The photochemical cisrtrans isomerization of urocanic acid UCA, Ž . Ž X X .
. E -3-1 H-imidazol-4 -yl propenoic acid was investigated using complete active space Ž . SCF CASSCF ab initio calculations. The singlet ground state and the triplet and the U Ž . singlet manifolds of the lowest-lying ª HOMO ª LUMO excitation of the neutral and the anionic UCA were calculated using the 6-31G U and the 6-31 q G U basis sets, respectively. The torsional barrier of the double bond of the propenoic acid moiety in UCA is observed to be considerably lower in the T and S excited states of the neutral 1 1
UCA and in the T but not in the S excited state of the anionic UCA, as compared to the 1 1 S state of the respective protonation form. The cis-isomer of both the neutral and the 0 anionic UCA is lower in energy than the trans-isomer in the S , T , and S states. This 0 1 1 energy difference is larger in the excited states than in the ground state, probably due to strengthening of the intramolecular hydrogen bond of cis-UCA as the molecule is excited.
The results of the calculations, interpreted in terms of the idea that UCA is deprotonated upon electronic excitation, led to construction of a new model for the photoisomerization mechanisms of UCA. According to this model, the trans-to-cis isomerization proceeds via both the triplet and the singlet manifolds in the deprotonated form of UCA. This isomerization may occur in the S state of the neutral UCA as well. The cis-to-trans 0 isomerization is suggested to proceed only in the S state of the neutral UCA. ᮊ 1999