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. Fu lly 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 frequenci es of aqueous NMA. HOH bend scale factors were found that are consiste nt 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 cal culated frequency difference between the two allowed NMA conformers co nfirms the assignment of the two observed amide II bands to the presen ce of these conformers in aqueous solution. An evaluation is made of t he assignments of other bands.