Ion Pairs or Neutral Molecule Adducts? Cooperativity in Hydrogen Bonding

Roger L. DeKock , Laura A. Schipper , Stephanie C. Dykhouse , Lee P. Heeringa and Benjamin M. Brandsen
Department of Chemistry and Biochemistry, Calvin College, Grand Rapids, MI 49546-4403
J. Chem. Educ., 2009, 86 (12), p 1459
DOI: 10.1021/ed086p1459
Publication Date (Web): December 1, 2009

Abstract

We performed theoretical studies on the systems NH3·HF·mH2O, NH3·HCl·mH2O, with m = 0, 1, 2, and 6. The molecules with m = 0 form hydrogen-bonded adducts with little tendency to form an ion-pair structure. The molecule NH3·HCl·H2O cannot be characterized as either a hydrogen-bond adduct or an ion-pair structure as it has a flat potential energy surface for motion of the hydrogen atom N · · · H · · · Cl. The molecule NH3·HCl·2H2O clearly behaves as an ion-pair NH4Cl·2H2O. There are sixteen arrangements of the cubic array NH3·HCl·6H2O and all sixteen exhibit the ion-pair NH4Cl·6H2O. The molecule NH3·HF·H2O does not form the ion-pair NH4F·H2O, nor does a second added water molecule facilitate ionization. However, of the sixteen arrangements of NH3·HF·6H2O, twelve are categorized as greater than 80% ion-pair character NH4F·6H2O, two are about 60% ion pair, and two are about 30% ion pair. We also have completed similar studies on the dimers and tetramers (NH3·HF)n and (NH3·HCl)n, where n = 2 or 4. Dimers (NH3·HCl)2 and tetramers (NH3·HCl)4 form ions (NH4Cl)2 and (NH4Cl)4, as do tetramers of (NH3·HF)4, which are therefore actually (NH4F)4. But the dimer (NH3·HF)2 remains as neutral molecule adduct. These results illustrate hydrogen-bond cooperativity in determining the outcome of a chemical reaction.

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History

  • Received: August 03, 2009

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