Ab initio MO calculation of force constants and dipole derivatives for formamide

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Ab initio SCF MO calculations have been carried out for the equilibrium geometry, vibrational frequencies, force constants, dipole moment and its derivatives of formamide. The energy gradient method was employed and the 4-31G basis set was used. For in-plane vibrations: (1) Calculated normal frequencies were 10–20% greater than the observed fundamental frequencies. (2) Isotope shifts (− d 0, − d 1, − d 2, and − d 3 species) were well reproduced. (3) The calculated dipole moment derivatives showed a good correspondence with the infrared intensity pattern. (4) The NH 2 rocking—OCN bending cross term, which should be zero in the Urey—Bradley force field, came out to be as large as −0.18 mdyne A. For out-of-plane vibrations, especially for the NH 2 wagging, it was found to be essential to include polarization functions for N, C and O atoms.