Studies of the electron density in the highest occupied molecular orbitals of PH3, PF3 and P(CH3)(3) by electron momentum spectroscopy and Hartree-Fock, MRSD-CI and DFT calculations

The spherically averaged momentum profiles for the highest occupied molecular orbitals of PF3 and P(CH3)(3) have been obtained by electron momentum spectroscopy. The measurements provide a stringent test of basis set effects and the quality of ab-initio methods in the description of these larger molecular systems. As in previous work on the methyl-substituted amines, intuitive arguments fail to predict the correct amount of s- and p-type contributions to the momentum profile while delocalized molecular orbital concepts provide a more adequate description of the HOMOs. The experimental momentum profiles have been compared with theoretical momentum profiles calculated at the level of the target Hartree-Fock approximation with a range of basis sets. New Hartree-Fock calculations are also presented for the HOMO of PH3 and compared to previously published experimental and theoretical momentum profiles. The experimental momentum profiles have further been compared to calculations at the level of the target Kohn-Sham approximation using density functional theory with the local density approximation and also with gradient corrected (non-local) exchange correlation potentials. In addition, total energies and dipole moments have been calculated for all three molecules by the various theoretical methods and compared to experimental values. Calculated `density difference maps' show the regions where the HOMO momentum and position electron densities of PF3 and P(CH3)(3) change relative to the corresponding HOMO density of PH3. The results suggest that methyl groups have an electron-attracting effect (relative to H) on the HOMO charge density in trimethyl phosphines. These conclusions are supported by a consideration of dipole moments and the P-31 NMR chemical shifts for PH3, PF3 and P(CH3)(3).