A study of the energetics of asymmetric OHN+/NHO+ hydrogen bridge formation using ab initio methods

The energetics of formation of heterocomplexes cations stabilised by asymmetric OHN+/NHO+ hydrogen bridges has been calculated by ab initio methods at the Restricted Hartree-Fock and Moller-Plesset levels using the Gaussian functional basis set 6-31G* in systems of protonated trimethylamine N-oxide as the proton donor and substituted pyridines as proton acceptors. The calculated energies, DeltaE(BHBi+), and Gibbs free energies, DeltaG(BHBi+), of the heterocomplexed cation formation were compared with the previously determined energies of formation of the cations stabilised by OHO+ (with amine N-oxides, i.e. substituted pyridine N-oxides and trimethylamine N-oxide) and NHN+ (with pyridine derivatives) bridges. The results showed that the OHN+/NHO+ bridges are intermediate between those of OHO+ and NHN+, with the lengths of the hydrogen bridge ranging, quantities have been correlated with basicities of the proton from 2.741 to 2.802 Angstrom. In addition, the DeltaE(BHBi+) and DeltaG(BHi+) acceptors in the gas phase expressed by the previously calculated proton ation energies, DeltaE(prot), and Gibbs free energies, DeltaG(prot), as well as with basicities of proton acceptors in solution expressed as pK(a) values of the protonated acceptors in acetonitrile. (C) 2003 Elsevier Science B.V. All rights reserved.