Topography of the electron density in pyrophosphate bonds

Abstract

Contour maps of the electron density in the phosphorus-oxygen bonds of inorganic ortho- and pyro-phosphate molecules and in the biologically important nucleotide adenosine-5′-diphosphate (ADP) are presented. The densities and overlap populations calculated from approximate molecular orbital methods are used to explain the nature of the so-called “energy-rich” phosphate bond. The P 3d orbitals are able to contribute more to the π bond strength of the terminal P-O bonds than to the P-OP bonds. Equations are introduced for calculating changes in valence electron distributions brought about by molecular formation and for comparing Mulliken populations with spatial distributions of electrons.