Ab initio calculation of amide carbonyl stretch vibrational frequencies insolution with modified basis sets. 1. N-methyl acetamide

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 a n efficient way for calculating amide I and amide II frequencies that are d irectly comparable to those commonly measured in solution, The calculationa l results are compared to experimentally measured FTIR spectra in gas and s olution 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 th e HF, CASSCF, MP2, QCISD, and CCD levels. The DFT(BPW91)/6-31G(d) level cal culation for the NMA . 3H(2)O hydrogen-bonded complex with an Onsager or CP CM reaction field yields amide I, II, and III frequencies comparable to the experiment in aqueous solution. The amide I and, to a smaller degree, amid e II frequencies are found to be sensitive to the exponent of the d functio n in the basis set. Use of more diffuse (smaller exponent) d functions in t he 6-31G(d) basis set results in a calculated amide I frequency closer to t he solution experimental values. Such modified. relatively small basis sets may provide a computationally efficient means of approximating the solvent effects on amide vibrational frequencies.