Dark catalytic electron transfer in solid-state hydrogen-bonded electron-donor–electron-acceptor complexes

Abstract

The mechanism for the catalysis of electron-transfer reactions in systems with weak donor-acceptor properties has been considered. According to this mechanism, the reaction proceeds via the low-lying excited states of the complexes. These states are generated as a result of multiorbital bonds between the molecules of the reagents. The significant p?rt played by hydrogen and other bridge bonds in the stabilization of the excited levels of the complexes is shown. The energetic characteristics of complexation (charge-transfer energy) and hydrogen-atom transfer (activation energy and reaction heat) are compared, taking as an example solid-phase hydrogen-bonded 7r complexes.One of the principal problems of the theory of chemical reactions in polymeric systems is elucidation of the nature of the potential barrier determining the reaction rate and the structure of the product. It is helpful to study the activated (transition) state of the reaction as one of the electronic states of the reacting system, which makes it possible to treat from a general standpoint the analysis of both catalytic and non-catalytic reactions. In ref.(1)-( 4) it has been stated that thermal reaction proceeds via the low-lying electronically excited state of a molecular complex of the reagents. In the framework of these concepts the similar character of photoand thermo-chemical reactions is established and electron excitation (transfer) is considered to be an elementary stage. The important role played by electron transfer in chemical reactions has been confirmed by experimentalThe present work deals with catalysis of the electron-transfer reaction in systems with weak donor-acceptor properties. This catalysis, in the general sense, is understood as the formation, by the multiorbital bonds between the molecules of the reaction system, of low-lying excited states. Thus the elucidation of the physical mechanisms for the stabilization of intermolecular electronic levels and their physical confirmation are important problems. Redox reactions of solid-phase hydrogenbonded 'TT complexes are considered to be suitable for catalytic electron transfer and are used for a comparison of the energetic characteristcs of the reaction and complexat ion. EXPERIMENTALThe spectral procedure for the investigation of solid complexes has been described previ~usly.~ The reflection spectra of complexes in the visible range were studied using an ATR attachment (fianite, 45") and the absorption spectra were studied in Vaseline oil. Low-temperature electron spectra were obtained using a vacuum quartz cell. For the kinetic measurements samples of the solid complexes were pressed in KBr discs. The kinetic curves were plotted according to the change in the intensity of the i.r. absorption bands corresponding to reagents or products. The e.p.r. spectra of the complexes were obtained using a Bruker

Vivifying properties of water in the light of new concepts of catalysis

“…inherent to matter (both organic and inorganic) and occur at every place of the Universe wherever these compounds are located. In this connection, we are glad to note that our ideas of mechanisms of chemical reactions agree with the views of Oparin (1977) on the role of the electron and hydrogen transfer reactions in biochemistry; in addition, these ideas were formulated independently and approximately at the same time (Kalninsh 1981a(Kalninsh , 1984.…”

“…At that time, no evidence was found in favour of proceeding chemical (biochemical) reactions via excited states of molecules and their complexes. Further development of the theory of chemical reactions was made in 1977 in our works (Kalninsh 1981a(Kalninsh , 1981b(Kalninsh , 1982(Kalninsh , 1984(Kalninsh , 1997. We have based our theory on completely different assumptions which were not related to biological systems, i.e.…”

“…Since our study is devoted to the role of water as a catalytic medium, it is important to discuss the works of biochemist Alexander Ivanovich Oparin (1977) (Kalninsh 1982(Kalninsh , 1984 who studied molecular mechanisms of emerging life in water. These works can be regarded as the most original and interesting.…”

Structure and Absorption Spectra of Polyhydroquinones

“…Crystalline quinone3hydroquinone complexes (so-called quinhydrone) undergo redox transformations with an increase in the difference between the hydroquinone ionization potential and quinone electron affinity [10]. It has been shown that transfer of two hydrogen atoms from hydroquinone to quinone occurs in the electron-excited state of quinhydrone [11].…”

Electron-Proton Effect in Catalysis

“…The excited states of such complexes are responsible for the thermal transfer of the hydrogen atom [5], an elementary step of many organic reactions. The excited states of such complexes are responsible for the thermal transfer of the hydrogen atom [5], an elementary step of many organic reactions.…”