A theoretical study of the conformers of trans- and cis-urocanic acid

Abstract

The geometry of both prototropic structures of trans-urocanic acid (( E)-3-(1 H-imidazol-4(5)-yl)propenoic acid) and of the corresponding cis isomer [( E)-3-(1 H-imidazol-4(5)-yl)propenoic acid] were studied by using quantum chemical ab initio and various semiempirical methods. A comparison of the computational methods shows that the AM1 Hamiltonian gives the best qualitative agreement with the 3-21G ab initio results for the potential energy surfaces of this class of molecules. The AM1 semiempirical method thus seems to be the best choice for locating the most interesting points on the potential energy surfaces of urocanic acid related molecules. The ab initio calculations can then be restricted to those regions of the surfaces. Both trans- and cis-urocanic acid are found to be planar. The most stable conformer of both trans-urocanic acid and cis-urocanic acid have the s-trans,s-cis conformation but they represent different prototropic structures. The barriers to torsion around all rotatable bonds are found to be quite high in both isomers of urocanic acid due to the π-bond conjugation which extends all over the molecules.