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 func tional theory (DFT) approximation are reported. The performance of four rec ent density functionals (BLYP, B3LYP, B3P86 and B3PW91) was compared with t he ab initio Hartree-Fock (HF) and post-HF methods with inclusion of electr on correlation at the MP2 level. With the basis sets of 6-31G*, or better q uality, the B3P86 and B3PW91 DFT calculated C-N, C=O bond lengths and C=O h armonic stretching vibrations nu (C=O) were predicted to be in good agreeme nt with available experimental and/or theoretical data. The agreement betwe en the DFT predicted dipole moment and the experiment was significantly bet ter than the corresponding MP2 results. The non-planar amide unit was predi cted 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 ge ometries of formamide. The sensitivity of Formamide nu (C=O) and GIAO NMR s hieldings towards structural distortions of the C=O bond length and OCNH di hedral angle, method of calculation, basis set size and quality was studied , too. (C) 2000 Elsevier Science B.V. All rights reserved.