HYDROGEN BOND STRENGTH DIFFERENCES IN THE $n\pi^{*}$ STATES OF THE DIAZINES AND TETRAZINE

Author Institution:Generally speaking, hydrogen bonds to the lone pairs of nitrogen heterocycles are expected to dissociate in $n\pi^{*}$ excited states. We have recently shown that this is not necessarily the case for pyrazine and tetrazine when bonded to very acidic alcohols. In the present study we present spectroscopic evidence that the expected hydrogen bond breaking does occur in the diazines pyridazine and pyrimidine. These results are surprising in that both of these molecules in the ground state are more basic and form stronger hydrogen bonds than either pyrazine or tetrazine. These differences suggest that there may be two different ""types"" of $n\pi^{*}$* electronic states in azines containing more than one nitrogen. A suggested explanation for these differences, based upon theoretical calculations for pyrazine, will be presented