NMR study of two multiple-conformer models of vitamin E

What is it about?

Overall conformations of both anomeric per-O-acetylated glucosyl derivatives of 2,2,5,7,8-pentamethylchroman-6-ol were studied in the context of their high conformational flexibility, based on NMR spectra in CDCl3 solution and related DFT as well as MP2 calculation results. The solvent effect was simulated using an IEF-PCM schema using the UFF radii; its other variants, including the use of IDSCRF radii, were applied for a few compact B3LYP-GD3BJ optimized structures showing a small imaginary vibrational frequency. All the main DFT findings were confirmed by additional results obtained at the MP2 level of theory.

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Why is it important?

Of the four DFT methods tested, only the application of the B3LYP/6-31+G(d,p) approximation provided the best outcome in the sets of 7 to 8 single forms for which the population-average values of the computed NMR parameters (δH, δC and some nJHH data) were in close agreement with those found experimentally. Three relevant statistical indices, i.e. Pearson's correlation coefficient square (r2), corrected root mean square error (CRMSE equal to RMSE) and corrected mean absolute error (CMAE), were used throughout the article as estimates of the uncertainty of the results. A higher value of r2 was considered the best indicator of a better fit between the correlated data sets.

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