Effect of adjoining aromatic ring upon excited state proton transfer, o-hydroxybenzaldehyde

The ground and first few excited states of o-hydroxybenzaldehyde (oHBA) are computed at the CIS and MP2/CIS levels with a 6-31 + G** basis set, with emphasis on its intramolecular H-bond. These results are compared with those for malonaldehyde, which differs from oHBA in that it lacks an adjoining benzene ring. In most respects, the addition of the latter aromatic system exerts surprisingly little influence upon the properties of malonaldehyde. With the exception of the (1)pi pi* state, electronic excitation weakens the H-bond and simultaneously raises the barrier to proton transfer in either system. Unlike the symmetric transfer potential in malonaldehyde, the enol and keto tautomers of oHBA are chemically distinct. pi --> pi* excitation reverses the preference for the enol tautomer in the ground state. This reversal is connected with the changing degree of aromaticity in the benzene ring of oHBA. The asymmetric transfer potential in oHBA leads to forward and reverse barriers of different magnitude. When this factor is accounted for by an averaging procedure, the transfer barriers in oHBA are remarkably similar to those of the corresponding states of malonaldehyde. (C) 1999 Elsevier Science B.V. All rights reserved.