A THEORETICAL-STUDY OF THE CONFORMERS OF TRANS-UROCANIC AND CIS-UROCANIC ACID

The geometry of both prototropic structures of trans-urocanic acid ((E )-3-(1H-imidazol-4(5)-yl)propenoic acid) and of the corresponding cis isomer [(Z)-3-(1H-imidazol-4(5)-yl)propenoic acid] were studied by usi ng quantum chemical ab initio and various semiempirical methods. A com parison of the computational methods shows that the AM1 Hamiltonian gi ves the best qualitative agreement with the 3-21G ab initio results fo r the potential energy surfaces of this class of molecules. The AM1 se miempirical method thus seems to be the best choice for locating the m ost interesting points on the potential energy surfaces of urocanic ac id related molecules. The ab initio calculations can then be restricte d to those regions of the surfaces. Both trans- and cis-urocanic acid are found to be planar. The most stable conformer of both trans-urocan ic acid and cis-urocanic acid have the s-trans,s-cis conformation but they represent different prototropic structures. The barriers to torsi on around all rotatable bonds are found to be quite high in both isome rs of urocanic acid due to the pi-bond conjugation which extends all o ver the molecules.