It was found previously that the proton‐transfer reaction between trimethylammonium ion and trimethylamine in aqueous solution takes place via water. In the present investigation the number of water molecules involved in this reaction was determined. Proton exchange rates were determined from NMR measurements in trimethylamine—trimethylammonium chloride buffer solutions in O17‐enriched water. It is concluded that one water molecule is involved in the transfer reaction.
In the appendix, theoretical equations are derived for the dependence of the observed spin‐echo decay rate on the 180° pulse rate for the case of fast exchange.
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The value given in Ref. 2 is at 22 °C. A recent determination6 of higher precision gives the value quoted.
9.
This statement might seem to be in contradiction to the fact that in the measurements reported in Refs. 1 and 6 is obtained from the broadening of the water line due to exchange between water and methylammonium ion. However, the latter measurements were made at a low pH, chosen to correspond to an exchange rate slow enough to give optimum broadening. In the measurements reported here, the pH is a few units higher, and the rate of proton exchange between methylammonium ion and water molecules is fast enough to give a single averaged line with negligible excess width. The chemical shift between the NH and water protons was found to be 216 cps. From this figure and the known value of a contribution to the water linewidth of about cps can be calculated under the conditions of the experiment. In actual measurements even this small contribution is eliminated because only line width differences between normal and enriched water entered the calculation and the above contribution is independent of concentration.
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Yeda Research and Development Company, The Weizmann Institute of Science, Rehovot, Israel.
13.
We admit to some inconsistency in the above procedure, as the value of J has been derived on the assumption of fast exchange. We feel, however, that no serious error is introduced, as the conditions for fast exchange are very nearly fulfilled and the result obtained for J is in good agreement with that previously reported.7
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© 1963 American Institute of Physics.
1963
American Institute of Physics
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