J. Phys. Chem. 1996, 100, 15695-15703
15695
Vibrational Analysis of the van der Waals Complexes between Vinyl Fluoride and Hydrogen Chloride in Liquefied Argon W. A. Herrebout and B. J. van der Veken* Department of Chemistry, UniVersitair Centrum Antwerpen, Groenenborgerlaan 171, B2020-Antwerpen, Belgium ReceiVed: April 10, 1996; In Final Form: July 11, 1996X
The mid-infrared spectra (4000-400 cm-1) of vinyl fluoride/HCl and vinyl fluoride/DCl mixtures, dissolved in liquefied argon at 105 K, were examined. In both cases, evidence was found for the occurrence of a 1:1 van der Waals complex between CH2dCHF and HCl (DCl). At higher concentrations of hydrogen chloride, absorption bands of a 1:2 species were also observed. By using spectra recorded at several temperatures between 95 and 115 K, the complexation enthalpies of CH2dCHF‚HCl and CH2dCHF‚(HCl)2 were determined to be -5.78 ( 0.13 and -9.85 ( 0.34 kJ mol-1, respectively. A structural study, using ab initio calculations at the MP2/6-31+G** level, indicates that the complexation can occur either via the fluorine atom or via the π bond. From a comparison of the experimental with the ab initio vibrational frequencies, it was concluded that all observed bands of the 1:1 complex are due to a species complexed via the fluorine atom.
Introduction Hydrogen chloride dissolved in liquefied argon has been observed to form van der Waals complexes with various alkyl halides1-6 and with molecules containing π bonds, such as ethylene.7 In the former case, a so-called σ complex is formed between the alkyl halogen atom and the hydrogen atom of HCl, while in the latter case the π electrons act as electron donors in the formation of a π complex. For the complex between methyl fluoride and HCl a complexation enthalpy ∆H° of -4.6 ( 0.4 kJ mol-1 has been measured,6 while for the complex between ethylene and HCl7 the ∆H° has been derived to be -6.30 ( 0.36 kJ mol-1. These data suggest that the bonding in the π complex is slightly stronger than that in the σ complex. In a molecule such as vinyl fluoride, both types of electron donors are present, and, from the preceding observation, competition in the formation of the two types of complexes must be expected. A similar argument, based on a consideration of the Legon-Millen rules and the Buckingham-Fowler electrostatic model, was used by Kisiel et al.8 to motivate their study of the supersonic jet FT microwave spectra of mixtures of these two compounds. In that study only the spectrum of the σ complex was observed. The authors put forward arguments to rationalize the observed preference to form the σ species, but the possible formation of the π complex in jets of a higher effective temperature was not ruled out.8 Similar complexes, for instance between vinyl fluoride and HF9 and between vinyl chloride and HCl,9,10 have been investigated by using matrix isolation infrared spectroscopy. In both instances only the σ complexes have been identified. The main drawback of the supersonic jet and matrix isolation techniques is that the presence of a complex in an experiment is not uniquely determined by the equilibrium thermodynamic properties of the species. Thus, from investigations using these techniques, no definitive conclusion on the relative stability of the π complex can be drawn. This, at least in principle, is possible from the study of the species in cryosolutions, as in these solutions the complexes are being formed under equilibrium conditions.11 Moreover, the two types of complexes must have different frequencies for a number of their vibrational X
Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)01071-4 CCC: $12.00
modes, making vibrational spectroscopy a recommended technique. Therefore, in this investigation solutions of vinyl fluoride and HCl in liquefied argon have been studied by using infrared spectroscopy. Corroborating evidence for the assignments made in this study was obtained from an ab initio study of the compounds. Experimental Section The sample of vinyl fluoride was obtained commercially (PCR Inc. 12200-2) and used without further purification. The HCl was synthesized in small amounts by hydrolyzing PCl3 with H2O and purified by pumping the reaction mixture through a 2-propanol slush at 180 K, followed by fractionation on a lowtemperature, low-pressure fractionation column. By using a similar procedure and D2O (Janssen Chimica 16.630.43) DCl was also synthesized. The argon used has a stated purity of 99.9999% and was used without further purification. The infrared spectra were recorded on a Bruker IFS 66v and a Bruker 113v Fourier transform spectrometer, using a Globar source in combination with a Ge/KBr beam splitter and a broadband MCT detector. The interferograms were averaged over 200 scans, Happ Genzel apodized, and Fourier transformed using a zero-filling factor of 4 to yield spectra at a resolution of 0.5 cm-1. A detailed description of the liquid noble gas setup was given in a previous study12 and will not be repeated here. To be able to distinguish the spectra of dissolved from those of undissolved vinyl fluoride, solid state spectra were obtained by condensing a small amount of the compound onto a window cooled to 10 K, followed by annealing until no further changes were observed in the infrared spectrum. For all solid state experiments, a Leybold ROK 10-300 double-stage cryostat with closed-cycle helium cooling was used. Ab Initio Calculations Computational Details. Ab initio calculations at the MP2/ 6-31+G** level were carried out using Gaussian 92.13 For all calculations, the correlation energy was calculated by using all molecular orbitals, while the Berny optimization14 was used with the tight convergence criteria. No restrictions due to possible symmetry of the species were imposed. © 1996 American Chemical Society
15696 J. Phys. Chem., Vol. 100, No. 39, 1996
Herrebout and van der Veken
TABLE 1: MP2/6-31+G** Structural Parameters (Bond Lengths in Angstroms, Bond Angles in Degrees) for the σ Complexes H2CdCHF‚HCl for CH2dCHF and HCl CH2dCHF‚HCl Ia
Ib
CH2dCHF
r(F1sC2) r(C2sC3) r(H4sC3) r(H5sC3) r(H6sC2) r(F1‚‚‚H7) r(H7sCl8)
1.3740 1.3264 1.0780 1.0773 1.0781 2.0295 1.2734
1.3734 1.3262 1.0778 1.0772 1.0787 2.0695 1.2733
1.3630 1.3280 1.0779 1.0771 1.0792
∠(C3sC2sF1) ∠(H4sC3sC2) ∠(H5sC3sC2) ∠(H6sC3sC2) ∠(H7‚‚‚F1sC2) ∠(Cl8sH7‚‚‚F1)
120.91 121.66 118.76 127.72 118.42 165.43
120.82 121.49 118.81 127.60 118.28 156.13
121.50 121.40 118.97 126.74
τ(H4sC3sC2sF1) τ(H5sC3sC2sF1) τ(H6sC2sC3sF1) τ(H7‚‚‚F1sC2sC3) τ(Cl8sH7‚‚‚F1sC2)
0.09 180.32 179.80 57.74 -20.37
0.00 179.99 180.01 178.56 156.13
0.00 180.00 180.00
dipole moment (D) energy (hartrees)
2.99 -637.581390
2.76 -637.581316
1.67 -177.357082
HCl
1.2689
1.22 -460.218345
TABLE 2: MP2/6-31+G** Structural Parameters (Bond Lengths in Angstroms, Bond Angles in Degrees) for π-CH2dCHF‚HCl (II)
Figure 1. MP2/6-31+G** equilibrium geometries for the 1:1 complexes between vinyl fluoride and HCl.
The complexation energies of the complexes were obtained by subtracting the calculated energies of the monomers from that of the complex, and these differences were corrected for basis set superposition error (BSSE) by using the full counterpoise correction method described by Boys et al.15 For all equilibrium geometries, the vibrational frequencies and the corresponding infrared intensities were calculated by using standard Cartesian harmonic force fields. The latter were calculated as the analytical second derivatives of the energy with respect to the coordinates. Equilibrium Geometry. For the σ complex, several structures were optimized by using standard convergence criteria, starting from geometries close to the one deduced by Kisiel et al.,8 using different initial values of the dihedral angle τ(CdCsF‚‚‚H). These optimizations resulted in two different equilibrium structures, which were consequently refined using the tight convergence criteria. These structures, described herein as conformers a and b of isomer I, are shown in Figure 1. Their structural parameters, and those of the monomers, are given in Table 1. The structure of the π complex, referred to as isomer II, was also calculated. Its structure is also shown in Figure 1 and its parameters are given in Table 2. As conformer Ia is closest to the experimental structure,8 we will use it for comparison. In the microwave analysis,8 the structure of the monomers was assumed to be unchanged by the complexation. This is supported by the calculations, except for the CsF and HsCl bond lengths. The latter increase significantly, signaling a weakening of the bonds in the complex. The calculated angle H7‚‚‚F1sC2, 118.4°, is close to the experimental8 value of 116.1 ( 0.1°. From the analysis of the quadrupole coupling it was deduced8 that the angle between
r(F1sC2) r(C2sC3) r(H4sC3) r(H5sC3) r(H6sC2) r(C2sX7)a r(X7‚‚‚H8) r(H8sCl9)
1.3600 1.3302 1.0787 1.0777 1.0796 0.8284 2.4566 1.2733
∠(C3sC2sF1) ∠(H4sC3sC2) ∠(H5sC3sC2) ∠(H6sC3sC2) ∠(C2sX7sC3)
121.31 121.42 118.93 126.79 180.00
dipole moment (D) energy (hartrees)
2.66 -637.579567
∠(C2sX7‚‚‚H8) ∠(X7‚‚‚H8sCl9)
90.00 180.04
τ(H4sC3sC2sF1) τ(H5sC3sC2sF1) τ(H6sC2sC3sF1) τ(H8‚‚‚X7sC2sF1) τ(H8‚‚‚X7sC2sH6) τ(H8‚‚‚X7sC3sH4) τ(H8‚‚‚X7sC3sH5) τ(C3‚X7sH8sC2) τ(C2‚X7sH8sCl9) τ(C3‚X7sH8sCl9)
1.56 179.79 179.50 -77.94 101.55 79.44 -102.27 180.00 -115.28 64.72
a X7 is a dummy atom situated in the CdC bond at the perpendicular projection of the H(Cl) atom.
the HsCl and hydrogen bond axes should be close to 10°; the present calculation leads to the slightly larger value of 14.6°. The rotational constants of Ia are close to the experimental values,8 while those of Ib are totally different. Unfortunately, the inertial defect calculated for Ia, -8.404 amu‚Å2, strongly differs from the experimental value of -0.269 amu‚Å2. This is due to the dihedral angle τ(H7‚‚‚F1sC2sC3), which is calculated at 57.7°, while the experimental data lead to a value of some 8°.8 The poor value calculated for this angle presumably is a consequence of the shallowness of the potential governing this dihedral angle. This potential was calculated and is shown in Figure 2. It can be seen that the barrier separating Ia from Ib is a mere 0.4 kJ mol-1; evidently, the calculated energy difference between the conformers is even lower. In the microwave spectra8 no lines due to a second conformer were detected, nor was the presence of one suggested by, for instance, tunneling splitting of the observed lines. Therefore, the accuracy of the potential shown in Figure 2 and, consequently, the value of the calculated dihedral angle must be questioned. From vinyl fluoride to the π-bonded complex, the CdC distance is calculated to increase from 1.3280 to 1.3302 Å, in agreement with the expectation that the CdC bond is slightly
CH2dCHF and HCl Complexes in Argon
J. Phys. Chem., Vol. 100, No. 39, 1996 15697 energies, ∆Ecorr, are given in Table 3. It can be seen that the π complex II is predicted to be less stable than the σ complex. This result agrees with the expectations of the Legon-Millen model.8 The energy difference, however, is small, so that in an environment in which complex formation between vinyl fluoride and HCl is taking place the occurrence of a measurable fraction of the complex molecules as isomer II cannot be excluded. Vibrational Spectra. In Table 4, the vibrational frequencies and infrared intensities calculated for Ia, Ib, II, and vinyl fluoride are given. Also given are the complexation shifts ∆ν, defined as νcomplex - νmonomer. These data will be used to identify the complex species observed in this study. Table 4 shows that, as could be expected for weakly bound van der Waals complexes, all of the intermolecular modes are predicted to give rise to bands in the far infrared. This region was not investigated, and thus only modes localized in the constituent molecules can be used in the identification. It can also be seen in Table 4 that the frequencies of some of the vibrational modes of the σ complex are conformer-dependent. Because the ab initio calculations poorly describe the potential generating the conformers, these frequencies may not be very accurate. Therefore, in the identification of the conformers, discussed in the following, only the complexation shifts that are largely conformer-independent will be used. Evidently, to be of diagnostic value, the complexation shift of a mode should be sufficiently different in the two types of complexes. Taking into account these conditions, inspection of Table 4 shows that for the σ complex the CsF stretching, the CdCsH out-ofplane, the CH2 rocking, and maybe also the CH2 twisting should occur red-shifted. For the π complex, the CH2 symmetric stretch and the CdC stretch must be red-shifted, while the CdCsH out-of-plane and CH2 twisting must be blue-shifted. For the CsF and CdC stretches, these predictions are in line with what could qualitatively be expected from the formation of a σ versus a π complex. As is clear from the preceding discussion, complex bands are expected to occur near the CsF and CdC stretches. Correct assignment of the observed features requires that the vibrational spectra in these regions are understood in detail. As vinyl fluoride has been used in its natural isotopic abundance, weak bands due to 13CH2dCHF and CH2d13CHF must be expected
Figure 2. Potential energy function for the internal rotation around the CsF bond in CH2dCHF‚HCl.
TABLE 3: MP2/6-31+G** Complexation Energies for the σ and π Complexes of Vinyl Fluoride with HCla ∆Eb (hartrees) ∆EBSSEc (hartrees) ∆Ecorrd (hartrees) ∆Ecorr (kJ mol-1)
Ia
Ib
II
-0.005963 -0.002368 -0.003592 -9.43
-0.005889 -0.002377 -0.003511 -9.22
-0.004140 -0.001423 -0.002717 -7.13
a The numbering of the complexes refers to Figure 1. b Calculated using the approximate expression: ∆E ) ECH2dCHCF‚HCl - ECH2dCHF EHCl. c Calculated using the full counterpoise correction.20 d Calculated using the expression ∆Ecorr ) ∆E - ∆EBSSE.
weakened in this complex. Table 2 also shows that the CsF bond length is calculated to decrease from 1.3630 to 1.3600 Å, while the HsCl bond length increases from 1.2689 to 1.2733 Å. This increase compares very well with that for the σ complex. In the π complex, the HCl molecule is situated in a plane very nearly perpendicular to the CdC bond. This plane is closer to the CH2 side than to the CHF side of the molecule. The HCl axis intersects the CdC bond and is rotated 10° in the direction of the fluorine atom. This orientation qualitatively agrees with that predicted from the Buckingham-Fowler model, as described by Kisiel et al.8 Relative Stability. The MP2/6-31+G** complexation energies of Ia, Ib, and II, the corresponding values for the basis set superposition error, ∆EBSSE, and the corrected complexation
TABLE 4: MP2/6-31+G** Vibrational Frequencies (cm-1) and Infrared Intensities (km mol-1) for the σ and π Complexes of CH2dCHF with HCla,b CH2dCHF‚HCl Ia
Ib
II
CH2dCHF
approximate description
νj
int.
∆νj
νj
int.
∆νj
νj
int.
∆νj
νj
int.
CH2 antisym stretch CH stretch CH2 sym stretch HsCl stretch* CdC stretch CdCsH in-plane def CH2 bending CsF stretch CdCsH out-of-plane def CH2 rocking CH2 wagging CH2 twist CdCsCl in-plane def
3384.2 3330.9 3270.7 3069.7 1726.7 1444.8 1353.4 1155.5 959.7 931.1 872.6 722.9 481.9 288.2 240.1 119.3 42.9 24.9
0.3 3.3 2.2 182.4 86.8 7.5 0.3 103.9 55.3 58.0 42.5 2.9 5.0 65.2 42.5 4.9 3.9 0.2
-0.2 14.0 0.1 -49.1 -2.8 -2.9 -2.7 -17.9 -5.6 -12.1 17.0 -3.9 0.0
3385.0 3325.1 3271.1 3073.1 1729.8 1444.9 1357.7 1155.8 960.2 932.2 865.5 721.7 485.0 298.8 216.5 109.1 31.8 29.1
0.1 2.4 0.1 175.5 90.9 5.5 1.0 102.2 54.8 62.4 45.6 3.3 6.4 87.4 45.6 4.1 3.8 0.4
0.6 8.2 0.5 -47.0 0.3 -2.8 1.6 -17.6 -5.1 -11.0 9.9 -5.1 3.1
3380.0 3315.5 3265.2 3055.8 1722.3 1448.8 1355.0 1178.3 977.0 947.9 877.4 740.0 483.9 272.6 240.8 86.8 74.5 24.0
0.1 3.7 2.6 213.7 91.9 6.7 1.1 93.8 58.2 42.2 65.8 1.9 4.5 27.4 27.0 0.2 4.0 1.8
-4.4 -1.9 -5.6 -60.3 -7.5 0.3 -1.2 4.9 11.7 4.7 21.8 13.2 2.0
3384.4 3316.9 3270.6
0.8 5.9 0.1
1729.5 1447.8 1356.2 1173.4 965.3 943.2 855.6 726.8 481.9
53.4 12.2 8.0 18.3 46.9 38.2 34.9 13.5 0.2
a
HCl monomer: 3120.1 cm-1, 26.3 km mol-1. b The numbering of the complexes refers to Figure 1.
15698 J. Phys. Chem., Vol. 100, No. 39, 1996
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TABLE 5: MP2/6-31+G** Frequencies (cm-1) for νCdC and νCsF in Vinyl Fluoride and Some Isotopic Derivatives CH2dCHF 13CH dCHF 2 CH2d13CHF
TABLE 6: Observeda Frequencies (cm-1) and Assignments for Vinyl Fluoride gasb
νCdC
νCsF
liq argon
rel int
1729.6 1700.6 1703.7
1173.4 1169.6 1153.7
3294 3282 3222 3211 3202 3137 3102 3094 3090 3056 3002 2994 2983 2945 2795 2745 2603 2569 2520 2450 2331 2290 2227 2219 2133 2068 2011 1857 1848 1790 1786 1730 1652 1632 1626
m w, sh vw w w s s s s m m m w w m w vw m m m w w w w w s vw m, sh s m m vvs m, sh m, sh
1654
1571 1424 1409 1375 1304 1148 1129 929 925 862 712 664 482
vs w m vs vs vvs w, sh s, sh vs vvs vs w s
1572
Figure 3. Infrared spectrum of vinyl fluoride dissolved in liquefied argon at 97 K.
near these two modes. Therefore, the frequencies of these isotopic species have been calculated by using the ab initio force field. The resulting values for the preceding stretches are summarized in Table 5. It can be seen that in CH2d13CHF the CsF stretching fundamental has undergone a red shift of 20 cm-1. In 13CH2dCHF, however, the CsF stretching is hardly affected. Moreover, for both CH2d13CHF and 13CH2dCHF the CdC stretching fundamental is calculated to red shift by more than 25 cm-1. Vibrational Spectra In Figure 3 a typical mid-infrared spectrum of vinyl fluoride dissolved in liquefied argon, at 101 K, is shown. Observed frequencies and their assignments, obtained by comparison with earlier studies on vinyl fluoride,16 are summarized in Table 6. On the low-frequency side of the intense νCsF at 1148 cm-1, a weak band appears at 1129 cm-1. In view of the preceding section, this band is assigned to CH2d13CHF. No separate band belonging to the 13CH2dCHF isotopomer was observed, presumably because it is strongly overlapped by the band of the mother isotope. Close to the main CdC stretching band at 1652 cm-1, a weak transition is observed at 1626 cm-1. We assign this band to the CdC stretching in both the CH2d13CHF and the 13CH dCHF isotopomers. The other weak band in this region, 2 situated at 1632 cm-1, is assigned to the combination of the CsF stretching and CdCsF in-plane deformation modes. In Figure 4, the HCl stretching region of an argon solution containing both vinyl fluoride and HCl, recorded at several temperatures between 98.0 and 116.2 K, and that of a solution containing only vinyl fluoride are compared. It can be seen that in the mixed solution new bands appear at 2848, 2838, and 2815 cm-1. The presence of new bands proves the formation of complex species. As expected, lowering of the temperature increases the relative intensity of these bands. The temperature variation hardly affects the intensity ratio of the bands at 2848 and 2838 cm-1, but the intensity ratio of the bands at 2838 and 2815 cm-1 changes remarkably. Therefore, the bands at 2848 and 2838 cm-1 must originate in the same species, while the band at 2815 cm-1 must be due to a second species. Similar phenomena were observed in the spectra of various alkyl chloride/HCl mixtures dissolved in liquefied argon.1-3 In
νi
approximate description 2ν4
3150 3115
ν1 ν2
3060
ν3
CH2 antisym stretch CH stretch ν6 + ν10 + ν11 ν6 + ν8 + ν10 CH2 antisym stretch
ν4 + ν6 ν4 + ν7 2ν5 2ν6 ν4 + ν9 ν5 + ν7 ν6 + ν7 2ν9 + ν12 2ν7
2798 2570 2453
2075 1857 1791 ν4
1380 1306 1156
ν5 ν6 ν7
929 863 713
ν8 ν9 ν10 ν11
483
ν12
ν4 + ν12 ν7 + ν9 ν7 + ν10 2ν8 2ν9 ν8 + ν10 ν6 + ν12 2ν10 CdC stretch ν7 + ν12 CdC stretch, H2Cd13CHF and H213CdCHF ν10 + ν11 2ν11 ν9 + ν12 CH2 bending CdCsH in-plane deformation CsF stretch 13CsF stretch CdCsH out of plane CH2 rocking CH2 wagging CH2 twist CO2 impurity CdCsF in plane
a
Abbreviations: vvs, very very strong; vs, very strong; s, strong; m, medium; w, weak; vw, very weak; sh, shoulder. b Taken from ref 16.
agreement with these results, the bands at 2848 and 2838 cm-1 are assigned to a 1:1 complex CH2dCHF‚HCl, while the band at 2815 cm-1 is assigned to a complex with 1:2 stoichiometry, CH2dCHF‚(HCl)2. As will be shown in the following, these assignments are confirmed by using concentration studies. For a 1:1 complex only a single HsCl stretching is expected. Therefore, we assign the more intense band at 2838 cm-1 to the stretching mode and the much weaker band at 2848 cm-1 to a combination of the stretching with a low-frequency mode of the complex, in agreement with previous assignments.1 The ab initio red shifts of the HCl stretching fundamental for the σ and π complexes are 49 and 60 cm-1, respectively. These values are much larger than the 33 cm-1 shift between the monomer band at 2871 cm-1 and the 2838 cm-1 complex band. Because of this, even with the shift for the σ complex closer to the observed one, assignment of the type of complex from the shift of the HsCl stretching does not appear justified.
CH2dCHF and HCl Complexes in Argon
Figure 4. HsCl stretching region of the spectra of CH2dCHF dissolved in liquefied argon at 98.1 K (e) and of a CH2dCHF/HCl mixture dissolved in liquefied argon at 97.8 (a), 102.8 (b), 109.4 (c), and at 116.5 K (d).
Measurable concentrations of the complexes are formed in concentrated solutions only. For these, most of the fundamental vinyl fluoride absorptions are fully saturated, preventing in all but one case the fundamental complex bands from being observed. Close scrutiny of the overtone regions of the spectra of mixed solutions, however, reveals the presence of several bands that must be assigned to complex species. The observed frequencies and their assignments, together with the frequencies of the corresponding monomer modes, are summarized in Table 5. It will be appreciated from Figure 4 that the 2815 cm-1 band is very weak at all times, so that the concentration of the second species must be very low. Therefore, apart from the 2815 cm-1 band, all observed complex bands are assigned to the 1:1 complex. For the σ complex, the ab initio shift of the CsF stretching of vinyl fluoride is some -18 cm-1. Hence, the corresponding fundamental of CH2dCHF‚HCl is expected to appear at 1148 - 18 ) 1130 cm-1, i.e., close to the 1129 cm-1 band of H2Cd13CHF. This region of the spectra is shown in Figure 5. It is clear that, as a consequence of the high intensity of the 1148 cm-1 band, the occurrence of a new band at 1130 cm-1 is obscured. In the spectra of the mixed solutions, however, another weak band appears at 1115 cm-1. The origin of this band will be discussed in the following. In Figure 6, the 2600-2200 cm-1 region of the spectrum of a solution containing vinyl fluoride and that of a solution containing both vinyl fluoride and HCl are compared. In the mixed solution, on the low-frequency side of the first overtone of the CsF stretching at 2290 cm-1 a weak band appears at 2261 cm-1. This band is assigned to the corresponding mode in CH2dCHF‚HCl. From this, the fundamental CsF stretching in the complex can be estimated to appear at 2261 × (1148/ 2290) ) 1133 cm-1. This prediction suggests that the fundamental complex band is obscured by the isotopic monomer band at 1129 cm-1. For the mixed solutions, complexes with CH2d13CHF also must be formed. If we assume that the CsF stretching in this isotopomer is shifted by the same amount as in the normal molecule, a weak complex band must be expected
J. Phys. Chem., Vol. 100, No. 39, 1996 15699
Figure 5. CsF stretching region of the spectra of CH2dCHF dissolved in liquefied argon at 98.1 K (e) and of a CH2dCHF/HCl mixture dissolved in liquefied argon at 97.8 (a), 102.8 (b), 107.1 (c), and at 111.9 K (d).
Figure 6. 2600-2200 cm-1 region of CH2dCHF dissolved in liquefied argon at 98.1 K (b) and of a CH2dCHF/HCl mixture dissolved in liquefied argon at 97.8 K (a).
at approximately 1129 - 14 ) 1115 cm-1. The band at 1115 cm-1 observed in the spectra of vinyl fluoride/HCl mixtures, therefore, is assigned to the CsF stretching fundamental in H2Cd13CHF‚HCl. The bands at 2450 and 2520 cm-1 in Figure 6 are assigned to the combination of the CsF stretching with the CdCsH in-plane deformation and the CH2 bending, respectively. In the mixed solutions, these bands are accompanied by complex bands at 2434 and 2503 cm-1, i.e., on the low-frequency side of the corresponding monomer bands. It can be seen in Table 4 that neither the CdCsH in-plane deformation nor the CH2 bending shows a significant complexation shift, and thus the observed frequencies of the combination bands must be determined by the complexation shift of the CsF stretching. It then follows that these complex bands must be assigned to the σ complex. In the spectra of monomer vinyl fluoride a band at 2068 cm-1 is assigned to the combination of the CsF stretching and the CH2 rocking. The corresponding complex band is found red-
15700 J. Phys. Chem., Vol. 100, No. 39, 1996
Herrebout and van der Veken
TABLE 7: Experimental Vibrational Frequencies (cm-1) for CH2dCHF‚HCl and the Corresponding Monomer Frequencies CH2dCHF‚HCl
CH2dCHF
1132a 1115 910a 869a 1400 1744 1829 2043 2261 2434 2503
1148 1129 925 862 1409 1730 1848 2068 2290 2450 2520
a
approximate description 12CsF
stretch stretch CH2 rocking CH2 wagging 13CsF
ν7 ν7′ ν9 ν10 ν9 + ν12 2ν10 2ν9 ν7 + ν9 2ν7 ν6 + ν7 ν5 + ν7
Frequency shifts derived from overtone frequencies.
The first overtone of the CH2 wagging is observed at 1730 cm-1 in the monomer and at 1744 cm-1 in the complex. For both types of complexes, the CH2 wagging is predicted to be blue-shifted. It is clear from Table 4 that the calculated shifts for the σ complex are strongly conformer-dependent. As a consequence, the 1744 cm-1 band is of little diagnostic value. The combination of the CH2 rocking and CdCsF out-ofplane deformation is observed at 1409 cm-1 in the monomer, while the corresponding complex mode is detected at 1400 cm-1. In this case Table 4 also indicates that the complex band must be due to the σ complex. By using averaged complexation shifts, the mode is predicted at 1396 cm-1, close to the observed band. In Figure 7, the νDCl region of a solution containing both vinyl fluoride and DCl and that of a solution containing only vinyl fluoride or DCl are compared. In the spectrum of the pure DCl solution, a broad, asymmetric band with well-defined P, Q, and R branches can be observed at 2081 cm-1. In the spectra of the mixture, a new band appears at 2054 cm-1. The 2848 and 2054 cm-1 bands show a frequency ratio of approximately 1.381. Because a similar ratio of 1.379 is observed for the HCl/DCl monomer bands, the 2029 cm-1 band is assigned to the DsCl stretching fundamental in CH2dCHF‚DCl. Stoichiometry of the Observed Complexes
Figure 7. DsCl stretching region of the spectra of CH2dCHF dissolved in liquefied argon at 98 K (c), of DCl dissolved in liquefied argon at 98 K (b), and of a CH2dCHF/DCl mixture dissolved in liquefied argon at 98 K (a).
cm-1.
shifted at 2043 Comparison with Table 4 shows that this shift is compatible only with the σ complex. By assuming that the complexation shift of a combination band equals the sum of the complexation shifts of the corresponding fundamentals, and by using ab initio complexation shifts averaged over the two conformers of the σ complex, the complex combination band is predicted at 2068 - (17.8 + 11.6) ) 2038.6 cm-1, in sufficient agreement with the observed frequency to justify the assignment of the latter to the proposed combination. In the spectra of vinyl fluoride, the overtone of the CdCsH out-of-plane deformation and the overtone of the CH2 rocking mode are assigned at 1857 and 1848 cm-1, respectively. In the mixed solution a complex band is observed at 1829 cm-1, i.e., on the low-frequency side of the preceding overtones. For the π complex the corresponding fundamentals are predicted to shift to higher frequencies. Consequently, the overtones must also be expected to blue shift. It follows that the complex band at 1829 cm-1 has to be assigned to the σ complex. For the CdCsH out-of-plane deformation, the predicted complexation shift is relatively small, -5 cm-1, so that its first overtone presumably coincides with the 1848 cm-1 monomer band. The calculated shift for the fundamental rocking mode, averaged over the two conformers, is -11.5 cm-1. If we assume that the first overtone shifts by twice this value, the complex band is expected at 1848 - 23 ) 1825 cm-1, which is very close to the observed complex band at 1829 cm-1. Therefore, the latter is assigned as the first overtone of the CH2 rocking mode.
The stoichiometries of the complexes proposed in a previous paragraph were confirmed by using a concentration study. To this end, infrared spectra of several solutions containing different concentrations of vinyl fluoride, varying from 0.2 × 10-3 to 6.8 × 10-3 M, and of HCl, varying from 0.4 × 10-3 to 1.0 × 10-2 M, were recorded at a constant temperature of 104.0 ( 0.3 K. The formation of a complex species (CH2dCHF)n‚(HCl)m between n vinyl fluoride molecules and m HCl molecules is described by the equilibrium reaction:
nCH2dCHF + mHCl h (CH2dCHF)n‚(HCl)m By assuming that the extinction coefficients do not vary with concentration, the equilibrium constant can be written as
Keq ) I(CH2dCHF)n‚(HCl)m/(ICH2dCHF)n(IHCl)m In this expression, I(CH2dCHF)n‚(HCl)m, ICH2dCHF, and IHCl are the integrated intensities of an absorption band of the complex, the vinyl fluoride monomer, and the HCl monomer, respectively. This equation shows that, at a constant temperature, the intensity of a complex band must vary linearly with the product of the monomer intensities, (ICH2dCHF)n(IHCl)m. This hypothesis can be tested by linear regression of the intensities, obtained at different concentrations, of a complex band against products of monomer intensities, (ICH2dCHF)p(IHCl)q, and this for different sets of p and q.12 For each set, the goodness of the fit is expressed via the χ2 value, and the p and q that lead to the lowest value of χ2 are accepted to correspond to the m and n of the complex. As can be seen in the expression for Keq, the analysis requires accurate values for the integrated intensity of a complex band and of bands due to HCl and vinyl fluoride. For monomer HCl, only the stretching mode near 2900 cm-1 is available, and it can be seen in Figure 4 that this band is severally overlapped by the complex HCl stretches and by bands due to vinyl fluoride. Its intensity was obtained as follows. At the same temperature at which the solution of the vinyl fluoride/ HCl mixture was investigated, the spectrum of a solution
CH2dCHF and HCl Complexes in Argon
J. Phys. Chem., Vol. 100, No. 39, 1996 15701 TABLE 8: χ2 Values for the Stoichiometry Analysis of the Complexes between Vinyl Fluoride and Hydrogen Chloride complex band
Figure 8. Contribution of the (CH2dCHF)x‚(HCl)y species to the HCl stretching region for several solutions containing vinyl fluoride and different amounts of HCl in liquefied argon at 104 K. From top to bottom, decreasing amounts of hydrogen chloride were dissolved.
proposed stoichiometry
2838 cm-1
2815 cm-1
1:1 1:2 2:1 2:2
0.002 240 0.068 219 0.080 964 0.192 815
0.001 608 0.000 028 0.000 895 0.057 777
which the spectrum was recorded contained higher concentrations of HCl and vinyl fluoride than those used in the concentration study. Even then, and taking into account that the spectra in Figure 6 are shown in transmittance, it can be seen that the relative intensities of the complex bands are very small. Therefore, for the present study, as monomer intensity the numerically integrated intensity of the 2569/2520 cm-1 multiplet was used without correcting for the complex bands. The stoichiometric analysis was performed by using both the 2838 and 2815 cm-1 complex bands, and linear regressions were made in each case by assuming a 1:1, 1:2, 2:1, and 2:2 stoichiometry of the complex. The resulting χ2 values are given in Table 8. From this it can be seen that the 2838 cm-1 band must originate in a 1:1 complex, while the 2815 cm-1 band must be due to a 1:2 complex, CH2dCHF‚(HCl)2. These results support the assignments made earlier, which were based on comparison with the spectra of other HCl complexes. Stability of the Complexes
containing only HCl was also recorded. The intensity of the HCl band of this solution was rescaled so as to accurately reproduce, in the region above 2850 cm-1, the HCl monomer contribution in the spectrum of the vinyl fluoride/HCl mixture. The numerically integrated intensity of the rescaled band was then used as the integrated intensity of the HCl monomer, IHCl. In Figure 8, some spectra resulting from the subtraction of the rescaled HCl spectrum from those of vinyl fluoride/HCl mixtures, at different concentrations, are shown. The HCl stretching bands due to the complexes appear at 2848, 2838, and 2815 cm-1, respectively. To obtain the integrated intensities of these bands, curve fitting was carried out using Gauss/Lorentz sum profiles. This yielded the intensities of the 2848 and 2838 cm-1 bands. It was found, however, that the agreement between the calculated and the experimental spectra in the region around 2815 cm-1 was less than could be desired. The reason for this presumably is the presence of weak spectral features in this region due to the HCl oligomers,12 incorrectly accounted for in the subtraction process. In addition, the νHCl bands may be slightly asymmetric due to the presence of excited state transitions. In the calculations, these contributions were not taken into account. Although of little consequence for the more intense bands at 2848 and 2838 cm-1, the less optimal reproduction of the experimental profile near 2815 cm-1 seriously affects the intensity calculated for the weak 2815 cm-1 band. For the analysis described in the following, therefore, it was preferable to use the absorbance at the band maximum as a measure for the intensity of the 2815 cm-1 band. As all measurements used in the concentration study were made at the same temperature, the bandwidth of the 2815 cm-1 band is the same in all spectra, and the approximation should be acceptable. The near-degeneracy of most of the vinyl fluoride monomer bands with the corresponding complex bands makes it difficult to accurately measure the monomer intensity. However, for the solutions used in the concentration study, the relative intensity of the complex bands was always very small. This is illustrated by the spectra in Figure 6: the mixed solution from
The stability of the 1:1 and 1:2 complexes, expressed as the complexation enthalpy ∆H°, was determined from a temperature study. By using the van’t Hoff relation and making the usual assumptions,12 it is easily shown that ln[ICH2dCHF‚(HCl)n/(ICH2dCHF(IHCl)n)] must be linearly related to 1/T and that the slope of the relation equals -∆H°/R. It is also clear that for the measurement of the enthalpy differences integrated intensities of a band of each of the species involved must be known. For HCl, the intensities were measured as described earlier. For the 1:1 complex, the numerically integrated intensity of the 2838 cm-1 band was used, while for the monomer an analogous procedure was used to obtain the intensity of the 2569/2520 cm-1 multiplet. For the 1:1 complex, spectra were recorded at several temperatures between 98.0 and 113.2 K for a solution containing approximately 3.0 × 10-3 M vinyl fluoride and 1.2 × 10-3 M HCl. The van’t Hoff plot constructed is shown in Figure 9a. The complexation enthalpy calculated from it is -5.85 ( 0.10 kJ mol-1. In addition, a van’t Hoff plot was constructed by using the intensity of the 1330/1270 cm-1 monomer multiplet as a measure of ICH2dCHF. From this plot, shown in Figure 9b, a complexation enthalpy of -5.71 ( 0.08 kJ mol-1 was obtained. Because of the good agreement between the two results, we propose to take the average value, -5.78 ( 0.12 kJ mol-1, as ∆H° of the 1:1 complex. The relative stability of CH2dCHF‚(HCl)2 was established by using the 2815 cm-1 complex band from spectra recorded at several temperatures between 98.0 and 108.0 K for a solution containing approximately 2.5 × 10-3 M vinyl fluoride and 4.5 × 10-3 M HCl. As the intensity of the 2815 cm-1 band the absorbance at the band maximum was again used. The resulting van’t Hoff plot is shown in Figure 10. From this, the complexation energy of the 1:2 complex was calculated to be -9.85 ( 0.34 kJ mol-1. Discussion All of the observed complex bands due to the 1:1 complex have frequencies compatible with the ab initio predictions for
15702 J. Phys. Chem., Vol. 100, No. 39, 1996
Figure 9. van’t Hoff plots for vinyl fluoride/HCl mixtures obtained by using the intensities of the 2569 and 2520 cm-1 bands (a) and the intensities of the 1330 and 1270 cm-1 bands (b) of vinyl fluoride.
Figure 10. van’t Hoff plot for the 1:2 complex between vinyl fluoride and HCl.
the σ complex. It is, therefore, concluded that in the cryosolutions the σ complex is being formed. The spectra were carefully inspected in the regions where diagnostic π complex bands are expected to occur, but none have been found. This leads to the conclusion that, in the solutions studied, the concentration of the π complex was below the detection limit. There is no obvious reason why the complexation entropy for the π complex should be dramatically different from that of the σ complex. Therefore, the absence of the π complex from the cryosolutions signals that this complex has a substantially higher energy than the σ complex. This is in contrast with the expectations discussed in the Introduction; we must, therefore, conclude that the fluorine atom in the R position of the double bond reduces the donor characteristics of the latter, making it less available for the formation of a π complex. It is tempting to interpret the 1:2 complex as one in which the first HCl molecule forms a σ complex and the second a π complex. However, the position of the HCl stretching assigned to this complex is very similar to that observed for the 1:2 complexes with alkyl halides,1-4 in which no π complex is possible. Therefore, we prefer the interpretation that the 1:2 complex has its second HCl attached to the chlorine atom of the first, similar to the 1:2 complexes with alkyl halides.1,2 The ∆H° for the 1:2 complex is 4.07 ( 0.26 kJ mol-1 higher than that for the 1:1 complex. This value favorably compares with that for the formation of the HCl dimer in cryosolution,12 -3.78 ( 0.33 kJ mol-1, which lends support to the proposed structure of the 1:2 complex. If the slightly larger ∆H° for the
Herrebout and van der Veken second step in the formation of the 1:2 complex is statistically significant, it reflects a weak cooperative effect,17 which causes the first hydrogen bond to be strengthened by the second. Comparison of Figure 4 with Figure 6 shows that the intensity of the HCl stretch of the 1:1 complex relative to that of the monomer is much higher than the intensity ratio for modes localized in the vinyl fluoride moiety. This effect was quantitatively assessed in the following way. The integrated intensities of 2νCsF of monomer vinyl fluoride, at 2290 cm-1, and of the 1:1 complex, at 2261 cm-1, were measured in the spectrum, recorded at 116 K, of a solution that was 0.0231 M in vinyl fluoride and 0.0077 M in HCl. The ratio of these intensities was determined to be 144. Similar values were obtained by using other complex bands. If it is assumed that the extinction coefficient of modes localized in vinyl fluoride do not change upon complexation, the preceding ratio shows that the fraction of the vinyl fluoride that was complexed is very small. Then, the complex concentration can be approximated as the analytical concentration of the vinyl fluoride divided by this fraction. By using this concentration and the analytical concentration of HCl, the ratio of the extinction coefficients for νHCl in the complex and in the monomer can be straightforwardly calculated. For the solution studied, this leads to an extinction coefficient for the complex that is 11 times that of the monomer. This important increase is borne out by the ab initio calculations: the data in Table 4 show a predicted increase by a factor of 6.9 and by 6.7 for conformers a and b of the σ complex, respectively. The intensity enhancement is a well-known effect and has been explained in terms of the reduced ability, in the complex, of the electron density to follow the HsCl stretching motion.18 The complexation enthalpy obtained in this study for the 1:1 complex cannot, a priori, be taken to measure the stability of the isolated complex because, even for solutions in liquefied argon, solvent influences are present.19 The magnitude of the latter can be estimated by using the Kirkwood-Onsager reaction field model. In this model, the solvent stabilization of a dipolar species is given by
-
(
)
r - 1 µ2 2r + 1 a3
in which µ is the dipole moment of the solute, a is the radius of the cavity it occupies, and r is the relative permittivity of the solvent. It has recently been shown that a reliable value for the cavity radius can be derived from ab initio calculations.20 For the present study, this definition was used, including the empirical correction of 0.5 Å, with calculations at the RHF/631G* level. It was shown earlier that the structure of the lowest energy conformer of the σ complex does not agree with the experimental one. Therefore, for the complex a structure based on the experimental data was used. In this structure, the CdCsF‚‚‚Cl dihedral angle was set at 8°, while the angle between the HCl axis and the HF bond was set at 10°, as derived by Kisiel et al.8 In this way, a cavity radius of 3.67 Å is derived. For the monomer molecules the cavity radii were also calculated starting from the experimental structures. The values obtained are 2.82 Å for HCl and 3.19 Å for vinyl fluoride. The experimental dipole moment of the complex has not yet been measured, and, therefore, the ab initio value of 3.04 D had to be used. For reasons of uniformity, the ab initio dipole moments for HCl and vinyl fluoride were also used. From the preceding expression, the solvent stabilizations were calculated to be 0.764 kJ mol-1 for HCl, 0.599 kJ mol-1 for vinyl fluoride, and 1.398 kJ mol-1 for the 1:1 complex. Thus, reaction field theory
CH2dCHF and HCl Complexes in Argon predicts that in liquefied argon the complex experiences an extra stabilization of 0.035 kJ mol-1. Although the individual stabilizations are nonnegliglible, for the present case the net stabilization of the complex is very small. Therefore, the vapor phase stability of the complex must be expected to be very similar to that determined here for the cryosolution. Acknowledgment. W.A.H. thanks the National Fund for Scientific Research (NFWO, Belgium) for successive appointments as Research Assisant (aspirant) and Postdoctoral Fellow. The N.F.W.O. is also thanked for financial help toward the spectroscopic equipment used in this study. References and Notes (1) Herrebout, W. A.; Van der Veken, B. J. J. Phys. Chem. 1993, 97, 10622. (2) Herrebout, W. A.; Van der Veken, B. J. J. Phys. Chem. 1994, 98, 2836. (3) Herrebout, W. A.; Van der Veken, B. J. J. Chem. Soc., Faraday Trans. 1994, 90, 3601. (4) Herrebout, W. A.; Van der Veken, B. J. J. Mol. Struct. (Theochem) 1995, 332, 231. (5) Kolomiitsova, T. D.; Milke, Z.; Tokhadze, K. G.; Shchepkin, D. N. Opt. Spectrosc. 1979, 46, 391. (6) Barri, M. F.; Tokhadze, K. G. Opt. Spectrosc. 1981, 51, 70. (7) Kimel’fel’d, Y. M. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: Amsterdam, 1992; Vol. 19, and references cited therein.
J. Phys. Chem., Vol. 100, No. 39, 1996 15703 (8) Kisiel, Z.; Fowler, P. W.; Legon, A. C. J. Chem. Phys. 1990, 90, 3054. (9) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J. Am. Chem. Soc. 1982, 104, 6180. (10) George, W. O.; Hirani, P. K.; Lewis, E. N.; Maddams, W. F.; Williams, D. A. J. Mol. Struct. 1986, 141, 227. (11) Bulanin, M. O. In Molecular Cryospectroscopy; Clark, R. J. H., Hester, R. E., Eds.; J. Wiley and Sons: Chichester, UK, 1995. (12) Van der Veken, B. J.; De Munck, F. R. J. Chem. Phys. 1992, 97, 3060. (13) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92, ReVision E.3; Gaussian, Inc.: Pittsburgh, PA, 1992. (14) Schlegel, H. B. J. Comput. Chem. 1982, 3, 214. (15) Boys, S. B.; Bernardi, F. Mol. Phys. 1970, 19, 553. (16) Smith, G. R.; Guillory, W. A. J. Chem. Phys. 1975, 63, 1311, and references cited therein. (17) Scheiner, S. In Theoretical Tretment of Large Molecules and Their Interactions; Maksic, Z. B., Ed.; Springer: Berlin, 1991; Vol. 15. (18) Szczesniak, M. M.; Kurnig, I. J.; Scheiner, S. J. Chem. Phys. 1988, 89, 3131. (19) Herrebout, W. A.; Van der Veken, B. J. J. Phys. Chem. 1996, 100, 9671. (20) Wong, M. W.; Frisch, M. J.; Wiberg, K. B. J. Am. Chem. Soc. 1991, 113, 4776.
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