The relationship between the density of the unpaired electron and reactivity of free radicals

Theoretical and Experimental Chemistry -     

Correlation between intramolecular bond lengths and K-shell σ-shape resonances in gas-phase molecules

Analysis of K-shell excitation spectra of gas-phase molecules containing at least two atoms of either B, C. N. O or F reveals the existence of a striking correlation between the bond length of the atomic pair. the sum of their atomic numbers and the associated α-shape resonance energy. Empirical rules are established which allow the derivation of intramolecular distances with accuracies reliably better than ± 0.05 A from the K-.shell absorption spectrum of one of the atoms in the Molecule.     

Prototropic mechanism of intramolecular migration of the unpaired electron in the radical-cation of 3,6-di-tert-butylpyrocatechol orthocarbonate

Conclusion The radical-cation of 3,6-di-tert-butylpyrocatechol orthocarbonate was obtained. It was established that the dynamic effects in its EPR spectra are due to the migration of a proton, attached by a hydrogen bond, from ligand to ligand.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1037–1039, May, 1988.     

Effect of intramolecular rotational reorganization of reagents on the ratio between free energies of activation and reaction

Conclusions The relationship between the free energies of activation G and reaction G_o for proton transfer processes have been analyzed, taking into account the effect of hindered rotation of the reagents. We have shown that the considered effect can considerably affect the shape of the G=f(G_o) curve.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 77–81, January 1989.     

Solvent-controlled regioselectivity in the ortho-palladation of benzylamines and intramolecular exchange in cyclopalladated ligands

Conclusions Cyclometallation of 1-(3,4-dimethoxyphenyl)-2-methyl-3-(4-nitrophenyl)-2-azapropane upon treatment with Pd(II) acetate occurs at the electron rich dimethoxy-substituted aromatic ring in CHCl₃ solution, and at the electron poor nitrophenyl ring in AcOH solution.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 175–181, January, 1988.     

Imidazole-substituted oxoverdazyl radical as a mediator of intramolecular and intermolecular exchange interaction

The 3-(2'-imidazolyl)-1,5-dimethyl-6-oxoverdazyl radical (imvd(*)) and the corresponding tetrazane H3imvd were prepared and structurally characterized, the former as two different hydrates. Reaction of imvd(*) with [M(hfac)2] led to the formation of monometallic complexes [M(hfac)2(imvd(*))] (M = Ni and Mn). They were characterized by single-crystal X-ray diffraction. In the solid state, all four radical-containing compounds exhibit imidazole-oxoverdazyl pi stacking. Following the structural analysis, imvd(*) behaves as an antiferromagnetic (AF) coupled chain with J = -100 cm(-1) (H = -J summation operator SiS(i+1)). The magnetic behavior of [M(hfac)2(imvd(*))] complexes is interpreted with a four-coupled spin model with a metal ion radical intramolecular interaction (JMn = -62.5 cm(-1) and JNi = 193 cm(-1); H = -JSMSimvd) and an AF intermolecular interaction (JMn' = -12.6 cm(-1) and JNi' = -4.3 cm(-1)) related to imidazole-oxoverdazyl pi stacking.     

Theory of kinetics of electron exchange reactions in a polar medium

Conclusions An estimate was made of the effective distance of charge transfer and the effective frequency of the solvent polarization fluctuations from an analysis of the experimental activation energies of a number of symmetrical redox reactions between complex ions.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2155–2157, September, 1973.     

Vibrational relaxation and coupling of two OH-stretch oscillators with an intramolecular hydrogen bond

We studied the vibrational dynamics of the OH-stretch oscillators of an alcohol with two vicinal OH groups using femtosecond midinfrared pump-probe spectroscopy. The absorption spectrum of pinacol (2,3-dimethyl-2,3-butanediol) in CDCl3 shows two OH-stretch peaks belonging to hydrogen bonded and free OH groups. The anharmonicities of the hydrogen-bonded and free OH-stretch vibrations are 180 and 160 cm(-1), respectively. The lifetime T1 of the OH-stretch vibration is found to be 3.5 +/- 0.4 ps for the hydrogen bonded and 7.4 +/- 0.5 ps for the free OH group. We observed sidebands in the transient spectra after excitation of the bonded OH group, which we attribute to a progression in a low-frequency hydrogen-bond mode. The sideband is redshifted 60 cm(-1) with respect to the 0 --> 1 transition. Due to the coupling between the two OH groups and the presence of the sidebands, simultaneous excitation of both OH-stretch vibrations leads to oscillations on the pump-probe signal with frequencies of 40 and 60 cm(-1).     

Facile generation of tripeptide radical cations in vacuo via intramolecular electron transfer in CuII tripeptide complexes containing sterically encumbered terpyridine ligands

Molecular radical cations of tripeptides of the form glycylglycyl(residue X) (GGX*+) are produced by the collision-induced, intramolecular one-electron transfer of [Cu(II)(L)GGX]*2+ complexes (L = triamine ligand). We demonstrate, for the first time, the formation of molecular radical cations of all of the aliphatic, basic, aromatic, acidic, and some heteroatom-bearing GGX tripeptides, albeit inefficiently in some cases, by altering the structure of the auxiliary polyamine ligand bound to the copper atom. The design of the ligand allows exquisite control over the nature of the dissociation pathway. Steric hindrance of bulky groups in the ligand affects the binding of the peptide to the copper ion; this interaction is an important factor in determining whether the electron transfer pathway predominates.     

Determination of the rate of intramolecular exchange between two complex spin systems from the NMR spectra

Reversible 1,2-shifts of the ethyl group in the 9-ethyl-9, 10-dimethylphenanthrenonium ion lead to a reversible change of the ring hydrogen states (A′B′C′D′ ? A″B″C″D″). Line shape analysis for the determination of the exchange rate was performed for this case of intramolecular exchange between the two multi-spin systems. The calculations are based on density matrix formalism.     

Interpreting the solvent dependence for the rate of electron exchange between tetracyanoquinodimethane and its anion radical

An approach previously devised is used in interpreting the observed solvent dependence of the rate of electron exchange between tetracyanoquinodimethane and its anion radical, as it incorporates effects from strong interaction in electrontransfer reactions involving intramolecular rearrangement. The kinetic characteristics have been derived, for which satisfactory values have not been obtained in the interpretation based on the traditional approach.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 3, pp. 353–357, May–June, 1987.I am indebted to L. D. Zusman for directing attention to [4].     

Effect of intramolecular hydrogen bonds on the kinetics of proton exchange of thiocarboxylic acids with phenols

The kinetics and mechanism of proton exchange between thiocarboxylic acids and o-substituted phenols with intramolecular hydrogen bonds (IMHB) were studied by dynamic PMR spectroscopy. Fast uncatalyzed exchange in cyclic binary complexes is observed for phenols with weak IMHB. In systems with strong IMHB fast exchange occurs only in the presence of an alkaline catalyst, the reaction of which with the phenol leads to cleavage of the IMHB. Concerted transfer of two protons and a cation is realized in the final step of the reaction. According to the proposed mechanism, the measured energy of activation of proton exchange is the sum of the energies of activation of the steps involving cleavage of the IMHB and strictly proton transfer.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 24, No. 1, pp. 49–56, January–February, 1988.     

The synthesis of carbohydrate-derived acylsilanes and their intramolecular free radical cyclizations with the formation of polyoxygenated cyclopentanes

A convenient way for the synthesis of acylsilanes from arabinose, lyxose, and ribose is developed. All the chiral centers of the carbohydrate templates are conserved, and only the reducing end is transformed into the acylsilane functional group. The non-reducing end of the templates can be converted into a bromide. These bromo acylsilanes undergo efficient intramolecular radical cyclizations to give polyoxygenated cyclopentanes.