High level ab initio molecular orbital study of the structures and vibrational spectra of CH_2Br and CH_2Br~+

摘要

The equilibrium structures and harmonic vibrational frequencies for CH_2Br and CH_2Br~+ have been determined using second-order Moller-Plesset perturbation theory (MP2), Becke's three parameter hybrid method employing the LYP correction functional (B3LYP) [A. D. Becke, J. Chem. Phys. 98, 5648 (1993)], and coupled-cluster theory with single and double excitations including perturbative corrections for the triple excitations CCSD(T) in conjunction with the triple-zeta double-polarized (TZ2P) and 6-311 ++ G(3df, 3pd) basis sets. Our computational results predict a very nearly planar structure for the CH_2Br radical. At the CCSD(T)/6-311 ++ G(3df, 3pd) level of theory bond lengths of 1.076 and 1.851 A are predicted for the C-H and C-Br bonds, and a 124.6 for the H-C-H angle in the CH_2Br radical, which are in good agreement with the experimental values of 1.086 A, 1.845 A, and 124 deg, respectively. The calculated rotational constant value of B + C at the same level is found to agree with experiment. Like CHBr~+ and CBr~+, the C-Br bond length in the CH_2Br~+ cation is found to be shorter than that of the neutral species, due to the reduction of repulsion between carbon and bromine atoms. The vibrational frequencies for the C-Br stretching are expected to increase by more than 160 cm~(-1) when the CH_2Br radical is ionized. The best estimate of the ionization potential for the CH_2Br radical is 196.6 kcal mol~(-1), which agrees very well with the experimental value of 198.5 +- 0.2 kcal mol~(-1).