A computational analysis of intramolecularity in proton transfer reactions.

Hydrogen bonds along which most proton transfers take place are analyzed regarding their contribution to the efficiency of intramolecular processes through the assessment of effective molarities by DFT calculations on rigid intra-molecular model systems in comparison with their intermolecular counterparts. A few cyclic systems in which there is no possibility of hydrogen bonding between reactants neither in the ground state nor in the product are identified by theoretical calculations as leading to important rate increases in intramolecular general-base catalysis constituting a new exception to the rule of low effective molarities (EMs) for proton transfers. The recovery of high efficiency in these systems is attributed to the absence of special features facilitating the corresponding intermolecular reactions. This work confirms that any explanation of EMs found in intramolecular systems also requires a careful analysis of the corresponding bimolecular systems.