Theoretical study of the force field and vibrational assignments of acetamide and deuterated analogues

The complete harmonic force constants of acetamide have been evaluated by ab initio calculations at the Hartree-Fock level with the 4-31G(d) basis set. The force field was scaled to compensate for the systematic overestimations of the Hartree-Fock-level force constants by empirical factors using the matrix isolation IR spectra of CH3CONH2 and CD3CONH2. A normal coordinate treatment has been carried out with the scaled force field to analyze the vibrational spectra of CH3CONH2. A normal coordinate treatment has been carried out with the scaled force field to analyze the vibrational spectra of CH3COND2, cis-CH3CONHD and trans-CH3CONHD. The effect of cis/trans isomerism of CH3CONHD on the fundamental bands was well reproduced by the calculations. The fundamental vibrations were also predicted for CD3COND2, cis-CD3CONHD and trans-CD3CONHD.