A theoretical study of conformational deuterium isotope effects and bond dissociation energies of diastereotopic hydrogens

In the conformations that correspond to the C6-X1-C2-X3-C4 moieties of 1,3-dioxane, 1,3-dithiane, and cyclohexane in their chair conformations, the model systems CH2(OH)2, CH2(SH)2, and gauche-butane contain "axial" and "equatorial" hydrogens. The vibrational frequencies of the diastereomeric monodeuterated CHD derivatives of these model compounds have been computed ab initio at the 6-31G*//6-31G* level of theory, scaled by 0.9, and used to calculate the conformational deuterium isotope effects in these systems, as well as the stretching and bending components of these isotope effects. The results are in very good agreement with the experimental observations and with some of the interpretations given by Anet for the six-membered rings. Using an equation suggested by McKean, the scaled isolated stretching frequencies have been used to predict the bond dissociation energies of the diastereotopic hydrogens of these molecules. The results suggest that the preference for radical abstraction of the axial C2 hydrogen in a 1,3-dioxane will be diminished in cyclohexane, and reversed in a 1,3-dithiane. Key words: stereochemistry, isotope effects, bond dissociation energies.