Ab Initio MRD-CI Study of the Electronic Spectrum of BrNO₂ and Photofragmentation

Abstract

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 1 A 1 r X 1 A 1 and 3 1 B 2 r X 1 A 1 , at 6.15 and 7.27 eV, respectively. At wavelengths that are atmospherically relevant to the BrNO 2 photolysis, 1 1 B 2 and 1 1 B 1 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 ClNO 2 demonstrates that the transition is red-shifted by 80 nm from ClNO 2 upon replacement of chlorine by bromine. Furthermore, the most intense transition 3 1 A 1 r X 1 A 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.

An ab initio investigation of the ground and low-lying singlet and triplet electronic states of XNO2 and XONO (X = Cl, Br, and I)

“…The triplet transitions in the BrNO 2 molecule cover wave-lengths out to nearly 380 nm and are comparable in intensity to the singlets, being an order of magnitude more intense than in ClNO 2 . The relative energies for the triplet states generally exhibit similar differences with the Lesar and Peyerimhoff values 18,84 as for the singlets. Note that the latter work did not compute oscillator strengths for the triplet transitions.…”

“…This is a common theme with all of the molecules of this work. CCSD(T)-F12b results for the equilibrium structures of BrNO 2 and INO 2 are shown in Table II where they are also compared to the previous CCSD(T) values for BrNO 2 of Lee, 23 Lesar et al, 84 and Tchana et al, 85 MP2 and B3LYP values from Papayannis and Kosmas, 86 and its experimental r m (2) structure. Due to the use of relatively small basis sets and the neglect of 3d correlation, most of the previous ab initio bond lengths for BrNO 2 are quite a bit longer than those of the present work.…”

Electronically excited states of XOONO (X=Cl, Br): theoretical studies

“…In addition both basis sets were enlarged by s-Rydberg functions located at the nitrogen and by a negative ion function for the halogen atom, thus giving the cc-pVDZ+sp and cc-pVTZ+sp basis sets. The exponents taken are α s (N ) = 0.028 α p (Cl) = 0.049 and α p (Br) = 0.032, which has been found to be suitable in previous calculations on chlorine and bromine species [31,37,38].…”

Electronic Spectrum and Photodissociation of ClONO in Comparison to BrONO

“…Double-ζ quality of the basis set was considered adequate for the species of interest. From our recent studies on halogen−nitrogen oxide compounds, ,, it is evident that the calculated excitation energies can have an error on the order of 0.3 eV.…”