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

Abstract

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 Møller–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, Δ E BHB 1 + , and Gibbs free energies, Δ G BHB 1 + , 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 from 2.741 to 2.802 Å. In addition, the Δ E BHB 1 + and Δ G BHB 1 + quantities have been correlated with basicities of the proton acceptors in the gas phase expressed by the previously calculated protonation energies, Δ E prot, and Gibbs free energies, Δ G prot, as well as with basicities of proton acceptors in solution expressed as p K a values of the protonated acceptors in acetonitrile.