An investigation of the pyranose ring interconversion path of α-l-idose calculated using density functional theory

The interconversion pathways of the pyranose ring conformation of α-l-idose from a 4C1 chair to other conformations were investigated using density functional calculations. From these calculations, four different ring interconversion paths and their transition state structures from the 4C1 chair to other conformations, such as B3,O, and 1S3, were obtained. These four transition-state conformations cover four possible combinations of the network patterns of the hydroxyl group hydrogen bonds (clockwise and counterclockwise) and the conformations of the primary alcohol group (tg and gg). The optimized conformations, transition states, and their intrinsic reaction coordinates (IRC) were all calculated at the B3LYP/6-31G∗∗ level. The energy differences among the structures obtained were evaluated at the B3LYP/6-311++G∗∗ level. The optimized conformations indicate that the conformers of 4C1, 2SO, and B3,O have similar energies, while 1S3 has a higher energy than the others. The comparison of the four transition states and their ring interconversion paths, which were confirmed using the IRC calculation, suggests that the most plausible ring interconversion of the α-l-idopyranose ring occurs between 4C1 and B3,O through the E3 envelope, which involves a 5.21 kcal/mol energy barrier.

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