Density Functional Theory Vibrational Frequencies of Amides and Amide Dimers

Abstract

Infrared spectra of amides and polypeptides can provide detailed information on conformation. An understanding of the amide group in model compounds is a vital step toward a deeper insight into the vibrational spectra of proteins. We show that in contrast to MP2 and the popular B3LYP functional, which overestimate amide I frequencies by 20−80 cm-1, the recently developed empirical density functional, EDF1, yields unscaled harmonic vibrational frequencies of monoamides in close agreement with experimental data, even using the relatively small 6-31+G* basis set. New calculations on several hydrogen-bonded amide dimers and the experimental data available for these dimers also support the conclusion that EDF1 yields frequencies in better agreement with experiment than MP2 or B3LYP. We present calculated minimum-energy structures and vibrational spectra of N-acetylglycine-N‘-methylamide and N-acetyl-l-alanine-N‘-methylamide at the EDF1/6-31+G* and B3LYP/6-31+G* levels.