Abstract
CNDO/force calculations are employed to evaluate theoretical force fields for formamide, acetamide, and urea. Since the stretching force constants are overestimated and other diagonal as well as off-diagonal force constants are predicted reasonably well by these calculations, the initial force fields are set up by transferring the stretching force constants from structurally related molecules and by including the other force constants from the CNDO force field. The experimental force fields are obtained for the condensed phases as well as monomeric species using vibrational frequencies by carrying out least squares refinement. Frequencies of HCONH2, HCOND2, DCONH2, DCOND2, HCONHD, and DCONHD for formamide, CH3CONH2, CH3COND2, CD3CONH2, CD3COND2, CH3CONHD, CD3CONHD, and CH3CO 15NH2 for acetamide, and NH2CONH2, ND2COND2, 15NH2CO 15NH2, 15ND2CO 15ND2, and NH2C 18ONH2 for urea are employed in refining the force fields. The force fields obtained are found to be reliable on the basis of the force constants for chemically related molecules, frequency fits, and potential energy distributions.
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