Faculty of Chemistry, University of Opole, Opole, Poland.
J Mol Model. 2012 Jun;18(6):2471-8. doi: 10.1007/s00894-011-1262-6. Epub 2011 Oct 21.
A linear correlation between harmonic and anharmonic frequencies of water calculated at B3LYP level of theory was observed with a number of basis sets. Similar relationships were found in both the gas phase and solution for several small molecules. The best correlation was found for C = O stretch mode in formaldehyde, formamide and N-methylacetamide. The average difference between B3LYP harmonic and anharmonic ν(C = O) frequencies calculated with several basis sets in these molecules was 30 cm(-1). The ad hoc correction of -30 cm(-1), added to harmonic frequencies of two different carbonyl groups present in a structure of a larger molecule was tested as a fast way of predicting anharmonic frequencies without elaborated calculations. The proposed approach was tested successfully on a larger molecule of E and Z isomers of N-acetyl-α,β-dehydrophenylalanine N',N'-dimethylamide [Ac-(E/Z)-ΔPhe-NMe(2)] and the estimated anharmonic ν(C = O) frequencies were close to directly calculated results.
观察到在 B3LYP 理论水平下，许多基组的水的谐波和非谐波频率之间存在线性相关性。在气相和溶液中，对于几种小分子，都发现了类似的关系。在甲醛、甲酰胺和 N-甲基乙酰胺中，C=O 伸缩模式的相关性最好。用几种基组计算这些分子中的 B3LYP 谐波和非谐波 ν(C=O)频率之间的平均差异为 30 cm(-1)。在一个较大分子的结构中，对于两个不同的羰基，添加 -30 cm(-1) 的特定校正值，作为一种无需详细计算即可预测非谐波频率的快速方法进行了测试。在 N-乙酰-α，β-去水苯丙氨酸 N'，N'-二甲基酰胺[Ac-(E/Z)-ΔPhe-NMe(2)]的较大分子上成功测试了该方法，估计的非谐波 ν(C=O)频率接近直接计算的结果。
