The Ionic Hydrogen Bond. 5. Polydentate and Solvent-Bridged Structures. Complexing of the Proton and the Hydronium Ion by Polyethers
Journal of the American Chemical Society
American Chemical Society
The thermochemistry associated with protonated complexes containing one or two glyme (MeOCH2CH2-OMe, (Gl)), diglyme (Me(OCHzCH2)20Me, (G2)), or triglyme (Me(OCH2CH2)30Me (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 MezCO 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 HaO+ 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 OH+.O bond contributes up to 1 13 kJ/mol(27 kcal/mol). Larger binding energies of 387-475 kJ/mol (93-99 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 H2O molecules form a protonated solvent bridge between ether groups in (G1*2HzO)H+ and (G3*2HzO)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 (04 kcal/mol). The solvent-bridged structures are models for water chains involved in proton transport in biomembranes.
The Ionic Hydrogen Bond. 5. Polydentate and Solvent-Bridged Structures. Complexing of the Proton and the Hydronium Ion by Polyethers M. Meot-Ner, L. W. Sieck, S. Scheiner, X. Duan J. Am. Chem. Soc. 1994 116 (17), 7848-7856.