Article pubs.acs.org/JPCA
Effect of Acid Identity on the Geometry of Intermolecular Complexes: The Microwave Spectrum and Molecular Structure of Vinyl Chloride− HF 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: The structure of the gas-phase bimolecular complex formed between vinyl chloride and hydrogen fluoride is determined using Fourier transform microwave spectroscopy from 6.3 to 21.4 GHz. Although all previous examples of complexes formed between protic acids and haloethylenes are observed to have similar modes of binding regardless of the specific identity of the acid, HF, HCl, or HCCH, the planar vinyl chloride−HF complex has HF located at the “top” of the vinyl chloride with the secondary interaction occurring with the cis hydrogen atom as opposed to the “side” binding configuration found for vinyl chloride−HCCH. Nevertheless, the details of the structure, such as hydrogen bond length (2.32 Å) and amount of deviation from linearity (19.8°), do reflect the strength of the interaction and show clear correlations with the gas-phase acidity. Comparison with analogous complexes allows the determination of the relative importance of electrostatic interactions and steric requirements in leading to the observed structures.
1. INTRODUCTION In the gas-phase complexes formed between fluorinesubstituted ethylenes and protic acids, such as HF, HCCH, and HCl, that have been studied by our group and the Legon group, two interactions are observed: the acid forms a primary hydrogen bond with a fluorine atom of the substituted ethylene subunit, and this bond bends to allow a secondary interaction between the nucleophilic portion of the acid and a hydrogen atom of the ethylene. The relative locations of the hydrogen and fluorine atoms of the fluoroethylene participating in the interactions depend on the number of fluorine substituents present. Specifically, for vinyl fluoride,1−3 1,1-difluoroethylene,4−6 and trans-1,2-difluoroethylene,7 the acid binds to a fluorine atom and a hydrogen atom located cis to each other in the fluoroethylene subunit, whereas in the presence of three fluorine substituents, as in 1,1,2-trifluoroethylene,8−10 the two atoms are connected to the same carbon atom. In all but the 1,1-difluoroethylene-containing complexes, both modes of binding are possible. The fact that only one, presumably the lower-energy, configuration is observed can be rationalized by consideration of the balance between electrostatic and steric factors in these complexes.7 Through this series of complexes, we have gained insight into how the presence of one or more fluorine atoms tunes the properties of a fluoroethylene subunit, thus affecting the nature of its interaction with a protic acid. We are now broadening our efforts to include the influence of chlorine substitution in a fluoroethylene. To date, our work on 1-chloro-1-fluoroethylene−HF,11 1-chloro-1-fluoroethylene−HCCH,12 and (E)-1-chloro-2-fluoroethylene−HF indicates that the acid binds to the fluorine and hydrogen atoms located across the © 2014 American Chemical Society
double bond of the chlorofluoroethylenes in a manner similar to their difluoroethylene counterparts. The mode of binding changes for (E)-1-chloro-1,2-difluoroethylene−HF: the acid is at the end of the double bond where one of the fluorine atoms and the hydrogen atom are positioned, analogous to its trifluoroethylene counterpart. It is interesting to note that although the chlorine atom in these complexes introduces additional possibilities for interactions, such as the acid forming a hydrogen bond with chlorine and a cis or geminal hydrogen, for the complexes studied so far, the hydrogen bond involves only a fluorine atom. To gain an understanding of how the less electronegative, more polarizable chlorine atom in a substituted ethylene would interact with an acid, we need to eliminate all fluorine atoms and turn to vinyl chloride. Here we report a microwave study of vinyl chloride−HF, where the acid is found to bind across the double bond, as it does in the case of vinyl fluoride−HF. An examination of the structural parameters of these vinyl halide complexes reveals the differences in the nature of the intermolecular interactions. This is in contrast to our previous work on vinyl chloride−HCCH, which shows a different binding motif: HCCH binds at one end of the double bond.13 These vinyl chloride complexes form a starting point from which we can compare both other vinyl chloride complexes and those chlorofluoroethylene complexes that exhibit direct involvement of chlorine in a van der Waals bond. Received: August 15, 2014 Revised: September 18, 2014 Published: September 19, 2014 9783
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2. AB INITIO CALCULATIONS We use GAUSSIAN 0914 at the MP2/6-311++G(2d,2p) level to locate the minima on the interaction potential energy surface formed between vinyl chloride and HF. Because the haloethylene complexes observed thus far are all planar, we similarly restrict our consideration of vinyl chloride−HF in our initial calculations. Three parameters are used to describe the structure of the complex: R is the distance between the center of the CC bond and H of HF, and θvc and θHF describe the angular orientations of the vinyl chloride and HF subunits, respectively. The geometries of the subunits are held at the average values in the ground vibrational states of the respective monomers.15,16 The value of θvc is varied from 5° to 355° in 10° steps whereas those for R and θHF are optimized to give a minimum energy path (Figure 1). There are four minima, and
Figure 1. Relaxed scan of the MP2/6-311++G(2d,2p) interaction potential beween HF and vinyl chloride. Only planar configurations of the complex are considered. The relevant structural parameters are shown in the inset and the labeled minima correspond to the structures shown in Figure 2.
Figure 2. (a)−(d) Structures corresponding to stationary points on the ab initio potential energy surface of the vinyl chloride−hydrogen fluoride complex when restricted to planar configurations (Figure 1). (e) Structure corresponding to the global minimum. (f) A higher energy nonplanar structure. The carbon atoms are dark gray, hydrogen atoms are light gray, the fluorine atom is light blue, and the chlorine atom is green.
all three geometric parameters corresponding to each minimum are then optimized and displayed in Figure 2. The energies of the fully optimized configurations are listed in Table 1 and are also included in Figure 2. Two structures, each containing a primary hydrogen bond and a secondary interaction, are considerably more stable than the others. The lowest-energy structure, structure b, has HF bound across the double bond of vinyl chloride, and is 41 cm−1 lower in energy than the configuration where HF lies at the Cl end of the double bond (structure a). Note that the lengths of the hydrogen bonds between HF and Cl predicted by theory are approximately the same (2.25 Å) in these two structures. The length of the secondary interaction, however, is significantly longer (by 0.24 Å) in structure a than in structure b. Additionally, even this longer secondary bond length in structure a requires a smaller CCl···H angle and a larger deviation of the HF bond from linearity, both of which are likely to contribute to steric strain. It is, therefore, not surprising that structure a is of higher energy. Structures c and d are of significantly higher energy than the two previously described structures. These latter two structures are formed by an interaction between F in HF and the two possible pairs of adjacent ethylene hydrogen atoms, respectively. The rotational constants for the four isomers are very different, making the task of determining the mode of bonding straightforward once the rotational spectrum is assigned.
Table 1. Relative Energies and Rotation Constants for Six Isomers of Vinyl Chloride−HF (Figure 2) Obtained from ab Initio Calculations structure
relative energies/cm−1
A/MHz
B/MHz
C/MHz
a b c d e f
41 0 1072 885 −0.5 104
8776 6095 24579 12378 6075 7390
2424 3196 1160 1623 3198 2403
1900 2097 1108 1435 2100 1936
We have also relaxed the restriction to planar configurations in our calculations. Three of the four minima, corresponding to structures a, c, and d, remain unchanged. However, we find a new global minimum with an energy 0.48 cm−1 lower than that of structure b and corresponding to a slightly nonplanar structure with HF making an angle of 8° with the vinyl chloride plane (Figure 2e). This suggests a shallow out-of-plane potential well with two equivalent configurations on either side of the plane where HF binds across the double bond, with structure b representing a saddle point. We do expect that when 9784
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the zero-point energy is taken into account, the average structure is effectively planar. It is worth noting that structures b and e are quite similar and that their rotational constants differ only slightly. We also find a second nonplanar structure that has the H of HF interacting with the CC bond (Figure 2f). The HF molecular axis is nearly normal to the vinyl chloride plane (82°), and the energy of this structure is 104 cm−1 higher than that of structure b. The coordinates for all atoms in structures a−f can be found in the Supporting Information.
3. EXPERIMENT The experiment is conducted in the 6.3−21.4 GHz region using a pulsed molecular beam, Balle-Flygare cavity Fourier transform microwave spectrometer. We observe the rotational spectra of two isotopologues of vinyl chloride−HF, the most abundant species and its naturally occurring 37Cl-containing isotopologue. As each rotational transition is split into several hyperfine components due to the chlorine nuclear quadrupole coupling interaction, and some of these components are further split by the HF nuclear spin−spin coupling interaction, the transitions are not intense enough for us to observe the naturally occurring 13 C-containing isotopologues. Additionally, we have chosen not to study the DF-containing isotopologues: our experience having shown that due to the different zero-point motions of HF and DF in complexes, the DF-containing isotopologues generally do not help in determining the structure of the HF counterpart. The complex is formed by expanding 1% of vinyl chloride diluted in argon at a stagnation pressure of approximately 2 atm through a 0.8 mm diameter pulsed nozzle mounted behind one of the mirrors that forms the Fabry−Pérot cavity. We have described the spectrometer previously,17 and we note here that for the most abundant species the time domain signal, after down conversion twice by a two-stage heterodyne detection system to a center frequency of 2.5 MHz, is typically digitized at a sampling frequency of 10 MHz for 4096 data points and corrected for background. The background corrected signal is averaged and then zero filled to a 8196-point record length before Fourier transformation to give a frequency domain signal with a resolution element of 1.22 kHz. Because of the lower abundance of the 37Cl-containing isotopologue, only half as many points are digitized and zero-filled, giving a resolution element of 2.44 kHz. For even weaker transitions in both isotopologues, the number of points is once again halved, resulting in a resolution element of 4.88 kHz. Because the molecular beam is parallel to the mirror axis, each transition appears as a Doppler doublet. The rest frequency of the transition is the mean frequency of the two Doppler components.
Figure 3. Two chlorine hyperfine components (F1 = 7/2−5/2 and 9/2−7/2) of the 313 − 212 rotational transition for the CH2CH35Cl− HF isotopologue over a range of 0.4 MHz. The additional splittings are a result of the hydrogen−fluorine nuclear spin−spin interaction. Due to the Doppler effect, each transition appears as a doublet.
CH2CH35Cl−HF are split into different spin−spin hyperfine components. Because the complex is a fairly asymmetric top (the asymmetry parameter is reported in Section 4.2), the spectrum is analyzed using the Watson A-reduced Hamiltonian in the Ir representation18 with the inclusion of the chlorine nuclear quadrupole coupling interaction and, when appropriate, the hydrogen−fluorine spin−spin coupling interaction. The angular momentum coupling scheme employed couples the rotational angular momentum of the complex (J) to the spin angular momentum of the chlorine nucleus (ICl) to give either the total angular momentum (F) for spectral lines that are not further split by the spin−spin coupling interaction or the intermediate angular momentum (F1) for other lines. In the latter case, F1 couples successively to the hydrogen and fluorine nuclear spins to account for the spin−spin coupling interaction, giving the total angular momentum. Using Pickett’s nonlinear leastsquares computer program, SPFIT,19 we fit, for each isotopologue, three rotational constants, five quartic centrifugal distortion constants, chlorine nuclear quadrupole coupling constants, and HF spin−spin coupling constants. The values of these spectroscopic constants, together with the numbers and types of transitions analyzed, are listed in Table 2. In addition, tables of observed and calculated transition frequencies with assignments for all isotopologues studied are available as Supporting Information. The rms deviation of each fit is about 2 kHz, a fraction of a typical line width. 4.2. Structure Determination. The values of the asymmetry parameters of the 35Cl- and 37Cl-containing isotopologues of vinyl chloride−HF are −0.457 and −0.436, and those of the inertial defect are −0.230 and −0.222 amu Å2, respectively. These values are consistent with a planar asymmetric top for which out-of-plane vibrational motions make the dominant contribution to the inertial defect. (The inertial defect values are smaller in magnitude than that predicted for structure e, which has a value of −0.566 amu Å2.) The experimentally determined rotational constants for the most abundant isotopologue agree with those for the two very similar lowest energy theoretical structures, structures b and e of Figure 2, within 2%, confirming that HF does bind to vinyl
4. RESULTS 4.1. Spectral Analysis. We have analyzed 29 and 24 rotational transitions, both a and b type, for the 35Cl- and 37Clcontaining isotopologues, respectively. These transitions cover a range of J values of 0−4, and Ka values of 0−2 or 3. They are split by nuclear quadrupole coupling interaction caused by the relevant chlorine nucleus and for many of them, are further split by hydrogen−fluorine nuclear spin−spin coupling interactions. A sample pattern of these splittings is shown in Figure 3, where the two 35Cl hyperfine Doppler doubled components (F1 = 7/2−5/2 and 9/2−7/2) of the 313−212 rotational transition for 9785
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mass in a vinyl chloride monomer parent. The location of the pseudo atom then approximates that of the F atom in HF. In the vinyl chloride monomer axis system, the magnitude for the Kraitchman20 b coordinate for the pseudo atom, |b|, is 3.1827(96) Å, whereas that for the a coordinate is imaginary. [The uncertainty of the coordinate is the Costain error, given by σ(r) = 0.0015 Å2/|r|.21] The unphysical a coordinate indicates that the pseudo atom is located practically along the b inertial axis of the monomer. It follows then that the different signs for the b coordinate give two possible locations of the pseudo atom, as shown in Figure 4, and each gives a reasonable distance to the chlorine atom: 3.40 and 3.25 Å, respectively, for the locations with a positive and negative b value. We thus turn to studies of complexes formed between HF and fluoroethylenes to help discern which sign reflects the actual structure of the complex. For HF complexes with vinyl fluoride,1 1,1difluoroethylene,4 trans-1,2-difluoroethylene,7 and 1,1,2-trifluoroethylene,8 the secondary interaction length between F in HF and a hydrogen atom in the ethylene subunit is in the range 2.6−2.8 Å, and we expect that the secondary interaction in vinyl chloride−HF will be similar. Of the two possible locations for the pseudo atom, the one with a positive sign (position i in Figure 4) for the b coordinate interacts most closely with the H atom geminal to Cl, whereas that with a negative sign (position ii in Figure, 4) is closest to the H atom cis to Cl. Specifically, they give secondary interaction distances of 1.8 and 2.4 Å, respectively. The former value represents an unreasonably small distance whereas the latter falls within the anticipated range; therefore, the pseudo atom, or more appropriately, F in HF must interact with the H atom located cis to Cl (position ii). A consideration of the rotational constants of the vinyl chloride−HF complex and of the vinyl chloride monomer shows how their principal axes are related. Because both species are planar, the molecular plane in both species defines the ab inertial plane, and the c inertial axis in each is normal to the plane. The value of the A rotational constant for the most abundant isotopologue of the vinyl chloride−HF complex, 6004.21946(39) MHz, is nearly the same as the value of the B rotational constant for the vinyl chloride monomer, 6029.941(14) MHz.15 Thus, the a axis of the complex is approximately the same as the b axis of the monomer. This is consistent with the fact that in the complex HF lies nearly on the b axis of the monomer as discussed above. The rotation of the inertial system of vinyl chloride by approximately 90° about the c axis upon complexation with HF is additionally supported by the coordinates of Cl. In the CH2CH35Cl monomer inertial axis system, aCl = −0.96282(10) Å and bCl = −0.08230(32) Å.15 In the complex, the substitution coordinates for Cl, obtained via a Kraitchman analysis20 of the rotational constants of CH2CH35Cl−HF and CH2CH37Cl−HF are found to be |a|Cl = 0.5501(27) Å and |b|Cl = 0.9966(15) Å. The similarity of the magnitudes of the a coordinate in the monomer axis system and the b coordinate in the dimer system is a further confirmation that the roles of the a and b axes are switched upon complexation of vinyl chloride with HF. So far we have established the qualitative structure of vinyl chloride−HF. It is necessary to determine more precisely the geometric parameters. Three parameters are necessary: the distance between the two subunits and the angular orientation of each subunit. Making the usual assumption that the structures of vinyl chloride15 and HF16 remain unchanged upon complexation, we initially attempted to determine these
Table 2. Spectroscopic Constants (MHz, unless Otherwise Noted) for Vinyl Chloride−HFa A B C ΔJ/10−3 ΔJK/10−3 ΔK/10−3 δJ/10−3 δK/10−3 χaa(Cl) χbb (Cl) χcc(Cl) |χab|(Cl)b Daa Dbb Dcc no. of rotational transitions no. of a type no. of b type no. of hyperfine components J range Ka range rms/kHz
CH2CH35Cl−HF
CH2CH37Cl−HF
6004.21946(39) 3131.82048(23) 2060.16791(16) 9.7195(69) 16.937(32) −15.354(79) 3.4465(38) 28.623(50) 11.1591(10) −44.1072(11) 32.94809(96) 41.62(22) −0.1387(19) 0.0130(25) 0.1257(22) 29 14 15 383 0−4 0−3 2.30
5870.23481(45) 3120.12202(36) 2039.09929(23) 9.385(12) 18.734(52) −18.148(95) 3.3444(58) 28.281(95) 9.7184(13) −35.7006(13) 25.9822(12) 32.42(31) −0.1378(20) 0.0152(26) 0.1226(21) 24 12 12 277 0−4 0−2 2.21
a
1σ standard deviations in the parameters are given in parentheses. The nuclear quadrupole coupling constants of chlorine and the hydrogen−fluorine spin−spin coupling constants are fitted as 1.5χaa and (χbb − χcc)/4 or 1.5Daa and (Dbb − Dcc)/4, respectively, and the Laplace condition is used to calculate the individual hyperfine constants. bThe signs of all χab values are determined to be positive. See section 4.3 for details.
chloride across the double bond. The same conclusion is reached via a Kraitchman substitution analysis,20 as described below. Vinyl chloride is a planar molecule and is displayed in Figure 4 in its inertial axis system.15 The manner with which it interacts with HF can be estimated by treating HF as a pseudo atom with a mass equivalent to the entire subunit and considering the complex as containing a single substitution of the pseudo atom that replaces a hypothetical one with zero-
Figure 4. Two possible locations of the pseudo atom with a mass equivalent to HF. Location i is too close to the hydrogen geminal to Cl and is not plausible, leaving location ii as the only physically reasonable configuration. 9786
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Table 3. Values of the Distance between Cl and H in HF as a Function of the Angle between C−Cl and the a Axisa angle formed by C−Cl and the a-axis/deg
C−Cl/Å
aCl/Å
bCl/Å
CCl···H/deg
Cl···HF/deg
60.0 60.5 61.0 61.5 62.0 62.5 63.0
2.3019(89) 2.3075(82) 2.3130(75) 2.3185(69) 2.3241(63) 2.3297(57) 2.3353(52)
−0.5235 −0.5319 −0.5403 −0.5487 −0.5571 −0.5655 −0.5739
−0.9963 −0.9953 −0.9941 −0.9929 −0.9916 −0.9902 −0.9888
102.94 102.77 102.59 102.42 102.25 102.07 101.90
161.13 160.81 160.48 160.16 159.83 159.50 159.17
a The HF orientation is fixed to reproduce nuclear spin−spin data. The coordinates of Cl, the angle CCl···H, and the angle Cl···HF are also given (the Cl is taken to be in the fourth quadrant).
axis makes an angle of ±61.79° with the a axis of the complex, with the sign depending on that of the nonzero off-diagonal component, χab. Assuming that the electric field gradient is not perturbed upon complexation, the z axes in both the complex and the vinyl chloride monomer should be identical, and work on other vinyl halides such as vinyl bromide and vinyl iodide suggests that this electric field gradient axis should make a small angle of about 0.5° from the C−Cl bond.15 Thus, the angle that C−Cl makes with the a axis should be very close to 61.79°. Fixing the HF orientation to be consistent with nuclear spin− spin data, we vary the value of the angle between C−Cl and the a axis from 60.0° to 63.0° and fit the distance between Cl and H in HF to Ia and Ic of the two isotopologues of vinyl chloride− HF. We find that the distance varies by only about 1.4% (Table 3). The angle that best reproduces the substitution coordinates of Cl (to 0.4%) is 61.5°, and the rms deviation of the fit is 0.91 amu Å2. We take this configuration that agrees with chlorine nuclear quadrupole coupling interactions, HF nuclear spin− spin coupling interactions, and the substitution coordinates of Cl to represent the structure of our complex. When translated to more chemically relevant parameters, this structure has a bond of 2.319(6) Å between H of HF and Cl of vinyl chloride, the CCl···H angle is 102.4(2)°, and HF deviates by 19.8° from linearity (Figure 5). 4.3. Chlorine Quadrupole Coupling Constants. With the exception of the sign of χab, the complete nuclear quadrupole coupling tensor has been determined for each of the two isotopologues of vinyl chloride−HF. We can unravel this missing information by recognizing that in the absence of electric field gradient perturbation by HF, the nuclear quadrupole coupling tensors for vinyl chloride and for its HF complex would be related by a simple rotation of the a and b inertial axes. Given the planarity of the structure for vinyl chloride−HF, there is no rotation of the c axis, and the values of χcc for the monomer and the complex should be the same. The difference of less than 4% between the monomer value of 31.74 MHz15 and the value in the complex determined here (32.95 MHz) could be merely an indication that the zero point motions are slightly different, although it might also be a result of a small degree of distortion in the electron distribution surrounding the chlorine nucleus accompanying the formation of the complex. There is support for this latter interpretation from ab initio calculation of the quadrupole coupling tensors of the two species using Gaussian 09 14 at the MP2/ 6-311++G(2d,2p) level, although the calculation seems to indicate a greater degree of perturbation than is actually observed. The predicted values for χcc are 29.07 and 31.37 MHz, respectively, for the monomer and the complex. Indeed, it is reasonable to expect that the stronger intermolecular interaction due to complexation with HF would have a greater
parameters by fitting them with Schwendeman’s STRFTQ program22 to two independent moments of inertia for each of the two isotopologues. We choose Ia and Ic because the former is typically sensitive to the angular orientation of the subunits in a complex, and the latter to intermolecular distance. Nevertheless, regardless of the specific geometric parameters used, or different choices for the pairs of moments of inertia, the geometric parameters are correlated and the fit does not converge. We thus turn to another source to provide additional structural information. One piece of information is furnished by the HF nuclear spin−spin coupling constants, as they depend only on the distance between the hydrogen and fluorine nuclei in HF istself and the orientation of the subunit in the complex. Continuing with the assumption that the HF bond length remains unchanged upon complexation, the spin−spin coupling constant along the g axis (where g = a, b, or c) in the complex, Dgg, is simply a ⟨P2⟩ projection of the spin−spin coupling constant of free HF, Dmonomer = −286.75(5) kHz:23,24 Dgg =
3cos2 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 of vinyl chloride−HF listed in Table 2, the values of θg (or more precisely, cos−1(⟨cos2 θg⟩)1/2 are 35.92(27)°, 56.59(36)°, and 78.31(75)°, respectively, for g = a, b, and c. Restricting the orientation of the HF subunit to give a value of 35.92° for θa, we once again attempted to fit the angular orientation of the vinyl chloride subunit and the distance between the two subunits to Ia and Ic of the two isotopologues but still found no convergence. Additional information, specifically concerning the orientation of the ethylene subunit, is available from the Cl nuclear quadrupole coupling tensor. We have determined the three diagonal components and the magnitude of the only nonzero, off-diagonal componment, χab, of the chlorine quadrupole coupling tensor for both isotopologues of the vinyl chloride complex. Diagonalization of the tensor provides the components in the principal electric field gradient axis system. For the most abundant species the two components that lie in the molecular plane, χyy and χzz, are 33.4824 and −66.4305 MHz, respectively. (The component perpendicular to the molecular plane, χxx, is the same as χcc, which is 32.9481 MHz.) The absolute value of the asymmetry parameter for the quadrupole coupling constants, |η| = |(χxx − χyy)/χzz| = 0.0080, indicates that there is only a very small deviation from a cylindrical charge distribution about the z electric field gradient axis. This 9787
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structure. Specifically, the Cl−F distance and the CCl···F angle are 3.2046(68) Å and 96.792(12)° in the experimental structure compared to 3.1607 Å and 96.732°, respectively, in the theoretical structure. From our work on complexes formed with fluoroethylenes, we have suggested that steric factors favor bonding of HF across the double bond or at the “top” of the ethylene molecule, whereas bonding at the end or at the “side” of the ethylene is driven by electrostatic considerations.7 That is, the HF molecule “fits” better when it is interacting with the vinyl fluoride’s fluorine atom and the hydrogen atom located cis to it on the other carbon atom, as in Figure 5a. The structure determined here for the vinyl chloride−HF complex suggests that the top binding configuration offers similar steric advantages when HF bonds to vinyl chloride. Using van der Waals radii of 1.20 Å for H, 1.47 Å for F, and 1.75 Å for Cl,25 and assuming that the hydrogen atom of HF is located within the van der Waals sphere of the fluorine atom, van der Waals contact distances between the F atom in HF and the Cl atom in vinyl chloride and between the F atom in HF and the F atom in vinyl fluoride are predicted to be 3.22 and 2.94 Å, respectively, whereas the secondary interaction in each case would have a predicted F−H van der Waals contact distance of 2.67 Å. Experimentally, we find that F−Cl separation in vinyl chloride− HF is equal to the van der Waals contact distance, whereas in the vinyl fluoride−HF complex, it is only 5% shorter. Similarly, the experimental F···H secondary interaction length is merely 3% shorter for the vinyl chloride complex and just 2% longer for the vinyl fluoride complex than predicted. The HF subunit deviates from linearity by a similar amount in the two complexes. Thus, the location of HF is well predicted by van der Waals distances. There is, however, a notable difference in the structures of the two complexes. In the vinyl chloride complex the CCl···H angle is 102.4° whereas in vinyl fluoride− HF the CF···H angle is 121.4°.1 If these angles indicate the location of greatest electron density in the plane of the ethylene subunit, then it is clear that chlorine, with its expanded valence shell, behaves differently than fluorine in a significant way. Indeed, mapping the electostatic potential of vinyl chloride onto its total electron density surface calculated at the MP2/6311++G(2d,2p) level shows that the most negative region of the potential is concentrated on a band centered about the Cl atom13 and approximately perpendicular to the C−Cl bond, supporting the small value of the observed CCl···H angle in the vinyl chloride−HF complex. When the same calculation is performed for vinyl fluoride, the most negative portion of the potential is directed away from the fluorine atom and along the C−F bond, supporting a greater value of the CF···H angle in the vinyl fluoride−HF complex. The binding modes to vinyl chloride are different for HF and HCCH (Figure 5) and we have rationalized the differences previously.13 We confine our comments here to noting that this is the first time we have found the binding mode of a protic acid to a haloethylene molecule to depend on acid identity. Despite the difference in binding mode, the 2.32 Å hydrogen bond length reflects a stronger interaction between vinyl chloride and HF than between vinyl chloride and acetylene, where the analogous bond is 3.01 Å long.13 Similarly, the deviation of the hydrogen bond from linearity, 19.8° here and 58.5° in vinyl chloride−HCCH13 is indicative of a stronger interaction in the present case. It would be most interesting to determine the binding mode for HCl, an acid intermediate in gas-phase strength to HF and HCCH to further unravel the
Figure 5. Structures of (a) vinyl chloride−HF (this work), (b) vinyl fluoride−HF (ref 1), (c) vinyl chloride−HCCH (ref 13) showing the chemically relevant geometric parameters: hydrogen bond length, hydrogen bond angle, deviation from linearity, and secondary interaction length. The principal inertial axes for vinyl chloride−HF are also indicated.
effect on the electron distribution of vinyl chloride than does the weaker interaction with acetylene, where a vanishingly small perturbation of the electric field gradient was observed.13 Nevertheless, proceeding for the moment by ignoring any possible disturbance to the electron distribution, the rotation matrix relating the quadrupole coupling tensors of vinyl chloride and its complex with HF is the eigenvector matrix for the inertia tensor of the dimer expressed in the monomer inertial axis system, which is readily obtained. We use the inverse of this matrix to rotate the quadrupole tensor of the complex into the monomer inertial axis system. We find that to best reproduce the previously reported diagonal values of the coupling tensor for vinyl chloride,15 the value of χab for each isotopologue of vinyl chloride−HF, with the positive directions of the inertial axes as indicated in Figure 5, must be positive and the value of χab for the corresponding vinyl chloride monomer, with axes given in Figure 4, must be negative. The value of χab for CH2CH35Cl determined in this manner is −31.8 MHz, which can be compared to the value of −34.7 MHz obtained in a similar fashion as for CH2CH35Cl−HCCH. This provides further evidence for some distortion of the electron arrangement in vinyl chloride caused by the interaction with HF.
5. DISCUSSION The experimentally determined average structure agrees remarkably well with structure b predicted by theory: the primary hydrogen bond is 0.071 Å longer in the average structure, and the CCl···H angle is 2.4° greater. There is a greater deviation of HF from linearity in the experimental structure compared to the case of the ab initio structure [19.8(3)° vs 11.23°], but this is the structural parameter that is the most affected by the zero-point motion of H in HF. Indeed, the location of the heavier F atom in HF in the experimental structure is quite similar to that given by the theoretical 9788
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factors that determine how vinyl chloride participates in intermolecular interactions.
ASSOCIATED CONTENT
S Supporting Information *
Tables of observed and calculated transition frequencies for all isotopologues of vinyl chloride−HF that are reported in this study are available as Supporting Information, as are the atomic coordinates for the structures shown in Figure 2. This material is available free of charge via the Internet at http://pubs.acs.org.
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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) 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. (3) Kisiel, Z.; Fowler, P. W.; Legon, A. C. Rotational Spectrum, Structure, and Chlorine Nuclear Quadrupole Tensor of the Vinyl Fluoride-Hydrogen Chloride Dimer. J. Chem. Phys. 1990, 93, 3054− 3062. (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.; 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. (8) 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. (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.; 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. (11) 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. (12) 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. (13) 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. (14) 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. (15) Hayashi, M.; Inagusa, T. Microwave Spectrum, Structure and Nuclear Quadrupole Coupling Constant Tensor of Ethyl Chloride and Vinyl Chloride. J. Mol. Struct. 1990, 220, 103−117. (16) Guelachvili, G. Absolute Wavenumber Measurements of 1−0, 2−0, HF and 2−0, H35Cl, H37Cl Absorption Bands. Opt. Commun. 1976, 19, 150−154. (17) 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. (18) Watson, J. K. G. In Aspects of Quartic and Sextic Centrifugal Effects on Rotational Energy Levels; Durig, J. R., Ed.; Elsevier Scientific Publishing: Amsterdam, 1977; pp 1−89. (19) Pickett, H. M. The Fitting and Prediction of Vibration-Rotation Spectra With Spin Interactions. J. Mol. Spectrosc. 1991, 148, 371−377. (20) Kraitchman, J. Determination of Molecular Structure From Microwave Spectroscopic Data. Am. J. Phys. 1953, 21, 17−24.
6. CONCLUSIONS As it does in forming a complex with vinyl fluoride, HF binds across the double bond of vinyl chloride, forming a planar, or essentially planar, species with a primary, hydrogen bonding interaction between the hydrogen atom of HF and the chlorine atom in vinyl chloride. A secondary interaction between the fluorine atom of the acid and the hydrogen atom cis to the chlorine provides further stability and causes the hydrogen bond to bend slightly from linearity. In addition to the expected longer hydrogen bond length due to the increased atomic size of chlorine compared to that of fluorine, differences in the electron distributions about the halogens lead to a 102.4° CCl···H angle in the vinyl chloride complex compared to 121.4° for the corresponding CF···H angle in the vinyl fluoride case.1 Consequently, the secondary interaction bond length is shorter in the vinyl chloride complex (2.59 Å) than for vinyl fluoride (2.73 Å),1 despite having nearly identical deviations from linearity in the hydrogen bond. Most significantly, the vinyl chloride−HF complex adopts a different binding motif than does the complex with acetylene, vinyl chloride−HCCH.13 In this latter complex with a weaker acid, the acetylene molecule binds to the “side” of the vinyl chloride, forging interactions with the chlorine atom and a geminal hydrogen atom (Figure 5). This represents the first case where we have observed different bonding motifs for a given haloethylene in forming a complex with different protic acids. For both vinyl fluoride−HF and vinyl chloride−HF, the formation of a favorable, strong hydrogen bond with a limited deviation from linearity creates a steric requirement that, though limiting the importance of the secondary interaction to the stability of the complex, nevertheless makes the “top” bonding configuration across the double bond the lowest energy configuration. The stability imparted by the hydrogen bond remains determinative to the structure in the formation of the vinyl fluoride−HCCH complex but loses its importance in vinyl chloride−HCCH so that what we have been calling the secondary interaction is optimized at the expense of the hydrogen bond to give the side-binding configuration.
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AUTHOR INFORMATION
Corresponding Authors
*H. O. Leung. Telephone: (413) 542-2660. Fax: (413) 5422735. E-mail:
[email protected]. *M. D. Marshall. Telephone: (413) 542-2006. Fax: (413) 5422735. E-mail:
[email protected]. 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-1111504. 9789
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