Catalytic Isomerization of Quadricyclane Using Fourier Transform Near-Infrared Absorption Spectroscopy:  Diffusion, Conversion, and Temperature Effect

Abstract

By using Fourier transform near-infrared (NIR) absorption spectroscopy, the kinetic behaviors of quadricyclane isomerization, as catalyzed by anhydrous CuSO(4) in chloroform mixture with and without agitation, are presented. Given the acquired NIR spectra, the concentration decay of quadricyclane with the reaction time is determined with the aid of partial least-squares analysis. When the mixture is not agitated, the diffusion coefficients in chloroform are evaluated to be (3.8 +/- 0.1) x 10(-5) cm(2) s(-1) at 27 degrees C and (4.4 +/- 0.1) x 10(-5) cm(2) s(-1) at 39 degrees C. In the size-dependent experiments of the catalyst, the one-site and two-site coordinated conversion rate constants are further determined to be (8.5 +/- 5.9) x 10(-6) s(-1) A(-1) and (2.2 +/- 0.8) x 10(-8) s(-1) A(-2), respectively, at 27 degrees C and (1.3 +/- 0.8) x 10(-5) s(-1) A(-1) and (1.92 +/- 0.01) x 10(-6) s(-1) A(-2), respectively, at 39 degrees C. A denotes the total catalyst surface area per unit effective volume of solvent. Accordingly, the activation energies for one-site and two-site coordination are evaluated to be 24.8 and 286.2 kJ mol(-1), respectively. The reaction is dominated by one-site coordination (1:1 complex) between the reactant and the catalyst. Unless temperature increases, the two-site coordinated reaction may be ignored. In contrast, when analogous experiments are performed in the stirred solution, the diffusion factor is ignored but the conversion rate constants rise due to the increase of collision frequency. For instance, the one-site and two-site coordinated rate constants are increased to (1.7 +/- 1.4) x 10(-5) s(-1) A(-1) and (1.27 +/- 0.06) x 10(-5) s(-1) A(-2) at 39 degrees C. The two-site coordinated reaction rate is enhanced by a factor of 10. Thus, isomerization may proceed via both 1:1 and 1:2 coordination between the reactant and the catalyst. The Arrhenius plot yields the corresponding activation energies to be 24 +/- 3 and 275 +/- 3 kJ mol(-1). The activation energies remain constant, no matter whether the solution is agitated or not.