Valence ionization potentials of anionic phosphate esters: An ab initio quantum mechanical study

Abstract

In earlier investigations N. S. Kim, P. R. LeBreton, J. Am. Chem. Soc. 118, 3694 (1996), and references therein, ultraviolet (UV) photoelectron data and ab initio molecular orbital calculations yielded information about nucleotide electronic structure by providing valence ionization potentials (IPs) of nucleotide bases and sugar model compounds. Here, model phosphate group ionization potentials have been evaluated by employing multireference, single and double excitation configuration interaction calculations with a complete active space self-consistent field (CASSCF) wave function. The five lowest energy IPs of H2PO4− and the four lowest energy IPs of (CH3)HPO4− and (CH3)2PO4− were evaluated. Calculations were performed using a (12, 9, 1)/[6, 4, 1] double-zeta basis set on phosphorus with a d-polarization function; (10, 7)/[4, 3] and (10, 6)/[4, 2] basis sets on oxygen and carbon, respectively; and a (6)/[3] basis set on hydrogen. Two types of CASSCF calculations were carried out. In one, denoted 8e8a/7e8a, the anions and radicals had 8 and 7 electrons, respectively, in 8 active orbitals. In the second, denoted 10e10a/9e10a, there were 9 and 10 electrons in 10 active orbitals. Ionization potentials of H2PO4−, (CH3)HPO4−, and (CH3)2PO4− were also obtained from second-order perturbation theory (CASPT2) calculations with the CASSCF reference functions. All of the ionization events examined are associated with removal of electrons from oxygen atom lone-pair orbitals on the closed-shell anions. For H2PO4− and (CH3)HPO4−, the lowest energy CASPT 2 ionization potentials obtained (4.29–4.36 eV and 4.12–4.27 eV, respectively) are smaller than corresponding IPs (4.89 and 4.69 eV) previously reported from results of 6-31+G* second-order Møller-Plesset (MP2) calculations. For the second through fifth IPs of H2PO4−, and the second through fourth IPs of (CH3)HPO4−, the values obtained from CASPT2 calculations are 0.42 to 1.95 eV smaller than values reported from the combined use of MP2 and configuration interaction singles calculations with the 6-31+G* basis set. A combination of results from MP2 and CASPT2 calculations yields values of 4.89, 5.42, 6.30, 6.64, and 7.41 eV for the five lowest IPs of H2PO4−; and values of 4.69, 5.43, 6.08, and 6.55 eV for the four lowest IPs of (CH3)HPO4−. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 1095–1106, 1997