Proton Transfer in Ground and Excited Electronic States of Glyoxalmonohydrazine
Journal of Physical Chemistry
American Chemical Society
The first two triplets and two lowest-lying excited singlet states of glyoxalmonohydrazine, along with its ground state, are studied by ab initio methods. The ground state proton transfer potential contains two wells, the lower of which corresponds to the N tautomer, with the bridging hydrogen of the intramolecular H bond attached to the N atom. The 0 tautomer is 9 kcaVmol higher in energy and exists in a shallow second well, separated from N by a barrier of less than 2 kcal/mol. When restrained to planarity, the h-z* and 3z-z* states have proton transfer potentials similar to the ground state in that the N tautomer is favored. Any second minimum corresponding to 0 lies in a very shallow well. The situation is reversed in the singlet and triplet n-z* states, where 0 is preferred to N, by 1 kcal/mol in h-z* and by 14 in the triplet. Unlike the malonaldehyde analogue, most of the excited states of glyoxalmonohydrazine favor a nonplanar geometry. The specific distortion mode is different for each state, as is the force toward nonplanarity, but each can be understood on the basis of bond indices within the OCCNN ring. Permitting full distortion has a profound influence upon the energetics of proton transfer, switching the relative stability of N and 0 in both excited singlets, as compared to the transfer in the planar case.
Proton Transfer in Ground and Excited Electronic States of Glyoxalmonohydrazine K. Luth, S. Scheiner J. Phys. Chem. 1995 99, 7352-7359.