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 (C ₁,C _s) 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.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 212–235, February, 1995.This work was carried out with the financial support of the Russian Foundation for Basic Research (Project No. 93-03-04075).     

Influence of Ring Annelation on the Mode Selectivity of the Ionized Diazabicycloheptenes and Corresponding Housanes: An ESR and ENDOR Study

The tricyclic azoalkanes, (1α,4α,4aα,7aα)‐4,4a,5,6,7,7a‐hexahydro‐1,4,8,8‐tetramethyl‐1,4‐methano‐1H‐cyclopenta[d]pyridazine ( 1c ), (1α,4α,4aα,6aα)‐4,4a,5,6,6a‐pentahydro‐1,4,7,7‐tetramethyl‐1,4‐methano‐1H‐cyclobuta[d]pyridazine ( 1d ), (1α,4α,4aα,6aα)‐4,4a,6a‐trihydro‐1,4,7,7‐tetramethyl‐1,4‐methano‐1H‐cyclobuta[d]pyridazine ( 1e ), and (1α,4α,4aα,5aα)‐4,4a,5,5a‐tetrahydro‐1,4,6,6‐tetramethyl‐1,4‐methano‐1H‐cyclopropa[d]pyridazine ( 1f ), as well as the corresponding housanes, the 2,3,3,4‐tetramethyl‐substituted tricyclo[3.3.0.0^2,4]octane ( 2c ), tricyclo[3.2.0.0^2,4]heptane ( 2d ), and tricyclo[3.2.0.0^2,4]hept‐6‐ene ( 2e ), were subjected to γ‐irradiation in Freon matrices. The reaction products were identified with the use of ESR and, in part, ENDOR spectroscopy. As expected, the strain on the C‐framework increases on going from the cyclopentane‐annelated azoalkanes and housanes ( 1c and 2c ) to those annelated by cyclobutane ( 1d and 2d ), by cyclobutene ( 1e and 2e ), and by cyclopropane ( 1f ). Accordingly, the products obtained from 1c and 2c in all three Freons used, CFCl₃, CF₃CCl₃, and CF₂ClCFCl₂, were the radical cations 3c ^.+ and 2c ^.+ of 2,3,4,4‐tetramethylbicyclo[3.3.0]oct‐2‐ene and 2,3,3,4‐tetramethylbicyclo[3.3.0]octane‐2,4‐diyl, respectively. In CFCl₃ and CF₃CCl₃ matrices, 1d and 2d yielded analogous products, namely the radical cations 3d ^.+ and 2d ^.+ of 2,3,4,4‐tetramethylbicyclo[3.2.0]hept‐2‐ene and 2,3,3,4‐tetramethylbicyclo[3.2.0]heptane‐2,4‐diyl. The radical cations 3c ^.+ and 3d ^.+ and 2c ^.+ and 2d ^.+ correspond to their non‐annelated counterparts 3a ^.+ and 3b ^.+, and 2a ^.+ and 2b ^.+ generated previously under the same conditions from 2,3‐diazabicyclo[2.2.1]hept‐2‐ene ( 1a ) and bicyclo[2.1.0]pentane ( 2a ), as well as from their 1,4‐dimethyl derivatives ( 1b and 2b ). However, in a CF₂ClCFCl₂ matrix, both 1d and 2d gave the radical cation 4d ^.+ of 2,3,3,4‐tetramethylcyclohepta‐1,4‐diene. Starting from 1e and 2e , the radical cations 4e ^.+ and 4e′ ^.+ of the isomeric 1,2,7,7‐ and 1,6,7,7‐tetramethylcyclohepta‐1,3,5‐trienes appeared as the corresponding products, while 1f was converted into the radical cation 4f ^.+ of 1,5,6,6‐tetramethylcyclohexa‐1,4‐diene which readily lost a proton to yield the corresponding cyclohexadienyl radical 4f ^.. Reaction mechanisms leading to the pertinent radical cations are discussed.     

The radical cation of hydrogen sulfide and related organic reactions

The electrochemical and chemical oxidation (by hinderedo-benzoquinones or NOClO₄) of H₂S in nonaqueous solutions (MeCN) proceeds with the donation of one electron. The formation of the unstable radical cation of hydrogen sulfide was detected by cyclic voltammetry. The radical cation decomposes to form H⁺ and the HS^. radical. The generation of the hydrogen sulfide radical cation was confirmed by ESR spectroscopy in a frozen Freon matrix. The possibility of using the hydrogen sulfide radical cation in the synthesis of organosulfur compounds under mild conditions was studied. The concept of the work was proposed by Prof. O. Yu. Okhlobystin. The first electrochemical experiments were performed when he was alive. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1182–1188, July, 2000.     

Single‐Electron Self‐Exchange between Cage Hydrocarbons and Their Radical Cations in the Gas Phase

We show that the radical cations of adamantane (C₁₀H₁₆^.+, 1 H^.+) and perdeuteroadamantane (C₁₀D₁₆^.+, 1 D^.+) are stable species in the gas phase. The radical cation of adamantylideneadamantane (C₂₀H₂₈^.+, 2 H^.+) is also stable (as in solution). By using the natural ¹³C abundances of the ions, we determine the rate constants for the reversible isergonic single‐electron transfer (SET) processes involving the dyads 1 H^.+/ 1 H, 1 D^.+/ 1 D and 2 H^.+/ 2 H. Rate constants for the reaction 1 H^.++ 1 D? 1 H+ 1 D^.+ are also determined and Marcus’ cross‐term equation is shown to hold in this case. The rate constants for the isergonic processes are extremely high, practically collision‐controlled. Ab initio computations of the electronic coupling (H_DA) and the reorganization energy (λ) allow rationalization of the mechanism of the process and give insights into the possible role of intermediate complexes in the reaction mechanism.     

Access to Stable Metalloradical Cations with Unsupported and Isomeric Metal–Metal Hemi‐Bonds

Metalloradical species [Co₂Fv(CO)₄]^.+ ( 1 ^.+, Fv=fulvalenediyl) and [Co₂Cp₂(CO)₄]^.+ ( 2 ^.+, Cp=η⁵‐C₅H₅), formed by one‐electron oxidations of piano‐stool cobalt carbonyl complexes, can be stabilized with weakly coordinating polyfluoroaluminate anions in the solid state. They feature a supported and an unsupported (i.e. unbridged) cobalt–cobalt three‐electron σ bond, respectively, each with a formal bond order of 0.5 (hemi‐bond). When Cp is replaced by bulkier Cp* (Cp*=η⁵‐C₅Me₅), an interchange between an unsupported radical [Co₂Cp*₂(CO)₄]^.+ (anti‐ 3 ^.+) and a supported radical [Co₂Cp*₂(μ‐CO)₂(CO)₂]^.+ (trans‐ 3 ^.+) is observed in solution, which cocrystallize and exist in the crystal phase. 2 ^.+ and anti‐ 3 ^.+ are the first stable thus isolable examples that feature an unsupported metal–metal hemi‐bond, and the coexistence of anti‐ 3 ^.+ and trans‐ 3 ^.+ in one crystal is unprecedented in the field of dinuclear metalloradical chemistry. The work suggests that more stable metalloradicals of metal–metal hemi‐bonds may be accessible by using metal carbonyls together with large and weakly coordinating polyfluoroaluminate anions.     

Effect of substituents on ionization potentials of phosphorus compounds. Conjugation in radical cations

The first vertical ionization potentials (I) of phosphorus compounds P(X_i)₃, OP(X_i)₃, SP(X_i)₃, (4-XC₆H₄)₃P, and PCX are related to the inductive, resonance, and polarizability parameters of inorganic, organic, and organometallic substituents X by dependences of the type I = I _H + a_I + b_R ⁺ + c, where I _H is the I value for X = H. The I values are also affected by hyperconjugation. The ratio of the contributions of the resonance (b_R ⁺) and polarizability (c) effects to the I value is determined by the degree of delocalization of the unpaired electron and the positive charge in the radical cations formed upon ionization of neutral molecules. The _R ⁺ resonance parameters of organosilicon, organogermanium, and organotin substituents bound to the P ^·+ radical cation center were calculated for the first time.     

Water-catalyzed isomerization of the glycine radical cation. From hydrogen-atom transfer to proton-transport catalysis

The isomerization reactions of the glycine radical cation, from [NH₂CH₂COOH]⁺, I, to [NH₃CHCOOH]⁺, II, or [NH₂CHC(OH)₂]⁺, III, in the presence of a water molecule have been studied theoretically. The water molecule reduces dramatically the energy barriers of the III and IIII tautomerizations owing to a change in the nature of the process. However, the role of the water molecule depends on the kind of isomerization, the catalytic effect being more important for the IIII reaction. As a consequence, the preferred mechanism for the interconversion of glycine radical cation I to the stablest isomer, III, is the direct one-step mechanism instead of the two step (III and IIIII) process found for isolated [NH₂CH₂COOH]⁺. When using ammonia as a solvent molecule, a spontaneous proton-transfer process from [NH₂CH₂COOH]⁺ to NH₃ is observed and so no tautomerization reactions take place. This behavior is the same as that observed in aqueous solution, as has been confirmed by continuum model calculations.Contribution to the Jacopo Tomasi Honorary Issue     

Reactivity of Diarylnitrenium Ions

Hydride abstraction from diarylamines with the trityl ion is explored in an attempt to generate a stable diarylnitrenium ion, Ar₂N⁺. Sequential H-atom abstraction reactions ensue. The first H-atom abstraction leads to intensely colored aminium radical cations, Ar₂NH^.+, some of which are quite stable. However, most undergo a second H-atom abstraction leading to ammonium ions, Ar₂NH₂⁺. In the absence of a ready source of H-atoms, a unique self-abstraction reaction occurs when Ar=Me₅C₆, leading to a novel iminium radical cation, Ar=N^.+Ar, which decays via a second self H-atom abstraction reaction to give a stable iminium ion, Ar=N⁺HAr. These products differ substantially from those derived via photochemically produced diarylnitrenium ions.     

The role of oxygen acidity on the side-chain fragmentation of ring methoxylated benzocycloalkenol radical cations

The reactivity of 2,2-dimethyl-5-methoxyindan-1-ol (1) and 2,2-dimethyl-6-methoxytetral-1-ol (2) radical cations has been studied both in acidic and basic solution. At pH≤4 both 1⁺ and 2⁺ undergo C_αH deprotonation as the exclusive reaction with k=4.6×10⁴ and 3.2×10⁴ s⁻¹, respectively. In basic solution 1⁺ and 2⁺ behave as oxygen acids undergoing ⁻OH-induced αOH deprotonation in a diffusion controlled process (k_−OH≈10¹⁰ M⁻¹ s⁻¹). An intermediate alkoxyl radical is formed which undergoes a 1,2-hydrogen atom shift in competition with CC β-scission (with 1⁺) or as the exclusive pathway (with 2⁺). A behavior which is interpreted in terms of the greater ease of ring-opening of a five membered ring as compared to a six-membered one.     

Cl-atom transfer from CFCl3 to the c-C6H11 radical. An indirect estimation of the CFCl2?Cl bond dissociation energy

The Cl-transfer reaction between CFCl₃ and c-C₆H₁₁ radicals (R) was studied in liquid cyclohexane (RH). The Arrhenius parameters for Cl abstraction were determined in the RH-CFCl₃ system versus the termination reaction between cyclohexyl radicals and competitively versus addition to C₂Cl₄ in the RH-CFCl₃-C₂Cl₄ system. The two sets of results are in very good agreement and give the following Arrhenius expression for the reaction R + CFCL₃ → RCl + CFCl₂ (2): where θ = 2.303RT in kcal/mol. Comparison with Cl-transfer data of other chloromethanes and chloroethanes shows that an increase in the C? Cl bond dissociation energy is the main cause of the reduced reactivity of CFCl₃. Based on a previously developed correlation, D(CFCl₂ ? Cl) is estimated to be equal to 74.4 kcal/mol.     

Formation of the Charge-Localized Dimer Radical Cation of 2-Ethyl-9,10-dimethoxyanthracene in Solution Phase

Although dimer radical ions of aromatic molecules in the liquid-solution phase have been intensely studied, the understanding of charge-localized dimers, in which the extra charge is localized in a single monomer unit instead of being shared between two monomer units, is still elusive. In this study, the formation of a charge-localized dimer radical cation of 2-ethyl-9,10-dimethoxyanthracene (DMA), (DMA)₂^.+ is investigated by transient absorption (TA) and time-resolved resonance Raman (TR³) spectroscopic methods combined with a pulse radiolysis technique. Visible- and near-IR TA signals in highly concentrated DMA solutions supported the formation of non-covalent (DMA)₂^.+ by association of DMA and DMA^.+. TR³ spectra obtained from 30 ns to 300 μs time delays showed that the major bands are quite similar to those of DMA except for small transient bands, even at 30 ns time delay, suggesting that the positive charge of non-covalent (DMA)₂^.+ is localized in a single monomer unit. From DFT calculations for (DMA)₂^.+, our TR³ spectra showed the best agreement with the calculated Raman spectrum of charge-localized edge-to-face T-shaped (DMA)₂^.+, termed DT^.+, although the charge-delocalized asymmetric π-stacked face-to-face (DMA)₂^.+, termed DF3^.+, is the most stable structure of (DMA)₂^.+ according to the energetics from DFT calculations. The calculated potential energy curves for the association between DMA^.+ and DMA showed that DT^.+ is likely to be efficiently formed and contribute significantly to the TR³ spectra as a result of the permanent charge-induced Coulombic interactions and a dynamic equilibrium between charge localized and delocalized structures.     

Direct IR spectroscopic study of the cyclopentadienyl radical

The products of vacuum pyrolysis of bis(cyclopentadienyl)nickel have been studied by matrix isolation infrared spetroscopy. Three IR bands at 3079, 1383 and 661 cm^–1 corresponding to the cyclopentadienyl radical have been observed. A comparison of the IR spectrum of the radical C₅H₅ with those of the ligand -C₅H₅ and free anion^–C₅H₅ has been performed.These results were presented for the first time in O. M. Nefedov's plenary lecture at X IUPAC Conference on Physical Organic Chemistry (Haifa, Israel, August 1990) (see ref. 1 and literature cited therein).Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1497–1499, August, 1993.     

First ionization potentials of amines and electronic effects in their radical cations

The possibility of application of linear free energy relationships for studying the effects of substituents on the first vertical ionization potentials of amines, I(n_N), was substantiated. The I(n_N) values depend on the inductive, resonance, and polarizability effects of substituents and are also affected by hyperconjugation. The _R ⁺ resonance parameters of substituents MR₃ (M = Si, Ge, Sn) and CH₂SiMe₃ bound to the N ^·+ radical cation center were calculated for the first time.     

Ionization potentials of halides. Substituent effects in Cl-, Br-, and I-centered radical cations

The first vertical ionization potentials (I) of halides HalX (Hal = Cl, Br, I; X is an inorganic or organic substituent) are linearly related to the inductive (_I), resonance (_R ⁺), and polarizability () constants of the substituents X (I = a + b_I + c_R ⁺ + d). As the atomic number of the Hal element in the Hal^·+X radical cations increases, the inductive interaction is strengthened while the polarizability interaction is weakened. Conjugation remains virtually independent of the Hal atom. The resonance _R ⁺-constants of the MX₃ (M = Si, Ge, Sn, Pb) substituents bound to the Hal^·+ radical cation centers were first calculated.     

OH-Induced shift from carbon to oxygen acidity in the side-chain deprotonation of 2-, 3- and 4-methoxybenzyl alcohol radical cations in aqueous solution: results from pulse radiolysis and DFT calculations

DFT calculations have been carried out for 2-, 3- and 4-methoxybenzyl alcohol radical cations (1⁺, 3⁺ and 4⁺, respectively) and the α-methyl derivatives 2⁺ and 5⁺ using the UB3LYP/6-31G(d) method. The theoretical results have been compared with the experimental rate constants for deprotonation of 1⁺-5⁺ under acidic and basic conditions. In acidic solution, the decay of 1⁺-5⁺ proceeds by cleavage of the C-H bond, while in the presence of ⁻OH all the radical cations undergo deprotonation from the α-OH group. This pH-dependent change in mechanism has been interpreted qualitatively in terms of simple frontier molecular orbital theory. The ⁻OH induced α-O-H deprotonation is consistent with a charge controlled reaction, whereas the C-H deprotonation, observed when the base is H₂O, appears to be affected by frontier orbital interactions.     

Spectroscopic studies of 1,2-diphenyl-3,3,4,4-tetramethylcyclobutene and 3,4-diphenyl-2,5-dimethyl-2,4-hexadiene radical cations and their possible interconversion

In order to study radical cation rearrangements, the species generated by γ-irradiation of 1,2-diphenyl-3,3,4, 4-tetramethylcyclobutene (CB) and 3,4-diphenyl-2,5-dimethyl-2,4-hexadiene (BD) in frozen Freon solutions were studied by monitoring their EPR and visible absorption spectra. The optical and EPR spectra are indicative of CB⁺ and BD⁺ Conversion of CB⁺ to BD⁺ is not a dominant process at low temperatures in CFCl₃ or the Freon mixture (50:50 CFCl₃:CF₂BrCF₂Br) but can be assisted photochemically.     

Structure and reactivity of the cysteine methyl ester radical cation

The structure and reactivity of the cysteine methyl ester radical cation, CysOMe^.+, have been examined in the gas phase using a combination of experiment and density functional theory (DFT) calculations. CysOMe^.+ undergoes rapid ion–molecule reactions with dimethyl disulfide, allyl bromide, and allyl iodide, but is unreactive towards allyl chloride. These reactions proceed by radical atom or group transfer and are consistent with CysOMe^.+ possessing structure 1 , in which the radical site is located on the sulfur atom and the amino group is protonated. This contrasts with DFT calculations that predict a captodative structure 2 , in which the radical site is positioned on the α carbon and the carbonyl group is protonated, and that is more stable than 1 by 13.0 kJ mol^?1. To resolve this apparent discrepancy the gas‐phase IR spectrum of CysOMe^.+ was experimentally determined and compared with the theoretically predicted IR spectra of a range of isomers. An excellent match was obtained for 1 . DFT calculations highlight that although 1 is thermodynamically less stable than 2 , it is kinetically stable with respect to rearrangement.     

Stoichiometry and mechanism of protonation of alkali metal salts of benzophenone radical anions by weak proton donors and its relevance to the base-catalyzed decomposition of benzopinacol

The Stoichiometry of the protonation of lithium and potassium salts of benzophenone radical anions and of the lithium salt of the fluorenone radical anion by methanol has been measured and found to be [(Ar₂C=O)^–]/[MeOH] =21. This result, which was obtained by the method of magnetic titration, implies that paramagnetism decays by the reaction between a ketyl anion and a ketyl radical (i.e., a protonated ketyl anion). The reactivities of alkali metal salts of fluorenone radical anions in relation to methanol exhibit a pronounced dependence on the nature of the counterion. No kinetic deuterium isotope effect has been found for the protonation of the lithium salt of the benzophenone radical anion in tetrahydrofuran (THF) bytert-pentyl alcohol. The lithium salt of the benzophenone radical anion inN,N,N,N-tetramethylethylenediamine (TMEDA) behaves markedly differently. Namely, its protonation by methanol exhibits 1 1 Stoichiometry and it reacts considerably more slowly withsec-butyl alkohol,K(THF)/K(TMEDA) = 2.5. Benzopinacol undergoes decomposition by an alkoxide base to diphenyl ketyl, which decays into an equimolar mixture of benzophenone and benzhydrol. The reaction follows second-order kinetics and the specific rate constants exhibit an inverse relationship with respect to the initial concentration of the alkoxide. With a very strong base benzopinacol decomposes into two diphenyl ketyl anions. On the basis of this information as well as on studies of products, relevant mechanisms are proposed for the protonation of ketyl anions and for the decomposition of aromatic pinacols in basic media.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 83–91, January, 1995.     

Remote substituent effect favoring the formation of syn-adducts in the chelation controlled radical reactions of γ-benzyloxy-α-methylenecarboxylic acid esters

The chelation controlled radical reactions of ethyl γ-benzyloxy-α-methylenecarboxylates bearing a bulky γ-substituent, such as CHMe₂, CHPh₂, c-C₆H₁₁ and CH(Ph)OTBDMS, with alkyl iodides gave the syn-adducts with high diastereoselectivities. However, the diastereoselectivity for the substrates bearing a γ-substituent CH(i-Pr)OTBDMS depended critically on the configuration of the substituent; the substrate bearing the OTBDMS group anti to the γ-benzyloxy group showed poor diastereoselectivity, but its diastereomer gave syn-adduct exclusively. The high syn-selectivitiy is referred to the H-atom transfer to the outside face of radical center in the sharply folded seven-membered chelate intermediate bearing the ethoxy group with Z-geometry. The corner flapping of the radical center atom of the global minimum energy conformer generates a local minimum conformer and the H-atom transfer to the outside face of the radical center of the newly formed structure gives the anti-adduct. The poor diastereoselectivity is due to the very small energy difference between the two conformers and consequently both the syn- and anti-adducts are yielded in nearly equal amounts.     

Structure and Reactivity of the Radical Cations of Methyl tert-Butyl Ether in Condensed Phase: An ESR and Quantum Chemical Study

The structure of the methyl tert-butyl ether (MTBE) radical cation and mechanism of its thermal and photochemical reactions in irradiated freons (CFCl₃, CF₂ClCFCl₂, and CF₃CCl₃) were studied. Radical products of MTBE radiolysis in the liquid phase were investigated by the spin trapping technique. The quantum-chemical calculations of the structure of MTBE radical cations and products of their transformations were carried out by density functional theory (DFT) and ab initioMP2 methods. The primary MTBE radical cations are stabilized in dilute solutions in CFCl₃and CF₃CCl₃. The ion–molecule reaction (proton transfer from the radical cation) was found to occur in concentrated solutions in CFCl₃immediately during irradiation. The action of light ( = 436 to 546 nm) at 77 K on the MTBE radical cation in CFCl₃and CF₃CCl₃matrices results in intramolecular migration of the methyl group to yield the distonic radical cation (CH₃)₂ ^.CO⁺(CH₃)₂. The primary MTBE radical cations undergo an irreversible transformation with methane elimination resulting in formation of the 2-methoxypropene radical cation ^.CH₂=⁺(₃)₃in CFCl₃and CF₃CCl₃matrices in the temperature range 110–130 K. In the case of CF₂ClCFCl₂matrix, such a reaction occurs during irradiation at 77 K. Using the spin trapping technique, it was shown that the liquid-phase radiolysis of the neat ether resulted in the formation of fragmentation products (^.CH₃,CH₃^., and t-BuO^. radicals) from the primary radical cations, as well as the products of their rearrangements and ion–molecule reactions.