Quantum-chemical calculations employing different theoretical methods and basis sets have been performed on borabenzene (C5H5B) as well as on its adducts to dinitrogen (N-2) 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 pi symmetry and a wide C-B-C bond angle (142.2 degrees). 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 N-2, 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 N-2 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.
