CIDNP-Assisted Assignment of ¹³C NMR Lines. Alkyl Phenyl Ketones¹

The reversible abstraction of hydrogen from hydrogen donors by photoexcited aromatic aldehydes and ketones leads to CIDNP in the 13C NMR spectra of these compounds. Additional information gained from the phase of the polarization in the CIDNP spectrum is used to assign the carbon-13 lines of alkyl phenyl ketones. The carbon-13 spectra of the ketones PhCOR with R = n-CxH2x+1, x = 0, 1, …, 6; C-CyH2y-1, y = 3, 4, 5, 6; as well as i-C3H7, i-C4H9, sec-C4H9, and t-C4H9 are completely assigned and their chemical shifts reported. The measurements show that for all ketones listed above the chemical shift of the aromatic ring carbon C-2’ (ortho) is upfield of the C-3’ (meta) regardless of whether R is a primary, secondary, or tertiary alkyl group. Only for benzaldehyde is the resonance of C-3’ found at lower shift values than that of C-2’. The application of the assignment technique to phenolic compounds is also discussed. © 1979, American Chemical Society. All rights reserved.