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Natural abundance 15 N chemical shifts in substituted benzamides and thiobenzamides
Author(s) -
Brownlee R. T. C.,
Sadek M.
Publication year - 1986
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/mrc.1260240917
Subject(s) - chemistry , substituent , chemical shift , nitrogen , oxygen , ab initio , resonance (particle physics) , ab initio quantum chemistry methods , computational chemistry , crystallography , stereochemistry , molecule , organic chemistry , atomic physics , physics
15 N Substituent chemical shifts (measured at natural abundantc at 0.4 M concentration) and calculated ( ab initio , STO‐3G) electron densities at the nitrogen and oxygen (or sulphur) are reported for series of para and meta ‐substituted benzamides and thiobenzamides with NMe 2 , NH 2 , OMe, NHCOMe, F, Cl, Br, I, Me, H, COMe, COOMe, CF 3 , CN, NO 2 , CONH 2 and CSNH 2 substituents. The 15 N shift are very sensitive to the substituent effects. The dual substituent parameter method shows excellent correlations of 15 N shifts against substituent field and resonance effects, with the sensitivity in the thiobenzamides being double that in the benzamides. In contrast, calculated π‐electron densities at the nitrogen show the same sensitivity to substituents in both series. The 15 N shifts correlate well with the calculated π‐electron densities at the nitrogen; however, estimates of the shielding based on local electron densities (described in Pople's independent electron model) cannot account for the increased sensitivity to the substituent effects observed for 15 N chemical shifts of thiobenzamides. It is suggested that non‐local shielding from sulphur and oxygen are responsible for the chemical shift trends.