Applications of ENDOR Spectroscopy to Radical Cations in Freon Matrices. Part 1. The radical cation of s-trans-buta-1,3-diene

Precise values of the proton coupling constants have been determined from the ENDOR spectra of the radical cation of s-trans-buta-1,3-diene generated from the neutral compound by γ irradiation in a CFCl₃ matrix at 77 K. These values are 1.119 and 1.050 mT for the pairs of exo- and endo-protons in the 1,4-positions, respectively, and 0.283 mT for the pair of protons in the 2,3-positions. A general TRIPLE resonance spectrum proves that all coupling constants have the same sign which should be negative by theory, Evidence by experiment and theory indicate that the s-trans-configuration of the neutral compound is retained upon ionization.     

MNDO-UHF study of the molecular and electronic structures of cation radicals derived from tetramethylgermane,hexamethyldigermane, and other related organogermanes

Calculations have been made, using the MNDO-UHF SCF method, of molecular and electronic structures of a range of neutral organogermanes and of the corresponding cation radicals. The cation radical (GeMe₄)⁺ is calculated to have D_2d symmetry as an isolated ion, while (Ge₂Me₆)⁺ is a σ radical in which the SOMO is strongly localised in the GeGe bond. The cation radicals (Me₃Ge)₂O⁺ and (Me₃Ge)₂NH⁺ are n-π radicals, while (Me₃Ge)₂CH₂⁺ dissociates to Me₃Ge⁺ and Me₃GeCH₂^•, which is planar at the radical centre. Both (Me₃Ge)₂O₂ and (Me₃Ge)₂S₂⁺ have trans-planar skeletons.     

Radical products from the radiolysis of 2,2,3,3-tetramethylbutane

Radiolysis of the pure title compound gives ESR signals only for neutral radical fragments, as estabhshed by Shiraishi et al.. and by Gerola et al. However, in the presence of a range of electron scavengers a novel species is detected by ESR spectroscopy which is tentatively identified as the parent cation, [Me₃C - CMe₃]⁺     

2-Phenylazo-1-alkene

Zusammenfassung Durch Umsetzung der Methylketon-phenylhydrazone1 mit J₂ in Pyridin werden die kristallinen 2-Phenylhydrazonoalkyl-pyridiniumjodide (2) synthetisiert und daraus mit Alkali unter 1,4-Eliminierung von Pyridin·HJ die in Substanz unbeständigen (E)-2-Phenylazo-1-alkene (4) hergestellt. Die spektroskopischen Untersuchungen (IR, Raman, EA und¹H-NMR) der orange-farbenen Öle4 beweisen dietrans-Konfiguration um die N=N-Bindung und außerdem, daß in Lösung4 a–d vorwiegend die s-trans (transoide) Konformation einnehmen, während4 e bevorzugt in der s-cis (cisoiden) Form vorliegt.

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2-phenylazo-1-alkenes

The reaction of the methyl ketone phenylhydrazones1 with I₂ in pyridine yields the crystalline 2-phenylhydrazonoalkyl-pyridinium iodides (2). Upon alkali induced 1,4-elimination of pyridine·HI the (E)-2-phenylazo-1-alkenes4 are obtained as orange liquids which are unstable when free of solvent. Spectroscopic investigations (ir, Raman, ea and¹H-nmr) reveal thetrans-configuration at the N=N-bond and moreover assign the s-trans (transoid) conformation to4 a–d whereas4 e is correlated to the s-cis (cisoid) conformer.

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Oxidationsprodukte von Arylhydrazon-Verbindungen,6. Mitt.; 5. Mitt.     

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.     

ESR spectra of C60 in concentrated sulfuric acid

On dissolution of C₆₀ in concentrated sulfuric acid, ESR spectra of paramagnetic species, which were identified as dimers (C₁₂₀ ⁺) or oligomers (nC₆₀)⁺, were recorded. The ESR spectra recorded upon the reaction of a toluene solution of C₆₀ with sulfuric acid were assigned to the radical cation C₆₀ ⁺. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2373–2376, November, 1998.     

Radical/Cation Transformation Polymerization and Its Application to the Preparation of Block Copolymers

Abstract

ESR study on the primary radicals obtained by decomposition of azo-compounds showed that primary radicals with electron donating substituents were transformed to the corresponding cations in the presence of electron acceptors such as ph₂I⁺PF^? ₆. Accordingly, propagating radicals are transformed to the corresponding cations in the polymerization of p-methoxy-styrene (MOS), n-butyl vinyl ether (BVE), and N-vinylcarbazole (VCZ) with azoinitiators such as AIBN in the presence of electron acceptors such as Ph₂I⁺PF^? ₆. In the case of BVE, the polymer formation was caused by cationic species produced by the transformation of the initiating radical. The polymerizations of MOS and VCZ were ascribed to the transformation of the growing radical to the corresponding cation during the propagation step which was classified as the radical/cation transformation polymerization. Block copolymers of MOS/cyclohexene oxide (CHO) and VCZ/CHO were effectively prepared by the radical/cation transformation polymerization of the appropriate monomers in the presence of AIBN, electron acceptor and CHO. The formation of block copolymers was characterized by turbidimetry, thin-layer chromatography, and solubility tests.     

The Radical Cation of Azatriquinane: An ESR Study

Azatriquinane (=10-azatricyclo[5.2.1.0^1,10]decane; 1 ) was oxidized to its radical cation 1 ^.+ by γ- irradiation in a CF₂ClCFCl₂ matrix at 77 K. A prominent feature of the ESR spectrum of 1 ^.+ is the ¹⁴N-hyperfine anisotropy which broadens the components with M_I(¹⁴N)=±1. The observed coupling constants are a_N=+ 2.5 and a_H=+ 4.0 mT for the ¹⁴N nucleus and the three methine β-protons, respectively. While the a_N value points to pyramidalization at the N-atom comparable to that in the radical cation of quinuclidine (=1-azabicyclo[2.2.2]octane; 4 ), an eclipsing of the singly occupied orbital at this atom by the three C−H_β bonds is indicated in view of the large a_H value. Theoretical calculations on the geometry of 1 ^.+ are in accord with the conclusions drawn from the experimental findings.     

Separation of different ion structures in atmospheric pressure photoionization-ion mobility spectrometry-mass spectrometry (APPI-IMS-MS)

This study demonstrates how positive ion atmospheric pressure photoionization-ion mobility spectrometry-mass spectrometry (APPI-IMS-MS) can be used to produce different ionic forms of an analy te and how these can be separated. When hexane:toluene (9:1) is used as a solvent, 2,6-di-tert-butylpyridine (2,6-DtBPyr) and 2,6-di-tert-4-methylpyridine (2,6-DtB-4-MPyr) efficiently produce radical cations [M]⁺ and protonated [M + H]⁺ molecules, whereas, when the sample solvent is hexane, protonated molecules are mainly formed. Interestingly, radical cations drift slower in the drift tube than the protonated molecules. It was observed that an oxygen adduct ion, [M + O₂]⁺, which was clearly seen in the mass spectra for hexane:toluene (9:1) solutions, shares the same mobility with radical cations, [M]⁺. Therefore, the observed mobility order is most likely explained by oxygen adduct formation, i.e., the radical cation forrning a heavier adduct. For pyridine and 2-tert-butylpyridine, only protonated molecules could be efficiently formed in the conditions used. For 1- and 2-naphthol it was observed that in hexane the protonated molecule typically had a higher intensity than the radical cation, whereas in hexane:toluene (9:1) the radical cation [M]⁺ typically had a higher intensity than the protonated molecule [M + H]⁺. Interestingly, the latter drifts slower than the radical cation [M]⁺, which is the opposite of the drift pattern seen for 2,6-DtBPyr and 2,6-DtB-4-MPyr.     

Radical Anions of Sterically Protected Polyenes: An ESR and ENDOR Study

Owing to the steric protection by four bulky substituents in the terminal positions 1 and n, several conjugated polyenes could be reduced with K or Cs metal in 1,2-dimethoxyethane (DME) or tetrahydrofuran (THF) to fairly persistent radical anions. These compounds, denoted here as 2 , 3 ,…︁ 7 (which corresponds to the number, \2 n=2, 3, …︁7, of their formal double bonds) are 1,1,n,n-tetra(tert-butyl) derivatives of buta-1,3-diene, hexa-1,3,5-triene, octa-1,3,5,7-tetraene, deca-1,3,5,7,9-pentaene, dodeca-1,3,5,7,9,11-hexaene, and tetradeca-1,3,5,7,9,11,13-heptaene. In addition to the six polyenes 2 – 7 with all-trans-configuration, the studies comprised an isomer of 3 , the trans,cis,trans-triene, c -3 . The radical anions 2 ^.− – 7 ^.− and c -3 ^.− were characterized by their hyperfine data acquired with ESR, ENDOR, and TRIPLE-resonance spectroscopy. The ¹H-coupling constants comply with the spin distribution predicted for the radical anions of such `linear' π-systems by simple MO models. Ion pairs formed with K⁺ in DME were loose but became tighter with Cs⁺ in THF. Propensity to ion pairing decreased with the lengthening of the π-system on going from 2 ^.− to 3 ^.− – 7 ^.−. Hyperfine data are likewise reported for the radical anions of all-trans-polyenes 8 and 9 , in which two tert-butyl substituents in one terminal position of 2 and 3 , respectively, were replaced by CN groups.     

Electron-Transfer-Catalyzed cis → trans Isomerization of 1,1′-Azonorborane. Prototype of a reversible two-stage storage system

ESR and cyclic voltammetry investigations show that isomerization of the radical cation of cis-1,1′-azonorbornane (cis- 1 ) to the trans-radical ion proceeds too fast in solution for direct investigation of the cis-radical ion even at ?78°. The facile isomerization of the radical cation is in agreement with PM 3 calculations proposing an activation barrier of only 17 kJ/mol. As a consequence, quantitative cis → trans isomerization of 1,1′-azonorbornane can effectively be accomplished by addition of catalytic amounts of one-electron oxidants. This is the first evidence for a radical-cation-catalyzed cis → trans isomerization of azo compounds.     

The methylamine radical cation [CH3NH2]+˙ and its stable isomer the methylenammonium radical cation [CH2NH3]+˙

The potential energy surface for the [CH₅N]^+˙ system has been investigated using ab initio molecular orbital calculations with large, polarization basis sets and incorporating valence-electron correlation. Two [CH₅N]^+˙ isomers can be distinguished: the well known methylamine radical cation, [CH₃NH₂]^+˙, and the less familiar methylenammonium radical cation, [CH₂NH₃]^+˙. The latter is calculated to lie 8 kJ mol^?1 lower in energy. A substantial barrier (176 kJ mol^?1) is predicted for rearrangement of [CH₂NH₃]^+˙ to [CH₃NH₂]^+˙. In addition, a large barrier (202 kJ mol^?1) is found for loss of a hydrogen radical from [CH₂NH₃]^+˙ via direct N—H bond cleavage to give the aminomethyl cation [CH₂NH₂]⁺. These results are consistent with the existence of the methylenammonium ion [CH₂NH₃]^+˙ as a stable observable species. The barrier to loss of a hydrogen radical from [CH₃NH₂]^+˙ is calculated to be 140 kJ mol^?1.     

Radical polymerization of methyl trans-β-vinylacrylate

Methyl trans-β-vinylacrylate (MVA) undergoes radical polymerization with α,α′-azobis(isobutyronitrile) (AIBN) in bulk and solution. The polymer obtained consists of 85% trans-1,4 and 15% trans-3,4 units. Poly(MVA) (PMVA) is readily soluble in common organic solvents, but insoluble in n-hexane and petroleum ether. PMVA exhibits a glass transition at 60°C, and loses no weight up to 300°C in nitrogen. The kinetics of MVA homopolymerization with AIBN was investigated in benzene. The rate of polymerization (R_p) can be expressed by R_p = k[AIBN]^0.5[MVA]^1.0, and the overall activation energy has been calculated to be 94 kJ/mol. The propagation radical of MVA at 80°C was detected by ESR spectroscopy, which indicated that the unpaired electron of the propagating radical was completely delocalized over the three allyl carbons. Furthermore, the steady-state concentration of the propagating radical of MVA at 60°C was determined by ESR spectroscopy, and the propagation rate constant (k_p) was calculated to be 1.25 X 10² L/mol ·s. Monomer reactivity ratios in copolymerization of MVA (M₂) with styrene (M₁) are r₁ = 0.16 and r₂ = 4.9, from which Q and e values of MVA are calculated as 4.2 and -0.32, respectively. © 1995 John Wiley & Sons, Inc.     

ESR.-Studies of Nonalternant Radicals Structurally Related to Phenalenyl

The radical anion and the radical cation of azuleno[1,2,3-cd]phenalene (III) have been investigated by ESR. spectroscopy, along with the radical anion of 2-phenylazulene (IV). Also studied has been the neutral radical obtained by one-electron reduction of cyclohepta[cd]phenalenium-cation (VI^⊕). Assignment of the proton coupling constants for the radical ions III. ·?, III ·⊕ and IV·⊕, and the radical VI · is supported by comparison with the ESR. spectra of specifically deuteriated derivatives III-d₅ ·?, III-d₅ ·⊕, IV-d₂ ·? and VI-d₁′. The experimental results are in full accord with qualitative topological arguments and predictions of HMO models. Whereas the radical anion III ·? exhibits α-spin distribution similar to that of IV ·?the corresponding radical cation III ·⊕ and the neutral radical VI · are related in this respect to phenalenyl (V·). It is noteworthy that oxidation of III by conc. H₂SO₄ yields a paramagnetic species (IIIa ·⊕) which has a similar – but not an identical – structure as the radical cation III ·⊕ produced from III with AlCl₃ in CH₃NO₂.     

A photoionization study of [C4H7]+ ions in the gas phase

The ionization and [C₄H₇]⁺ appearance energies for a series of C₄H₇CI and C₄H₇Br isomers have been measured by dissociative photoionization mass spectrometry. Cationic heats of formation, based on the stationary electron convention, are derived. No threshold ion is observed with a heat of formation corresponding to the trans-1-methylallyl cation, although there is evidence for formation of the less stable cis isomer. A 298 K heat of formation of 871 kJ mol^?1 is obtained for the cyclopropylcarbinyl cation, with the cyclobutyl cation having a higher value of 886 kJ mol^?1. At the HF/6-31G^** level, ab initio molecular orbital calculations show the 2-butenyl, isobutenyl and homoallyl cations to be stable forms of [C₄H₇]⁺, being less stable than the trans-1-methylallyl cation by 101 kJ mol^?1, 159 kJ mol^?1 and 164 kJ mol^?1, respectively. However, threshold formation is not observed for any of these ions, the fragmentation of appropriate precursor molecules producing [C₄H₇]⁺ ions with lower energy structures.     

Initiation of cationic polymerization of n-butyl vinyl ether by stable cation radicals of substituted phenothiazines

3,7-Diethyl- 10-phenylphenothiazine (DEPPT), a phenothiazine derivative whose 3,7- and 10-positions are blocked, was synthesized. Potentiostatic electrolysis of DEPPT in acetonitrile (ACN) in the presence of 0.1M of LiClO₄ at 0.7 V (vs. Ag/Ag/Cl) yielded the stable cation radical of DEPPT (DEPPT^+·) which was characterized by cyclic voltammetry, UV-visible spectroscopy, and ESR spectrometry. Stable cation radicals of 10-phenylphenothiazine and 3,7-diethyl-10-methylphenothiazine were also prepared. The cationic polymerization of n-butyl vinyl ether was initiated by these cation radicals, including DEPPT^·+. The electron transfer mechanism for the initiation step, which we proposed previously, was supported by the fact that DEPPT^·+ was capable of initiating the polymerization; dimerization of DEPPT^·+ by releasing protons is precluded because 3,7- and 10-positions are all blocked. © 1994 John Wiley & Sons, Inc.     

ESR study of adducts of diisopropylphosphoryl radicals with C60C[P(O)(OEt)2]2 2 isomers

The radical adducts of the P^·(O)(OPrⁱ)₂ (R^·) radicals with C₆₀C[P(O)(OEt)₂]_{2 2} fullerene derivatives were studed by ESR spectroscopy. The number of stable regioisomers of phosphorylfullerenyl radicals formed by addition of the phosphoryl radicals to the C₆₀C[P(O)(OEt)₂]_{2 2} isomers depends on the mutual position of the organophosphorus groups and decreases in the series trans-2 > trans-3 trans-4 > e. The rate constants for addition of the R^· radicals to the trans-3 regioisomer (k = 1.7·10⁸ L mol^–1 s^–1) were determined.     

A 4 K matrix ESR study of the rearrangement and fragmentation of molecular radical cations of alkyl formates and acetates: Direct evidence for a McLafferty rearrangement

A 4 K matrix ESR study shows that the molecular radical cations of isopropyl formate and acetate, produced radiolytically in halocarbon matrices at 4.2 K, undergo spontaneous rearrangement due to a selective intramolecular hydrogen shift from the tertiary CH bond in the isopropyl group to the carbonyl oxygen atom giving RC⁺(OH)OC(CH₃)₂, where R = H or CH₃. The radical cation of tert-butyl acetate undergoes further fragmentation at the ester CO bond following a similar rearrangement to give an isobutene radical cation in CFCl₃.     

ESR spectra of fullerence C70 in concentrated sulfuric acid

It was established by ESR spectroscopy that the reaction of C₇₀ with concentrated H₂SO₄ resulted in the formation of isomeric dimers C₁₄₀ ⁺, and the action of H₂SO₄ on a toluene solution of C₇₀ ⁺ gave the C₇₀ radical cation. The structure of dimers was discussed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 208–210, January, 1999.     

Substitution reactions under NH3 chemical ionization conditions in cis-/trans-1,2-dihydroxybenzosuberans

The nucleophilic substitution reaction under NH₃ chemical ionization (CI) conditions in cis- and trans-1,2-dihydroxybenzosuberans (1–4) has been studied with the help of ND₃ CI and metastable data. The results indicate that in the parent diols 1 (cis) and 2 (trans), the substitution ion [M_sH]⁺, is produced mainly by the loss of H₂O from the [MNH₄]⁺ ion (S_Ni reaction) while in their 7-methoxy derivatives 3 and 4, the ion-molecule reaction between [M? OH]⁺ and NH₃ seems to be the major pathway for the formation of [M_sH]⁺. The substitution ion from 1 and 2 and the [MH]⁺ ion from trans-1-amino-2-hydroxybenzosuberan give similar collision-induced dissociation mass-analysed ion kinetic energy spectra. Interestingly, their diacetates do not undergo the substitution reaction.