High-resolution electron spectroscopy and structures of lithium-nucleobase (adenine, uracil, and thymine) complexes

摘要

Li complexes of adenine, uracil, and thymine were produced by laser vaporization of rods made of Li and nucleobase powders in a metal-cluster beam source and studied by pulsed-field-ionization zero-electron-kinetic-energy (ZEKE) spectroscopy and density functional theory calculations. The ZEKE measurements determined the adiabatic ionization energies of the three neutral complexes and frequencies of several vibrational modes for the metal-adenine and -uracil ions. The measured spectra were compared with spectral simulations to determine the preferred metal binding sites. For adenine, the most stable structure is formed by Li/Li+ bidentately binding to both the N7 atom of the imidazole ring and the NH2 group of the pyrimidine ring. For uracil and thymine, the ideal site for Li/Li+ coordination is the O4 atom. Although it has only a small effect on the geometries of uracil and thymine, lithium coordination forces the rotation of the NH2 group out of the adenine plane. The adiabatic ionization energies of the three complexes follow the trend of uracil (33910±5 cm−1)>thymine (33386±5 cm−1)>adenine (32240±5 cm−1), whereas their metal-ligand bond dissociation energies are about the same, (92–97) ±6 kJ mol−1. For all three complexes, the neutral bond energies are smaller than those of the corresponding ions due to a weaker electrostatic interaction and stronger electron repulsion. © 2010 American Institute of Physics Article Outline INTRODUCTION EXPERIMENTAL AND COMPUTATIONAL METHODS RESULTS AND DISCUSSION PIE spectra of Li-adenine, Li-thymine, and Li-uracil ZEKE spectrum and structure of Li-adenine ZEKE spectra and structures of Li-uracil and Li-thymine Ionization and bond dissociation energies of Li-adenine, Li-uracil, and Li-thymine CONCLUSIONS