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Natural bond orbital (NBO) theory has been applied to analyze stereoelectronic preferences of the gg, tg and tt stationary states and two connecting transition states of the dimethylphosphate (DMP−) anion. In going from the compact gg to the extended tt state, the Oa–P–Oa angle closes as phosphoryl anionic oxygen, P–Oa, bonds are weakened by negative hyperconjugation. Phosphoryl ester oxygen, P–Oe, bonds are strengthened, however, due to increased π-overreach, largely a result of delocalization of ester oxygen lone pair density. In a ‘closing scissors effect’, contraction of the Oe–P–Oe angle between these stronger bonds also results, in this case due to the dominance of repulsive forces among the lone pairs. Counterintuitive arrangements in the transition states between gg and tg, and between two equivalent, twisted tt stationary states result, again, from dominant repulsions of oxygen lone pairs. Complexation of DMP− with water, Na+, or Mg+2 ions is accompanied by significant charge transfer to the ligand, thus imparting a degree of covalency to the anion–ligand bond. H-bonds between water and the two Oa oxygens lead to delocalization of charge through lone pairs at the docking site of DMP− into σ∗(Ow–H) antibonds. For the ion-pairs, charge is transferred by a similar mechanism into Rydberg orbitals on the cation. Rearrangement of electron density within DMP− in the complexes replenishes losses from Oa lone pairs and increases the magnitude of the anomeric effect involving Oe lone pairs. NBO theory provides a quantitative description of the complex balance of interactions that dictate the conformational features of this biologically significant molecular functionality. |
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