Quantum-Chemical Ab Initio Calculations on Borabenzene (C5H5B) and its Adducts with Ne, Ar, Kr, and N2. Could Free Borabenzene be Observed in Rare Gas Matrices?

Abstract

Quantum-chemical calculations employing different theoretical methods and basis sets have been performed on borabenzene (C5H5B) as well as on its adducts to dinitrogen (N2) and the rare gases Ne, Ar, and Kr. In agreement with previous calculations, the ground state of borabenzene was found to be a planar singlet with six electrons in molecular orbitals of π symmetry and a wide C-B-C bond angle (142.2°). Depending on the method (PUMP2, SAC-CI, CASPT2(8,8)), the lowest triplet state was found to be 28 to 46 kcal mol–1 (1 kcal mol–1 = 4.186 kJ mol–1) higher in energy. The energies associated with the formation of the adducts with N2, Ne, Ar, and Kr were calculated as –14.9, –0.5, –1.4, and –3.5 kcal mol–1 respectively. Our calculated spectrum of the normal modes as well as the electronic excitation spectrum of the N2 adduct reproduce qualitatively the characteristic features of the IR and the UV-vis spectra described by experimentalists. The corresponding calculated spectra (normal modes, UV-vis) of the rare gas adducts were found to be very similar to those of free borabenzene.