Correlation of substituent – inducted chemical shifts of aromatic protons: Substituted benzonitriles and methyl benzoates

Abstract

Proton NMR spectra in deuteriochloroform are reported for 53 meta- and para-substituted benzonitriles and 61 methyl benzoates. The substituent-induced chemical shifts (SCS) were correlated with dual substituent parameters (DSP), with 13C SCS of the adjoining carbon, and with the other 1H SCS using the principal component analysis (PCA). They are controlled by different factors in each position to the variable substituent. In the position 4 the long-range polar effects, as expressed by DSP, are decisive, the conjugative component being more important. Remarkable is also very close correlation of 13C and 1H SCS in this position which has no analogy in the other position. In the position 3 there is an additional specific effect of heavier halogens. In the position 2 the short-range effect can be quantified by special constants, only for a subgroup of simple donor substituents it is proportional to DSP. In all positions SCS are practically independent of the second substituent, so that the total chemical shifts can be calculated by an additive scheme. The coupling constants J(2,3), J(5,6), and J(2,6) depend on substitution in the same manner, probably are controlled mainly by substituent electronegativity.