F Interactions in the Vinyl Fluoride···Difluoromethane Complex

Article pubs.acs.org/JPCA

Rotational Spectroscopic Studies of C−H···F Interactions in the Vinyl Fluoride···Difluoromethane Complex Cori L. Christenholz, Daniel A. Obenchain, Rebecca A. Peebles, and Sean A. Peebles* Department of Chemistry, Eastern Illinois University, 600 Lincoln Avenue, Charleston, Illinois 61920, United States S Supporting Information *

ABSTRACT: Rotational spectra of the normal isotopic species and three 13C isotopologues of the 1:1 complex between vinyl fluoride (CH2CHF) and difluoromethane (CH2F2) have been measured using 480 MHz bandwidth chirped-pulse Fourier-transform microwave spectroscopy in the 6.5−20 GHz region. A structure for this dimer has been determined by fitting the moments of inertia of all isotopologues and confirmed by calculation of Kraitchman single isotopic substitution coordinates. The structure is consistent with that determined by ab initio geometry optimization at the MP2/6-311++G(2d,2p) level and has the difluoromethane subunit located on the CHF side of the vinyl fluoride subunit with three C−H···F contacts and with the hydrogen atoms of the CH2F2 straddling the vinyl fluoride symmetry plane.

1. INTRODUCTION In our series of studies of halogenated methane dimers with unsaturated molecules (such as HCCH complexed with CH2ClF,1 CHClF2,2 CHBrF2,3 and CH2F24) to explore variation of structural parameters and the preference for identity of the halogen acceptor atom, a logical progression was to extend to complexes of ethylene. This led us to examine the ethylene complex with CH2F2 (difluoromethane, DFM hereafter) and an initial investigation of this rotational spectrum revealed small splittings, presumably arising from internal rotation of the ethylene subunit about one or more of its axes.5 Although analysis of these transitions is ongoing, preliminary rotational constants appear consistent with a structure in which DFM is located above the ethylene plane (such that the C2 axis of DFM lies in the symmetry plane bisecting the CC bond). One C−H bond of DFM sits over the CC bond and the two F atoms point down toward the H atoms on adjacent carbons of ethylene. The structure of the ethylene···CH2ClF complex5 appears to be similar and that spectrum also shows indication of large amplitude motions of ethylene. Substitution of one hydrogen atom of ethylene with a fluorine atom (to give vinyl fluoride) breaks the symmetry of the H2CCH2 subunit, thereby preventing internal motions interconverting between equivalent structures, but also significantly changing the electronic charge distribution (the fluorine atom can be thought of as deactivating the CC unsaturated bond) and therefore presents a new set of possible sites for complexation. In addition to the π system, vinyl fluoride (VF, hereafter) possesses lone pairs with which an electrophilic site may interact. DFM complexes with the related molecules 1,1difluoroethylene (DFE) and 1,1,2-trifluoroethylene (TFE) have already been characterized by Tatamitani et al.,6 who showed that DFM aligns along the top of the DFE subunit (in DFE··· DFM) and along the side of the TFE subunit (in TFE···DFM). In the top-bonded DFE···DFM, the DFM is situated alongside the CC bond and binds to the fluorine atom on one carbon © 2014 American Chemical Society

and the cis-hydrogen atom on the adjacent carbon, with the hydrogen atoms of DFM located out of the DFE plane and with one of the fluorine atoms of DFM interacting with the DFE hydrogen atom. In the side-bonded TFE···DFM, the DFM subunit is located between the H and F atoms that are bound to the same carbon atom of TFE. The F atoms from DFM extend out of the plane of TFE, with two H···F contacts in which DFM is the acceptor and an additional H···F contact with DFM as the donor. Interestingly, structures in which DFM is located above the fluoroethylene plane, such as that observed for ethylene···DFM, do not appear to be energetically favorable for fluorinated ethylenes. There has been significant work by the groups of Legon on VF···HX complexes (X = F,7 Cl,8 Br,9 and CCH10) and of Leung and Marshall exploring the effects that differing degrees of fluorination have on the complexation of fluoroethylenes with HX (X = F,11−13 Cl,14 and CCH15,16). The complexes of fluoroethylenes with HX molecules again allow two possibilities for the binding of HX: top-binding and side-binding; both options allow for secondary interactions between the X atom of HX and an H atom of the fluoroethylene, resulting in hydrogen bond interactions that are often quite nonlinear. HX is typically found to bind along the top with secondary interactions between the X atom and the H atom cis to the fluoroethylene F atom. In this article, we examine the complex between VF and DFM, the DFM having two options to bind, either along the top (similar to DFE···DFM6) or at the side (similar to TFE··· DFM6). The goal of the present work is therefore to characterize the remaining unstudied complex in the series of DFM complexed with fluoroethylenes, namely, the VF···DFM complex, where chemical intuition would favor the side-bonded Received: January 10, 2014 Revised: February 12, 2014 Published: February 24, 2014 1610

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abundance) were carried out on a resonant-cavity FTMW spectrometer of the Balle−Flygare design.19,20 Spectra were initially viewed and assigned using the AABS package of Kisiel,21 and transition frequencies were fitted/predicted using the SPFIT/SPCAT programs of Pickett,22 with a Watson Areduced Hamiltonian in the Ir representation.23 Stark effect experiments were performed on the resonantcavity instrument by application of voltages up to ±5 kV to a pair of parallel steel mesh plates placed within the vacuum chamber to straddle the molecular expansion. Calibration of the applied electric field was carried out by measurement of the J = 1 ← 0 transition of OCS and assuming a dipole moment of μ = 0.71519(3) D.24

form because of the more electropositive geminal H atom in VF.

2. AB INITIO CALCULATIONS Geometry optimizations were carried out on various configurations of VF···DFM using Gaussian0917 to identify the lowest energy structure and to provide predicted rotational spectra to aid in spectral assignment. These optimizations used the CALCALL option and were done at the MP2/6-311+ +G(2d,2p) level with corrections for basis set superposition error (BSSE) made using the COUNTERPOISE keyword. Zero-point energy (ZPE) corrections were determined from harmonic frequency calculations. Two different classes of structures of Cs symmetry were considered (corresponding to the DFM subunit placed either along the top or side of the VF monomer). Two different initial orientations were used as starting points for optimization of each class of structure, with either the F or H atoms straddling the VF plane, giving a total of four possible structures (I−IV; Figure 1). Attempts to locate

4. RESULTS 4.1. Ab Initio Calculations. Table 1 lists the results of geometry optimizations for the five structures (I−V, Figure 1) Table 1. MP2/6-311++G(2d,2p) Predicted Spectroscopic Parameters, Dipole Moment Components, and Relative Energiesa for Five Structures of VF···DFM; the Structures Are Shown in Figure 1 A (MHz) B (MHz) C (MHz) μa (D) μb (D) μc (D) ΔEBSSE (cm−1) ΔEBSSE+ZPE (cm−1)

I

II

III

IV

V

11690 1017 942 1.97 0.38 0 0.0 0.0

7224 1178 1020 2.63 0.62 0 26 23

6232 1309 1271 0.39 0 0.46 68 61

4945 1385 1350 0.66 0.77 0 138 116

5861 1209 1056 1.52 0.10 1.36 136 155

a

Relative energies are either corrected for basis set superposition error (ΔEBSSE) or are both basis set superposition error and zero-point energy corrected (ΔEBSSE+ZPE).

Figure 1. MP2/6-311++G(2d,2p) stationary points for the vinyl fluoride···difluoromethane (VF···DFM) complex. Structures I−IV have all three carbon atoms in the plane of the paper, while in structure V the DFM carbon atom is located ∼2.56 Å above the plane of the vinyl fluoride. The associated relative energies, rotational constants, and dipole moment components are listed in Table 1. All structures are minima, except for III and IV, which each have a single imaginary frequency (see text for more discussion).

in decreasing stability order, with structure I determined to be the most stable. In the first four structures, the in-plane C−H··· F distances show a variation of only about 0.1 Å, while the outof-plane distances show even smaller (∼0.06 Å) variation (Figure 1); ab initio principal axis coordinates for structures I− V are available as Supporting Information. The two lowest energy structures (I (side-bonded) and II (top-bonded), Figure 1) are predicted to lie only about 23 cm−1 apart based on relative energies corrected for both BSSE and ZPE, although no indication of any spectrum belonging to structure II was observed in our scan. Recent studies of vinyl fluoride complexed with CO225 did reveal the existence of both top- and side-bonded geometries; however, in that case, the two structures were determined to have only a 6 cm−1 difference in energy at the same level of calculation, so absence of a second structure for VF···DFM in the molecular expansion is not unexpected. Structures III and IV (the structures with the F atoms of DFM straddling the plane of the VF) both possessed one imaginary frequency at this level of calculation, being classified therefore as transition states rather than minima. Although this effectively eliminates them as possible experimental structures, these structures are still included for comparison purposes since whether or not a particular geometry is found to possess imaginary frequencies can depend to some extent on the level of theory and optimization criteria used during the calculations. The energetic preference of structures I and II over III and IV can be rationalized if the

a stationary point with DFM above the plane of VF (in a geometry similar to that observed for ethylene···DFM) led to convergence back to structures I or II, except in the case of structure V where the DFM sits above the H−CC−H pocket of the VF monomer.

3. EXPERIMENTAL SECTION A sample of 1% CH2F2 (99.7%, Sigma Aldrich) and 1% CHF CH2 (98%, Synquest Laboratories) in a He/Ne carrier gas (17.5% He/82.5% Ne, BOC Gases) was delivered at a pressure of 2 atm to a General Valve Series 9 pulsed nozzle with a 0.8 mm orifice. The rotational spectrum was measured on a 480 MHz bandwidth chirped-pulse Fourier-transform microwave (FTMW) spectrometer18 over the 6.5−20 GHz region, in 240 MHz overlapping segments. A total of 2000 free induction decays were averaged and Fourier-transformed for each frequency segment, and a LabVIEW program was used to determine absolute frequencies and assemble the frequency domain sections into a full spectrum. Additional measurements of weaker transitions and the three 13C species (in natural 1611

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Table 2. Spectroscopic Parameters for the Normal and Three 13C-Substituted Species of the VF···DFM Dimer parametera

CH2CHF···CH2F2

CH2CHF···13CH2F2

CH213CHF···CH2F2

A (MHz) B (MHz) C (MHz) ΔJ (kHz) ΔJK (kHz) δJ (kHz) Nc Δνrms (kHz)d Paa (u Å2)e Pbb (u Å2) Pcc (u Å2)

11824.0527(14) 1035.12491(26) 960.47855(20) 0.8254(10) −1.011(26) 0.0549(10) 31 2.3 485.8313(1) 40.3429(1) 2.3987(1)

11801.5(32) 1029.6599(4) 955.6112(4) 0.8254b −1.011b 0.0549b 9 2.6 488.426(6) 40.428(6) 2.3952(6)

11798(5) 1026.2173(6) 952.6818(6) 0.8254b −1.011b 0.0549b 8 3.8 490.057(9) 40.424(9) 2.411(9)

CH2CHF···CH2F2

13

11821(6) 1011.6785(6) 940.1406(7) 0.8254b −1.011b 0.0549b 9 4.3 497.175(11) 40.382(11) 2.370(11)

a Errors in parentheses are standard errors and reflect one standard deviation. bFixed at values from the normal isotopologue. cNumber of fitted transitions. dΔνrms = [∑(νobs − νcalcd)2/N]1/2. ePlanar moments Paa = 0.5(Ib + Ic − Ia) = ∑imiai2, etc.

Table 3. Stark Coefficientsa and Dipole Moment Components for the VF···DFM Complex

molecular electric quadrupole moments for VF and DFM are considered; only in structures I and II (where H atoms straddle the VF plane) are the out-of-plane quadrupole moment components for the two monomers of opposite signs and hence result in favorable quadrupole−quadrupole interactions (Qcc(DFM) = +7.3(20) × 10−40 C m2, Qcc(VF) = −9.7(10) × 10−40 C m2).26,27 Structure V is determined to be the least stable, lying some 155 cm−1 above the most stable geometry and will not be considered further. 4.2. Rotational Spectrum. Both a- and b-type rotational transitions were observed, although the a-type transitions were considerably more intense, in accordance with dipole moment components predicted for the lowest energy structures (I and II, Table 1). Spectroscopic parameters obtained from fitting the measured rotational transitions for each isotopic species are given in Table 2. The A rotational constant is more poorly determined for the 13C substituted species (which were measured in natural abundance) due to an absence of the considerably less intense b-type transitions within the fitted data set. The large (∼11.8 GHz) value of the A experimental rotational constant (Table 2) is only consistent with Structure I (Table 1); MP2 values underestimate the experimental rotational constants by between 1 and 2%. 4.3. Dipole Moment. Second-order perturbation theory (implemented in the ASYSPEC program28) was used to calculate Stark coefficients, which were then utilized in a leastsquares fitting program to obtain the dipole moment components that best reproduced the observed Stark shifts. Experimental and calculated Stark coefficients are listed in Table 3, along with fitted dipole moment components. A comparison with the MP2 predictions for μa and μb given in Table 1 shows that ab initio calculations for structure I overestimated μa and μb by approximately 4% and 6%, respectively. 4.4. Structure Determination. The closeness of experimental rotational constants and dipole moment components to the MP2 predicted values points convincingly to structure I as the observed geometry. An inspection of the invariance of out-of-plane planar moments (Pcc = ∑imici2 = 0.5(Ia + Ib − Ic)) for all measured isotopologues (Table 2) further confirms all isotopic substitution occurred within the ab symmetry plane. The small value of Pcc (∼2.4 u Å2) in all species (Table 2) is consistent with only hydrogen atoms lying out of the symmetry plane, again favoring structure I. Structure III (the other sidebonded structure) has fluorine atoms lying out of this plane, and a considerably higher value (∼44 u Å2) of Pcc would

JKaKc′ ← JKaKc″ 404 ← 303

|M|

0 1 2 3 303 ← 202 0 1 313 ← 212 0 414 ← 313 0 1 μa = 1.896(5) a

105 Δν/E2 (obs)

105 Δν/E2 (calcd)

105 Δν/E2 (obs − calcd)

−0.4933 −0.4790 −0.0143 −0.3125 −0.3091 −0.0034 0.2064 0.2004 0.0060 1.0578 1.0495 0.0083 −1.1764 −1.1816 0.0052 −0.3415 −0.3391 −0.0024 −0.2873 −0.2811 −0.0062 0.3185 0.3080 0.0105 1.5241 1.5355 −0.0114 D, μb = 0.3576(22) D, μtotal = 1.930(5) D

Stark coefficients are in units of MHz V−2 cm2.

therefore be expected.29 The observed value of 2.4 u Å2 for Pcc of the complex is a little larger than the literature value for Pbb of DFM (1.69 u Å2);29 this seems to be fairly typical behavior for a weakly bound complex exhibiting out-of-plane vibrational motions.30 Moments of inertia obtained from the fits of rotational transition frequencies for all isotopic species were used in the STRFITQ program31 to allow determination of a ground state (r0) structure for this complex. Fixing the geometries of the DFM29 and VF32 monomers at their literature structures (Table 4) and assuming an ab plane of symmetry leaves three structural parameters to describe the dimer geometry: an intermolecular distance (chosen here to be the H6···F11 separation) and two angles (defined as θ (H6···F11−C7) and ϕ (C4−H6···F11); see Figure 2). Table 5 lists results of fits of the Ib and Ic moments of inertia for all four isotopic species, with the fit either including (the Ia, Ib, Ic column) or excluding (the Ib, Ic column) the Ia moment of inertia for the normal isotopologue. Since the uncertainty in the A rotational constant is high for the 13C substituted species, these constants were not employed in any fit. Ab initio values of the structural parameters for structure I are included in the last column of Table 5 for comparison. The fit of Ib, Ic only is preferred since it gives the lower standard deviation (0.061 u Å2 versus 0.478 u Å2); however, both inertial fits agree to within the experimental uncertainties in the structural parameters. Independent determination of some structural parameters is also possible using single isotopic substitution data obtained from rotational constants measured for the 13C substituted 1612

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Table 4. Literature Structures for the Difluoromethane29 and Vinyl Fluoride32 Monomers Used in Determination of Structure of the Complex; Atom Numbering Refers to Figure 2

Table 6. Kraitchman Coordinates (in Å) for the Substituted Carbon Atomsa in the VF···DFM Complex and a Comparison with ab Initio (MP2/6-311++G(2d,2p)) and Inertial Fit Derived (Ib, Ic) Principal Axis Coordinates

CH2F2 R(C7−H8,9) (Å) R(C7−F10,11) (Å) ∠(H8−C7−H9) (deg) ∠(F10−C7−F11) (deg) CH2CHF

1.093 1.357 113.7 108.3

R(C2−H3) (Å) R(C2−H1) (Å) R(C4−F5) (Å) R(C4−H6) (Å) R(C2C4) (Å) ∠(H6−C4C2) (deg) ∠(H3−C2C4) (deg) ∠(H1−C2C4) (deg) ∠(F5−C4C2) (deg)

1.077 1.087 1.347 1.082 1.329 129.2 119.0 120.9 120.8

Kraitchman |a| |b| |c| Inertial Fit a b c Ab Initio a b c a

C2

C4

C7

3.3795(13) 0.201(22) 0.17(3)i

2.0624(20) 0.286(15) 0.112(38)

1.6162(23) 0.294(14) 0.06(7)i

−3.3898 0.2648 0.0000

−2.0609 0.2819 0.0000

1.6365 −0.3144 0.0000

−3.4116 0.2752 0.0000

−2.0877 0.2762 0.0000

1.6430 −0.3068 0.0000

Atom numbering follows that of Figure 2.

coordinates are all small (none are greater than 0.3 Å, Table 6), even this agreement can be considered reasonable. These results serve as a confirmation of our inertial fit structure (the coordinates of which are no more than a few hundredths of an Angstrom different from those obtained from the Kraitchman analysis and ab initio calculations). Of course, it should be noted that the ab initio parameters refer to an equilibrium geometry but it nonetheless provides an excellent estimate of the ground vibrational state averaged (r0) structure in this case. Determination of the CC bond distance in vinyl fluoride using the carbon atom substitution coordinates gives a CC bond length of 1.320(3) Å, in excellent agreement with the literature value32 of 1.329 Å. Using the structure obtained from fitting the Ib and Ic moments of inertia, it is possible to project the monomer dipole moments (μb(DFM) = 1.96(2) D;35 μa(VF) = 1.284(4) D and μb(VF) = 0.712(12) D36) into the principal axis coordinate system of the dimer, to estimate dipole moment values of μa = 1.81 D and μb = 0.41 D for the complex. These are very close to the experimental dipole moment components derived from Stark effect measurements (Table 3), with a small difference of