Theory Of Electronic Transitions Research Articles

Making Molecules Vibrate: Interactive Web Environment for the Teaching of Infrared Spectroscopy

Infrared spectroscopy (IR) is a staple structural elucidation and characterization technique because of its ability to identify functional groups and its ease of use. Interestingly, it allows the capture of electronic effects via their influence on the bond strength of “probes”, such as the carbonyl group and also offers a wealth of examples for discussion on the theory of electronic transitions. For this reason, IR spectroscopy is typically taught both in theoretical classes and in applied structural analysis courses. In practice, there is rarely a link between those courses, and both suffer from the lack of exploratory learning, that is, tools with which students can explore the interplay between symmetry and selection rules, as well as electronic effects and vibrational frequencies─with almost immediate feedback. In practice, this might lead to students that are well skilled in looking up vibrational frequencies in lookup tables but do not understand the links to electronic effects and reactivity. Here, we introduce a web app that leverages semiempirical quantum mechanical (or force-field based) calculations, performed on a web service, in an interactive interface to provide an environment in which students can explore how slight changes to the structure manifest in changes of the spectrum. This approach avoids the time-consuming handling of potentially hazardous materials that might not be readily available and invites students to play with spectroscopy─to “see” and “test” electronic effects that are so commonplace in organic chemistry education. As a “side effect”, our web app also provides a powerful aid for research scientists to investigate how different structural modifications, such as substitution, isomerism, or steric strain, would manifest in the infrared spectrum.

Theory of intermolecular electron transfer in nanoscale biological structures

Macromolecular biological systems performing directed electron transfer are nano-sized structures. The distance between carrier molecules (cofactors), which represent practically isolated electron localization centers, reaches tens of angstroms. The electron transfer theory based on the concept of delocalized electron states, which is conventionally used in biophysics, is unable to adequately interpret the results of concrete observations in many cases. On the basis of the theory of electronic transitions in the case of localized states, developed in the physics of disordered matter, a mechanism of long-distance electron transfer in biological systems is suggested. The molecular relaxation of the microenvironment of electron localization centers that accompanies the electron transfer process is also considered.

Application of the standard theory of electronic transitions to the description of oscillations in the kinetics of electron transfer in reaction centers of purple bacteria

The standard theory of the electron transfer between donor and acceptor molecules was used to describe oscillations in the reduction kinetics of the intermediate electron acceptor BA and the primary electron acceptor HA. The kinetics of the reduction of BA and HA were simulated on the basis of the model in which one and two accepting modes were used. A principal experiment is offered for the selection of the suitable theory for adequate description of oscillations in the kinetics of electron transfer in the reaction centers of purple bacteria.

Electronic States and Photoprocesses in Bichromophore Systems

We present the results of quantum‐chemical investigation of energy transfer in organic molecules and systems and the inferences drawn. The Forster theory has been subjected to a critical analysis in order that the energy transfer could be described in the context of the current theory of nonradiative transitions and the incorrectness of the basic premises of the Forster theory has been demonstrated. A new variant of the mechanism of electronic energy transfer on the basis of the theory of electron transitions and of the quantum mechanics of molecules has been suggested. It is shown that the interaction of the molecules of the donor and acceptor perturbs the electronic states of isolated molecules even before the excitation of the donor molecule. A characteristic feature of the manifestation of intermolecular interaction is the spatial delocalization of the wave functions of the electronic states of interacting molecules, leading to the possibility of occurrence of conventional photophysical processes with participation of the electronic states of various molecules of the bimolecular system. In experimental investigations, the result of the intermolecular nonradiative transition is recorded as evidence of the spatial transfer of the energy of electronic excitation from the donor molecule to the acceptor molecule.

Theory of electronic transitions in slow atomic collisions

This review deals with quantitative descriptions of electronic transitions in atom-atom and ion-atom collisions. In one type of description, the nuclear motion is treated classically or semiclassically, and a wave function for the electrons satisfies a time-dependent Schr\"odinger equation. Expansion of this wave function in a suitable basis leads to time-dependent coupled equations. The role played by electron-translation factors in this expansion is noted, and their effects upon transition amplitudes are discussed. In a fully quantum-mechanical framework there is a wave function describing the motion of electrons and nuclei. Expansion of this wave function in a basis which spans the space of electron variables leads to quantum-mechanical close-coupled equations. In the conventional formulation, known as perturbed-stationary-states theory, certain difficulties arise because scattering boundary conditions cannot be exactly satisfied within a finite basis. These difficulties are examined, and a theory is developed which surmounts them. This theory is based upon an intersecting-curved-wave picture. The use of rotating or space-fixed electronic basis sets is discussed. Various bases are classified by Hund's cases (a)-(e). For rotating basis sets, the angular motion of the nuclei is best described using symmetric-top eigenfunctions, and an example of partial-wave analysis in such functions is developed. Definitions of adiabatic and diabatic representations are given, and rules for choosing a good representation are presented. Finally, representations and excitation mechanisms for specific systems are reviewed. Processes discussed include spin-flip transitions, rotational coupling transitions, inner-shell excitations, covalent-ionic transitions, resonant and near-resonant charge exchange, fine-structure transitions, and collisional autoionization and electron detachment.

Molecular orbital theory of electronic transitions at the vacancy in diamond

Abstract In diamond, a series of optical absorption lines lying between 1·66 and 3·0 eV, which have been labelled GR1 to GR8, are commonly observed. These lines are fairly sharp, and the symmetry of some of the states from which they are derived have been deduced from piezo-spectroscopic studies. Furthermore, photoconductivity indicates that the absorption lines GR2 to GR8 are associated with hole production. A model for the absorption lines is proposed and investigated. This model assumes the transitions take place around a vacancy, the vacancy being represented by a defect molecule as originally suggested by Coulson and Kearsley (1957). The lines GR2 to GR8 arise from transitions from the neutral vacancy to states associated with the negative vacancy and a hole located in one of the nearby molecular orbitals. The relative intensities of the transitions are calculated. Using results of a cluster model, the electronic orbitals occupied by the hole are located at the top of the valence band and are found t...

Calculations of potential energy surfaces in the complex plane

We calculate the energies of the 2∑+ and 2Π states of the HF+ system by the ab initio restricted Hartree-Fock method at a number of real and complex values of the internuclear separation. The energies of the associated spin-orbit states are calculated via a semiempirical scheme. The complex intersection point of these energy curves, an integral component of a semiclassical theory of electronic transitions, is determined by the ab initio calculations. The intersection point is also determined, for comparative purposes, by other methods of analytic continuation, i.e., Gaussian and rational-fraction fits to the real-valued energy calculations. Semiclassical dynamical calculations of the cross section for the process F(2 P 3/2)+H+ → F(2 P 1/2)+H+ using the various intersection points yield differences up to 20%.

Theory of Electronic Transitions in Slow Atomic Collisions

The near-adiabatic transition probability between a pair of pseudocrossing adiabatic (molecular) electronic states is studied be generalizing Zener's time-dependent treatment and describing the heavy-particle motion classically. This more rigorous formalism, based however on the simple Landau-Zener model, leads to the well-known Landau-Zener formula in a first approximation. From a reasonable definition of the width of the finite region over which the transitions take place on the trajectory, a precise condition which inherently limits the validity of the formula is derived. This condition may be compared with similar ones obtained in recent quite different derivations and removes some apparent difficulties which have been raised in connection with earlier treatments. The first correction to the Landau-Zener formula, associated with the finiteness of the transition region, is studied analytically and found to be important and to have interesting properties. This leads to further information about the validity of the simple Landau-Zener expression.

Theory of Electronic Transitions in Slow Atomic Collisions

The near-adiabatic transition probability between a pair of pseudocrossing adiabatic (molecular) electronic states is studied be generalizing Zener's time-dependent treatment and describing the heavy-particle motion classically. This more rigorous formalism, based however on the simple Landau-Zener model, leads to the well-known Landau-Zener formula in a first approximation. From a reasonable definition of the width of the finite region over which the transitions take place on the trajectory, a precise condition which inherently limits the validity of the formula is derived. This condition may be compared with similar ones obtained in recent quite different derivations and removes some apparent difficulties which have been raised in connection with earlier treatments. The first correction to the Landau-Zener formula, associated with the finiteness of the transition region, is studied analytically and found to be important and to have interesting properties. This leads to further information about the validity of the simple Landau-Zener expression.