NEW STUDIES OF HYDROGEN BONDED $\eta\Pi^{*}$ EXCITED STATES

$^{1}$H. Baba, L. Goodman, and P. Valenti, J. Am. Chem, Soc. 88, 5410 (1966); J. A, Stikeleather, Chem. Phys, Lett. 21, 326 (1973). $^{2}$ J. Rose and R. S. Drago, J. Am. Chem. Soc. 81, 6138 (1959).Author Institution: Department of Chemistry, Williams CollegeThere has been considerable study of the effect of hydrogen bonding on the energy and shape of $\eta\Pi^{*}$ electronic absorption and luminescence bands. In several studies involving azaaromatic molecules, it is found that while the absorption spectrum is significantly shifted by the presence of a hydrogen bond, the fluorescence is not. It is therefore concluded that hydrogen bonds are broken in $\eta\Pi^{*}$ excited $states.^{1}$ In this paper we would like to report on evidence which suggests that for some systems this may not be true. In particular, the lowest $\eta\Pi^{*}$ absorption system of pyrazine retains considerable vibronic structure when singly hydrogen bonded by any one of several fluorinated alcohols. A generalized Rose-Drago $procedure^{2}$ for separating out the overlapping absorption spectra of singly, doubly, and non-hydrogen bonded spectra, and obtaining thermo-dynamic parameters for the hydrogen bonded complex will be described. The effect of these strongly hydrogen bonded alcohols on the spectra of other azaaromatic molecules will also be reported