Influence of steric hindrance in molecules of substituted 2-phenyl-1,1,3,3-tetramethylguanidines on the kinetics and isotope effects of their proton transfer reactions with 4-nitrophenylnitromethane in acetonitrile solvent

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Abstract
Rate constants have been determined for the proton and deuteron transfer reactions of 4-nitrophenylnitromethane, NO2C6H4CH2NO2, with three phenyl substituted derivatives of 2-phenyl-1,1,3,3-tetramethylguanidine, X—C6H4—N=C[N(CH3)2]2, where X = 4-Cl, 2-Cl, and 2-Br, in the temperature range 278–308 K in acetonitrile solvent. In the concentration ranges used, the reaction to form an ion-paired product is first order in acid and first order in base. The reaction was followed spectrophotometrically using the stopped-flow technique. The reaction is fastest for the 4-Cl substituted base and slowest for the 2-Br analogue. The observed deuterium kinetic isotope effects show a clear dependence on the position and the size of the substituent in the base molecule. Kinetic isotope effects at 25 °C, kH/kD = 9.7 ± 0.3, 12.5 ± 0.3, and 15.6 ± 0.3, were observed for 4-Cl, 2-Cl, and 2-Br substituted bases respectively. The appropriate ratios of Arrhenius preexponential factors, AH/AD = 0.98 ± 0.10, 0.87 ± 0.10, and 0.77 ± 0.10, may indicate increasing importance of tunnelling of the proton when the substituent is moved from the para to the ortho position or when the size of ortho substituent increases. Values of tunnelling correction factors were calculated using Bell's equation by means of the Caldor and Saunders method.

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