摘要：

The T → S luminescence spectra at 77° in several rigid glasses are reported for pyrimidine and pyrimidine hydrochloride. The emission is assigned 2Bi (n, π*) → 1A1 from the mirror image relationship with the 1A → 1B1(n, π*) absorption spectrum in pyrimidine. The o,o band is observed at 28290 ± 30 cm.-1 yielding an experimental singlet-triplet interval of 2630 cm.-1 between o,o bands. Both the observed singlet-triplet interval and the observed principal vibrational progression (1080 cm.-1) in the spectrum (corresponding to the 1066 cm.-1 angular ring distortion) suggest the σ orbitale of the nitrogen atoms are not in sp2 hybridization in the excited state. The observed pyrimidine hydrochloride emission proves that the N...H hydrogen bond is broken or is very weak in the excited state. A similar conclusion is drawn from the pyrazine hydrochloride emission (also reported), thus placing the well known n → π* absorption blue shift on a sound basis. A relationship between H-bond energy and the absorption blue shift is proposed and tested on the three diazines.

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