Aleksandr M. Kuznetsov

Resonance and Environmental Fluctuation Effects in STM Currents through Large Adsorbed Molecules

Abstract Scanning tunnelling microscopy (STM) involving electron tunnelling through large adsorbate molecules with discrete electronic levels accessible at low bias voltage, exhibits conceptual and physical analogies to other thermal and optical multi-level electronic processes. The analogies are most conspicuous if the adsorbate levels are strongly coupled to the environmental molecular, conformational or solvent nuclear motion but interact weakly with the conducting substrate and tip. These conditions would be appropriate for example for adsorbed large transition metal complexes or redox metalloproteins. In these limits electronic-vibrational coupling induces resonance between the local adsorbate level and either the substrate or the tip levels, and the STM current-voltage relations can be approached by methods known from the theory of related electronic transitions such as long-range molecular electron transfer and multi-photon optical processes. We provide a new theoretical frame for STM processes in this limit. The formalism rests on perturbative coupling of the adsorbate levels to the substrate and tip. Specific models incorporate strong coupling to the adsorbate and environmental nuclear motion, vibrational relaxation features, and the continuous electronic spectra of the substrate and tip. All these features are directly and transparently reflected in the current-voltage relations of the STM process.

Self-consistent environmental fluctuation effects on the electronic tunnel factor and the activation Gibbs energy in long-range electron transfer

Abstract Electronic—vibrational interaction including the solvent environment in long-range electron transfer is disregarded in most applications of electron transfer theory and the electron tunnel factor associated with a constant electron density. We provide here a formalism for self-consistent variational calculation of both the electron tunnel factor and the polarization configuration at the crossing of the reactants and products potential Gibbs energy surfaces for diabatic processes. The scheme applies to arbitrary localized electronic wave function forms and leads in general to modulation of the electronic wave functions by the environment, to non-Condon effects, and to lower activation Gibbs energy compared with the Condon limit of constant electron density. We show specifically how the formalism can be combined with a set of LCAO MOs. We also show that single-exponential wave functions lead to significantly larger tunnel factors which reach asymptotic limiting values at long electron transfer distances where the orbital exponent has dropped to 60–75% of its value at polarization equilibrium.

Dissipative relaxation of a low-energy intermediate electronic state in three-level electron transfer

Abstract Long-range electron transfer (ET) in molecular and solid state systems always involves intermediate “environmental” electronic states. These participate in the superexchange mode if their energies are high, or sequentially when the energy is low and the states temporarily populated. The intermediate state nuclear motion can, however, be partially unrelaxed prior to the second ET step and the latter mechanism therefore differs from that of consecutive independent ET steps. We have analyzed the effect of intermediate state vibrational damping in a three-level process by a one-dimensional model and simple trajectory calculations. Damping is most reflected in the diabatic limit. Complete absence of damping gives a quadratic dependence on the electron exchange matrix elements. This differs from the fourth-order dependence obtained by second-order perturbation theory and a single reactive attempt at the intermediate-final state crossing. Vibrational damping drastically modifies this and the dependence on the electronic factor can now be of either second or fourth order, depending on the energies of the two crossing regions. This can have profound effects on the reaction free energy profile, external field dependence, etc. We finally discuss the first two ET steps in the bacterial photosynthetic reaction centre in terms of these views.

Bioluminescent method in studying the complex effect of sewage components.

The inhibition of bacterial luminescence has been used in testing industrial enterprises sewage. The toxicity of the sewage is less than the total toxicity of separate components due to neutralization of quinone products of polyphenol oxidation in the reactions with the other phenol components of sewage. Toxicity increase is due to their influence on the cell membrane. Studies of cell ultrastructure confirm this fact. The studied mechanism of the complex effect allowed a more accurate forecast of the ecological situation during the discharge of phenol compounds and metals. It also showed the necessity of taking into account the complex effect of sewage components on contaminant discharge into water reservoirs.

Electron tunnelling in electrochemical processes and in situ scanning tunnel microscopy of structurally organized systems

Abstract The electronic tunnel factor in homogeneous long-range electron transfer (ET) systems has been characterized experimentally and theoretically in great detail. Outstanding questions, however, remain, for example interference in multiple ET routes and environmentally induced barrier fluctuations. Well characterized electrochemical long-range ET systems have also become available where the tunnel factor can be accurately deconvoluted from the total current. Intriguing new effects here are associated with the electrode charge or overpotential effects on the tunnel factor and with the feasibility of observation of critical current behaviour near phase transitions in thin films across which electron tunnelling occurs. In situ scanning tunnel microscopy (STM) offers a conceptual and technical new frame for mapping of molecular ET routes through large adsorbate molecules with low-lying local fluctuating levels. Such a configuration would be representative of large transition metal complexes or redox metalloproteins. High-resolution images and new theoretical approaches to STM mapping of large adsorbates are, moreover reported. The independent in situ electrode potential control provides new, non-monotonous features in the current-voltage relations which differ from electrochemical ET behaviour at a single interface.

Scanning tunnelling microscopy currents through large adsorbate molecules as a molecular three-centre electronic process

Abstract We have investigated theoretically the tunnel features and curerent-voltage relations of scanning microscopy (STM) processes involving large adsorbate molecules in which the electronic levels are strongly coupled to the environmental nuclear motion. The theoretical formalism resembles electrochemical electron transfer theory but differs in both conceptual and formal respects. The tunnel features of high energy adsorbate levels can be approached by multilevel equations of motion or by simpler tunnel theory extended to fluctuating barriers. Low energy adsorbate levels induce several mechanisms where the levels are temporarily populated. The mechanisms extend to weak (diabetic) and strong (adiabatic) adsorbate-electrode interaction and full or partial vibrational relaxation in the intermediate state of the adsorbate electron or hole population. Some perspectives related to fluctuationally induced STM current pulses and Coulomb blockade monitored by STM are discussed.

DYNAMICS OF LOW-BARRIER PROTON TRANSFER IN POLAR SOLVENTS AND PROTEIN MEDIA

Abstract Proton transfer in chemical, physical, and biological systems frequently involves low and shallow barriers. This is caused by strong donor-acceptor interaction which also imposes fully adiabatic character on the process. Low or shallow barriers are not covered straightforwardly by tunnel views otherwise central in proton transfer theory. We introduce a formalism focused on a shallow enough proton transfer mode that the proton dynamics is indistinguishable from classical behaviour, strongly coupled to stochastic environmental nuclear motion. We derive rate constants for three cases, namely fast solvent relaxation, fast proton equilibration, and proton transfer followed by fast decay of the donor-acceptor complex. These cases may constitute useful descriptions of several kinds of processes including fast intramolecular proton transfer, hydrolytic and redox enzyme catalysis, and physical or biological proton conductivity.

Simple schemes in chemical electron transfer formalism beyond single-mode quadratic forms: environmental vibrational dispersion and anharmonic nuclear motion

Simple quadratic rate constant forms, sometimes combined with a single high-frequency harmonic nuclear mode are commonly used in electron transfer data analysis. Features such as vibrational frequency dispersion, nuclear equilibrium displacement combined with vibrational frequency changes, and local mode anharmonicity are, however, important in many contexts but are not covered by the simplest formalism. We provide here a new and simple parametric scheme, with a single running parameter for inclusion of these effects in the diabatic and weakly adiabatic limits of electron transfer. The parameter coincides with the symmetry factor in all cases of harmonic motion but with the local mode coordinate at the crossing of the potential surfaces when a local mode is significantly anharmonic. The scheme is numerical but easy to use and does not rely on extensive computational efforts. The effects of vibrational frequency changes, nuclear tunnelling, and local mode anharmonicity are illustrated by calculation of free energy relations of electron transfer in homogeneous solution and current–overvoltage relations in electrochemical electron transfer.

Fluctuational self-consistency of the theory of the elementary act of electrochemical reactions in the neighbourhood of the equilibrium state

Abstract The analysis of the theory of the elementary act of electrochemical reactions in relation to the fluctuation–dissipation theorem shows that in the neighbourhood of the equilibrium state the fluctuational self-consistency of the theory holds if an integral number of electrons is transferred in the elementary reaction act. The fluctuational self-consistency must also exist for the electrode processes with partial charge transfer. However, this requirement results in a non-zero correlation between fluctuations of the electrochemical reaction rate and the fluctuations of the number of electrons involved in the elementary act.

Fluctuational tunnel effects and vibrational dispersion in primary electron transfer processes of bacterial photosynthesis

Abstract We have investigated forward electron transfer (ET) from reduced bacteriopheophytin (bph − ) to the primary quinone (Q A ), and the recombination ET from Q A − to the oxidized special pair (bch) 2 + in the photosynthetic reaction centre of Rh . sphaeroides . The same reactant, Q A , is here engaged in two ET reactions of widely different distances. The processes can also be followed over broad temperature ranges. Focus is on two elements not commonly considered in ET theory. One is continuous environmental dielectric dispersion which is both formally and physically important. The other one is environmental fluctuational modulation of the tunnel factor which is important when the donor and acceptor are separated by environmental matter. The analysis shows, first that a broad dielectric dispersion is in line with the ET data up to ≈180°K. Fluctuational modulation of the tunnel factor is, secondly, unimportant in shorter-range forward ET from bph − to Q A (13 A centre-to-centre distance) but could be important in the much longer-range recombination ET from Q A − to (bch) 2 + (28 A).