The Optical Rotation of Glucose Prototypes: A Local or a Global Property?

Finding reliable methodology for optical rotation and correct predictions of (s)-methyloxirane and (1R,5R)-β-pinene

Theoretical calculations of optical rotation (OR), although important to predict absolute configurations (ACs) and corroborate experiments, require efficient methodology able to reproduce enantiomer specificity and real OR values. Also, troublesome molecules are recurring in the literature, such as (S)-methyloxirane and (1R,5R)-β-pinene. This study evaluates DFT functionals B3LYP, CAM-B3LYP, ωB97X-D, M06-2X, and PBE0 considering basis sets aug-cc-pVDZ, aug-cc-pVTZ, 6-311++G(2d,p), and 6-311++G(3df,2p) in OR prediction of 42 rigid organic molecules assessing cases with wrong enantiomeric determination comparing to available experimental data at wavelengths 355, 589, and 633 nm. Functionals CAM-B3LYP and ωB97X-D with aug-cc-pVTZ are indicated here to reproduce experimental values more accurately considering fewer number of wrong AC predictions, normalized RMSD values below 0.70, and a good approximation to experimental values in hierarchical cluster analysis. Methyloxirane AC was reproduced in CAM-B3LYP and PBE0, with [ α ] 355 = 6 . 94 $$ {\left[\alpha \right]}_{355}=6.94 $$ for CAM-B3LYP/aug-cc-pVTZ close to experimental value [ α ] 355 = 7 . 49 ± 0 . 30 $$ {\left[\alpha \right]}_{355}=7.49\pm 0.30 $$ . Good results were found for AC of β-pinene in M06-2X, CAM-B3LYP, and ωB97X-D while the latter in 6-311++G(3df,2p) obtained OR values of [ α ] 589 = 3 . 44 $$ {\left[\alpha \right]}_{589}=3.44 $$ and [ α ] 689 = 4 . 20 $$ {\left[\alpha \right]}_{689}=4.20 $$ close to experimental values [ α ] 589 = 2 . 8 $$ {\left[\alpha \right]}_{589}=2.8 $$ and [ α ] 689 = 4 . 66 ± 0 . 60 $$ {\left[\alpha \right]}_{689}=4.66\pm 0.60 $$ . The two molecules aforementioned are, for the first time, reported to give valid theoretical OR values in such simple methodologies. OR calculations were all performed after geometry optimization at the same level of theory, and analysis of different functional combinations for each step in β-pinene showed it can interfere with AC prediction even in rigid molecules.

Specific rotation of monosaccharides: a global property bringing local information

Carbohydrates generally occur in several conformations that may differ among themselves by energy values that are smaller than the accuracy of the most sophisticated theoretical methods used to determine them. In addition, the preferential orientations of the hydroxyl groups of these molecules cannot be identified by any experimental technique. Therefore, a method that is able to validate the absolute conformations (i.e., consisting of the orientations of the hydroxyl groups) of carbohydrates would be helpful to improve our knowledge about monosaccharides. SR has been used for this purpose, and here, we present a test to measure the specific rotation (SR) ability of a molecule that possesses not only many conformations, but also four adjacent chiral centers. The results show that the final SR value is a weighted average of a global property (obtained for each conformation), and the latter by its turn is influenced by each chiral center in a multi chiral system. By comparing the SR values calculated for the most abundant anomers of xylopyranose with those of the corresponding monochiral analogs obtained by saturation of three different chiral centers each time, the influence of each center on the global property is confirmed.

Specific rotation as a property to validate monosaccharide conformations

Specific rotation ([α](D)) values were calculated for the 15 conformations of xylopyranose that are the most stable in the gas phase and in aqueous solution. The effects of different theoretical descriptions and the medium on the geometry of the conformers and the [α](D) values are evaluated. Differences in [α](D) values found for the same conformer in all descriptions used were smaller than those found between any two different conformers in the same description. The difference between [α](D) values is prominent, even for two conformations that are distinguished from each other only by the orientation of one secondary hydroxyl group. This finding suggests that [α](D) values may potentially be used in the validation of monosaccharide conformations that are theoretically sampled.