Pentacoordinated phosphoranes, PH4X, substituted with a full range of firstand 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 firstand 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. meinschaft, Fonds der Chemischen Industrie, and the Convex Computer Corporation. Y. Zhang was supported by an Alexander von Humboldt Foundation fellowship. We thank H. Xie for technical assistance.
Geometry optimizations at the HF/3-21G(*) and HF/6-31G* levels of ab initio theory have been carried out for various isomers of model disubstituted phosphoranes PH:>XY(X, Y=OH, CH3, NH2, and SH). Reasonable agreement was obtained between the optimized geometries and available crystal structure data for analogous compounds. The isomers were further characterized by frequency calculations. The MP2/6-31G*//6-31G* + ZPE energy data reveal that the interactions between the ligands are relatively small (0-4 kcal mol-') for the most stable conformations of the isomers. Hence, for these conformations the apicophilicities (based upon monosubstituted phosphoranes) are approximately additive. The less stable PHuXY conformations are in general transition states or higher-order saddle points, and their interligand interactions are larger in magnitude (up to 10 kcal mol-'); the results with these conformations suggest that apicophilicities may not be as additive for some highly substituted phosphoranes.
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