A b i n i t i o investigation of geometry changes during inversion of NH3, NH2F, NHF2, NF3 and PH3, PH2F, PHF2, PF3

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Abstract
Ab initio SCF molecular orbital calculations with complete geometry optimization have been performed using consistent basis sets for each member of the series NH3, NH2F, NHF2, NF3 and PH3, PH2F, PHF2, PF3. The calculations were repeated using the same basis sets with the molecules constrained to the planar configuration which is the transition state for inversion, all other geometrical parameters being energy optimized. In the planar configuration, shorter N–F, N–H, and P–H bonds resulted as well as a change from the nearly equal pyramidal bond angles to highly asymmetric planar bond angles with &HXH≳ &HXF≳ &FXF. Omission of polarization functions from the basis set produced longer P–N and N–F bonds. Regardless of the inclusion or exclusion of d atomic orbitals in the basis set, substitution of fluorine for hydrogen yields regular increases in inversion barrier heights. Exclusion of d atomic orbitals results in inversion barriers that are smaller by nearly the same amount for all molecules in a series. The increase in barrier with substitution of fluorine for hydrogen is correlated with an increase of s character in the lone pair. Dipole moment and components, bond moments, and lone pair moments are discussed. Effects of varying the d orbital exponent coefficient on the central atom were examined.