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

Authors

Michael Meot-Ner, National Institute of Standards and Technology
L. Wayne Sieck, National Institute of Standards and Technology
Steve Scheiner, Utah State UniversityFollow
Xiaofeng Duan, Southern Illinois University Carbondale

Document Type

Article

Journal/Book Title

Journal of the American Chemical Society

Publication Date

8-1994

Publisher

American Chemical Society

Volume

116

Abstract

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 H₂O molecules was measured by pulsed high-pressure mass spectrometry. Comparison of polyether, crown ether, and acetone complexes with H⁺ and H₃O⁺ 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 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 H₃O⁺ 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^_.2H₂O)H⁺ and (G3^_.2H₂O)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.

Comments

Originally published in the Journal of the American Chemical Society by the American Chemical Society . Publisher’s PDF available through remote link. DOI: 10.1021/ja00096a047

Recommended Citation

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.