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.