N. D. Chuvylkin

ESR AND QUANTUM CHEMICAL STUDIES OF THE STRUCTURES AND THERMAL TRANSFORMATIONS OF THE RADICAL CATIONS OF VINYLCYCLOPROPANE IN IRRADIATED FROZEN FREON MATRICES. SIMULATION OF RADICAL PROCESSES IN SOLIDS

Thermal transformations of vinylcyclopropane radical cations (VCP.+) in X-ray-irradiated frozen Freon matrices (CFCl2CF2Cl and CFCl3) were studied by ESR; radical processes involving VCP.+ in solid VCP were simulated.Gauche- andanti-VCP.+ were found to be the primary radical cations, however, the former, unlike the latter, is stable only under “gas-phase” conditions. The thermodynamic equilibrium betweenanti-VCP.+ and its less stable distonic form,dist(90,0)-C5H8.+, is established in frozen Freon matrices and the VCP host matrix; the structure of dist(90,0)****-C5H8.+ is stabilized by a molecule ofanti-VCP. In CFC3, along with dist(90,0)-C5H8.+,π-dimeric resonance [anti-VCP]2.+ complex was detected. A general scheme of the transformations of VCP.+ in the solid phase has been proposed.

ESR and quantum-chemical studies of the structure and thermal transformations of vinylcyclopropane radical cations in irradiated frozen Freon matrices. Simulation of radical processes in the gas phase

Thermal trasfomations of vinylcyclopropane (VCP) radical cations (RC) in X-ray irradiated frozen Freon matrices, CFCl2CF2Cl and CFCl3, were studed by ESR. Radical processes involving VCP.+ in very rarefied and moderately thickened gaseous VCP were simulated. Monomolecular cleavage of the cyclopropane ring ofgauche-VCP.+ (1) occurs to give the more thermally stable distonic radical cationdist(0.90)-C5H8.+ (3). As the density of VCP increases RC3 adds at the double bond ofanti-VCP to give the distonic RC,.CH2CH2CHCH(CH2)3CHCHCH2+ (5). Under the same conditions, the less thermally stableanti-VCP.+(2) undergoes monomolecular isomerization into RC1 or reacts withanti-VCP with the rearrangement (as in the condensed phase) to give its distonic form,dist(90.0)-C5H8.+ (4). The MNDO-UHF method was adapted for quantum-chemical analysis of the constants of isotropic hyperfine coupling with1H and13C nuclei in neutral and charged hydrocabon radicals, since the standard version of this method inadequately reproduces the structural parameters of low-symmetry (C1,Cs) paramagnetic species. A quantum-chemical analysis of the radiospectroscopic information and of the stereoelectronic control of thermal transformations of conformers of RC1 and2 into their structurally nonequivalent distonic forms3 and4, respectively, was carried out.

Quantum chemical procedure of automatic correction of optimized geometric parameters of free radicals

Experimental geometric parameters of the HCO σ-electron radical are compared with those optimized for total energy within the scope of widely distributed DFT, ab initio and semiempirical quantum chemical schemes. The comparative analysis of the whole set of geometric and magnetic resonance characteristics of the theoretical and experimental radical structures points to the possibility of systematical correction of the optimized geometric parameters of free radicals on the basis of ESR data. A specialized automatic computing procedure allowing to put into practice such a correction is developed and evaluated.

Quantum-chemical and spectroscopic studies of structures of 1 : 2 complexes of carbonyl compounds with aluminum halides

Quantum-chemical calculations of model systems, namely, benzaldehyde and its 1 : 1 and 1 : 2 complexes with AlCl3, were carried out by the MNDO method. In the 1 : 2 complex, a bridging Al-Cl-Al bond occurs. Apparently, this complex is stabilized through the Coulomb interaction between the positively charged C atom of the carbonyl group and the AlCl3 fragment, which carries an excessive negative charge and which is not involved in donor-acceptor bonding with the carbonyl O atom. The IR spectra of the 1 : 1 and 1 : 2 complexes of benzophenone with AlBr3 were recorded, and the differences in the low-frequency regions of these spectra, which are indicative of the presence of the Br2Al-Br-AlBr3 fragment in the 1 : 2 complex, are discussed.

A simple method for quantitative estimation of C-H bond lengths in hydrocarbon radical cations

A quantitative correlation between deviations of the C-H bond lengths of hydrocarbon molecules during their adiabatic single ionization and isotropic hyperfine coupling constants with protons of their primary radical cations (RC) was established. A simple method for estimation of the C-H bond lengths of hydrocarbon RC was proposed on the basis of the correlation found. Specific features of the structure of the RC of methane and severaln-alkanes of the general formula [H*(CH2nH*]+,n = 0 to 12, were analyzed. A widely used empirical rule, according to which deprotonation of hydrocarbon RC during the ion-molecular process is determined by the proton possessing the highest spin density, was refined and geometrically substantiated.

Correlation methods for analyzing and predicting reactivities in nucleophilic substitution processes

Various approaches used for analyzing and predicting reactivity in nucleophilic substitution reactions at saturated carbon atoms were systematized and compared. The similarities between these approaches, their scopes, advantages, and disadvantages were established. The trends and outlooks in the further development of qualitative and quantitative models for describing dependences of the reactivities of substances on the physicochemical properties of substrates and nucleophiles were analyzed.

CORRELATIONS BETWEEN MAGNETIC RESONANCE PARAMETERS OF ESR AND NMR SPECTRA

Empirically established correlations between magnetic resonance parameters of free radicals (g-factors, isotropic hyperfine coupling constans) and isostructural molecules (chemical shifts, isotropic spin-spin coupling constants) or structurally similar ligands in paramagnetic transition metal complexes (isotropic chemical shifts) are systematized and critically discussed. Quantum-chemical analysis of the suggested spin distribution damping coefficients in model systems and structurally similar chemical compounds is performed. Based on the results obtained, physicochemical interpretation of the observed correlations between the parameters of ESR and NMR spectra is given.

A comparative quantum-chemical analysis of electronic structures and spectroscopic parameters of cycloalkanes and cyclopolysilanes

A comparative quantum-chemical analysis of the electronic structures and spectroscopic parameters of the cycloalkanes C3H6, C4H8, C5H10, and C6H12 and their silicon analogs Si3H6, Si4H8, Si5H10 and Si6H12 was performed in the framework of the SCF MO LCAO method in the INDO approximation. Qualitative interpretation of “abnormal” ionization potentials and energies of electronic absorption spectra of cyclopolysilanes has been given.