13C NMR Investigation of Electronic Interactions in 5-Substituted 1-Naphthonitriles

✍ Scribed by Ingeborg I. Schuster

• Publisher: John Wiley and Sons
• Year: 1996
• Tongue: English
• Weight: 809 KB
• Volume: 34
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/(sici)1097-458x(199604)34:4<301::aid-omr879>3.0.co;2-6

✦ Synopsis

Carbon-13 NMR chemical shifts of 5-Z-substituted l-naphthonitriles (1; Z = H, F, CI, Br, NH, , NMe,, CN, NO,, OMe, CHO, C0,Me) in deuteriochloroform and in neat trifluoroacetic acid (TFA) are reported. The C N 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 x polarization of the C N multiple bond, due primarily to differences in the throughspace field effects of the various Z. The effect diminishes for 1 in neat TFA because of the greater contribution of dipolar ArC+=N-to the resonance hybrid. Deviations of the aromatic carbon shifts from substituent chemical shift additivities are small, yet show distinct patterns for many of the carbon resonances. The deviations of the C-1-CN &so 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 C N 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 electrondonating groups, Z, in response to the CN-induced charge depletion within the adjacent aromatic ring. From the location of data for 5-methoxy-l-naphthonitrile (1; Z = QMe) 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.