Qualitative characterization of the P-C bonds in ylides of phosphorus

The aim of this paper is qualitative understanding of the behavior of electrons in the ylides of phosphorus by means of the sharing amplitudes and the volume-point sharing indices (Fulton, R. L. J. Phys. Chem. 1993, 97, 7516). This paper supplements a previously carried out quantitative analysis (Mitrasinovic, P. M. J. Comput. Chem. 2001, 22, 1387-1395) of the P-C bonds in the ylides of phosphorus that was based solely on the sharing indices. The sharing amplitude and the sharing indices are quantitative measures of the degree to which an electron, as a wave, is shared between two spatial points in a many electron system. As a quantitative measure of the degree of sharing of an electron between two disjoint regions, the delocalization index (Fulton, R. L. J. Phys. Chem. 1993, 97, 7516) is used to display the shell structure in phosphorus. The delocalization shell structure displays spatial regions from which electrons are greatly delocalized as well as regions in which electrons are essentially localized. By varying the positions of the fixed points in the valence region of the phosphorus, the sharing amplitudes exhibit a more diffuse character on the phosphorus and are tightly placed on the carbon in the complexes PH 3 CH 2 and PHCH 2 . Since the nodes formed in the sharing amplitudes appear only as remnants of nodes in the volume-point sharing indices, the circular shape of the minimum of the volume-point sharing index from the carbon atom in the vicinity of the phosphorus in PH 3 CH 2 , having the radius of about 1.7 au (within the valence shell of the phosphorus), indicates a remnant of a 3s or a 3p contribution. For the positions of the fixed points along the P-C bonds, the shapes of the nodal lines of the sharing amplitudes indicate an increase of the 2p character on the carbon in PH 3 CH 2 in comparison to that in PHCH 2 . The increase of 2p character on the carbon in PH 3 CH 2 is in agreement with the values of the P-C interbasin sharing indices of 0.92 and 1.30 for PH 3 CH 2 and PHCH 2 , respectively. The values of 0.87 and 1.54 correspond to a classical C-C single bond and a classical C=C double bond (Mitrasinovic, P. M. J. Comput. Chem. 2001, 22, 1387-1395) (see Tables 1-3 given below). The volume-point sharing index from the carbon atom, which describes the sharing emanating from a basin microscopically, shows the delocalization of an electron over the carbon, phosphorus and hydrogen (bonded to the phosphorus) basins in PHCH 2 and PH 3 CH 2 . This delocalization was reflected in ref 2 by the values of 0.13 and 0.17 for the sharing indices between the carbons and hydrogens on phosphorus in PH 3 CH 2 and PHCH 2 , respectively. This substantial delocalization of an electron over the carbon, phosphorus and hydrogen basins in PH 3 CH 2 and PHCH 2 indicates the special stabilization of the carbanion. The electron density does not mimic the volume-point sharing index from the carbon basin in PH 3 CH 2 . The volume-point sharing index from the phosphorus basin resembles the electron density in PH 3 CH 2 within the core of the phosphorus basin. The volume-point sharing index from the carbon basin in PHCH 2 displays a shape very similar to that in PH 3 CH 2 qualitatively, especially in terms of the electron delocalization over the C, P, and H (attached to the phosphorus) basins.