Computational study of three dimensional potential energy surfaces in intermolecular hydrogen bonding of cis-urocanic acid

The interamolecular hydrogen bond (IMHB) of the neutral, cationic, and anionic cis-urocanic acid (cis-UCA, (Z)-3-(I’H-imidazol-4’(5’)-yl)propenoic acid) were investigated in the gas phase within ab-initio calculations. To determine the energy and molecular structures, all the compounds were initially optimized at B3LYP and MP2 methods by use of 6-31G* and 6-31G** level of theory. Suitable single-point calculations have been subsequently done. Two dimensional energy curves have been obtained, and by adding a geometrical parameter, the potential energy surface is also plotted. The three-dimensional potential energy surface for proton motion within the molecule in the direction posses the lowest possible energy has been determined. These results show that the properties of hydrogen bonds are significantly different in the three forms of the investigated cis-urocanic acid. For instance, the bond strength in ionic molecules (cationic and anionic) is about twice of that of the neutral molecule. Evaluation of the potential energy versus distance curve is performed by mentioned methods and a single potential barrier curve which is relatively symmetrical is gained. The removal of potential barrier can be attributed to the high magnitude of hydrogen bonding strength.