High level ab initio molecular orbital study of the structures and vibrational spectra of CH2Br and CH2Br+

Abstract

The equilibrium structures and harmonic vibrational frequencies for CH2Br and CH2Br+ have been determined using second-order Mo/ller–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 CH2Br radical. At the CCSD(T)/6-311++G(3df,3pd) level of theory bond lengths of 1.076 and 1.851 Å are predicted for the C–H and C–Br bonds, and a 124.6° for the H–C–H angle in the CH2Br radical, which are in good agreement with the experimental values of 1.086 Å, 1.845 Å, and 124°, 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 CH2Br+ 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 CH2Br radical is ionized. The best estimate of the ionization potential for the CH2Br radical is 196.6 kcal mol−1, which agrees very well with the experimental value of 198.5±0.2 kcal mol−1.