Force field calculations for formamide, acetamide, and urea using the CNDO/force method

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 CH₃CO¹⁵NH₂ for acetamide, and NH₂CONH₂, ND₂COND₂, ¹⁵NH₂CO¹⁵NH₂, ¹⁵ND₂CO¹⁵ND₂, and NH₂C¹⁸ONH₂ 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.