The Influence of Inter-and Intramolecular Hydrogen Bonding upon the Structure and Photochemistry of 3-(2-Pyridyl)propenamides

The molecular structures and photochemical behavior of (E)-and (Z)-N-methyl-3-(2-pyridyl)-propenamide have been investigated in the solid state and in solution. While the molecular conformation of the thermodynamically more stable E isomer is independent of phase or solvent, the conformation of the Z isomer is dependent upon its environment, adopting an s-trans intramolecular hydrogen-bonded conformation in chlorocarbon solvents, an s-cis conformation in methyl sulfoxide solution, and an orthogonal intermolecular hydrogen-bonded conformation in the solid state. The electronic structure of these molecules has been investigated using the semiemperical INDO/SSCF-CI (ZINDO) algorithm. The two lowest energy absorption bands are attributed to allowed Π→π* transitions; however, the existence of lower energy singlet Π,π* states is predicted. Neither isomer undergoes photoisomerization in the solid state. Photoisomerization of the E isomer is efficient in nonpolar solvents but occurs with decreased efficiency in solvents which are good hydrogen bond acceptors. The Z isomer undergoes photoisomerization only in strong hydrogen bond acceptor solvents which disrupt the intramolecular hydrogen bond. Thus, either inter-or intramolecular hydrogen bonding can influence the efficiency of photoisomerization. Similar results are obtained with several related N-substituted 3-(2-pyridyl)propenamides. These results provide the first example of counterthermodynamic one-way E →Z photoisomerization based upon intramolecular hydrogen bonding.