The photochemistry of propanone and its derivatives as studied using flash-photolysis ESR; electron spin polarization (CIDEP) in radical products

Abstract

By direct observation of the transient radical products of the flash photolysis of ketones related to propanone, three different primary reactions have been demonstrated. Propanone and butanone exhibit both Norrish type 1 and hydrogen-abstraction reactions whose relative contributions depend upon the solvent used, but 3,3-dimethyl-2-butanone exhibits a dominant bond scission (Norrish type 1) reaction. Ketones with hydroxyl substituents exhibit only the latter reaction, as it appears do those with chlorine substituents, but in these, in contrast to the others, it is the 13 bond which breaks. This might suggest a change in the character of the triplet state from which the radicals are produced from n~* to ~rt*. The CIDEP observed is consistent with this interpretation. Most spectra exhibit net polarization whose phase varies with the nature of the substituent, and which originates in the triplet mechanism. The significance of the observations to the controversial polarization observed in the propanone ketyl radical produced in the reactions of propanone with a suitable hydrogen source is discussed. Unusual observations of acyl radicals are reported, with some resolved hyperfine structure,

Triplet and reversed triplet mechanism CIDEP studied by quenching experiments

“…The acetone result is in agreement with previous quantitative attempts to measure the size of the net polarisation [9,12] and an investigation of triplet acetone polarisation by fast reaction with t¡ [28]. However it is in disagreement with some qualitative experiments which suggested the TM was operative [3,14]. In particular it has been argued that TREPR spectrometers are too insensitive to detect Boltzmann signals, the current results show this view is too pessimistic.…”

“…The resulting 2-hydroxy-2-propyl radicals are observed in an emissiord absorption (E/A) multiplet overlaid upon a net A contribution. The E/A pattern is accepted to arise from the Radical Pair Mechanism (RPM), however the origin of the net A contribution has been fiercely debated [3,[9][10][11][12][13][14]. Variation of the triplet lifetime (rv) by addition of triplet quenchers should provide a simple qualitative means to distinguish between the two mechanisms.…”

Origin of an Absorptive Electron Spin Polarization Observed in Photochemical Hydrogen Abstraction Reaction by Benzophenone Derivatives. CW and Pulsed EPR Studies

“…Paul proposed that the spin-equilibrated triplet state played an important role for net absorptive contribution in spin-polarized EPR spectra of the transient radical when it was created by laser pulse irradiation to acetone in propan-2-ol. , By application of the time-resolved Fourier transform EPR (FT-EPR) method, a similar result was obtained. , However, McLauchlan et al concluded that the net contribution results from the Norrish type 1 reaction . The reaction yields spin-polarized radicals from the TM.…”

Time-Resolved CIDNP Investigation of the Cross-Relaxation Mechanism of 1H Nuclear Polarization

“…Additionally, the temperature correlation of the CIDNP and CIDEP appears in error since the ST - mixing causing the inverse of polarization of the X-band CIDEP effects 32 would not be expected to occur at the high field of the NMR experiment. The explanation of Salzmann's results required the CIDEP to be in E, which would be unexpected for an aliphatic ketone . Finally, the photolysis of 2,2,4,4-tetramethylpentan-3-one producing tert -butyl radicals is a well-investigated model system for time-resolved CIDNP, and here, a cross-relaxation effect has not been reported 12-15 although from the above it would be expected.…”

“…This is an important result with respect to the ongoing and unresolved debate upon the origin of the A contribution in the CIDEP of radicals created from aliphatic ketones. It has been variously assigned to the TM, , the radical's Boltzmann signal, , and the TM arising from a photoreduction of the enol product …”