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ERIC Number: EJ863203
Record Type: Journal
Publication Date: 2009-Dec
Pages: 6
Abstractor: As Provided
Reference Count: 26
ISSN: ISSN-0021-9584
Ion Pairs or Neutral Molecule Adducts? Cooperativity in Hydrogen Bonding
DeKock, Roger L.; Schipper, Laura A.; Dykhouse, Stephanie C.; Heeringa, Lee P.; Brandsen, Benjamin M.
Journal of Chemical Education, v86 n12 p1459-1464 Dec 2009
We performed theoretical studies on the systems NH[subscript 3] times HF times mH[subscript 2]O, NH[subscript 3] times HCl times mH[subscript 2]O, 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 NH[subscript 3] times HCl times H[subscript 2]O 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 NH[subscript 3] times HCl times 2H[subscript 2]O clearly behaves as an ion-pair NH[subscript 4]Cl times 2H[subscript 2]O. There are sixteen arrangements of the cubic array NH[subscript 3] times HCl times 6H[subscript 2]O and all sixteen exhibit the ion-pair NH[subscript 4]Cl times 6H[subscript 2]O. The molecule NH[subscript 3] times HF times H[subscript 2]O does not form the ion-pair NH[subscript 4]F times H[subscript 2]O, nor does a second added water molecule facilitate ionization. However, of the sixteen arrangements of NH[subscript 3] times F times 6H[subscript 2]O, twelve are categorized as greater than 80% ion-pair character NH[subscript 4]F times 6H[subscript 2]O, two are about 60% ion pair, and two are about 30% ion pair. We also have completed similar studies on the dimers and tetramers (NH[subscript 3] times HF)[subscript n] and (NH[subscript 3] times HCl)[subscript n], where n = 2 or 4. Dimers (NH[subscript 3] times HCl)[subscript 2] and tetramers (NH[subscript 3] times HCl)[subscript 4] form ions (NH[subscript 4]Cl)[subscript 2] and (NH[subscript 4]Cl)[subscript 4], as do tetramers of (NH[subscript 3] times HF)[subscript 4], which are therefore actually (NH[subscript 4]F)[subscript 4]. But the dimer (NH[subscript 3] times HF)[subscript 2] remains as neutral molecule adduct. These results illustrate hydrogen-bond cooperativity in determining the outcome of a chemical reaction. (Contains 1 table, 8 figures, and 7 notes.)
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Publication Type: Journal Articles; Reports - Research
Education Level: N/A
Audience: N/A
Language: English
Sponsor: N/A
Authoring Institution: N/A