Electronic absorption spectra of B3 and B3− in neon matrices and ab initio analysis of the vibronic structure

Mass selected B3− ions have been isolated in 6 K neon matrices and their absorption spectra measured. A band system with origin at 467 nm is assigned as the E′1←X 1A1′ electronic transition of the cyclic anion. After photobleaching, the 1 2E′←X 2A1′ and 2 2E′←X 2A1′ band systems of neutral cyclic B3 are observed which start around 736 and 458 nm, respectively. Large scale ab initio calculations have provided potential energy surfaces for a variational treatment of the vibrational motion. Calculated band origins leave no doubt about the electronic symmetry assignments. The complex vibrational structure in the 1 2E′ state, which is due to relatively strong Jahn–Teller distortions, appears to be closely reproduced by the calculated vibrational energies and intensities, if the first observed stronger line is identified with the first vibrationally excited state, placing the “true” band origin of the 1 2E′ state at 775 nm where no signal with significant strength is apparent. The 2 2E′ state undergoes only a relatively weak Jahn–Teller distortion and shows a short progression with an observed frequency of 981(10) cm−1 that compares favorably with the theoretical frequency of 973 cm−1. The E′1 system of B3− shows a Jahn–Teller activity comparable to that of the 1 2E′ state of B3.