, G.; Gambi, A.; Gauss, J. Rotational spectra of 1-chloro-2-fluoroethylene. II. Equilibrium structures of the cis and trans isomer. J. Chem. Phys. 2006, 125, 054307. (13) Guelachvili, G. Absolute wavenumber measurements of 1−0, 2− 0, HF and 2−0, H35Cl, H37Cl absorption bands. Opt. Commun. 1976, 19, 150−154. (14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b07252. Tables of observed and calculated transition frequencies for all isotopologues of vinyl chloride−HF that are reported in this study, atomic coordinates for the structures shown in Figure 2 and the complete citation for Gaussian 09. (PDF) 7945


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The Journal of Physical Chemistry A B.; Petersson, G. A., et al. Gaussian 09, Revision B.01; Gaussian, Inc: Wallingford, CT, 2009. (15) Leung, H. O.; Marshall, M. D.; Ray, M. R.; Kang, J. T. Rotational spectroscopy and molecular structure of the 1,1,2trifluoroethylene-hydrogen chloride complex. J. Phys. Chem. A 2010, 114, 10975−10980. (16) Leung, H. O.; Marshall, M. D.; Cashion, W. T.; Chen, V. L. Rotational spectroscopy and molecular structure of the 1,1,2trifluoroethylene-acetylene complex. J. Chem. Phys. 2008, 128, 064315. (17) Leung, H. O.; Marshall, M. D.; Amberger, B. K. Fourier transform microwave spectroscopy and molecular structure of the trans-1,2-difluoroethylene−hydrogen fluoride complex. J. Chem. Phys. 2009, 131, 204302. (18) Leung, H. O.; Marshall, M. D.; Drake, T. L.; Pudlik, T.; Savji, N.; McCune, D. W. Fourier transform microwave spectroscopy and molecular structure of the 1,1-difluoroethylene−hydrogen fluoride complex. J. Chem. Phys. 2009, 131, 204301. (19) Kisiel, Z.; Fowler, P. W.; Legon, A. C. Investigation of the rotational spectrum of the hydrogen-bonded dimer CF2CH2•••HCl. J. Chem. Soc., Faraday Trans. 1992, 88, 3385−3391. (20) Leung, H. O.; Marshall, M. D. Rotational spectroscopy and molecular structure of the 1,1-difluoroethylene-acetylene complex. J. Chem. Phys. 2006, 125, 154301. (21) Leung, H. O.; Gangwani, D.; Grabow, J. U. Nuclear quadrupole hyperfine structure in the microwave spectrum of Ar-N2O. J. Mol. Spectrosc. 1997, 184, 106−112. (22) Watson, J. K. G. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier Scientific Publishing: Amsterdam, 1977; Vol. 1, pp 1−89. (23) Pickett, H. M. The fitting and prediction of vibration-rotation spectra with spin interactions. J. Mol. Spectrosc. 1991, 148, 371−377. (24) Kraitchman, J. Determination of molecular structure from microwave spectroscopic data. Am. J. Phys. 1953, 21, 17−24. (25) Muenter, J. S.; Klemperer, W. Hyperfine constants of HF and DF. J. Chem. Phys. 1970, 52, 6033−6037. (26) Shea, J. A.; Flygare, W. H. The rotational spectrum and molecular structure of the ethylene-HF complex. J. Chem. Phys. 1982, 76, 4857−4864. (27) Costain, C. C. Determination of Molecular Structures from Ground State Rotational Constants. J. Chem. Phys. 1958, 29, 864−874.

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