Electronic and spatial structure of dimethyl orthophosphate and orthophosphate

The applicability of different variants of the semiempirical CNDO method to calculate the electronic structure of different conformations of all possible ionic forms of dimethyl orthophosphate and orthophosphate is considered. The CNDO/BW (Boyd-Whitehead parameterization) approximation with selected parameters for the P-O bond is shown to provide the best qualitative and sometimes quantitative agreement with ab initio methods. The dependences of energy and P-O bond strengths in P-O-CH₃ (P-O-H) groups on torsion angles at P-O bonds are obtained in this approximation for dimethyl orthophosphate and orthophosphate. The rotation of these groups is found to stremgthen one P-O bond stronger and labilize another. The energy minima of dimethyl orthophosphate and orthophosphate anions are shown to correspond to conformations where the strengths of the studied P-O bonds are almost the same, i.e., none of the bonds is weakened to a minimum. Protonation of these compounds increases the strength of P-O bonds and decreases the dependence of bond strength on torsion angles. The di-and trianions of dimethyl orthophosphate and orthophosphate are also studied. The growth of negative charge is shown to progressively weaken the P-O bond. The dependence of bond strength on torsion angle for the dianion is less pronounced than that for the monoanion. Calculation results are compared with experimental data known from literature. The significance of the data obtained for revealing essential features of the enzyme cleaving and forming the P-O bond is discussed.