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 ofLi and nucleobase powders in a metal-cluster beam source and studied by pulsed-field-ionizationzero-electron-kinetic-energy (ZEKE) spectroscopy and density functional theory calculations. TheZEKE measurements determined the adiabatic ionization energies of the three neutral complexesand frequencies of several vibrational modes for the metal-adenine and -uracil ions. The measuredspectra were compared with spectral simulations to determine the preferred metal binding sites. Foradenine, the most stable structure is formed by Li/Li~+ bidentately binding to both the N7 atomof the imidazole ring and the NH2 group of the pyrimidine ring. For uracil and thymine, theideal site for Li/Li~+coordination is the O4 atom. Although it has only a small effect on thegeometries of uracil and thymine, lithium coordination forces the rotation of the NH_2groupout of the adenine plane. The adiabatic ionization energies of the three complexes follow the trendof uracil (33910 ± 5 cm~(-1)) > thymine (33386 ± 5 cm~(-adenine 32240 1)),whereastheir metal-ligand bond dissociation energies are about the same, (92-97) ±6 kJ mol~(-1).For allthree complexes, the neutral bond energies are smaller than those of the corresponding ions due toa weaker electrostatic interaction and stronger electron repulsion.