Multi-conformer molecules in solutions: an NMR-based DFT/MP2 conformational study of two glucopyranosides of a vitamin E model compound
Data
2016Autor
Nazarski, Ryszard Bolesław
Wałejko, Piotr
Witkowski, Stanisław
Metadata
Pokaż pełny rekordStreszczenie
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 flexibility, on the basis of NMR spectra in CDCl3
solution and related DFT calculation results. A few computational protocols were used, including diverse
density functional/basis set combinations with a special emphasis on accounting (at various steps of the
study) for the impact of intramolecular London-dispersion (LD) effects on geometries and relative Gibbs
free energies (ΔGs) of different conformers coexisting in solution. The solvent effect was simulated by an
IEF-PCM approach with the UFF radii; its other variants, including the use of the recently introduced
IDSCRF radii, were employed for a few compact B3LYP-GD3BJ optimized structures showing one small
imaginary vibrational frequency. The advantage of using IDSCRF radii for such purposes was shown. Of
the four tested DFT methods, only the application of the B3LYP/6-31+G(d,p) approximation afforded
ensembles of 7–8 single forms for which population-average values of computed NMR parameters (δH,
δC and some nJHH data) were in close agreement with those measured experimentally; binuclear (δH,C
1 : 1) correlations, rH,C
2 = 0.9998. The associated individual ΔG values, corrected for LD interactions by
applying Grimme’s DFT-D3 terms, afforded relative contents of different contributors to the analyzed
conformational families in much better agreement with pertinent DFT/NMR-derived populations (i.e.,
both data sets were found to be practically equal within the limits of estimated errors) than those calculated
from dispersion uncorrected ΔGs. All these main findings were confirmed by additional results
obtained at the MP2 level of theory. Various other aspects of the study such as the crystal vs. solution
structure, gg/gt rotamer ratio, diagnostic (de)shielding effects, dihydrogen C–H⋯H–C contacts, and
doubtful applicability of some specialized DFT functionals (M06-2X, ωB97X-D and B3LYP-GD3BJ) for the
description of highly flexible molecules are also discussed in detail.
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