Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055, USA.
J Chem Phys. 2010 Jan 28;132(4):044304. doi: 10.1063/1.3299278.
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 NH(2) 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 NH(2) 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.
腺嘌呤、尿嘧啶和胸腺嘧啶的 Li 配合物是通过激光蒸发 Li 和碱基粉末制成的棒在金属团簇束源中产生的，并通过脉冲场电离零电子动能（ZEKE）光谱和密度泛函理论计算进行了研究。ZEKE 测量确定了三种中性配合物的绝热电离能以及金属-腺嘌呤和-尿嘧啶离子的几个振动模式的频率。测量的光谱与光谱模拟进行了比较，以确定首选的金属结合位点。对于腺嘌呤，最稳定的结构是由 Li/Li(+) 以双齿方式与咪唑环的 N7 原子和嘧啶环的 NH(2)基团结合形成的。对于尿嘧啶和胸腺嘧啶，Li/Li(+) 配位的理想位点是 O4 原子。尽管它对尿嘧啶和胸腺嘧啶的几何形状只有很小的影响，但锂配位迫使 NH(2)基团从腺嘌呤平面旋转。三种配合物的绝热电离能遵循尿嘧啶（33910+/-5 cm(-1))>胸腺嘧啶（33386+/-5 cm(-1))>腺嘌呤（32240+/-5 cm(-1))的趋势，而它们的金属-配体键解离能大致相同，（92-97) +/-6 kJ mol(-1)。对于所有三种配合物，由于静电相互作用较弱和电子排斥较强，中性键能都小于相应离子的键能。
