I. V. Mikhailov

Simulation of photochemical processes and calculation of the quantum yields of isomerization reactions of substituted dienes

The possibility of construction of a semiempirical method for simulation of photochemical processes and calculation of the quantum yields of reactions has been studied. The practicability of the approach was exemplified with the photochemical transformations of some diene compounds into their cyclic isomers, namely, 2,4-dimethylpentadiene-1,3 → trimethylcyclobutene, 2,3-dimethylbutadiene-1,3 → dimethylcyclobutene, pentadiene-1,3 → 3-methylcyclobutene, cis-butadiene-1,3 → cyclobutene, and 2-methylbutadiene-1,3 → 1-methylcyclobutene. The calculated quantum yields of the reactions are in qualitative and quantitative agreement with experimental data.

A method for simulation of photochemical processes and calculation of quantum yields of reactions

The possibility of construction of a semiempirical method for simulation of photochemical processes and calculation of quantum yields of reactions has been studied. The practicability of the approach was demonstrated for the o-xylene → m-xylene, m-xylene → p-xylene, m-xylene → o-xylene, and o-diethylbenzene → m-diethylbenzene photoisomerization reactions as an example. The calculated quantum yields of the reactions are in qualitative agreement with experimental data.

Calculation of the Quantum Yield of the Photochemical Isomerization Reaction Methoxybutadiene Methoxycyclobutene

The methoxybutadiene → methoxycyclobutene photoisomerization process was simulated and the quantum yield of the product was calculated. The validity of the theoretical ideas and model concepts used in the proposed method for the calculation of quantum yields of photochemical reactions was confirmed. It was shown that the empirical parameter used in the model (broadening parameter) is stable in value and has the inherent property of transferability over a series of related molecules and analogous reactions. The cause of a substantial difference between the experimental quantum yields of reactions of the same type for a number of molecules bearing different substituents was revealed: this is a change in the form of vibrations and/or in the magnitude of structural rearrangement that directly involves the substituent.

Simulation of the cyclopropanecarbaldehyde and cyclopropylethanone photoisomerization processes and calculation of the quantum yields of the reactions

The cyclopropanecarbaldehyde → 2-butenal and cyclopropylethanone → 3-pentenone-2 photoisomerization processes were simulated. The calculated quantum yields of the products are in quantitative agreement with experimental data. The validity of the method proposed for the calculation of quantum yields of photochemical reactions was confirmed. It was found that if the condition of relative smallness of the optical transition probabilities as compared to the quantum beat frequency is met, the quantum yields can be quantitatively estimated with satisfactory accuracy directly from the transition probabilities without running the full calculation of the phototransformation kinetics. It was shown that the integral characteristics of photochemical reactions (quantum yields) are highly responsive to the conformational state of the molecules involved in the process.

Simulation of photochemical reactions of butadiene and calculation of the quantum yields

The photochemical reactions of butadiene-1,3 have been simulated. The calculated quantum yields of the products quantitatively agree with experimental data (deviation from the experimental data in on average less than 30%). The efficacy of the method in predicting photochemical processes of various types (isomerization, decomposition) occurring simultaneously via different pathways has been shown. It has been confirmed that the quantum beat frequency of resonating states can be used as a parameter of the method and, as such, allows obtainable quantitative estimates to be refined.

The arrow of time at the early stages of biosphere evolution. Determinism and pluralism

The modern theory of molecular processes provides a natural explanation for some clue observed features of biosphere formation during the early stages of the appearance of the molecular world. It was shown that a mere increase in the complexity of molecular objects is accompanied by the first manifestations of the fundamental properties that become subsequently predominant in much more complex objects: viruses, cells, bacteria, etc.

A modified method for calculating quantum yields of photochemical reactions

The previously proposed approach to calculation of quantum yields of photochemical reactions has been improved. The new version uses a different procedure of accounting for the asymmetry (anharmonicity) of potential wells of the molecules involved in the photochemical conversion. The form of a doubleminimum potential energy surface has been considered in a more correct manner. It has been shown that the number of model parameters is reduced by two thirds compared with the model used previously, adequately reflecting the characteristics of structural transformations of polyatomic molecules in which a relatively small part of the total number of coordinates are transformed. The quantum yields of photochemical transformations of six dienes into their cyclic isomers have been calculated. Quantitative agreement of the calculated values with the experimental data has been achieved for all the reactions. It has been shown that the model parameters have good transferability in a series of related molecules, thereby rendering the simulation of photochemical processes by the method in question predictive.

Processes of the development of the molecular world at early stages of its formation: Evidence from the investigation of the behavior of the vibrational entropy of molecules

We analyzed the possibility of explaining empirical relations in the evolution of the molecular world on the basis of fundamental physical laws and theories of the behavior of the entropy of a system. Quantitative expressions were evaluated describing the entropy of internal motion of atoms in a molecular ensemble (vibrational entropy), which is a term of total entropy with a clear physical meaning. It was shown that vibrational entropy is controlled primarily by the structure of the compound rather than the number and identity of atoms in the molecule. During the early stages of evolution, the appearance of acyclic structures is more advantageous than that of cyclic structures. An increase in the complexity of the molecular world inevitably leads to the appearance of molecules with a complex skeleton formed mainly by bonds of the same elasticity as C–C. The necessity of helical structures, whose spatial elasticity is provided by hydrogen bonds, was explained.

On the possibility of a priori quantitative prediction of the quantum yield of a photochemical reaction

The possibility of performing predictive calculations of the kinetics and quantum yields of photochemical transformations of polyatomic molecules with satisfactory accuracy has been shown. Taking into account the physically grounded requirements imposed and rather stringent constraints on the prediction error (less than 50%), a satisfactory theoretical result has been obtained for all of the considered reactions of three chemically different types. In most cases (eight out of ten reactions), the deviation from the experimental data was less than 20%. Model parameters have high transferability in a series of similar reactions, thereby ensuring the predictive character of simulation of photochemical processes using the method developed earlier. In complicated cases, it is possible to a priori theoretically assess the type of the reaction of interest and its physicochemical characteristics.

The feasibility of standardless molecular spectral analysis of mixtures under conditions of photochemical transformations of molecules

The problem of standardless molecular spectral analysis of multicomponent mixtures with allowance for photochemical reactions of their molecules has been formulated and its formal procedure has been developed. The case when the time dependence of the spectrum of the mixture is determined only by the change in the concentrations of the components with time and the spectra of the individual components remain constant has been considered. The corresponding requirements for the physicochemical properties of the molecules have been identified and shown to be met for quite a wide range of polyatomic molecules and their reactions. The exception is molecules with metastable excited electronic states. A large set of model calculations relevant to the actual molecular entities and their experimentally observed photochemical transformations have shown the practicability of the approach. The solution of the problem converges to exact values and is resistant to uncertainty in setting the initial spectral data due to the peculiarities of the experimental procedure, the choice of theoretical models, and other factors. The resulting theoretical values of the concentrations meet the least squares test and their errors are measured by the variance per unit weight. The variance makes it possible to evaluate the noise level of the spectral data used in the analysis and reliability of the result and can serve as a criterion for the correctness of the initial hypotheses about the qualitative composition of a multicomponent mixture.