The charge distribution in amides and thioamides by nuclear quadrupole coupling, dipole moments and electronic structure calculations

The nuclear quadrupole coupling constants from microwave spectroscopy (MW) and quadrupole resonance (NQR) for amides and thioamides are discussed in relation to Hartree-Fock calculations with and without Moller-Plesset correlation effects. The view that the larger dipole moments from thioamides than the corresponding amides is a function of enhanced resonance in the former is discussed and (in effect) confirmed by the present procedures. The principal mechamism seems to be the push/pull pi/sigma effects of the N atom with respect to the CO and CS groups, with S being a better sigma-donor than O; however, the effect is still present with formamidine where no electronegativity effects are important, so the overall effect is the 2,1,1 pi-electron contribution to the allylic system from N, C, O(S). The me of localised MO's and NO's is described, and the centroid positions are discussed in relation to the polarity of the bonds. The LMO's largely truncate the contributions to each NQCC to the three attached bonds (or 2 bends + a lone pair orbital at O or S), as is used in the Townes-Dailey procedures. More distant LMO's generally contribute <0.05 a. u. to the EFG, simplifying the analysis. The effects of O(or Sf-protonation of urea and thiourea is discussed.