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

Abstract

The thermochemistry associated with protonated complexes containing one or two glyme (McOCH{sub 2}CH{sub 2}-OMe, (G1)), diglyme (Me(OCH{sub 2}CH{sub 2}){sub 2}OMe, (G2)), or triglyme (Me(OCH{sub 2}CH{sub 2}){sub 3}OMe (G3)) molecules and 0-3 H{sub 2}O molecules was measured by pulsed high-pressure mass spectrometry. Comparison of polyether, crown ether, and acetone complexes with H{sup +} and H{sub 3}O{sup +} 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 Me{sub 2}CO 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){sub 2}H{sup +} and (G2){sub 2}H{sup +} dimers, respectively. The thermochemistry of H{sub 3}O{sup +} 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{sup +}.O bond contributes up to 113 kJ/mol (27 kcal/mol). 34 refs., 11 figs., 3 tabs.