Ab Initio Calculation of Amide Carbonyl Stretch Vibrational Frequencies in Solution with Modified Basis Sets. 1. N-Methyl Acetamide

Abstract

Density functional theory DFT(BPW91) level calculations with modified 6-31G(d) basis sets are tested for a small amide, N-methyl acetamide (NMA), as an efficient way for calculating amide I and amide II frequencies that are directly comparable to those commonly measured in solution. The calculational results are compared to experimentally measured FTIR spectra in gas and solution phases. The 6-31G(d) basis set at the DFT level yields vibrational frequencies that have the best agreement with the gas-phase experiment, as compared to amide I and II frequencies calculated with the same basis at the HF, CASSCF, MP2, QCISD, and CCD levels. The DFT(BPW91)/6-31G(d) level calculation for the NMA·3H2O hydrogen-bonded complex with an Onsager or CPCM reaction field yields amide I, II, and III frequencies comparable to the experiment in aqueous solution. The amide I and, to a smaller degree, amide II frequencies are found to be sensitive to the exponent of the d function in the basis set. Use of more diffuse (smaller exponent) d functions in the 6-31G(d) basis set results in a calculated amide I frequency closer to the solution experimental values. Such modified, relatively small basis sets may provide a computationally efficient means of approximating the solvent effects on amide vibrational frequencies.