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

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.