The Effects of the First- and Second-Row Substituents on the Structures and Energies of PH4X Phosphoranes. An ab initio Study

: Pentacoordinated phosphoranes, PH4X, substituted with a full range of first- and second-row groups have been studied with ab initio computations. The apically and equatorially substituted trigonal-bipyramidal (TB) and apically substituted square-pyramidal (SP) conformations were fully optimized with the 3-21G(*) and 6-31G* basis sets. Frequency calculations were performed at 6-31G* on the optimized structures. Correlation corrections through the MP4 level were carried out on these 6-31G* structures. Energies calculated with zero-point energy corrections, i.e., at MP4SDTQ/6-31G* + ZPE, provide relative energies of various isomers. The apically substituted SP structures for PH4X (X = Li, Na, BeH, and MgH) are the most stable. The intrinsic apicophilicities of the first- and second-row groups are derived and are compared to available experimental data. The apicophilicities (in kcal mol"1) are OH (0.4) > SH (-0.1) > CH3 (-0.9) > PH2 (-3.3) > NH2 (-7.2) > SiH3 (-8.6). Due to their high degree of ion pair character, PH4F and PH4C1 are unsuitable as models for relative energy comparisons, interaction energies were evaluated by calculating various PH4X (X = NH2, PH2, OH, SH) conformations. Natural bond orbital (NBO) analysis on these conformers shows that the interaction is due chiefly to nx — _ . Inductive and bonding contributions are estimated for the first-row group substituents. The effects of substitution (e.g., relative energies) that are dominated by inductive interactions correlate linearly with group electronegativities. where F is the Fock operator and e, and are NBO orbital energies. We will use NBO analysis to examine the role of d orbitals in PF15 and the effects of some substituents. The conclusions quantify concepts established by previous workers. Table lists the natural charges of the PH4X species. These are based on improved natural (localized) orbitals and overcome the large basis set sensitivity that mars Mulliken population analysis. In this study, we consider these fundamental structural types: the apically substituted TP structure a, the equatorially substituted TP structure e, and the apically substituted SP structure s. The combination of the atomic number with these symbols represents the corresponding substituted conformation (for example, 3a, 3e, 3s for PH4Li). For some substituents, other conformations also were investigated, for example, to examine ir-bonding effects. Abstract: A new molecular similarity index, called the number of overlapping electrons (NOEL), is proposed. This similarity index can be computed very rapidly from the natural orbitals and their occupation numbers of the molecules under comparison. The low computational cost makes it possible to optimize the mutual orientation of molecules by maximizing NOEL. The magnitude of NOEL is related to the number of electrons in the molecular fragment common to both molecules. The new approach is illustrated on the examples of benzene, aniline, nitrobenzene, and 4-nitroaniline molecules and the acetate, isoxazole 3-oxide and isoxazole 5-oxide anions.