Ab initia MO calculation in terms of localized orbitals of the 14N electric field gradient in nitriles

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The contributions of each localized orbital of the CN group and of the molecular remainder to the N electric field gradient (EFG) were calculated for nitriles with R=H, F, CH3, OH, NH2, CCH, COH, NO, CN and the E and Z forms of iminoacetonitrile. The changes in the EFG introduced by substituents resulted mainly from the polarization and conjugation of the C-N bonds, with a small contribution from the N lone pair. The total EFG has a large and rather constant contribution from the N lone pair, which is modified by the contributions from the three C-N bonds and the molecular remainder. The resulting EFG is determined by the signs and magnitudes of the C-N bonds' contributions for each substituent and to a lesser degree by the molecular remainders. The asymmetry parameter of the EFG is mainly determined by the inequivalence of the C-N bonds with a very small contribution from the molecular remainder. The molecular remainder and the crystal field in solids contribute more to the largest component of the EFG than to the asymmetry parameter. All these results have allowed us to discuss the validity of the assumptions made in the Townes and Dailey theory when it is applied to nitriles.