The Ionic Hydrogen Bond. 5. Polydentate and Solvent-Bridged Structures. Complexing of the Proton and the Hydronium Ion by Polyethers

The thermochemistry associated with protonated complexes containing one or two glyme (MeOCH2CH2- OMe, (G1)), diglyme (Me(OCH2CH2)2OMe, (G2)), or triglyme (Me(OCH2CH2)3OMe (G3)) molecules and 0-3 H20 molecules was measured by pulsed high-pressure mass spectrometry.Comparison of polyether, crown ether, and acetone complexes with H+ and H30+ shows increasing binding energies with increasing flexibility in the ligands.For example, in protonated clusters containing ligands with a total of four polar groups, the proton is bonded by a total energy of (kJ/mol (kcal/mol)): four Me2CO molecules, 1044.8 (249.7);two G1 molecules, 987.4 (236.0);one G3 molecule, 962.3 (230.0);12-crown-4, 941.4 (225.0).Stabilization of the proton by dipoles of the free ether groups contributes significantly; for example, 17 and 54 kJ/mol (4 and 13 kcal/mol) in the binding energy within (G1)2H+ and (G2)2H+ dimers, respectively.The thermochemistry of H30+ binding indicates bidentate complexes with one each of G2, G3, and 15-crown-5 molecules and two G1 molecules, with binding energies of 310-352 kJ/mol (74-84 kcal/ mol).In these complexes the second 0 +•0 bond contributes up to 113 kJ/mol (27 kcal/mol).Larger binding energies of 387-475 kJ/mol (93)(94)(95)(96)(97)(98)(99) kcal/mol) indicate tridentate complexes of H30+ with two G1 and two G3 molecules, as well as with 18-crown-6, which is the best complexing agent due to entropy effects.In complexes containing additional water molecules, the thermochemistry suggests that two H20 molecules form a protonated solvent bridge between ether groups in (G1-2H20)H+ and (G3-2H20)H+.Ab initio calculations show that open and solvent-bridged structures have comparable energies (within 16 kJ/mol (4 kcal/mol)).The calculated barriers to direct and solvent-mediated proton transfer between functional groups are 0-16 kJ/mol (0-4 kcal/mol).The solvent-bridged structures are models for water chains involved in proton transport in biomembranes.