Ab Initio Studies on the Photophysics of Uric Acid and Its Monohydrates: Role of the Water Molecule

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2014-02, Vol.118 (6), p.1132-1141
• Main Authors: Yamazaki, Shohei, Urashima, Shu-hei, Saigusa, Hiroyuki, Taketsugu, Tetsuya
• Format: Article
• Language: English
• Subjects: Energy of dissociation; Excitation; Hydrogen Bonding; Ionization; Isomers; Mathematical analysis; Migration; Models, Molecular; Molecular Conformation; Motion; Potential energy; Quantum Theory; Thermodynamics; Uric acid; Uric Acid - chemistry; Water - chemistry
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp411880z

The photophysical behavior of three lowest-energy tautomers of uric acid and seven most stable isomers of uric acid monohydrate is comprehensively studied by ab initio calculations. Ground-state energies are calculated with the CCSD(T) method, while excitation and ionization energies as well as excited-state potential energy profiles of photoinduced processes are calculated with the CC2 method. For the 1ππ* state, it is found that the excitation energy of the monohydrate cluster is significantly lower than that of isolated uric acid when the water molecule is hydrogen-bonded at a specific carbonyl group. The calculated excited-state potential energy profiles suggest that some monohydrate isomers can undergo a migration of the water molecule from one site to another site in the 1ππ* state with a small energy barrier. It is also found for both uric acid and its monohydrate that nonradiative decay via the NH bond dissociation in the 1πσ* state is likely to occur at higher excitation energies. On the basis of the computational results, possible mechanisms for the absence of specific isomers of uric acid monohydrate from the resonant two-photon ionization spectrum are discussed.