Orbital interactions in stable and metastable conformations of the dimethylphosphate anion

Natural bond orbital (NBO) theory has been applied to analyze stereoelectro nic preferences of the gg, tg and tt stationary states and two connecting t ransition states of the dimethylphosphate (DMP-) anion. In going from the c ompact gg to the extended tt state, the O-a-P-O-a angle closes as phosphory l anionic oxygen, P-O-a, bonds are weakened by negative hyperconjugation. P hosphoryl ester oxygen, P-O-e, bonds are strengthened, however, due to incr eased pi -overreach, largely a result of delocalization of ester oxygen lon e pair density. In a 'closing scissors effect', contraction of the O-e-P-O- e angle between these stronger bonds also results, in this case due to the dominance of repulsive forces among the lone pairs. Counterintuitive arrang ements in the transition states between gg and tg, and between two equivale nt, twisted tt stationary states result, again, from dominant repulsions of oxygen lone pairs. Complexation of DMP- with water, Na+, or Mg+2 ions is a ccompanied by significant charge transfer to the ligand, thus imparting a d egree of covalency to the anion-ligand bond. H-bonds between water and the two O-a oxygens lead to delocalization of charge through lone pairs at the docking site of DMP- into sigma*(O-w-H) antibonds. For the ion-pairs, charg e is transferred by a similar mechanism into Rydberg, orbitals on the catio n. Rearrangement of electron density within DMP- in the complexes replenish es losses from O-a lone pairs and increases the magnitude of the anomeric e ffect involving O-e lone pairs. NBO theory provides a quantitative descript ion of the complex balance of interactions that dictate the conformational features of this biologically significant molecular functionality. (C) 2001 Elsevier Science B.V. All rights reserved.