Theoretical studies are reported for conformational energies and bond energies of compounds X-H, X-O, X-OH, X-NO, X-NO2 X-ONO, XO-NO, and XN(O)-O with X = CH3, tert-butyl, and C13H21, which are templates for a radical sire of a hydrogen-terminated diamond C(111) surface. All structures are fully optimized using density functional theory (DFT) based on the B3LYP functional. For X = CH3 calibration calculations are done in detail using the coupled-cluster approach (CCSD(T)) and do support use of the B3LYP functional for rotational barriers and bond energies. Bond energies X-O and X-OH for X = tert-butyl are close to those computed for X = C13H21, but bond energies X-NO, X-NO2, X-ONO, and XO-NO differ by 5-11 kcal/mol and show that the effect of second nearest neighbors of the surface is significant in these cases. Combining the trends observed for the small cluster models yields our best values for adsorption energies, on an active site of hydrogenated C(111) : Bond energies, decrease in the sequence C-EI (98 kcal/ mol) > C-OH(95 kcal/mol) > C-O (94 kcal/mol) > trans C-ONO (53 kcal/mol) > C-NO2 (53 kcal/mol) > cis C-ONO (45 kcal/mol) > C-NO (30 kcal/mol). Dissociation energies of cis CO-NO and trans CO-NO are small (26 kcal/mol, 33 kcal/mol). All barriers computed for internal rotation along C-N and C-O are less than 1 kcal/mol, which shows that the rotation of adsorbed species is essentially free.