Chinese Journal of Magnetic Resonance

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REVISIT TO HYDROGEN EXCHANGE BETWEEN AMMONIUM AND WATER: Diffusion Independent Chemical Exchange

ZHANG Yong-hong,MAO Xi-an*   

  1. State Key Laboratory of Magnetic Resonance and Atomic Molecular Physics, Wuhan Institute of Physics and Mathematics, The Chinese Academy of Sciences P.O. Box, 71010, Wuhan 430071, China
  • Received:2002-02-26 Revised:2002-04-23 Published:2002-09-05 Online:2002-09-05
  • Supported by:

    国家自然科学基金资助项目(29725307).

Abstract:

Hydrogen exchange between ammonium and water is revisited. Exchange rate constants, diffusion coefficients and relaxation times for solutions with var ied pH, temperature and concentration of a coexisting salt are measured using NM Rspectroscopy. It is demonstrated that the hydrogen exchange in ammonium soluti ons is independent of apparent diffusion. The mechanism involves chemical-bond/ h ydrogen-bond exchange. In solutions, water molecules, with the help of hydrogen - bonding, serve as abridge to connect the ammonium with its base partner ammonia . During hydrogen exchange, the ammonia accepts a hydrogen from water while the a mmonium donates a hydrogen to water, or ahydrogen jumps from water to ammonium  and at the same time another hydrogen jumps back from ammonium to water. There is nonet hydrogen transfer between ammonium and water. At lower pH, the concentr ation of ammonia becomes lower and less ammonium-water-ammonia complex is form ed . As aresult, the probability of the hydrogen exchange is reduced. This explain s why at lower pH hydrogen exchange is slow, a common phenomenon observed in bio logical hydrogen exchange systems.

Key words: ammonium aqueous solution, hydrogen exchange, hydrogen bond, chemical bond, self-diffusion

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