Conformational behavior of simple furanosides studied by optical rotation

Abstract

Experimental and theoretical specific optical rotations (OR) of anhydro, epithio, and epiminoderivatives of methyl tetrofuranosides in chloroform solutions have been compared and used as a tool for exploring their conformational behavior. The potential energy surfaces of these saccharides with reduced flexibility were examined with the density functional theory and the MP2 and CCSD(T) wavefunctions methods. Theoretical ORs were obtained by Boltzmann averaging of values calculated for local minima. Resultant rotations could be used to assess the quality of the DFT and MP2 relative conformer energies. OR values calculated for equilibrium geometries in vacuum were significantly improved when the solvent was accounted for by a polarizable continuum model and first and diagonal second OR derivatives were used for an anharmonic vibrational averaging. The DFT used as a default method reproduced the experimental data fairly well. A modified B3LYP functional containing 70% of HF exchange further improved the results. Because of the strong dependence of OR on the conformation, not only the absolute configuration could be determined, but also the conformational populations were estimated. Likewise, the predicted dependence of OR on the light wavelength well agreed with experiment. The increasing precision of the contemporary computational methods thus makes it possible to relate the specific rotation to more detailed features in molecular structure.

Vibrational averaging of the chemical shift in crystalline α‐glycine

“…For 300 K, the Boltzmann quantum kT ∼ 200 cm −1 , and only the lowest mode anharmonic contribution (dominated by σ ii ) significantly changes with the temperature. On the other hand, no rule about the total contribution of individual modes to the vibrational averaging of the crystal NMR parameters is apparent, which is consistent with previous NMR[32] and ORD[79] averaging studies. In practical computations, we thus need to consider all fundamental molecular or crystal cell vibrations.…”

Polarimetry as a Tool for the Study of Solutions of Chiral Solutes

“…The consistent Boltzmann population values obtained with both basis sets is noticeable. Therefore, for the fourth time, [28][29][30] this approach was proven to be useful to validate conformational sampling of carbohydrates and carbohydrate derivatives.…”

“…Each conformer contributes to the final property value, and this latter value was calculated as a weighted average of the individual values. [28][29][30] All calculations were performed with Gaussian 03 computational code.…”

“…[55][56][57] Although their arguments may have proper foundations in the systems they have in-vestigated, the SR values obtained theoretically for conformers of some monosaccharides have been shown to correspond to the experimental values. [28][29][30]…”

The importance of orientation of exocyclic groups in a naphthoxyloside: A specific rotation calculation study

“…Recently, the possibility of using a theoretical description to reproduce specific rotation (SR or [ α ] 589 ) values of monosaccharides in aqueous solution has been suggested in the literature to validate monosaccharide conformational sampling . This procedure has been proven to be effective for glucose, xylose, levoglucosan, and other carbohydrate systems …”