QUANTUM-CHEMICAL GAS-PHASE CALCULATIONS ON THE PROTONATION FORMS OF TRANS-UROCANIC AND CIS-UROCANIC ACID

The neutral, cationic, and anionic structures of both prototropic taut omers of trans-and cis-urocanic acid [(E)- and (Z)-3-(1'H-imidazol-4'( 5')-yl)propenoic acid, respectively] were studied by using semiempiric al and ab initio gas-phase calculations. Potential energy surfaces of the structures were calculated by using the semiempirical AM1 method, and the geometries corresponding to global minima on these surfaces we re optimized up to the MP2/6-31G level of theory. The calculated prot onation forms of each urocanic acid isomer have a planar molecular str ucture due to a delocalized pi-electron system, and all of them prefer the s-trans conformation with respect to the bond between the imidazo le and the propenoic acid moieties. The cis-urocanic acid structures a re stabilized by an intramolecular hydrogen bond. The charged cis-uroc anic acid isomers have a lower molecular energy than the corresponding trans-isomers, whereas the neutral molecules have, after inclusion of thermodynamic corrections, approximately the same energy. The cationi c urocanic acid structures have about 2500 kJ mol(-1) lower energy tha n the anionic ones and about 1000 kJ mol(-1) lower energy than the neu tral ones. The nonzwitterionic forms of the neutral urocanic acid isom ers have about 200 kJ mol(-1) lower energy than the zwitterionic ones. These energy differences are explained by the proton affinities of th e imidazole and the propenoic acid moieties of the urocanic acid struc tures.