A. Dransfeld et Pv. Schleyer, ANALYSIS OF SUBSTITUENT EFFECTS ON P-31 NMR CHEMICAL-SHIFTS - PX2Y MOLECULES, Magnetic resonance in chemistry, 36, 1998, pp. 29-43
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.