N. I. Raevskii

Electron structure and reactivity of organofluorine compounds. 3. Mndo and ami calculations of the ground state of perfluoroalkyl chlorides, bromides, and iodides

The semiempirical quantum chemical MNDO and AMI methods were used to determine the equilibrium geometries and electron properties of molecules of perfluoroalkyl halides (RFX): CF3X, CF3CF2X, (CF3)2CFX, (CF3)3CX for X=Cl, Br, and I. It was determined that the effective charge on the Cl atom in RFCl is negative, positive on the I atom in RFI, and depends on RF for the Br atom in RFBr. The CF3 group can act as either an electron acceptor or donor in various perfluoroalkyl halides. The strongest Cα−I bond in the perfluoroalkyl halides occurs with a tertiary RF group.

Electron structure and reactivity of organofluorine compounds. 4. AMI calculations of anion radicals of primary, secondary, and tertiary perfluoroalkyl chlorides, bromides, and iodides

Calculations were carried out using the semiempirical quantum chemical AMI method for anion radicals (AR) of the perfluoroalkyl halides (RFX): CF3X, CF3CF2X, (CF3)2-CFX, and (CF3)3CX for X=Cl, Br, and I. All the ARs studied are thermally stable. The electron affinity of the perfluoroalkyl halides, and consequently, the thermal stability of their ARs increases in the series from F-methyl to F-tertbutyl halides and from the chlorides to bromides and iodides. During formation of an AR the spin density is preferentially localized on the σ* orbital of the Cα−X bond which leads to an increase in the distance between these atoms. Dissociation of the AR of tert-perfluorobutyl iodide to a perfluorocarbanion and an I atom is thermodynamically more favorable than dissociation with formation of a perfluoroalkyl radical and I−.

ISOMERIZATION OF DICHLORO- AND DIFLUOROBENZENES: A QUANTUM-CHEMICAL CONSIDERATION

The AM-1 method with full optimization of geometry was used to study several proposed isomerization mechanisms for dichloro- and difluorobenzenes on active centers as proton donors. Preferred positions of proton addition in dihalobenzenes were studied and protonation and activation energies for intramolecular migration of hydrogen and halogen atoms were determined. It was shown that the isomerization mechanisms are the same for dichloro-and difluorobenzenes and result from a 1,2-shift of the halogen. The activation energy for migration of a fluorine atom in difluorobenzenonium cations is substantially higher than for a chlorine atom in dichlorobenzenonium cations. The data obtained on the mechanism have experimental verification.

Dependence of the through-space19F-19F spin-spin coupling constant on the internuclear distance for polyfluorinated allyl cations

Conclusions1.The MNDO LCAO MO method was used to obtain the structural parameters of three polyfluoroallyl cations.2.The dependence of through-space contribution to the19F-19F spin-spin coupling constant through four bonds for polyfluoroallyl cations on the internuclear distance was experimentally established.

A quantum-chemical study of regioselectivity of indolization of some unsymmetrical ketone phenylhydrazones

The heat of formation (ΔHf) of unsymmetrical ketone phenylhydrazones and their enehydrazine tautomers was calculated by the semi-empirical quantum-chemical AM1 method. It is concluded that from ΔHf values it is possible to judge the regioselectivity of indolization of unsymmetrical ketone phenylhydrazones.

Paths for the formation of dioxin-type xenobiotics: Quantum-chemical treatment 1. Formation of polychlorobiphenylenes from polychlorobenzenes

The heats of formation of polychloroarynes (PCAs) from polychlorobenzenes (PCBs) have been calculated by the AMI method as a function of the composition and structure of the PCBs according to a two-step scheme including deprotonation with the formation of carbanions (PCCAs, ΔH1) and dechlorination to PCAs (ΔH2). The influence of the solvent on ΔH1 and ΔH2 has been evaluated in the framework of the solvaton model. It has been shown that the value of ΔH1 decreases as the number of C1 atoms in the PCB is increased, while ΔH2 increases along the same series due to the increase in the relative stability of the PCCA as the number of C1 atoms is increased. Although the dimerization of PCAs with the formation of polychlorobiphenylenes (PCBPs) is forbidden by orbital symmetry rules, calculations of fragments of the potential-energy surface have shown that this reaction takes place with an energy barrier amounting to about 1 kcal/mole.

Pathways of formation of xenobiotics of the dioxin type: Quantum-chemical consideration. 2. 1,2-Ketocarbenes as intermediates in the conversions of polychlorophenols to dioxin

The AM1 method was used to calculate the enthalpies of formation of polychlorinated 1,2-ketocarbenes (PCKC) from polychlorophenols (PCP) according to a two-step scheme — deprotonation with the formation of polychlorophenolate anions (PCPA, ΔH1) and dechlorination to PCKC (ΔH2). It was shown that the biradical state of PCKC has a lower energy than the singlet state. The influence of the solvent on ΔH1 and ΔH2 was estimated within the framework of the solvation model. It was shown that ΔH1 decreases with increasing number of Cl atoms in PCP, which is evidence of an increase in the acidity, whereas ΔH2 increases, which is due to an increase in the relative stability of PCPA with increasing number of Cl atoms. A calculation of the dimerization of PCKC in the singlet state showed that this process occurs with virtually no activation energy. The electronic state of 1,2-ketocarbene and its analogs in the singlet state was studied by a nonempirical MO LCAO SCF method. A comparison with 1,2-thioketocarbene and its analogs was performed.

Electronic structure and models of monooxygenase inductor receptor from a number of polychlorinated polycyclic compounds. 5. MNDO calculations of halogen derivatives of azoxybenzenes

The MNDO method has been used to calculate the electronic and geometric structure of 3,3′,4,4′-tetraehloroazoxybenzene (TCAOB), 3,3′,4,4′-tetrachloro-6-hydroxyazobenzene (TCHAB), and 2,3,7,8-tetrachlorodibenzofuran (TCDF). The TCAOB exists in the gas phase in the form of two configurations, one nonplanar and one nearly planar. The latter is approximately 4 kcal/mole less stable than the first. The oxidation of 3,3′,4,4′-tetrachloroazobenzene (TCAB) to TCAOB is endothermic only in the case of the acton of hydrogen peroxide, not molecular oxygen. The isomerization of TCAOB to TCHAB proceeds with a gain in energy (−62 kcal/mole); however, the reaction from the ground state of the TCAOB molecule is symmetry-forbidden and is possible only from an excited state of the TCAOB. The process of oxidation of TCAOB and TCHAB by molecular oxygen to form TCDF or 2,3,7,8-tetrachlorobenzo-p-dioxin is energy-favorable; this is important in judging the biological action of TCAOB.

Electronic structure and models of receptor of monooxygenase inductors from a number of polychlorinated polycyclic compounds. IV. MNDO calculations of halogen-substituted azobenzenes

The electron and geometric structures of the cis and trans isomers of 3,3′,4,4′-tetrachloroazobenzene (3,3′,4,4′-TCAB) and the trans isomers of 3,3′,5,5′-TCAB and 3,3′-dichloro-4,4′-difluoroazobenzene were calculated by the MNDO method. It was established that the trans isomer, which has a planar structure, is most stable for 3,3′,4,4′-TCAB. Change in the position of the Cl atoms in the azobenzene, i.e., the transition from 3,3′,4,4′-TCAB to 3,3′,5,5′-TCAB, does not lead to appreciable change in the formation energy, the position of the electronic levels, the nature of the frontier orbitals, or the charge distribution in the molecules. This gives reason to suggest that the biological activity of 3,3′,4,4′-TCAB is due to the metabolism products and not to the action of the substrates themselves.

Electronic structure and models of receptor of monooxygenase inductors from a number of polychlorinated polycyclic compounds. 3. Mndo calculations of chlorine and fluorine derivatives of dibenzo-p-dioxins

MNDO calculations for dibenzo-para-dioxin (DD) and its chlorine- and fluorine-substituted derivatives have shown that the geometric structure of the molecules and the Π-donor properties are very little dependent on the type of halogen (chlorine or fluorine) or on the position of the halogen in the molecule. The chlorine and fluorine derivatives of the DD do exhibit substantial differences in the charges on the C atoms directly connected to the halogens. The higher atomic charges on the C atoms connected to F atoms suggests that the fluorine-containing DDs will be capable of metabolism to mercapturic acids.