The specific rate constant k′ of the surface-mediated hydride-transfer reaction of 1.4-cyclohexadiene with triphenylmethylium is strongly dependent on the nature of the solid acid catalyst used. The catalysis of this hydride-transfer reaction by 30-moderately strong solid acid catalysts, e.g., silicas, aluminas, aluminosilicates, and titanium dioxide particles, has been studied. Generation of the triphenylmethylium when chlorotriphenylmethane is chemisorbed to the solid acid catalyst was used for the kinetic measurement. The individual, pseudo-first-order rate constant, k, of the hydride-transfer reaction increases linearly with the amount of solid acid catalyst used. The specific rate constant, k′, based on the surface area of the solid acid, was used to quantifiy the catalytic activity of the solid acids. k′ increases in the order silicas < aluminosilicates < titanium dioxides = aluminas. Ln k′ can be be correlated with the corresponding surface polarity parameters of the solid acid, e.g., Reichardt's E T (30), Gutmann's acceptor number AN, and Kamlet-Taft's parameters R (hydrogen-bond-donating acidity) and π* (dipolarity/polarizability). It is shown that the catalytic activity of a moderately strong solid acid catalyst can be characterized by Gutmann's acceptor number AN or the Kamlet-Taft R value. The different classes of solid acids, e.g., silicas, aluminas, and aluminosilicates, respectively, give different dependencies of k′ as a function of their solid acid acidity parameters.

The transition energy of the charge-transfer UV/Vis absorption maxima (# max, CT ) and the structure of the Michler's ketone (MK)-tetracyanoethene (TCNE) electron donor-acceptor (EDA) complex are remarkably solvent dependent. The UV/Vis spectra of the MK-TCNE complex were measured in 20 non-protic and seven protic solvents. In non-protic solvents, # max, CT of the EDA complex is quantitatively described by a multiple LSE relationship using the Kamlet-Taft dipolarity/polarizability (p*) and basicity (b) parameters of the solvents. The influence of the two terms b and p* on # max, CT is opposite, indicating a qualitatively different solvent-induced stabilization of the electronic ground and excited states of the complex. As expected, increasing dipolarity/ dipolarizability of the solvent causes a bathochromic band shift (positive solvatochromism), whereas the basicity of solvents is responsible for a hypsochromic band shift due to specific solvation of the TCNE site. In protic solvents, the complex formation is associated with the formation of an ionic species (n max = 19 800 cm À1 ) derived from Michler's ketone due to coordination of TCNE at the carbonyl oxygen of MK (called an n-complex). The solvent-induced switching of the p-complex into the n-complex is demonstrated for mixtures of DCE with protic solvents and for silica surfaces.

The apparent rate constants k′ of different surface-mediated reactions of three nucleophiles 1,4-cyclohexadiene, triethylsilane, and isobutylvinyl ether with triphenylmethylium ions have been determined for three different solid acid catalysts and various triphenylmethylium precursors (R 1 R 2 R 3 C-X, X ) SCN, OH, Cl, or Br). Generation of triphenylmethylium ions [R 1 R 2 R 3 C + ] was used for kinetic measurements when R 1 R 2 R 3 C-X derivates are chemisorbed to the solid acid catalysts. The catalysis of those surface-mediated reactions by a silica, an alumina, and an aluminosilicate has been studied in a slurry of dichloromethane at ambient temperature. The value of k′ increases in the order OH -< SCN -< Cl -< Brand silica < alumina < aluminosilicate. The log k′ can be correlated with the pK s of the corresponding HX from R 1 R 2 R 3 C-X or the acidity of the solid acid (

The transition energy of the charge‐transfer UV/Vis absorption maxima (νmax, CT) and the structure of the Michler's ketone (MK)–tetracyanoethene (TCNE) electron donor–acceptor (EDA) complex are remarkably solvent dependent. The UV/Vis spectra of the MK–TCNE complex were measured in 20 non‐protic and seven protic solvents. In non‐protic solvents, νmax, CT of the EDA complex is quantitatively described by a multiple LSE relationship using the Kamlet–Taft dipolarity/polarizability (π*) and basicity (β) parameters of the solvents. The influence of the two terms β and π* on νmax, CT is opposite, indicating a qualitatively different solvent‐induced stabilization of the electronic ground and excited states of the complex. As expected, increasing dipolarity/dipolarizability of the solvent causes a bathochromic band shift (positive solvatochromism), whereas the basicity of solvents is responsible for a hypsochromic band shift due to specific solvation of the TCNE site. In protic solvents, the complex formation is associated with the formation of an ionic species (νmax = 19 800 cm−1) derived from Michler's ketone due to coordination of TCNE at the carbonyl oxygen of MK (called an n‐complex). The solvent‐induced switching of the π‐complex into the n‐complex is demonstrated for mixtures of DCE with protic solvents and for silica surfaces. Copyright © 1999 John Wiley & Sons, Ltd.