Intramolecular electric field effect on a (1)J(C,H) NMR spin-spin couplingconstant. An experimental and theoretical study

An experimental study of the effects of intramolecular electric fields on ( 1)J(C,H) coupling constants involving a formyl proton in a series of 5-X-sa licylaldehydes is reported. The particularly large electric field component along the C-c-H-f bond, calculated for X = NO2, leads to a measured substi tuent effect of 7.36 Hz. For other X substituents a linear correlation was found between the calculated substituent electric field component along the C-c-H-f bond and the substituent effect on (1)J(C-c,H-f). To determine if 5-X substituents affect the intramolecular C=O ... H-O hydrogen bond, NBO a nalyses were carried out on 5-NO2- and 5-H-salicylaldehyde, and they were c ompared with those corresponding to benzaldehyde and phenol. The effects of interactions other than the electrostatic one on (1)J(C-c,H-f) couplings w ere ruled out. The experimental results are in agreement with theoretical p redictions published previously. Copyright (C) 1999 John Wiley & Sons, Ltd.