An ab initio study of the structures, barriers for internal rotation, vibrational frequencies, and thermodynamic functions of the hydrochlorofluorocarbon CH₃CF₂Cl and the corresponding radical CH₂CF₂Cl

Abstract

Ab initio molecular orbital calculations were performed using the GAUSSIAN 90 system of programs at the HF/6-31G* level of theory, on the hydrochlorofluorocarbon (HCFC) 1-chloro-1,1-difluoroethane and the 1-chloro-1,1-difluoroethyl radical. Equilibrium geometries, barriers for internal rotation, and harmonic vibrational frequencies were thus calculated. A single conformational minimum in the potential energy surface was located for both the radical and the parent molecule. The radical center in CH2CF2Cl was found to be nonplanar. Transition structures for internal rotation about the C—C bond were located for both the molecule and the radical using analytical methods. The rotation barriers, evaluated at the fourth-order Møller–Plesset perturbation theory ((U)MP4/6-311G**/6-31G*). were calculated after inclusion of zero-point vibrational energy differences to be 1.11 and 4.12 kcal/mol for the radical and the parent molecule, respectively. Computed thermodynamic properties including heat capacity, entropy, enthalpy, and free energy functions are reported as a function of temperature. Using an experimentally measured heat of formation of CH3CF2Cl at 298 K, the heat of formation of CH2CF2Cl was calculated to be −74.3 ± 1.7 kcal/mol. Tabulations of ΔH0f,T, ΔG0f,T, and log10Kf,T over the temperature range of 0–1500 K are also reported for both species.

Structures, Vibrational Frequencies, Thermodynamic Properties, and Bond Dissociation Energies of the Bromomethanes and Bromomethyl Radicals: an Ab Initio Study

“…Thermodynamic Properties. Previous work has demonstrated the reliability of calculating the heat of formation of both molecules , and radicals − by coupling ab initio theoretically computed energy changes for homodesmic and isodesmic reactions with known heats of formation of other reaction components. As was mentioned earlier, the standard enthalpies of formation of the bromomethanes and bromomethyl radicals are not well established with the exception of methyl bromide.…”

CHF2Cl and CH3CF2Cl Detection by Coherent Anti-Stokes Raman Scattering and Photoacoustic Raman Spectroscopy

“…The ν 1 and ν 2 bands have been assigned to the CH 3 antisymmetric and symmetric stretches, respectively, while the ν 3 to the antisymmetric CH 3 deformation. 17,19,22 The CH 3 CF 2 Cl spectra do not show any shifted peaks that can be attributed to the CH 3 CF 2 37 Cl isotopomer. This behavior differs from that of CHF 2 Cl, but agrees very well with very recent predicted vibrational frequencies, 17 calculated by ab initio at the MP2/6-311G** level.…”

“…From the general appearance of this band it seems that it corresponds to an A-type band. The ν 1 and ν 2 bands have been assigned to the CH 3 antisymmetric and symmetric stretches, respectively, while the ν 3 to the antisymmetric CH 3 deformation. ,, …”

“…Because of the direct environmental relevance, the interest in these compounds is increasing and so is the number of their studies. Recent research is directed toward kinetics and dynamics measurements of their reactions with O( 1 D) , OH, − and Cl, to photodissociation dynamics studies, − and to detailed theoretical and spectroscopic studies to obtain a basis for their environmental monitoring. − …”

“…Recent research is directed toward kinetics and dynamics measurements of their reactions with O( 1 D) 4,5 OH, [6][7][8][9] and Cl, 10 to photodissociation dynamics studies, [11][12][13] and to detailed theoretical and spectroscopic studies to obtain a basis for their environmental monitoring. [14][15][16][17][18][19][20][21][22] Extensive measurements of microwave and infrared spectra revealed spectroscopic constants for the ground state and different vibrational levels of HCFC compounds. [14][15][16][17][18][19][20][21] In the past, the Raman spectra of several HCFC's have been recorded by a three-prism glass spectrograph 23 and by mercury arc excitation.…”