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Analysis of substituent effects on 31 P NMR chemical shifts: PX 2 Y molecules
Author(s) -
Dransfeld Alk,
Ragué Schleyer Paul von
Publication year - 1998
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/(sici)1097-458x(199806)36:13<s29::aid-omr288>3.0.co;2-t
Subject(s) - electronegativity , chemistry , substituent , chemical shift , molecule , crystallography , basis set , stereochemistry , computational chemistry , hyperconjugation , organic chemistry
Tricoordinate phosphorus NMR chemical shifts are computed (GIAO/6–311+G**//RMP2(fc)/6–31+G*) and analyzed for PX 2 Y molecules (X, Y=EH n : F, OH, NH 2 , CH 3 , BH 2 , BeH, Li and H=‘simple first row substituents’), the ‘normal’ δ( 31 P) relationship with the substituent electronegativity sum, ∑ EN (∑ i =1 3 EN ( E i ); downfield shift with increasing ∑ EN ) is substantiated for experimentally important molecules by the ab initio results. The ‘inverse’ δ( 31 P)–∑ EN trend obtained for X=F is related to negative hyperconjugation ( Lp (X)→σ* (P—X′) orbital interaction). The P(OH) 2 Y molecules display ‘normal’ ∑ EN –shift relationships when the electronegativity of Y is low and ‘inverse’ when the electronegativity of Y is high. For some PX 2 Y phosphanes with simple substituents, the δ( 31 P) values are well related to ∑ EN . The best correlation (cc: 0.955) between δ( 31 P) and ∑ EN is found for the set of monosubstituted phosphanes, PH 2 X. The δ( 31 P)– EN (Y) slope (in ppm per EN ‘unit’), ranges from 162 (X=BeH), 141 (X=H) and 98 (X=CH 3 and X=BH 2 ) to ‐105 (X=F). The δ( 31 P) values of the PX 2 Y molecules are representative for the complete set of PXYZ with simple substituents. Since there is no ‘simple, general relationship’ of δ( 31 P) for phosphanes with less or much more electronegative substituents, EN (E)< EN (P)≪ EN (E), consequently, it does not exist in the complete set. © 1998 John Wiley & Sons, Ltd.