Carbon-13 and proton NMR parameters of substituted aromatics and steriods

Abstract

The 1H NMR spectra of a series of l-methoxy-4-X-naphthalenes have been determined in dilute solutions of carbon tetrachloride. The chemical shifts ortho to the varying substituent correlate well with the empirical parameter, Q; whereas, the peri chemical shifts show a complex behavior which is discussed in terms of electronic, anisotropic, and steric contributions. The other chemical shifts in the unsubstituted ring are relatively insensitive to substituent variations but were shown to correlate with the parameters sigma1,6, sigma1,7, and sigma1,8. Several characteristic inter-ring couplings were discovered and found to be useful in assigning chemical shifts in the unsubstituted ring. The 13C chemical shifts were determined for nine monosubstituted benzenes and eleven ortho substituted fluorobenzenes in 33% cyclohexane solutions. The chemical shift assignments were made by referring to substituent additivity relationships and by observing characteristic 13C-19F coupling constants. The multiple regression analysis of the aromatic chemical shifts with respect to the separated field (F) and resonance (R) parameters were compared to the results obtained from the multiple regression results in relation to F, R, and Q. The F test was applied and it was found that the addition of Q to the regression gave significant increases in the correlation only at the C-X and ortho aromatic carbons. The 1H and 13C chemical shifts ortho to the varying substituent were plotted against one another and deviations from the predicted behavior were discussed in terms of steric inhibition of resonance, sterically induced polarization of the ortho C-H bond, and substituent anisotropy. Unequivocal chemical shift assignments were made for all the resonances in cholesterol, i-cholesterol, epi-i-cholesterol, and i-cholestan-6-one by considering substituent effects, off-resonance coupling data, and the lanthanide shift reagent data from Eu(DPM)3 and Yb(DPM). A computer program designed to correlate the observed pseudo-contact shifts to the molecular coordinates is described and various applications discussed. The induced shifts from Yb(DPM)3 were found to be primarily due to pseudo-contact contributions; whereas, Eu(DPM)3 induced shifts did not lend to computer analysis because of significant contact shift contributions. The Eu(DPM) contact shift magnitudes were analyzed and related to structure characteristics.