I. Matrix Isolation of 1,1-Diazenes. II. Distance, Temperature, and Dynamic Solvent Effects on Electron Transfer Reactions
<p>Part I: Investigations of the reactive intermediates known as 1, 1-diazenes or aminonitrenes are reported. These unstable species were generated by UV photolysis of appropriately substituted carbamoyl azides under matrix isolation conditions. Four systems were investigated: three cyclic dia...
| Summary: | <p>Part I: Investigations of the reactive intermediates known as 1, 1-diazenes or aminonitrenes are reported. These unstable species were generated by UV photolysis of appropriately substituted carbamoyl azides under matrix isolation conditions. Four systems were investigated: three cyclic dialkyl 1,1-diazenes (1,1-tetramethylenediazene 1, 1, 1-trimethylenediazene 2, and 3,4-dehydro-1, 1-tetramethylenediazene 15) and the first diaryl 1,1-diazene, 1,1-diphenyldiazene 22. 1,1-Diazene 1 could be generated by broad band UV photolysis (200-400 nm) of the carbamoyl azide, but 2 and 22 required narrow band photolysis (290-310 nm) and gave poorer yields. 1, 1-Diazene 15 could not be isolated even at 10 K. The chemical and spectroscopic properties of the 1, 1-diazenes were investigated in some detail, and the results were analyzed with regard to the stability of the various 1,1-diazenes and the different reactive pathways available to 1,1-diazenes with different substituents.</p>
<p>Part II: The dependence of intramolecular electron transfer rates for porphyrin-quinone compounds on distance and temperature was studied. It was found that the rates depend exponentially on the edge-to-edge donor acceptor distance R<sub>e</sub> as </p>
<p>k<sub>ET</sub> = k<sub>o</sub> exp [-α R<sub>e</sub>]</p>
<p>with α values of 1.10 to 1.25 in different solvents. The temperature dependence studies revealed that the electron transfer rates are not activated in a classical sense, but instead depend on the dynamic relaxation properties of the solvent. In 2-methyltetrahydrofuran, the rates are nearly independent of temperature at high
temperatures (200-300 K), then begin to decrease with decreasing temperature. In toluene, the rates increase with decreasing temperature, while in deuterated toluene the rates initially increase with decreasing temperature, then go through a maximum around 215 K, and finally decrease. Apart from the unusual solvent isotope effect in the toluene and toluene-<sub>8</sub> data, this appears to be the first observation of the rate turnover with solvent "friction" predicted by Kramers. The results were analyzed in terms of theoretical predictions of the dependence of electron transfer rates on the longitudinal relaxation time of the solvent τ<sub>L</sub> using the equation </p>
<p>k<sub>ET</sub> = k<sub>NA</sub> / 1.00 + (α / K<sub>NA</sub>τ<sub>L</sub>) + γk<sub>NA</sub>τ<sub>L</sub> </p>
<p>where k<sub>NA</sub> is a maximum rate and α and γ are fitted parameters. This equation gave reasonable fits to the data for all three solvents.</p> |
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