Faculty of Chemistry, University of Opole, 48 Oleska Street, 45-052, Opole, Poland.
J Mol Model. 2011 Sep;17(9):2265-74. doi: 10.1007/s00894-010-0944-9. Epub 2011 Jan 26.
Formamide harmonic and anharmonic frequencies of fundamental vibrations in the gas phase and in several solvents were successfully estimated in the B3LYP Kohn-Sham complete basis set limit (KS CBS). CBS results were obtained by extrapolating a power function (two-parameter formula) to the results calculated with polarization-consistent basis sets. Anharmonic corrections using the second order perturbation treatment (PT2) and hybrid B3LYP functional combined with polarization consistent pc-n (n=0, 1, 2, 3, 4) and several Pople's basis sets were analyzed for all fundamental formamide vibrational modes in the gas phase and solution. Solvent effects were modeled within a PCM method. The anharmonic frequency of diagnostic amide vibration C=O in the gas phase and the CCl(4) solution calculated with the VPT2 method was significantly closer to experimental data than the corresponding harmonic frequency. Both harmonic and anharmonic frequencies of C=O stretching mode decreased linearly with solvent polarity, expressed by relative environment permittivity (ε) ratio (ε-1)/(2ε+1). However, an unphysical behavior of solvent dependence of some low frequency anharmonic amide modes of formamide (e.g., CN stretch, NH(2) scissoring, and NH(2) in plane bend) was observed, probably due to the presence of severe anharmonicity and Fermi resonance.
在 B3LYP Kohn-Sham 完全基组极限 (KS CBS) 中，成功估计了气相中和几种溶剂中甲酰胺基频和非频的谐波和非谐波频率。CBS 结果通过将幂函数（双参数公式）外推到用极化一致基组计算的结果来获得。对气相和溶液中所有甲酰胺基频振动模式，使用二阶微扰处理（PT2）和混合 B3LYP 函数与极化一致 pc-n（n=0,1,2,3,4）和几个 Pople 基组进行了非谐修正分析。采用 PCM 方法模拟溶剂效应。用 VPT2 方法计算的气相和 CCl(4)溶液中诊断酰胺振动 C=O 的非谐频率与实验数据更接近，而相应的谐频则不然。C=O 伸缩模式的谐频和非谐频都随溶剂极性线性降低，用相对环境介电常数（ε）比（ε-1）/（2ε+1）表示。然而，观察到甲酰胺一些低频非谐酰胺模式（例如，CN 伸缩、NH(2) 剪断和 NH(2) 面内弯曲）对溶剂的依赖存在非物理行为，这可能是由于存在严重的非谐性和费米共振。
