Knowledge Bank

Generally, it is assumed that hydrogen bonds to non-bonding orbitals are broken when the electrons in those orbitals are promoted to an $n\pi \ast$ excited state. In fact, considerable evidence has been accumulated to support this contention. Last year we reported that the large amount of vibronic structure which remained when pyrazine $(C4N2H4)$ was hydrogen-bonded with fluorinated alcohols at room temperature suggested a stable hydrogen-bonded, $n\pi \ast$ excited state for the singly hydrogen-bonded species, but that the broader spectrum of the doubly hydrogen-bonded species indicated dissociation. New measurements reveal Chat both the singly and doubly hydrogen-bonded pyrazine species show essentially the same fluorescence spectrum which differs markedly from that of the unbonded species. This finding strongly supports the notion that the doubly bonded $n\pi \ast$ excited state species dissociates to the singly bonded species prior to fluorescence but that the singly bonded species has an excited state lifetime comparable to the fluorescence lifetime.