Electronic structure and characterization of the spectra of trans/cis tautomers of urocanic acid isomers: A diagnostic tool

Abstract

The gas-phase information of ionization on the fundamental properties of biological molecules is essential for understanding numerous biological phenomena, as it provides insight into the physicochemical origin of molecular properties. In this work, conformational and tautomerism analysis of urocanic acid (UA) were performed by two methods of B3LYP and M062X at 6-311++g (d,p) basis set for all structures in two isomers of cis/trans in the gas phase. Then, stable structures were selected to calculate the ionization potential. The symmetry-adapted-cluster configuration interaction (SAC-CI General-R) method was used for calculating ionization energies and the photoelectron spectra. The NBO calculations were performed to determine the contribution of different natural bonding orbitals to the canonical molecular orbitals involved in ionization processes, which was required for spectral band assignment. The results of NBO calculation indicated that the ionization bands corresponding to ionizations from orbitals localized the imidazole ring had a larger contribution to the ionization process than the CC and CO bands. The most important point about the ionization of all tautomers is the removal of an electron from the imidazole ring. Koopmans’ approximation is only applicable for describing the first lowest-energy orbital ionization (photoelectron band), and others may make a linear combination of HF determinations.