S1–S0 electronic absorption and supersonic-jet fluorescence spectra of catecholborane: puckering potential for the five-membered ring

Abstract

S₁–S₀ fluorescence spectra of catecholborane in a supersonic jet show that a band in the gas-phase absorption spectrum, previously assigned as a sequence band, is really 24²₀, where ν₂₄ is the puckering vibration of the five-membered ring. The fact that bands with Δv₂₄ even, only, are observed in the fluorescence spectra confirms the planarity of the molecule in S₀. Assignments in the high-resolution electronic absorption spectrum have enabled us to obtain the three lowest levels of ν₂₄ in both S₁ and S₀ for the ¹¹B and ¹⁰B isotopomers and hence, assuming a reduced mass of 100 u for CB–¹¹B, the puckering potential functions for both electronic states. The molecule is planar in S₁ also. The vibrational levels of ν₂₄ in S₀ show that previous assignments in the gas-phase far-infrared spectrum are incorrect. A group of bands attributed to the butterfly vibration ν₂₃ is shown to be due to ν₂₄ of the ¹¹B isotopomer, while a weaker group is shown to be due to ν₂₄ of the ¹⁰B isotopomer. Assignments of bands in the SVLF spectra involving ν₂₃ result in a wavenumber of ca. 310 cm^–1 in the S₀ state.