Analysis of substituent effects on31P 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’ δ(31P) relationship with the substituent electronegativity sum, ∑ EN (∑i=13 EN(Ei); downfield shift with increasing ∑ EN) is substantiated for experimentally important molecules by the ab initio results. The ‘inverse’ δ(31P)–∑ EN trend obtained for X=F is related to negative hyperconjugation (Lp(X)→σ* (P—X′) orbital interaction). The P(OH)2Y molecules display ‘normal’ ∑ EN–shift relationships when the electronegativity of Y is low and ‘inverse’ when the electronegativity of Y is high. For some PX2Y phosphanes with simple substituents, the δ(31P) values are well related to ∑ EN. The best correlation (cc: 0.955) between δ(31P) and ∑ EN is found for the set of monosubstituted phosphanes, PH2X. The δ(31P)–EN(Y) slope (in ppm per EN ‘unit’), ranges from 162 (X=BeH), 141 (X=H) and 98 (X=CH3 and X=BH2) to -105 (X=F). The δ(31P) values of the PX2Y molecules are representative for the complete set of PXYZ with simple substituents. Since there is no ‘simple, general relationship’ of δ(31P) for phosphanes with less or much more electronegative substituents, EN(E)