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

Abstract

Reported here is a theoretical study of the entire series of bromomethanes (CH4 - n Br n ) and bromomethyl radicals (CH3 - m Br m ) establishing a self-consistent set of structural and thermodynamic information. Ab initio molecular orbital calculations were performed to compute equilibrium geometries for the molecules and radicals initially at the (U)HF/6-31G* and (R)HF/6-31G* levels, respectively, and then refined at the MP2/6-31G* level. Vibrational frequencies were determined for all species at the HF/6-31G* level and comparison with infrared measurements and matrix isolation studies is favorable. Electron correlation contributions were performed by single-point calculations using fourth-order Møller−Plesset perturbation theory for derived MP2/6-31G* geometries. Enthalpies of formation were obtained from a consideration of applicable isodesmic reactions using the derived MP4/6-31G**//MP2/6-31G* total energies in conjunction with experimentally established enthalpies of formation for CH3Br, CH4, and CH3 •. The calculations predict the following standard enthalpies of formation in kilocalories per mole (at 298 K and 1 atm): CH2Br2, 1.07 ± 0.6; CHBr3, 12.16 ± 0.7; CBr4, 25.23 ± 0.8; CH2Br•, 41.63 ± 0.4; CHBr2 •, 48.11 ± 0.6; and CBr3 •, 55.36 ± 0.7. These data are then used to tabulate ΔH°f,T, ΔG°f,T, and K f,T for all species over the temperature range 0−1500 K. Comparison is made to existing thermochemical data through calculation of C−H and C−Br bond dissociation energies.

A Computational Study of the Thermochemistry of Bromine- and Iodine-Containing Methanes and Methyl Radicals

“…A similar trend applies to the CHBr 2 data, although the value of 227.3 kJ mol -1 is clearly too high. 6 Our computed enthalpy of 191.7 kJ mol -1 is up to 6 kJ mol -1 higher than the most recent measurements, 15 21,24 (iV) Iodomethyl Radicals. Our value of 217.2 kJ mol -1 for CH 2 I is close to the electron impact result of de Corpo et al, 16 but about 12 kJ mol -1 smaller than other experimental determinations (see Table 4 and Figure 2D).…”

“…Our calculated value lies between contradictory earlier calculations of 119.2 ( 4.2 and 105.6 ( 3.3 kJ mol -1 , where presumably at least one set of error limits has been underestimated. 21,24 "Experimental" data for the remaining bromomethanes, CHBr 3 and CH 2 Br 2 , are …”

Molecular, thermodynamic, and kinetic parameters of iodomethanes and iodomethyl radicals: ab initio calculations

“…A complete theoretical calculation for bromomethanes and bromomethyl radicals was performed even earlier [6]. In this work, we carried out a similar theoretical study of iodomethanes and iodomethyl radicals.…”

“…It is essential that the normal mode frequencies yielded by our ab initio calculations do not require further multiplication by a scaled factor, by contrast to the previous studies [6,10,11] which neglect intramolecular electron correlations. Table 5.…”

Ab initio direct dynamics studies on the hydrogen abstraction reactions of CF2H2 and CF3H with F atom

“…The destruction of stratospheric ozone, which results from the UV photodissociation of chlorofluorocarbons (CFCs), is by now well known and has led international agreements to limit the production and use of these compounds [1][2][3]. In light of the possible widespread use of HFCs and HCFCs, information concerning their atmospheric fate and environmental impact is needed.…”