Ab Initio Structures of Phosphorus Acids and Esters. 1. Phosphinic, Phosphonic and Phosphoric Acids.

摘要

The molecular structures of all stable conformations of phosphinic, phosphonic, and phosphoric acid have been computed by employing complete geometry optimization in a variety of basis sets. Phosphinic acid, the species with the fewest torsional degrees of freedom, was examined in detail to determine the origin of forces giving rise to its preferred conformation. These forces are due almost entirely to electrostatic effects such as dipole-dipole iterations, with stereoelectronic anomeric and steric effects each being at most 12% as large. Bond lengths and angles generally agree with the limited prior theoretical and experimental values. In each acid one or more additional higher energy stable conformations are found in the STO basis set, but these usually do not appear when larger basis sets are employed. All three acids exhibit a minimum-energy conformation in which the acidic hydrogens are oriented toward the phosphoryl oxygen and a distance from it, typically 2.7-2.8 A, which is nearly the same in each case. The conformations of all three acids may be described solely on the basis of dipole-dipole interactions plus relatively small internal hydrogen bonding and steric effects. In contrast to earlier studies little conformational evidence of anomeric effects is found in any of these compounds. (Author)