Density functional study of a model amide. Prediction of formamide geometry, dipole moment, IR harmonic vibration nu(C=O) and GIAO NMR shieldings

The results of extended MO calculations on formamide using the density functional theory (DFT) approximation are reported. The performance of four recent density functionals (BLYP, B3LYP, B3P86 and B3PW91) was compared with the ab initio Hartree-Fock (HF) and post-HF methods with inclusion of electron correlation at the MP2 level. With the basis sets of 6-31G*, or better quality, the B3P86 and B3PW91 DFT calculated C-N, C=O bond lengths and C=O harmonic stretching vibrations nu (C=O) were predicted to be in good agreement with available experimental and/or theoretical data. The agreement between the DFT predicted dipole moment and the experiment was significantly better than the corresponding MP2 results. The non-planar amide unit was predicted at the MP2 level for basis sets including only polarization, and both polarization and diffuse functions. The GIAO NMR shieldings were calculated for experimental and theoretical geometries of formamide. The sensitivity of Formamide nu (C=O) and GIAO NMR shieldings towards structural distortions of the C=O bond length and OCNH dihedral angle, method of calculation, basis set size and quality was studied, too. (C) 2000 Elsevier Science B.V. All rights reserved. Journal of Molecular Structure-Theochem