PUSHING PYRAZINE TO THE (STATISTICAL) LIMIT

The hydrogen-bonded complex of pyrazine and 1,1,1,3,3,3-hexafluoro-2-p ropanol (HFP) is strongly bound in the ground and first excited n pi singlet states. Ground-state vapor-phase FTIR and supersonic-jet fluor escence excitation spectroscopy were utilized to obtain estimates of t he binding energies in both states. The results, accurate to approxima tely 200 cm(-1), are 3400 cm(-1) in S-0 (40 +/- 2 kJ/mol) and 3000 cm( -1) in S-1 (n pi, 35 +/- 2 kJ/mol). The 0(0)(0) and two excited vibra tional bands of the complex, 6a(0)(1) at 583 and 10a(0)(2) at 790 cm(- 1), were observed in the electronic spectrum. Pyrazine is a well-known example of an intermediate case molecule whose fluorescence decay cur ve when excited to SI displays a fast and a slow component. However, t he complex in the vibrationless level of S-1 displays ''statistical li mit'', or large-molecule radiationless decay, with no slow component i n its fluorescence decay curve.