Electron transfer reactions

According to the Marcus theory of electron transfer, which was proposed by R.A. Marcus in 1965 and is discussed fully in Section 24.11, the rates of electron transfer (from ground or excited states) depend on: 1 The distance between the donor and acceptor, with electron transfer becoming more efficient as the distance between donor and acceptor decreases. 2 The reaction Gibbs energy, ∆_rG, with electron transfer becoming more efficient as the reaction becomes more exergonic. For example, efficient photooxidation of S requires that the reduction potential of S* be lower than the reduction potential of Q. 3 The reorganization energy, the energy cost incurred by molecular rearrange ments of donor, acceptor, and medium during electron transfer. The electron trans fer rate is predicted to increase as this reorganization energy is matched closely by the reaction Gibbs energy. Electron transfer can also be studied by time-resolved spectroscopy (Section 14.6). The oxidized and reduced products often have electronic absorption spectra distinct from those of their neutral parent compounds. Therefore, the rapid appearance of such known features in the absorption spectrum after excitation by a laser pulse may be taken as indication of quenching by electron transfer.

The Earth’s atmosphere contains primarily N2 and O2 gas, with low concentrations of a large number of other species of both natural and anthropogenic origins. Indeed, many of the natural trace constituents of our atmosphere participate in complex chemical reactions that have contributed to the proliferation of life on the planet. The development of industrial societies has added new components to the Earth’s atmo sphere and has led to significant changes in the concentrations of some natural trace species. The negative consequences of these changes for the environment are either already being felt or, more disturbingly, are yet to be felt in the next few decades (see, for example, the discussion of global warming in Impact I13.2). Careful kinetic ana lysis allows us to understand the origins of our complex atmosphere and point to ways in which environmental problems can be solved or avoided. The Earth’s atmosphere consists of layers, as shown in Fig. 23.17. The pressure decreases as altitude increases (see Problems 1.27 and 16.20), but the variation of temperature with altitude is complex, owing to processes that capture radiant energy from the Sun. We focus on the stratosphere, a region spanning from 15 km to 50 km above the surface of the Earth, and on the chemistry of the trace component ozone, O3. In the troposphere, the region between the Earth’s surface and the stratosphere, temperature decreases with increasing altitude. This behaviour may be understood in terms of a model in which the boundary between the troposphere and the stratosphere, also called the tropopause, is considered adiabatic. Then we know from Section 2.6 that, as atmospheric gases are allowed to expand from layers close to the surface to higher layers, the temperature varies with pressure, and hence height, as

Fig. 23.17 The temperature profile through the atmosphere and some of the reactions that occur in each region.

The model predicts a decrease in temperature with increasing altitude because Cp, m/ CV, m ≈ for air. In the stratosphere, a temperature inversion is observed because of photochemical chain reactions that produce ozone from O₂. The Chapman model accounts for ozone formation and destruction in an atmosphere that contains only O₂:

Initiation: O2+hν→O+O 185 nm < λ < 220 nm v=k1[O₂]

Propagation: O +O2+M→O3+M* ∆_rH=−106.6 kJ mol⁻¹ v=k₂[O][O₂] [M]

O₃+hν→O₂+O    210 nm < λ < 300 nm v=k₃[O₃]

Termination: O +O₃→O₂+O₂ ∆rH=−391.9 kJ mol⁻¹ v =k₄[O][O₃]

O+O+M→O₂+M*    v =k₅[O]₂[M]

where M is an arbitrary third body, such as O2 in an ‘oxygen-only’ atmosphere, which helps to remove excess energy from the products of combination and recombination reactions. The mechanism shows that absorption of radiation by O2 and O3 during the daytime leads to the production of reactive O atoms, which, in turn, participate in exothermic reactions that are responsible for the heating of the stratosphere. Using values of the rate constants that are applicable to stratospheric conditions, the Chapman model predicts a net formation of trace amounts of ozone, as seen in Fig. 23.18 (see also Problem 23.33). However, the model overestimates the concentration of ozone in the stratosphere because other trace species X contribute to catalytic enhancement of the termination step O₃ + O → O₂ + O₂ according to

X+O₃→XO+O₂

 XO+O→X+O₂

Fig. 23.18 Net formation of ozone via the Chapman model in a stratospheric model containing only O2, O, andO3. The rate constants are consistent with reasonable stratospheric conditions. (a) Early reaction period after irradiation begins at t = 0. (b) Late reaction period, showing that the concentration of O atoms begins to level off after about 4 hours of continuous irradiation. (c) Late reaction period, showing that the ozone concentration also begins to level offsimilarly. For details of the calculation, see Problem 23.33 and M.P. Cady and C.A. Trapp, A Mathcad primer for physical chemistry. Oxford University Press (1999).

The catalyst X can be H, OH, NO, or Cl. Chlorine atoms are produced by photolysis of CH3Cl which, in turn, is a by-product of reactions between Cl− and decaying vegetation in oceans. Nitric oxide, NO, is produced in the stratosphere from reaction between excited oxygen atoms and N2O, which is formed mainly by microbial denitrification processes in soil. The hydroxyl radical is a product, along with the methyl radical, of the reaction between excited oxygen atoms and methane gas, which is a by-product of a number of natural processes (such as digestion of cellulose in ruminant animals, anaerobic decomposition of organic waste matter) and industrial processes (such as food production and fossil fuel use). In spite of the presence of these catalysts, a natural stratosphere is still capable to maintain a low concentration of ozone.

The chemistry outlined above shows that the photochemical reactions of the Chapman model account for absorption of a significant portion of solar ultraviolet radiation in the stratosphere. Hence, the surface of the Earth is bathed by lower energy radiation, which does not damage biological tissue (see Impact I23.3). However, some pollutants can lower the concentration of stratospheric ozone. For example, chlorofluorocarbons (CFCs) have been used as propellants and refrigerants over many years. As CFC molecules diffuse slowly into the middle stratosphere, they are finally photolysed by ultraviolet radiation. For CF₂Cl₂, also known as CFC^-12, the reaction is:

CF₂Cl₂ + hν→CF₂Cl+Cl

We already know that the resulting Cl atoms can participate in the decomposition of ozone according to the catalytic cycle shown in Fig. 23.19. A number of experimental observations have linked this chemistry of CFCs to a dangerously rapid decline in the concentration of stratospheric ozone over the last three decades. Ozone depletion has increased the amount of ultraviolet radiation at the Earth’s surface, particularly radiation in the ‘UVB range’, 290–320 nm. The physiological consequences of prolonged exposure to UVB radiation include DNA damage, princip ally by photodimerization of adjacent thymine bases to yield either a cyclobutane thymine dimer (5) or a so-called 6–4 photoproduct (6). The former has been linked directly to cell death and the latter may lead to DNA mutations and, consequently, to the formation of tumours. There are several natural mechanisms for protection from and repair of photochemical damage. For example, the enzyme DNA photolyase, present in organisms from all kingdoms but not in humans, catalyses the destruction of cyclobutane thymine dimers. Also, ultraviolet radiation can induce the production of the pigment melanin (in a process more commonly known as ‘tanning’), which shields the skin from damage. However, repair and protective mechanisms become increasingly less effective with persistent and prolonged exposure to solar radiation. Consequently, there is concern that the depletion of stratospheric ozone may lead to an increase in mortality not only of animals but also the plants and lower organisms that form the base of the food chain. Chlorofluorocarbons are being phased out according to international agreements and alternatives, such as the hydrofluorocarbon CH₂FCH₃, are already being used. However, the temperature inversion shown in Fig. 23.17 leads to trapping of gases in the troposphere, so CFCs are likely to continue to cause ozone depletion over many decades as the molecules diffuse slowly into the middle stratosphere, where they are photolysed by intense solar UV radiation.

Fig. 23.19 A catalytic cycle showing the propagation of ozone decomposition by chlorine atoms.

A large proportion of solar radiation with wavelengths below 400 nm and above 1000 nm is absorbed by atmospheric gases such as ozone and O₂, which absorb ultra violet radiation (Impact I23.1), and CO₂ and H₂O, which absorb infrared radiation (Impact I13.2). As a result, plants, algae, and some species of bacteria evolved photo synthetic apparatus that capture visible and near-infrared radiation. Plants use radiation in the wavelength range of 400–700 nm to drive the endergonic reduction of CO₂ to glucose, with concomitant oxidation of water to O2 (∆rG^⊕=+2880 kJ mol⁻¹), in essence the reverse of glycolysis and the citric acid cycle (Impact I7.2):

Electrons flow from reductant to oxidant via a series of electrochemical reactions that are coupled to the synthesis of ATP. The process takes place in the chloroplast, a special organelle of the plant cell, where chlorophylls a and b (7) and carotenoids (of which β-carotene, 8, is an example) bind to integral proteins called light harvesting complexes, which absorb solar energy and transfer it to protein complexes known as reaction centres, where light-induced electron transfer reactions occur. The combination of a light harvesting complex and a reaction centre complex is called a photosystem. Plants have two photosystems that drive the reduction of NADP+ (9) by water:

It is clear that energy from light is required to drive this reaction because, in the dark, E^⊕=−1.135V and ∆rG^⊕=+438.0 kJ mol⁻¹. Light harvesting complexes bind large numbers of pigments in order to provide a sufficiently large area for capture of radiation. In photosystems I and II, absorption of a photon raises a chlorophyll or carotenoid molecule to an excited singlet state and within 0.1–5 ps the energy hops to a nearby pigment via the Förster mechanism (Section 23.7e). About 100–200 ps later, which corresponds to thousands of hops within the light harvesting complex, more than 90 per cent of the absorbed energy reaches the reaction centre. There, a chlorophyll a dimer becomes electronically

excited and initiates ultrafast electron transfer reactions. For example, the transfer of an electron from the excited singlet state of P680, the chlorophyll dimer of the photo system II reaction centre, to its immediate electron acceptor, a phaeophytin a molecule (a chlorophyll a molecule where the central Mg2+ ion is replaced by two protons, which are bound to two of the pyrrole nitrogens in the ring), occurs within 3 ps. Once the excited state of P680 has been quenched efficiently by this first reaction, subsequent steps that lead to the oxidation of water occur more slowly, with reaction times varying from 200 ps to 1 m_s . The electrochemical reactions within the photosystem I reaction centre also occur in this time interval. We see that the initial energy and electron transfer events of photosynthesis are under tight kinetic control. Photosynthesis captures solar energy efficiently because the excited singlet state of chlorophyll is quenched rapidly by processes that occur with relaxation times that are much shorter than the fluorescence lifetime, which is typically about 1 ns in organic solvents at room temperature. Working together, photosystem I and the enzyme ferredoxin: NADP+ oxidoreductase catalyse the light-induced oxidation of NADP +to NADPH. The electrons required for this process come initially from P700 in its excited state. The resulting P700+ is then reduced by the mobile carrier plastocyanin (Pc), a protein in which the bound copper ion can exist in oxidation states +2 and +1. The net reaction is

Oxidized plastocyanin accepts electrons from reduced plastoquinone (PQ, 10). The process is catalysed by the cytochrome b6 f complex, a membrane protein complex that resembles complex III of mitochondria (Impact I7.2):

Fig. 23.20 In plant photosynthesis, light induced electron transfer processes lead to the oxidation of water to O2 and the reduction of NADP+ to NADPH, with concomitant production of ATP. The energy stored in ATP and NADPH is used to reduce CO2 to carbohydrate in a separate set of reactions. The scheme summarizes the general patterns of electron f low and does not show all the intermediate electron carriers in photosystems I and II, the cytochrome b6f complex, and ferredoxin: NADP+ oxidoreductase.

This reaction is sufficiently exergonic to drive the synthesis of ATP in the process known as photophosphorylation. Plastoquinone is reduced by water in a process catalysed by light and photosystem II. The electrons required for the reduction of plastoquinone come initially from P680 in its excited state. The resulting P680+ is then reduced ultimately by water. The net reaction is

In this way, plant photosynthesis uses an abundant source of electrons (water) and of energy (the Sun) to drive the endergonic reduction of NADP+, with concomitant synthesis of ATP (Fig. 23.20). Experiments show that, for each molecule of NADPH formed in the chloroplast of green plants, one molecule of ATP is synthesized. The ATP and NADPH molecules formed by the light-induced electron transfer re actions of plant photosynthesis participate directly in the reduction of CO₂ to glucose in the chloroplast:

6 CO₂ +12 NADPH +12 ATP +12 H⁺→ C₆H₁₂O₆ +12 NADP++12 ADP +12 Pi+6H₂O

In summary, plant photosynthesis uses solar energy to transfer electrons from a poor reductant (water) to carbon dioxide. In the process, high energy molecules (carbohydrates, such as glucose) are synthesized in the cell. Animals feed on the carbo hydrates derived from photosynthesis. During aerobic metabolism, the O₂ released by photosynthesis as a waste product is used to oxidize carbohydrates to CO₂, driving biological processes, such as biosynthesis, muscle contraction, cell division, and nerve conduction. Hence, the sustenance of life on Earth depends on a tightly regulated carbon–oxygen cycle that is driven by solar energy.

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مواضيع ذات صلة

Experimental probes of reactive collisions

Photosensitization

Rate laws of complex photochemical reactions

The overall quantum yield of a photochemical reaction

Resonance energy transfer

The primary quantum yield

Timescales of photophysical processes

The catalytic efficiency of enzymes

The Michaelis–Menten mechanism of enzyme catalysis

Enzymes

Application of the techniques

Monitoring the progress of a reaction