Theoretical Studies of Ribose and Its Radicals Produced by Hydrogen Abstraction from Ring Carbons

The energies and structures of ribose and its radicals produced by hydrogen abstraction from each of the four ribose ring carbons were studied by ab initio quantum chemical methods including geometry optimization at the HF/6-31G level. Two types of the sugar ring pseudorotational states (N-type and S-type) and different orientations of the hydroxyl groups on C2 and C3 were identified. Three energy minima are found for the N-type conformation of ribose with different orientations of the two hydroxy groups while only two energy minima are found for the S-type conformation. The N-type pseudorotamer is more stable than the S-type. The radicals formed by H abstraction from the carbons of ribose all show two forms that can be classified as N-type and S-type conformers. The ring of the radicals is flattened compared to that of ribose, with the C2 radical showing the most significant reduction in puckering amplitude. The possibility of radical inversion adds additional complexity to the conformational properties of ribose radicals. In the C1 and C4 radicals, the inverted orientations are minima more stable than the original ones. The bond dissociation energies of the various hydrogens lie in a range of 87−93 kcal/mol and, in distinction from deoxyribose, they are all of comparable strength.