APICOPHILICITIES OF SUBSTITUTED OXYPHOSPHORANES - [P(OH)(4)X, PO-(OH)(3)X]

Axial- and equatorial-substituted pentacoordinate oxyphosphoranes, P(O H)(4)X, and oxyphosphorane monoanions, PO-(OH)(3)X, where X = OH, H, F , CH3, and NH2, have been studied using ab initio quantum chemical met hods, leading to complete optimized structures, total energies, and ha rmonic vibrational frequencies for the most stable conformer of each c ompound. Thermodynamic results including gas-phase apicophilicities fo r each substituent as a function of protonation state along with estim ates of the gas-phase proton affinity for each substituted oxyphosphor ane monoanion are also presented. Split-valence basis sets ranging in quality from 3-21+G(d) to 6-311+G(d,p) are employed, and the effects o f electron correlation on the structure and thermodynamics are estimat ed through the use of Moller-Plesset perturbation theory [MP2-MP4(SDTQ )]. The effect of aqueous solvation on the gas-phase apicophilicities is also estimated through the use of a continuum reaction field model. Final predictions for the gas-phase apicophilicities (kJ mol(-1)) det ermined at the MP4(SDTQ) 6-31+G(d)/MP2 6-31+G(d)+ZPVE level are F(+25. 5) > OH(0.0) > H(-5.7) > CH3(-17.7) > NH2(-28.0) for the neutral oxyph osphoranes and F(+30.2) > H(+1.6) approximate to [CH3] approximate to OH(0.0) > NH2(-18.7) for the oxyphosphorane monoanions. Certain substi tuted oxyphosphoranes, including X = Cl and SH, are found to be unstab le with respect to dissociation and thus are not considered. The effec t of aqueous solvation, as estimated by a continuum reaction field, is modest and does not change the overall apicophilicity ordering signif icantly in either the neutral or anionic systems. Comparison is made w ith the corresponding hydrogen phosphoranes as well as the empirically based model of Holmes.