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

The equilibrium structures and harmonic vibrational frequencies for CH2Br and CH2Br+ 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 CH2Br radical. At the CCSD(T)/6-311++G(3df,3pd) level of theory bond lengths of 1.076 and 1.851 Angstrom are predicted for the C-H and C-Br bonds, and a 124.6 degrees for the H-C-H angle in the CH2Br radical, which are in good agreement with the experimental values of 1.086 Angstrom, 1.845 Angstrom, and 124 degrees, 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). (C) 1999 American Institute of Physics. [S0021-9606(99)31501-4].