Ab initio vibrational analysis of isotopic derivatives of aqueous hydrogen-bonded trans-N-methylacetamide

Normal modes have been calculated for a CH3CONHCH3-(H2O)(2) [NMA-(H2O)(2)] system and its CCD3, NCD3, (CN)-C-13-N-15, and ND derivatives. Fully optimized structures were obtained with the 6-31+G* basis set. The scale factors for the NMA force constants were obtained by optimizing the scale factors for the isolated molecule to experimental frequencies of aqueous NMA. HOH bend scale factors were found that are consistent with observed resonance Raman enhancements and infrared intensities of the components of the amide I doublet. Its frequency splitting can be reproduced quantitatively by a reaction field calculation. The calculated frequency difference between the two allowed NMA conformers confirms the assignment of the two observed amide II bands to the presence of these conformers in aqueous solution. An evaluation is made of the assignments of other bands.