ANALYSIS OF SUBSTITUENT EFFECTS ON P-31 NMR CHEMICAL-SHIFTS - PX2Y MOLECULES

Tricoordinate phosphorus NMR chemical shifts are computed (GIAO/6-311 + G*//RMP2(fc)/6-31 + G*) and analyzed for PX2Y molecules (X, Y = EHn : F, OH; NH2, CH3, BH2, BeH, Li and H = 'simple first row substituents '), the 'normal' delta(P-31) relationship with the substituent electro negativity sum, Sigma EN (Sigma(i=1)(3), EN(E-i); downfield shift with increasing Sigma EN) is substantiated for experimentally important mo lecules by the ab initio results. The 'inverse' delta(P-31)-Sigma EN t rend obtained for X = F is related to negative hyperconjugation (Lp(X) --> sigma (P-X') orbital interaction). The P(OH)(2)Y molecules displ ay 'normal' Sigma EN-shift relationships when the electronegativity of Y is low and 'inverse' when the electronegativity of Y is high. For s ome PX2Y phosphanes with simple substituents, the delta((31) P) values are well related to Sigma EN. The best correlation (cc: 0.955) betwee n delta(P-31) and Sigma EN is found for the set of monosubstituted pho sphanes, PH2X. The delta(P-31)-EN(Y) slope (in ppm per EN 'unit'), ran ges from 162 (X = BeH), 141 (X = H) and 98 (X = CH, and X = BH2) to - 105 (X = F). The delta(P-31) values of the PX,Y molecules are represen tative for the complete set of PXYZ with simple substituents. Since th ere is no 'simple, general relationship' of delta(P-31) for phosphanes with less or much more electronegative substituents, EN(E) < EN(P) mu ch less than EN(E), consequently, it does not exist in the complete se t. (C) 1998 John Wiley & Sons, Ltd.