Absorption Spectrum of the 2000 Å System of Borazine in the Gas Phase

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Abstract
The optical absorption spectrum of borazine vapor, at pressures between 0.1 and 5 mm Hg, has been obtained in the wavelength region 2015 Å–1800 Å on a 2.0 m vacuum spectrograph both photoelectrically and photographically. On the basis of vibronic analysis, observed absorption features are consistent with a single π‐π*, dipole‐forbidden, electronic transition, 1A1′←1A1′, assuming the D3h point group is a valid description for both ground and excited electronic states of borazine. Absorption and emission to the red of this state have been searched for but neither have been observed under current sensitivity conditions. It is concluded that the excited 1A1′ state is most likely the lowest singlet excited state of borazine. This spectrum is discussed in comparison with that of benzene; the 1A1′(D3h) state of borazine corresponds to the 1B1u(D6h) state of benzene. In particular, the 3B1u←1A1g spectrum is compared to the 1A1′←1A1′ absorption. Arguments are presented suggesting the Jahn‐Teller nature of the 1A1′ excited state of borazine. In this framework, various possible excited state vibrational assignments are presented.