Ab Initio MRD-CI Study of the Electronic Spectrum of BrNO_2 and Photofragmentation

摘要

Multireference configuration interaction,MRD-CI,methods with the cc-pVDZ+sp and cc-pVTZ+sp basis sets were employed to determine the low-lying singlet and triplet electronic states of nitryl bromide,BrNO_2.The calculations predict two very strong transitions,3~1A_1<-X~1A_1 and 3~1B_2<-X~1A_1,at 6.15 and 7.27 eV,respectively.At wavelengths that are atmospherically relevant to the BrNO_2 photolysis,1 ~1B_2 and 1 ~1B,singlet states with vertical transition energies of 3.43 and 3.85 eV,respectively,are computed.The corresponding triplet states are lower in energy by 0.27 and 0.36 eV,respectively.Both singlet and triplet states are found to be highly repulsive along the Br-N coordinate and they dissociate to the ground-state fragments Br and NO_2.The comparison between the first singlet excited state of BrNO_2 and C1NO_2 demonstrates that the transition is red-shifted by 80 nm from C1NO_2 upon replacement of chlorine by bromine.Furthermore,the most intense transition 3~1A_1 <-X~1A_1 of BrNO_2 has a 0.9 eV lower energy than that of ClNO_2.These energy differences can be explained by the large halogen character of the orbitals involved in the electron excitations.