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13 C NMR Investigation of Electronic Interactions in 5‐Substituted 1‐Naphthonitriles
Author(s) -
Schuster Ingeborg I.
Publication year - 1996
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(199604)34:4<301::aid-omr879>3.0.co;2-6
Subject(s) - chemistry , substituent , trifluoroacetic acid , chemical shift , electronic effect , resonance (particle physics) , stereochemistry , organic chemistry , steric effects , atomic physics , physics
Carbon‐13 NMR chemical shifts of 5‐Z‐substituted 1‐naphthonitriles (1; Z = H, F, Cl, Br, NH 2 , NMe 2 , CN, NO 2 , OMe, CHO, CO 2 Me) in deuteriochloroform and in neat trifluoroacetic acid (TFA) are reported. The CN carbon shifts are found to correlate well with the dual substituent parameters (DSPs). Negative values of the transmission coefficients in the DSP correlation give evidence of a reverse substituent electronic effect, which is associated with variations in π polarization of the CN multiple bond, due primarily to differences in the through‐space field effects of the various Z. The effect diminishes for 1 in neat TFA because of the greater contribution ofdipolar ArC + ξN − to the resonance hybrid. Deviations of the aromatic carbon shifts from substituent chemicalshift additivities are small, yet show distinct patterns for many of the carbon resonances. The deviations of the C‐1—CN ipso carbon shifts of 1 in neutral solvents and in TFA correlate roughly with the DSPs. They are attributable to changes in charge density at C‐1 that arise as a consequence of substituent‐induced changes in the polarity of the CN bond. The greater than expected shielding that is observed for the C‐6 and C‐8 resonances accords with reduced electron withdrawal by + R substituents and increased mesomeric activity by electron‐donating groups, Z, in response to the CN‐induced charge depletion within the adjacent aromatic ring. From the location of data for 5‐methoxy‐1‐naphthonitrile (1; Z = OMe) in the chemical shift correlations of 1 in neat TFA, one can conclude that the methoxy group of this compound, unlike that of 1‐methoxynaphthalene itself, is not significantly hydrogen‐bonded by TFA.