Vibrational Frequencies of Amides and Amide Dimers: The Assessment of PW91_XC Functional

Abstract

PW91XC functional has recently been assessed for the interaction energies of selected dimers in which the nonbonded interactions play a critical role. In this study, we assess the PW91XC functional with the 6-31+G* basis set for the vibrational spectra of amide and amide dimers. The set of molecules chosen consists of (a) the monomeric amides: formamide, acetamide, cis-NMF, trans-NMF, N,N-DMF, cis-NMA, trans-NMA, (b) the covalent dipeptides N-acetyl−glycine-N‘-methylamide in C7eq and C5 conformations and the C7ax, C7eq, C5ext, β2, αp conformers of N-acetyl-l-alanine-N‘-methylamide, (c) the dimers cis-NMA cyclic dimer, two conformations of trans-NMA dimer and four conformations of formamide dimer. This set has been also used for the assessment of the EDF1 functional for the prediction of vibrational spectra. Comparison of PW91XC/6-31+G* results with EDF1, B3LYP, and MP2 values with the same basis set accentuate PW91XC as the best performing functional for amide I and II modes while for amide III mode EDF1 performs better. Overall, PW91XC has the better performance with EDF1 following close. We propose that PW91 may be used for the study of proteins and polypeptidic chains.