A Hartree–Fock, MP2 and DFT computational study of the structures and energies of ″b 2 ions derived from deprotonated peptides. A comparison of method and basis set used on relative product stabilities

A detailed computational study of possible ″b 2 ion structures derived by nominal elimination of neutral glycine from deprotonated triglycine along with the structures and energies of the corresponding neutrals has been carried out. These computations of structures and energies have been carried out at the RHF/3-21G, RHF/6-31G(d), B3LYP/6-31G(d), B3LYP/6-31+G(d), MP2(fc)/6-31G(d) and MP2(fc)/6-31+G(d) levels of theory. With the exception of the RHF/3-21G level of theory, all levels gave structures and relative energies of both neutrals and anions, which were in agreement within the expected uncertainty. However, the gas-phase acidities calculated with inclusion of diffuse functions, as in the zero-point coorected B3LYP/6-31+G(d) and MP2/6-31+G(d) levels, were 12–15kcalmol−1 lower than those calculated without inclusion of diffuse functions. The inclusion of diffuse functions lowers the absolute energies of both neutrals and anions but has a considerably greater effect on the anions. Nine possible stable structures for ″b 2 ions have been identified. Of these, five are of relatively low energy and are candidate structures for the ions formed in the fragmentation of deprotonated peptides. Possible pathways, as suggested by experimental studies, to the various computed anion structures are discussed.