Conformational analysis from statistical treatment of 13C NMR chemical shifts

• We study the conformational equilibria of several aliphatic and aromatic compounds.
• We calculate shielding constants for all conformers of conformational equilibria.
• We do regressions between experimental 13C NMR chemical shifts and shielding constants.
• Regressions allow the estimation of the conformational equilibrium populations.
• The results are in agreement with calculated energies and published data.

Statistical treatment of experimental 13C NMR chemical shifts of different compounds and calculated isotropic shielding constants of their respective conformers has been carried out in two different ways. The first method was a ridge linear regression between experimental chemical shifts and calculated shielding constants where the calculated coefficients represent the mole fraction of each conformer. Consequently, the sum of all coefficients has to be restricted to 1. The second method was a linear regression between experimental and calculated internal chemical shifts using the same restriction. In general, both methods gave similar results although the second one had a larger standard deviation. The results showed that, in nearly all cases, there is a significant correlation between experimental and calculated data for, at least, one conformer, being this the major one present in the conformational equilibrium. For planar aromatic compounds the conformational equilibrium has been fully characterized when other conformers have been found statistically significant. The statistical analysis on cyclic aliphatic molecules always yielded a conformational composition comparable with the published values. The advantages and drawbacks of the methodology are discussed.