Absorption Spectra and Termination Kinetics of Transient Free Radicals in Solution Observed by UV./VIS. Spectroscopy with modulated excitation

Abstract

SummaryFree radicals are generated in liquid solutions by harmonically modulated photolysis of suitable substrates. Harmonic analysis of the absorbance as functions of wavelength and modulation frequency yields the optical spectra and the decay kinetics of the transient species. The experimental technique and the analysis are described in detail. Results on t-butyl, 2-propyl and benzyl radicals generated by photolysis of the corresponding dialkyl resp. dibenzyl ketones are reported. They confirm previous spectral assignments and show that the termination reactions are diffusion controlled.

Modulation spectroscopy: A technique for the study of free radicals in solution

“…The first optical modulation spectrometer for the study of free radicals in solution was designed by Fmggenberger and Fischer [13]. irradiated by a mercury-xenon lamp, A.…”

“…This aspect is particularly important for the analysis of second order kinetics where, for accuracy, a homogeneous radical distribution is essential. It also means that the rate of radical generation can be measured by several methods including product analysis [13]. These advantages are not normally realized in either flash photolysis or pulse radiolysis and hence these techniques rarely yield reliable estimates of bimolecular rate constants.…”

“…The spectrometer is extremely sensitive. Excellent spectra can be obtained at radical concentrations of 10 -7 to 10 -8 M. Moreover, it is inexpensive to build since many of its components are standard items of laboratory equipment [13,14]. …”

“…The phase relationships between these three concentrations are such that, at appropriate modulation frequencies, the spectram due to any one can be recorded without contributions from the others (vide. infra) [13].…”

Effiziente Photoionisierung eines Norrish‐II‐ Diradikals

“…Selbst das dürfte in unserem Fall sehr unwahrscheinlich sein, weil seine Absorption zu schwach ist (der Extinktionskoeffizient des tert-Butylradikals bei 308 nm ist nur 200 m À1 cm À1 ). [9] Im Unterschied dazu haben sowohl das Ketylradikal als auch seine deprotonierte Form, das Radikalanion, bei 308 nm mittelstarke bis starke Absorptionen, und ihre Photoionisierungen sind energetisch erheblich günstiger als die eines Alkylradikals, weil sich neben dem Elektron eine Spezies mit einer abgeschlossenen Valenzschale bildet, nämlich die Carbonylgruppe wiedergewonnen wird. Bedeutsam für die Elektronenausbeute ist die Rotverschiebung der Absorptionsbande durch die Deprotonierung, [10] die zu einer erheblichen Vergrößerung des Extinktionskoeffizienten bei 308 nm führt.…”