Effect of Chlorine Substitution in Modulating the Relative Importance of

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Effect of Chlorine Substitution in Modulating the Relative Importance of Two Intermolecular Interactions: The Microwave Spectrum and Molecular Structure of (E)‑1-Chloro-2-fluoroethylene− HCl Helen O. Leung* and Mark D. Marshall* Department of Chemistry, Amherst College, P.O. Box 5000, Amherst, Massachusetts 01002-5000, United States S Supporting Information *

ABSTRACT: Fourier transform microwave rotational spectroscopy is used to determine the structure of the gas-phase bimolecular complex formed between (E)-1-chloro-2-fluoroethylene and hydrogen chloride. Extensively split by nuclear quadrupole hyperfine structure and isotopic dilution, the spectrum is first identified via weak features observed using a broadband chirped pulse spectrometer in the 5.6−18.1 GHz range and studied in detail with greater sensitivity and resolution over 6.0−20.0 GHz with a Balle−Flygare, narrowband instrument. The complex has a geometry similar to that of vinyl fluoride−HCl, with HCl binding across the CC double bond, forming a hydrogen bond to the fluorine atom of the haloethylene and bending to allow a secondary interaction to develop with the hydrogen atom in the cis position. Further consideration of structural details among the complexes of hydrogen fluoride and hydrogen chloride with (E)-1-chloro-2-fluoroethylene and vinyl fluoride suggests that the addition of a trans Cl atom in vinyl fluoride enhances the significance of the secondary interaction while deemphasizing that of the hydrogen bond.

1. INTRODUCTION

currently extending our investigations to other examples of this type of complex. The situation is rather different for vinyl chloride complexes. Although vinyl chloride−HF (ref 12) adopts a binding mode similar to that of its vinyl fluoride counterpart, vinyl chloride− HCl is nonplanar,13,14 with HCl binding across the double bond, and the complex exhibits a tunneling motion between the two equivalent configurations with HCl located on either side of the vinyl chloride plane. Vinyl chloride−HCCH adopts yet another motif:11 a side binding mode with the acid interacting with Cl and with the H atom bonded to the same carbon. We have been able to rationalize the structures of the HF and HCCH complexes by considering the electron distribution at Cl in vinyl chloride as well as electrostatic and steric factors in the complexes. Analysis for the vinyl chloride−HCl complex is ongoing. We also have information on two HF complexes with ethylenes that contain one F and one Cl atom. In 1-chloro-1fluoroethylene−HF (ref 15) and (E)-1-chloro-2-fluoroethylene−HF,16 HF adopts a top binding mode, interacting with F and H cis to each other in the ethylene. Clearly then, if we consider these ethylene subunits as vinyl fluoride with Cl as a second substitutent, then the presence of Cl does not alter the

In our efforts to examine how changes in the electron density of a molecule affect its ability to form intermolecular interactions, we have been studying molecular complexes that contain haloethylenes and protic acids. Specifically, the electronic environment of the ethylene subunit is tuned by using a different number and type of halogen substituent and observing how it binds to three gas-phase acids of decreasing strength: HF, HCl, and HCCH. Many of these complexes with ethylenes containing fluorine atoms and at least one hydrogen atom have already been studied.1−10 To summarize, these complexes are all effectively planar, with the acid forming a hydrogen bond to an F atom in the ethylene subunit. A bend in the hydrogen bond allows a secondary interaction between the nucleophilic portion of the acid and an H atom in the ethylene. It is interesting to note that the binding mode in each fluoroethylene complex does not depend on the identity of the acid, but the mode changes from “top” binding (with HX binding across the double bond) in vinyl fluoride and 1,1-difluoroethylene complexes to “side” binding (with HX binding at one end of the double bond) in complexes with 1,1,2-trifluoroethylene. The only 1,2-difluoroethylene−HX complex studied thus far is trans-1,2-difluoroethylene−HF, and it adopts a “top” binding configuration. Of course, for each ethylene, the values of the geometric parameters do depend on which acid is present, and trends have been summarized elsewhere.11 We are © 2016 American Chemical Society

Received: August 1, 2016 Revised: September 21, 2016 Published: September 22, 2016 7955

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Figure 1. Coordinate system used to describe the position of H in HCl 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 Cl in HCl are optimized. Four minima are labeled on the minimum energy path. The carbon atoms are dark gray, the hydrogen atoms are light gray, the fluorine atom is light blue, and the chlorine atoms are green.

Figure 2. Optimized structures (structures a−d) corresponding to the four minima found in the relaxed potential scan for (E)-1-chloro-2fluoroethylene−HCl showing chemically relevant geometric parameters. A fifth structure with HCl bonded with H and F across the double bond, forming a planar complex is shown in (e). The carbon atoms are dark gray, the hydrogen atoms are light gray, the fluorine atom is light blue, and the chlorine atoms are green.

the less electronegative Cl atom instead of F. It is natural then to examine how HCl, the acid of intermediate strength (in the gas phase) to HF and HCCH, binds to haloethylenes. Given that vinyl fluoride−HCl is planar and top binding and vinyl chloride−HCl is nonplanar, it is of interest for us to investigate how HCl interacts with ethylene when both F and Cl are present. We choose (E)- and (Z)-1-chloro-2-fluoroethylene as partners and investigate how the locations of F and Cl affect intermolecular interactions. We report here our work on (E)-1-

binding mode of the acid HF. If, however, we view these ethylenes as derived from the addition of an F atom to vinyl chloride, then the binding mode of HF changes significantly: it no longer resembles that of vinyl chloride−HF. That is, HF chooses to bind to F, not Cl in these cases. The acid HCCH offers interesting comparisons. Though HCCH binds to the F atom in 1-chloro-1-fluoroethylene in a top binding fashion,17 it interacts with Cl in (Z)-1-chloro-2fluoroethylene in a side binding manner,18 representing the first haloethylene−protic acid complex where the acid interacts with 7956

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The Journal of Physical Chemistry A chloro-2-fluoroethylene−HCl. The work on the Z isomer is currently underway.

the global minimum (Figure 2 and Table 1). The primary hydrogen bond of structure e is 0.028 Å longer while the secondary bond is 0.172 Å shorter than the corresponding ones in structure c. Interestingly, the order of the energies for these two isomers is switched when we take into account basis set superposition error: Structure e is lower in energy than structure c by 7 cm−1. Given the modest level of theory used, this small difference is, of course, too small for any significance to be attached to it, and it is mentioned only for completeness sake. The ordering of energy does not change for the higher energy species. It is worth noting that structure c as illustrated represents only one of two equivalent structures where HCl lies on one or the other side of the ethylene plane, and the average of these two structures is therefore planar, much like structure e. It is likely that the zero-point energy of the complex is higher than the energy difference between structures c and e; we thus expect to see an effectively planar structure of the complex under our experimental conditions. Furthermore, the dipole moment components for the planar configuration, structure (e), along the a and b axes are calculated to be 1.31 and 0.99 D and we should therefore observe both a and b type transitions. For an effectively planar complex, there are no c type rotational transitions. Those transitions due to the oscillating, instantaneous dipole along the c axis due to the out of plane motion would be considered vibrational transitions. It is worth noting that structure c has a dipole moment component along the c axis calculated to be 0.14 D, which would lead to observable c type transitions for this geometry. The Cartesian coordinates in the principal inertial axis system for all structures in Figure 2 are available as Supporting Information.

2. AB INITIO CALCULATIONS We carry out ab initio calculations at the MP2/6-311+ +G(2d,2p) level with GAUSSIAN 0919 to guide our spectroscopic search for the (E)-1-chloro-2-fluoroethylene− HCl complex. The structures of the subunits are fixed to the effective zero-point (r0) structures of the free monomers,20,21 and the coordinate system employed is shown in Figure 1. The origin is at the middle of the double bond and the x and z axes are in the ethylene plane, with the z axis along the CC bond. The position of the H atom in HCl is described by the spherical polar coordinates R, θ, and ϕ. A series of relaxed scans is performed where the values of R, ϕ, and the position of Cl in HCl is optimized at fixed values of θ. The minimum energy path is shown in Figure 1. Four minima are observed, and the corresponding optimized structures [(a)−(d)] are shown in Figure 2, and their energies and rotational constants are presented in Table 1. The minimum-energy structures Table 1. Relative Energies, without and with BSSE Correction, and Rotation Constants for Five Isomers of (E)1-Chloro-2-fluoroethylene−HCl (Shown in Figure 2) Obtained from ab Initio Calculations relative energy/cm−1

structure a b c d e expa

no BSSE correction

with BSSE correction

A/ MHz

B/ MHz

C/ MHz

inertial defect/ u Å2

220 114 0 82 7

185 146 7 58 0

3726 2576 4644 8021 4844 4469

996 1435 928 701 894 922

789 944 780 645 755 764

−2.643 −12.712 −5.483 0.000 0.000 +0.273

3. EXPERIMENT We use a broadband, chirped pulse22−24 and a narrowband, Balle−Flygare,23,25 Fourier transform microwave spectrometer for this work. Both spectrometers have been described previously and only specific experimental details are reported here. The complex is formed by expanding a gas mixture containing 1% each of (E)-1-chloro-2-fluoroethylene (SynQuest, Achala, FL, Lot Q16B-14) and HCl (Matheson) in Ar (Matheson), with a backing pressure of 1−2 atm. The Balle− Flygare spectrometer uses one pulsed valve with a 0.8 mm diameter nozzle, whereas the chirped pulse spectrometer uses two. The 5.6−18.1 GHz spectrum is first collected, in 4.0−4.5 GHz segments, using the chirped pulse spectrometer. The chirped microwave polarization pulse duration is 4 μs with 20− 25 W of power, and the resulting 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 two pulsed valves which typically operate at 4 Hz, although this is reduced to 0.8 Hz for overnight operation. A total of 624 000− 792 000 FIDs are averaged for each segment, and as described previously,23 the average is Fourier transformed to give a frequency domain spectrum with a resolution element of 23.84 kHz and typical line widths (fwhm) of 225 kHz. We observe several weak features that are extensively split, as is typical when multiple quadrupolar nuclei are present, and are able to tentatively assign them to the most abundant isotopologue of the complex. These weak features are then studied in the Balle−Flygare spectrometer where they reveal nuclear quadrupole hyperfine structure due to the two chlorine nuclei. In this instrument, the time domain signals are digitized for 2048 data points and zero-filled to a 4096-point record length and Fourier

a

For the most abundant isotopologue. See Table 2 for the constants with full precision and associated uncertainty.

correspond to HCl binding to the four possible pairings of adjacent hydrogen and halogen atoms in the ethylene, and only one of them, structure d, is planar. The two highest energy structures [(a) and (b)] both involve HCl binding to Cl, with binding across the double bond the much more stable of the two. The two lowest energy structures [(c) and (d)] show HCl forming a hydrogen bond with F in the ethylene, with the lowest energy attained when the secondary interaction forms between the Cl atom in HCl and the H atom cis to F. The energy increases to 82 cm−1 when the secondary interaction involves the geminal H atom. Both the primary and secondary bonds in the global minimum structure are shorter, by 0.009 and 0.120 Å, respectively, than the side binding structure to F. The values for the CF···H angle are 116° and 110°, and the deviations of the hydrogen bond from linearity are 18° and 22°, respectively, for the global minimum and the F side binding structures; thus, the angles are less strained in the former configuration, conferring a greater degree of stability to it. Because the question of the planarity of the complex is central to the present investigation, it is useful to examine the complex when HCl is bound to the F and H pair across the ethylene bond, as in the global minimum configuration, but restricting the structure to a planar geometry. The resulting configuration, structure e, is only 7 cm−1 higher in energy than 7957

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deviation of each fit is about 1 kHz, less than the resolution element and is only a fraction of a typical line width. B. Structure Determination. With an asymmetry parameter between −0.918 and −0.915 for the isotopologues, the (E)-1-chloro-2-fluoroethylene−HCl complex is a near prolate asymmetric top. In fact, the values of the rotational constants of the most abundant species compare very well, within 8%, to those of structures c and e. To ascertain the planarity of the complex, we turn to the inertial defect. The values are between 0.273 and 0.279 u Å2; these small, positive values are consistent with a planar complex where an in-plane vibrational contribution is dominant. Indeed, a molecule with a nonplanar structure, i.e., having atoms with nonzero values for the c coordinate in the principal axis system, will necessarily have a negative inertial defect. (The inertial defect is equal to −2Pc = −2∑i mici2, where Pc is the planar second moment.) For a rigidly planar species all atoms have c = 0, and Pc = 0 as is the inertial defect. Out-of-plane vibrational motion in a nonrigid structure makes a negative contribution to the inertial defect, whereas in-plane motion makes a positive one.28 Thus, the effective structure of the complex is indeed planar, as we deduced from the ab initio work. As a result, only two moments of inertia of each isotopologue are independent. Continuing to assume that the monomer structures are unchanged upon complexation, three geometric parameters are necessary to describe the structure of the heterodimer: the distance between the subunits and the angular orientation of each subunit. We fixed the structures of the subunits to be the same as the r 0 structures of the corresponding free monomers,20,21 and choosing Ia and Ic as the two independent moments of inertia for each of the three isotopologues, fit to them the values of the geometric parameters. These two are normally chosen as Ia is particularly sensitive to the angular orientation of the two monomers and Ic to the distance between their centers of mass. The angular orientations of the subunits were highly correlated in this fit. We thus turn to the nuclear quadrupole coupling constants of the acid Cl nucleus to supply the angular information for HCl. Assuming that there is no electric field gradient perturbation for HCl upon complexation, its quadrupole coupling tensor can be rotated into the inertial axis system of the complex. Direct consideration of the transformation (Supporting Information) shows that the value of the χaa component of the tensor for the HCl chlorine atom in the complex is a ⟨P2⟩ projection of that for the free HCl monomer29 and is unaffected by any out-ofplane vibrational averaging. That is,

transformed after background correction to give a frequency domain signal with a resolution element of 2.4 kHz. The greater resolution and higher sensitivity of the Balle−Flygare spectrometer allows the observation of many more transitions and in the end, the spectra of the most abundant form of (E)-1chloro-2-fluoroethylene−HCl and two isotopologues, each singly substituted with 37Cl, are obtained in the 6.0−20.0 GHz region.

4. RESULTS A. Spectral Analysis. We observe between 32 and 61 aand b-type rotational transitions from the three isotopologues of (E)-1-chloro-2-fluoroethylene−HCl, with J ranging from 1 to at least 9, and Ka from 0 to at least 2. Each transition is split by the chlorine nuclei into many hyperfine components, and an example spectrum for the most abundant isotopologue is shown in Figure 3, where 7 of the 16 observed Doppler

Figure 3. Portion of the 606−505 transition over a 0.6 MHz range showing some of the nuclear quadrupole hyperfine components resulting from two chlorine nuclei in (E)-CH35ClCHF−H35Cl. The Doppler doublets of each hyperfine component are connected by a comb and labeled by F1′F′−F1″F″.

doubled components of the 606−505 transition are displayed over a 0.6 MHz region. The narrowband spectrum of each isotopologue is analyzed using the Watson A-reduced Hamiltonian26 with the inclusion of nuclear quadrupole coupling interactions due to the two chlorine nuclei. The angular momentum coupling scheme first couples the rotational angular momentum with the spin angular momentum of the ethylene Cl nucleus to give an intermediate angular momentum, F1, which then couples with the spin angular momentum of the acid Cl to give the total angular momentum, F. Using Pickett’s nonlinear SPFIT program27 for each species, we have fitted three rotational constants, five quartic centrifugal distortion constants, and for each Cl nucleus, the diagonal components and the magnitude of one off-diagonal component of the nuclear quadrupole coupling tensor. These constants are tabulated in Table 2. We have also included tables of observed and calculated transition frequencies with assignments for all isotopologues studied as Supporting Information. The rms

χaa ,complex = χaa ,HCl

3⟨cos2 θa⟩ − 1 2

where θa is the angle between the a axis and the HCl axis in the complex. The bb and cc components of the tensor as well as the one, nonzero off-diagonal component, χab, are affected by outof-plane vibrational averaging, so that direct diagonalization of the measured tensor will not give the desired result. (For example, upon diagonalization, one finds that χxx ≠ χyy, which would imply that the tensor is no longer cylindrically symmetric about the HCl bond axis. Although this could reflect a small amount of field gradient perturbation, corresponding to a value of |η| = 0.0034, it is more likely an indication that in-plane and out-of-plane vibrational averaging, as expected, are not equivalent.) 7958

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The Journal of Physical Chemistry A Table 2. Spectroscopic Constants (MHz, unless Otherwise Noted) for Three Isotopologues of the (E)-1-Chloro-2fluoroethylene−HCl Complexa,b A B C ΔJ/10−3 ΔJK/10−3 ΔK/10−3 δJ/10−3 δK/10−3 χaa (ethylene Cl) χbb (ethylene Cl) χcc (ethylene Cl) |χab| (ethylene Cl)c χaa (acid Cl) χbb (acid Cl) χcc (acid Cl) |χab| (acid Cl)d no. of rotational transitions no. of a type no. of b type no. of hyperfine components J range Ka range rms/kHz

CH35ClCHF−H35Cl

CH37ClCHF−H35Cl

CH35ClCHF−H37Cl

4469.453617(63) 921.936900(38) 763.968932(28) 1.45211(20) −17.4640(14) 114.2597(65) 0.376518(70) 3.324(14) −57.48365(95) 22.44715(81) 35.03651(82) 39.987(23) 12.3950(10) −39.49003(90) 27.09505(81) 31.528(29) 61 31 30 840 1−11 0−3 1.25

4431.40085(31) 900.810323(93) 748.320092(86) 1.38222(44) −16.9607(40) 112.753(62) 0.35527(15) 3.215(37) −45.8108(21) 18.1913(13) 27.6196(15) 31.049(28) 12.8998(17) −39.9954(12) 27.0957(12) 31.156(29) 32 18 14 373 1−9 0−2 0.94

4435.36298(33) 895.685569(99) 744.891758(93) 1.38466(47) −16.8768(41) 111.986(64) 0.35284(16) 3.238(40) −56.8027(19) 21.7658(12) 35.0370(14) 40.657(23) 9.3615(19) −30.7194(13) 21.3579(14) 25.246(35) 32 18 14 391 1−9 0−2 1.03

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. cGiven the choice for the positive directions of the inertial axes indicated in Figure 5b, the signs of these χab values are determined to be positive. See Section 4C for details. dGiven the choice for the positive directions of the inertial axes indicated in Figure 5b, the signs of these χab values are determined to be negative. See Section 4C for details. a

The projection formula gives a value of 62.647° for θa (or −1

quadrupole coupling tensor for each Cl nucleus, the magnitude of which has been determined through the analysis of the observed transition frequencies. With the structure of the complex determined, the sign of this component can also be resolved, relative to the assigned directions of the inertial axes. The complete nuclear quadrupole coupling tensors for several isotopologues of (E)-1-chloro-2-fluoroethylene have been precisely determined16 and the nonzero components for the most abundant species and its 37Cl isotopologue are listed in Table 4. The sign of each χab component is negative when the inertial axes are arranged as shown in Figure 5a. Applying the same rotation matrix that converts the inertial tensor of the ethylene (Figure 5a) to that of its HCl complex (Figure 5b) to the quadrupole coupling tensor of the ethylene, we are able to determine that the sign of χab corresponding to the complex with the inertial axes arrangement shown in Figure 5b is positive. When a similar procedure is carried out for HCl, a negative value for χab is obtained for the acid Cl in the complex. These values are indeed relative to the assumed (and arbitrary) specification of the positive directions for the a and b inertial axes, but then again so is the sense of rotation in making the transformation from the monomer axis system to that of the complex. A consistent choice of signs is necessary when the quadrupole tensor is used for structural information. Diagonalizing the nuclear quadrupole coupling tensor for the ethylene Cl in the most abundant species of the complex yields the quadrupole coupling components χxx, χyy, and χzz along the principal electric field gradient axes, and the corresponding values are 39.0168, 35.0365, and −74.0534 MHz, respectively. The y axis is the same as the c axis whereas the x and z axes are in the molecular plane, with z being the axis of greatest

2

⟨cos θa⟩ ). Fixing the HCl subunit to more precisely, cos reproduce this angle, we are able to determine the two remaining geometric parameters using Schwendeman’s STRFTQ program30 and, therefore, the structure of the complex. The rms deviation of the fit is 0.046 u Å2, and the chemically relevant parameters are shown in Figure 4a. The values of the coordinates for two chlorine atoms agree very well with those of their Kraitchman coordinates31 (Table 3), differing between 0.3 and 0.8%. The uncertainties in structural parameters shown in Figure 4 represent the statistical uncertainties arising in the least-squares fit. Model errors and the effects of vibrational averaging, as indicated by the nonzero value for the inertial defect, certainly introduce additional uncertainty. Some estimate of these may be obtained by repeating the fits with alternate choices for the two independent moments of inertia. Doing so provides bond lengths that are consistent to a few thousandths of an Ångstrom and angle to a few hundredths of a degree, which may be considered realistic uncertainties in the parameters. As can also be determined from the tensor rotation, χbb ,complex − χcc ,complex =

3 χ ⟨sin 2 θa⟩⟨cos 2ϕoop⟩ 2 aa ,HCl

where ϕoop is the rotation of the HCl molecule out of plane. Using ⟨sin2 θ⟩ = 1 − ⟨cos2 θ⟩ and using cos−1 ⟨cos 2ϕoop⟩/2, we calculate a value of 16.8° for ϕoop as a measure of out-of-plane vibrational motion. C. Nuclear Quadrupole Coupling Constants. Because the complex is effectively planar in the ground state, there is only one nonzero off-diagonal component (χab) in the nuclear 7959

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Figure 5. Inertial axes of (E)-CH35ClCHF (drawn from the structure given in ref 20) and (E)-CH35ClCHF−HCl (this work). The carbon atoms are dark gray, the hydrogen atoms are light gray, the fluorine atom is light blue, and the chlorine atoms are green.

magnitude of the quadrupole coupling component. The absolute value of the asymmetry parameter for the quadrupole coupling constants, |η| = |(χxx − χyy)/χzz|, is 0.0537, indicating that the charge distribution about the z axis deviates slightly from cylindrical. The z axis forms an angle of 22.51° with the a axis, which is similar to the angle of 21.18° formed by the C−Cl bond with the a axis. Thus, the z axis lies almost along the C− Cl bond. The absolute value of the asymmetry parameter for the ethylene Cl in (E)-1-chloro-2-fluoroethylene−H35Cl is nearly equal to, but slightly smaller than, that for its HF counterpart (|η| = 0.0556).16 Both are greater than that for the (E)-1chloro-2-fluoroethylene monomer (|η| = 0.0475).16 Because the acid (HF or HCl) in each case is binding to the fluorine atom three bonds removed, it is unlikely that the electric field gradient about the Cl atom is perturbed to any significant extent. Consequently, this variation in the asymmetry parameter serves to indicate that the amounts of zero-point motion in the HCl complex and in the HF complex are similar, and both are greater than that in the (E)-1-chloro-2fluoroethylene monomer. Our deduction regarding the zeropoint motions can also be examined using the values of χyy for the three species, and we use those for the most abundant isotopologues as an example: 35.7836(21) MHz for (E)CH 35 ClCHF (ref 16), 35.34659(97) MHz for (E)CH35ClCHF−HF (ref 16), and 35.03651(82) MHz for (E)CH35ClCHF−H35Cl. Because the y (and c) axes for these species are the same (perpendicular to the ethylene plane), the values of χyy would be identical if the molecules were rigid. In fact, for the HF and HCl complexes, the values are 1.2% and 2.1%, respectively, smaller than that in the monomer, once again suggesting similar vibrational motions of the ethylene subunit in the two complexes.

Figure 4. Structures of (a) (E)-1-chloro-2-fluoroethylene−HCl (this work), (b) (E)-1-chloro-2-fluoroethylene−HF,16 (c) vinyl fluoride− HCl,2,32 and (d) vinyl fluoride−HF.1 The carbon atoms are dark gray, the hydrogen atoms are light gray, the fluorine atoms are light blue, and the chlorine atoms are green. The uncertainties in structural parameters represent the statistical uncertainty arising in the leastsquares fit. Consideration of the effects of vibrational averaging and other model errors suggest uncertainties of ±0.005 Å for bond lengths and 0.05° for angles.

Table 3. Coordinates of Cl Atoms Determined from a Kraitchman Analysis, a Structural Fit, and ab Initio Calculation ethylene Cl |a|/Å |b|/Å a/Å b/Å a/Å b/Å a

(i) Substitution Coordinatesa 2.55614(59) 0.7136(21) (ii) From Structural Fit 2.5628 −0.7090 (iii) Ab Initio for Structure (e) 2.6148 −0.6663

acid Cl 2.85782(52) 0.6775(22) −2.8712 −0.6718 −2.9055 −0.6521

Costain errors34 in the parameters are given in parentheses.

Table 4. Nonzero Nuclear Quadrupole Coupling Constants for Two Isotopologues of (E)-1-Chloro-2-fluoroethylene16 χaa χbb χcc χaba

CH35ClCHF

CH37ClCHF

−63.6484(18) 27.8649(21) 35.7836(21) −34.4(11)

−50.2162(16) 22.0143(20) 28.2020(20) −26.30(93)

5. DISCUSSION The experimental structure of (E)-1-chloro-2-fluoroethylene− HCl, shown in Figure 4, has the same binding motif as (E)-1chloro-2-fluoroethylene−HF (Figure 4b),16 vinyl fluoride−HCl (Figure 4c),2,32 and vinyl fluoride−HF (Figure 4d).1 All four structures involve an acid interacting with the F and H atoms located across the double bond in the ethylene subunit. The values of the CF···H angle are similar, approximately 120°,

a

The sign of this component is appropriate to the axis choice made for (E)-1-chloro-2-fluoroethylene in Figure 5a.

7960

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The Journal of Physical Chemistry A suggesting that the addition of Cl trans to F in vinyl fluoride does not change the direction of the lone pair on the F atom interacting with the acid and, furthermore, does not change the manner with which HCl and HF interact with the ethylene. In other words, the acids prefer to bind to F instead of Cl when both are present trans to each other in the ethylene subunit. On the contrary, turning this around, one observes that the addition of an F atom to vinyl chloride does change the binding mode of HCl and HF significantly. We first examine how the structural parameters depend on the ethylene partners. The length of the hydrogen bond in (E)1-chloro-2-fluoroethylene−HCl is 2.19481(34) Å and bends by 27.37° from linearity to allow the acid Cl to form a secondary interaction with a length of 2.9011(5) Å to the H atom cis to F. When the ethylene partner is vinyl fluoride, the primary bond becomes 3.3% (or 0.072 Å) shorter, with a lesser bend (by 9.1°) from linearity and a secondary bond longer by 9.0% (or 0.261 Å). The longer hydrogen bond in the (E)-1-chloro-2fluoroethylene complex and the greater distortion from linearity indicate that this bond is weaker than that in the vinyl fluoride counterpart. However, the secondary interaction length in the (E)-1-chloro-2-fluoroethylene complex is similar to the sum of the van der Waals radii of H and Cl (2.95 Å33) and is shorter and therefore stronger than the one in the vinyl fluoride complex. It follows then that with a stronger hydrogen bond, the secondary interaction plays a lesser role in stabilizing the vinyl fluoride complex. Conversely, for the (E)-1-chloro-2fluoroethylene complex, because the hydrogen bond is weaker, the secondary interaction plays a more important role. The structures of the two HCl complexes suggest that the presence of Cl trans to the F atom and geminal to the H atom that participates in the secondary interaction renders the F atom less nucleophilic while the H atom becomes more electropositive. This effect can also be seen by mapping the electrostatic potential onto an electron density isosurface for each ethylene subunit, (E)-1-chloro-2-fluoroethylene (Figure 6a) and vinyl fluoride (Figure 6b). It is interesting to note that

trifluoroethylene−HCl, (109.7°),8 then for (E)-1-chloro-2fluoroethylene−HCl to have a side binding motif, the hydrogen bond would have to deviate from linearity by 47.6°, much greater than the value of 27.37° derived earlier for the experimental structure. This large angle would be much more destabilizing, and thus, it is not observed as the most stable structure for the complex. Next we consider the effects of different acid partners. A comparison between (E)-1-chloro-2-fluoroethylene−HCl and its HF counterpart shows that the hydrogen bond length in the HCl complex is significantly longer, by 0.25 Å (approximately 12%) and bends by 2.6° more from linearity, giving a secondary interaction 0.45 Å longer than that in the HF complex (Figure 4). Thus, changing the acid from HCl to the stronger HF affects the primary bond to a much greater extent than adding Cl to vinyl fluoride, and the primary bond is much weaker in the HCl complex. This lengthening of the secondary bond in the HCl complex compared to that in the HF complex, however, is greater than the difference between the van der Waals radii of Cl and F (0.28 Å),33 suggesting that the secondary bond is also weaker in the HCl complex. It is therefore not surprising that the (E)-1-chloro-2-fluoroethylene subunit exhibits a greater zero-point motion when bound to HCl (as shown by the deviation of the value for χyy in the complex from that of the ethylene monomer). Additionally, although comparison of hyperfine constants (spin−spin for HF and quadrupole coupling for HCl) between the free molecule and in the respective complexes shows that both HF and HCl exhibit similar vibrational excursions from the molecular plane, the fact that HF has a rotational constant twice that of HCl means that a greater anisotropy, and thus a stronger interaction, is required to quench the free rotation of HF and supports the deduction that HCl forms a weaker complex with (E)-1-chloro2-fluoroethylene. It is interesting to note that very similar differences as observed for the HCl and HF complexes of (E)-1-chloro-2fluoroethylene are also found for the complexes with vinyl fluoride (Figure 4). The primary hydrogen bond and the secondary bond in the vinyl fluoride−HCl complex are longer than those in the HF complex by 0.23 and 0.43 Å, nearly the same as observed and each shorter by the same 0.02 Å than observed for their (E)-1-chloro-2-fluoroethylene counterparts. On the contrary, the deviations from linearity for the hydrogen bonds in the vinyl fluoride complexes are effectively the same [18.3(1)° in the HCl complex and 18.7(15)° in the HF complex]. However, at 3.162 Å, the secondary interaction in vinyl fluoride−HCl is significantly weaker than that in (E)-1chloro-2-fluoroethylene−HCl and likely cannot compensate for the loss in stability resulting from an increased deviation from linearity as it can in the (E)-1-chloro-2-fluoroethylene complex.

Figure 6. Electrostatic potential, mapped onto a total electron density isosurface for (a) (E)-1-chloro-2-fluoroethylene and (b) vinyl fluoride. The same value of electron density is used for the isosurface in both molecules, and identical color scales are used. Blue represents positive electrostatic potential, and red, negative electrostatic potential.

6. CONCLUSION In the gas-phase heterodimer with (E)-1-chloro-2-fluoroethylene, hydrogen chloride binds across the CC double bond, forming a primary, hydrogen bonding interaction with the F atom of the substituted ethylene and a secondary interaction between the Cl atom of the hydrogen chloride and the H atom of the ethylene that is cis to the F atom. When compared to the case of the analogous vinyl fluoride complex, the substitution of Cl for H trans to the hydrogen bonding F atom does not change the fundamental nature of the interaction between the two species. On the contrary, viewing (E)-1-chloro-2-fluoroethylene as having an F atom replacing a

the surface for (E)-1-chloro-2-fluoroethylene shows that the H atom geminal to the F atom is also very electropositive. However, for HCl to bind to the H, F pair connected to C-2 in the ethylene subunit while maintaining the same primary and secondary bond lengths as in the observed top binding geometry would demand a large deviation from linearity. To illustrate this more clearly, if we assume the value of the CFH angle to be the same as that for the side binding complex 1,1,27961

DOI: 10.1021/acs.jpca.6b07739 J. Phys. Chem. A 2016, 120, 7955−7963

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The Journal of Physical Chemistry A

(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.; 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. (5) 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. (6) Leung, H. O.; Marshall, M. D. Rotational Spectroscopy and Molecular Structure of the 1,1-Difluoroethylene-Acetylene Complex. J. Chem. Phys. 2006, 125, 154301. (7) 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. (8) Leung, H. O.; Marshall, M. D.; Ray, M. R.; Kang, J. T. Rotational Spectroscopy and Molecular Structure of the 1,1,2-TrifluoroethyleneHydrogen Chloride Complex. J. Phys. Chem. A 2010, 114, 10975− 10980. (9) 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. (10) 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. (11) 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. (12) 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. (13) 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. The 69th International Symposium on Molecular Spectroscopy, Urbana-Champaign, IL, 2014; Talk TE07. (14) Leung, H. O.; Marshall, M. D.; Messinger, J. P. Chlorine Nuclear Quadrupole Hyperfine Structure in the Vinyl ChlorideHydrogen Chloride Complex. The 70th International Symposium on Molecular Spectroscopy, Urbana-Champaign, IL, 2015; Talk WJ06. (15) 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. (16) Leung, H. O.; Marshall, M. D.; Lee, A. J. The Microwave Spectrum and Molecular Structure of (E)-1-Chloro-2-Fluoroethylene−HF: Revealing the Balance Among Electrostatics, Sterics, and Resonance in Intermolecular Interactions. J. Phys. Chem. A 2016, DOI: 10.1021/acs.jpca.6b07252. (17) Leung, H. O.; Marshall, M. D.; Grimes, D. D. Rotational Spectroscopy and Molecular Structure of the 1-Chloro-1-Fluoroethylene-Acetylene Complex. J. Chem. Phys. 2011, 134, 034303. (18) Khan, N. D.; Leung, H. O.; Marshall, M. D. The Microwave Spectrum and Unexpected Structure of the Bimolecular Complex Formed Between Acetylene and (Z)-1-Chloro-2-Fluoroethylene. The 70th International Symposium on Molecular Spectroscopy, UrbanaChampaign, IL, 2015; Talk WJ05. (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision B.01; Gaussian, Inc.: Wallingford, CT, 2009.

hydrogen trans to chlorine in vinyl chloride and comparing the structures of the heterodimers of these two haloethylenes with HCl and HF reveals the significant impact of fluorine substitution. Apparently, the presence of the F atom determines the gross structure of these species due to the strength of the hydrogen bond it can form. The location of the electron density surrounding the F atom, and the desirability for a small deviation from linearity in the hydrogen bond then leads to a steric preference for what we denote as top binding and formation of the secondary interaction with the hydrogen cis to the fluorine. Nevertheless, the presence of the Cl atom does have an impact on the nature of the interaction with the F atom. As also seen in (E)-1-chloro-2-fluoroethylene−HF,16 the Cl atom renders the F atom less nucleophilic relative to vinyl fluoride while making the H atom bonded to the same carbon (i.e., geminal to the Cl atom) more electropositive. We have argued elsewhere that this is the result of both inductive and resonance withdrawal of electron density by the Cl atom.16 As a result of this effect, the importance of the secondary interaction in the (E)-1-chloro-2-fluoroethylene complexes, although still relatively minor, is enhanced relative to that in vinyl fluoride while that of the primary interaction is reduced. The hydrogen bond in (E)-1-chloro-2-fluoroethylene−HCl has a greater deviation from linearity than (E)-1-chloro-2-fluoroethylene−HF whereas the analogous vinyl fluoride species have essentially identical deviations even though the changes in primary and secondary interaction bond lengths in going from HF to HCl are comparable for the two different haloethylenes.

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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.6b07739. Tables of observed and calculated transition frequencies for all isotopologues of (E)-1-chloro-2-fluoroethylene− HCl that are reported in this study, the atomic coordinates for the structures shown in Figure 2, and a discussion of the rotation of the quadrupole coupling tensor of HCl (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*H. O. Leung. Fax: +1-413-542-2735. E-mail: hleung@amherst. edu *M. D. Marshall. Fax: +1-413-542-2735. E-mail: mdmarshall@ amherst.edu. 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.6b07739 J. Phys. Chem. A 2016, 120, 7955−7963