Effects of Intermolecular Hydrogen-Bonding Interactions on the Amide I Mode of N-Methylacetamide:  Matrix-Isolation Infrared Studies and ab Initio Molecular Orbital Calculations

• Published in: The journal of physical chemistry. B Vol. 102; no. 1; pp. 309 - 314
• Main Authors: Torii, Hajime, Tatsumi, Tomoaki, Kanazawa, Takanori, Tasumi, Mitsuo
• Format: Journal Article
• Language: English
• Published: American Chemical Society 01.01.1998
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp972879j

Effects of intermolecular hydrogen-bonding interactions on the amide I mode of N-methylacetamide (NMA) are studied by matrix-isolation infrared (IR) spectroscopy and ab initio molecular orbital calculations. The wavenumbers of the amide I IR bands of NMA in Ar and N2 matrixes with various NMA/matrix gas mixing ratios are compared with those calculated for the monomer, dimers, and trimers of trans-NMA and the monomer and dimer of cis-NMA. The band at 1708 (1706) cm-1 in Ar (N2) matrixes is assigned to the amide I mode of the monomer of trans-NMA. The band observed at 1686 (1681) cm-1 in Ar (N2) matrixes with the NMA/matrix gas mixing ratio larger than 1/500 is assigned to the amide I band due to the dimers of trans-NMA. The bands observed at lower wavenumbers for samples with the NMA/matrix gas mixing ratio as large as 1/100 are assigned to the amide I bands of the trimers and larger clusters of trans-NMA. It is likely that the band observed at 1695 (1693) cm-1 in the Ar (N2) matrix arises from the dimer of cis-NMA, which is as stable as the trans-NMA dimers because of the formation of two hydrogen bonds in a cyclic form. Although there are two amide I modes in an NMA dimer and three in an NMA trimer, only one mode is strongly IR active in each species. The intrinsic amide I wavenumbers of individual peptide groups in NMA clusters, i.e., the amide I wavenumbers in the case where there is no resonant vibrational coupling between the peptide groups, are examined by calculating the amide I wavenumbers for the dimers and trimers whose constituent molecules other than the target molecule have the CO group(s) substituted with 13C and 18O. It is shown that the shifts of the amide I band to lower wavenumbers induced by hydrogen bonding to the CO group are 20−25 cm-1, while those induced by hydrogen bonding to the N−H group are 15−20 cm-1. These shifts are approximately in line with the changes in the CO bond lengths and are approximately additive if both the CO and N−H groups of a peptide group are hydrogen bonded.