Electron Transfer in Supercritical Carbon Dioxide: Ultraexothermic Charge Recombination at the End of the “Inverted Region”

Abstract

Charge-recombination rates in contact radical-ion pairs, formed between aromatic hydrocarbons and nitriles in supercritical CO(2) and heptane, decrease with the exothermicity of the reactions until they reach -70 kcal mol(-1), but from there on an increase is observed. The first decrease in rate is typical of the "inverted region" of electron-transfer reactions. The change to an increase in the rate for ultra-exothermic electron transfer indicates a new free-energy relationship. We show that the resulting "double-inverted region" is not due to a change in mechanism. It is an intrinsic property of electron-transfer reactions, and it is due to the increase of the reorganisation energy with the reaction exothermicity.

Temperature Dependence of Ultra‐Exothermic Charge Recombinations

“…We have shown in detail that the exciplexes formed between these species have nearly quantitative charge separations [15] and that in n-heptane, the fluorescence and triplet-state formation are minor decay channels. The fluorescence quantum yields are 0.102 and 0.0044 for the Py/FN and Na/FN radical ion pairs, respectively, and the corresponding triplet quantum yields are 0.06 for Py/FN and < 0.01 for Na/FN.…”

“…The fluorescence quantum yields are 0.102 and 0.0044 for the Py/FN and Na/FN radical ion pairs, respectively, and the corresponding triplet quantum yields are 0.06 for Py/FN and < 0.01 for Na/FN. [15] The energy balance obtained in photoacoustic calorimetry measurements reveal that the quantum yields for the formation of other products must be less than 0.01. [15] Consequently, the charge recombination quantum yields within the Py/FN and Na/FN radical ion pairs are 0.84 and 0.996, respectively.…”

“…[15] Moreover, as the separation of these ions is endoenergetic, in n-heptane and isopropyl ether they exist as nearly pure contact radical ion pairs (CRIP) with a very weak (exciplex) emission, and their decays are dominated by the charge recombination. [15] These systems give us the opportunity to focus on the temperature dependence of the reorganization energies because the free-energy dependence of the rates is negligible within AE 5 kcal mol…”

(More) About biphenyl first excited triplet state energy

“…A rather comprehensive study of alkene triplet energies was done by Caldwell and co-workers, and permitted to assess the energy differences between relaxed and spectroscopic triplets [23][24][25]. We have also used this technique to study the triplet energies of organic polymers [26,27], the enthalpy of electron transfer reactions [22,28], and of the photocyclization of phenylamines [29,30].…”

A chemical understanding for the enhanced hydrogen tunnelling in hydroperoxidation of linoleic acid catalysed by soybean lipoxygenase‐1

“…[42][43][44] In the case of SLO catalysis of the hydroperoxidation of linoleic acid, the p D -Fe A distance obtained from docking calculations is 5.69 Å and the Fe A -O bond length is 1.86, [4] which lead to an edge-to-edge electron donor-acceptor distance of Dr % 4 Å . Edge-to-edge distance, rather than centre-to-centre distances, are employed for consistency with earlier work on electron transfer reactions.…”