A DFT study of the dissociation, ionization, and UV/Visible spectra of methyl hypobromite

The energetics and the molecular structures of CH3OBr in the ground electronic, S-0, second excited, S-2, sextet excited, S-6, and cationic ground, D-0(+) states have been investigated theoretically. The HBr elimination of CH3OBr is found to have two dissociation pathways involving two transition states with an energy difference of 5.28 kcal/mol. The B3LYP optimization of the S-2 state, which is found to be 1.95 eV above the So state, shows a tendency of O-Br bond dissociation, resulting in CH3O and Br formation. The optimization of the S-6 state, which is found to be 2.35 eV above the So state, shows a tendency of C-O bond dissociation, forming CH3 and OBr. The ionization energy of the anticipated dissociation products was calculated and found to agree well with the values reported experimentally. For CH3OBr, which has no experimental value for the ionization energy, our ionization energy calculations predicted a value of 9.98 eV. Utilizing the excited states calculations and the ionization energy calculations, a design of a pump probe experiment has been provided in order to explore the ultrafast dissociation dynamics of CH3OBr through S-2 and S-6 states. The vertical excitation energy calculations were carried out to explore the UV/Visible spectrum of CH3OBr, and the frequency calculations were performed to explore its IR spectrum. The excitation and ionization of CH3OBr result in significant molecular changes including bond lengths, bond angles, dihedral angles, atomic charges, dipole moments, and rotational constants. The excitation to the S-2 results in a significant elongation of the O-Br bond, which reflects its tendency for the dissociation through this bond. The excitation to the S-6 state results in a remarkable elongation in the C-O bond, reflecting its tendency to dissociate along this bond. The calculations of atomic charges and dipole moment indicate noticeable changes due to excitation and ionization. The electron density contour of HOMO's indicates the specific site of the HOMO from which the excitation and ionization takes place. The calculations of rotational constants A, B, and C indicated the structural changes through the three mutually perpendicular axes x, y, and z. The effect of ionization to D-0(+) state on CH3O, CH3, and OBr was investigated following the same thread used for the mother molecule, CH3OBr. CH3O was found to suffer dramatic changes in the molecular structure. It was found to form the CH2OH isomer upon ionization, in which the C-O bond has more double bond character. Moreover, the ionization induces a significant change in the planarity of the CH3O, which changes to the fully planar CH2OH isomer. (C) 2016 Elsevier B.V. All rights reserved.