Kinetics of Rapid Reactions in Liquid Ammonia. The Ammonia—Amide Proton Exchange

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Abstract
Highly purified liquid ammonia was prepared and the proton magnetic resonance spectrum was recorded as a function of temperature from about −70° to about 35°C. The line shapes were shown to be given quantitatively by the equation derived by Pople and precise values for the nitrogen—proton coupling constant and the transition probabilities connecting the 14N spin states were measured. The Bloch equations pertinent to pure 14NH3 were extended to include the possibility of chemical exchange and a means was devised to extract precise values for the rate constants for the exchange process. Solutions of KNH2 in NH3 were prepared by the decomposition of potassium—ammonia solutions with platinum black and the kinetics of the ammonia—amide proton exchange were studied as a function of temperature and concentration. The bimolecular rate constant for the exchange was found to be 1.51×107 liter/mole·sec at 25°C, the enthalpy of activation is 4.0±0.5 kcal/mole, and the entropy of activation is −12±2 eu. This large negative entropy is interpreted in terms of a sizeable reorganization of the solvent structure in the activated complex.