Tian Shan Xi
Hefei National Laboratory for Physical Sciences at Microscale, Laboratory of Bond Selective Chemistry, Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China.
J Chem Phys. 2005 Dec 22;123(24):244310. doi: 10.1063/1.2139996.
The low-lying conformers of N-/O-methylglycine are studied by ab initio calculations at the B3LYP, MP3, and MP4(SDQ) levels of theory with the aug-cc-pVDZ basis set. The conformers having the intramolecular hydrogen bonds N-H...O=C or O-H...N are more stable than the others. Vertical ionization energies for the valence molecular orbitals of each conformer predicted with the electron propagator theory in the partial third-order quasiparticle approximation are in good agreement with the experimental data available in the literatures. The relative energies of the conformers and comparison between the simulated and the experimental photoelectron spectra demonstrate that there are at least three and two conformers of N- and O-methylglycine, respectively, in the gas-phase experiments. The intramolecular hydrogen bonding O-H...N effects on the molecular electronic structures are discussed for the glycine methyl derivatives, on the basis of the ab initio electronic structure calculations, natural orbital bond, and atoms-in-molecules analyses. The intramolecular hydrogen bonding O-H...N interactions hardly affect the electronic structures of the O-NH2-CH2-C(=O)-O-CH3 and alpha-methylated NH2-CH2-C(CH3)OOH conformers, while the similar intramolecular interactions lead to the significantly lower-energy levels of the highest occupied molecular orbitals for the N-(CH3-NH-CH2-COOH) and beta-methylated (NH2-CH2-CH2-COOH) conformers.
采用B3LYP、MP3和MP4(SDQ)理论水平以及aug-cc-pVDZ基组，通过从头算对N-/O-甲基甘氨酸的低位构象异构体进行了研究。具有分子内氢键N-H...O=C或O-H...N的构象异构体比其他异构体更稳定。用电子传播理论在部分三阶准粒子近似下预测的每个构象异构体价分子轨道的垂直电离能与文献中可用的实验数据吻合良好。构象异构体的相对能量以及模拟光电子能谱与实验光电子能谱的比较表明，在气相实验中，N-甲基甘氨酸和O-甲基甘氨酸分别至少有三种和两种构象异构体。基于从头算电子结构计算、自然轨道键和分子中的原子分析，讨论了甘氨酸甲基衍生物中分子内氢键O-H...N对分子电子结构的影响。分子内氢键O-H...N相互作用几乎不影响O-NH2-CH2-C(=O)-O-CH3和α-甲基化NH2-CH2-C(CH3)OOH构象异构体的电子结构，而类似的分子内相互作用导致N-(CH3-NH-CH2-COOH)和β-甲基化(NH2-CH2-CH2-COOH)构象异构体的最高占据分子轨道能级显著降低。
