31P chemical shielding as a function of bond angle: phosphorus chemical shielding surface of phosphinoborane R2PBR'2

Abstract

The phosphorus chemical shielding surface of phosphinoborane R2PBR'2 has been investigated via molecular orbital theory calculation and experimental measurement of selected derivatives. Ab initio calculations of phosphorus chemical shielding tensors were determined for the phosphinoboranes H2PBH2 and (CH3)2PB(CH3)2. Changes in the angle from planarity, i.e., that between the P-BR'2 plane and the bisector of the RPR angle, are reflected in the orientations and magnitudes of the three principal components of the phosphorus chemical shielding tensor. To determine the validity of the calculated phosphorus chemical shielding surface, three phosphinoborane compounds with different angles from planarity were synthesized and studied by solid-state 31P NMR spectroscopy. The 31P NMR powder patterns provided experimental magnitudes of the principal components of the phosphorus chemical shielding tensor for each compound, which compared well to the calculated predictions; where orientations of these tensors could be determined experimentally, they also agreed well with the calculated results. The combined experimental and theoretical results provide a good description of the effects on changes in bond angle on phosphorus chemical shielding as the molecule is distorted from a planar to folded geometry.Key words: chemical shielding, solid-state NMR, phoshinoborane, ab initio calculations.