Jeffrey C. Evans

Electron spin resonance studies of Ziegler-type catalysts. Part I. Characterisation of a vanadium–aluminium complex obtained on mixing dichlorobis(η-cyclopentadienyl)vanadium with ethylaluminium dichloride

Mixtures of dichlorobis(η-cyclopentadienyl)vanadium and ethylaluminium dichloride have been examined in methylene dichloride–heptane at Al : V ratios of 7 : 1. This system catalyses polymerisation of ethylene to give a polymer with good characteristics. Three species which exhibit e.s.r. spectra are present in this catalyst system, and one of these has been studied in detail by e.s.r. (room and glass temperature) and u.v. spectroscopy. The results show that this species, [Cl(cp)2V(µ-Cl)2AlCl2](which is not catalytically active), has a trigonal-bipyramidal structure.

Reactions of radical anions. Part 3.—Disproportionation of benzil radical anion

The disproportionation reaction of the benzil radical anion in tetrahydrofuran has been examined by electron spin resonance spectroscopy. The following equilibrium has been studied: [graphic omitted] The radical anion concentration, the total dianion concentration (cis + trans) and the neutral benzil concentration has been determined and the equilibrium constant, K1=[dianion(cis+ trans)][benzil]/[radical anion]2 measured at different temperatures for the gegen ions Li+, Na+, K+, and Cs+. As the gegen ion changes from Li+ to Cs+ the equilibrium constant K1 increases. Evidence for ion-pair formation is found from the metal splitting in the e.s.r. spectrum.

Study of bipyridyl radical cations. Part 5. Effect of structure on the dimerisation equilibrium

We have studied the e.s.r. spectra of methanol solutions of radical cations of the form (1; R = H, Me, Prn, Bun, or PhCH2), over a range of temperature from +40 to –90 °C, and determined the values of the splitting constants. It is found that as the temperature decreases, the concentration of the radical cation decreases, until at –90 °C there is practically no e.s.r. spectrum. This process is reversible. Concentration experiments show that the radical cations are in equilibrium with a dimeric diamagnetic species. The thermodynamic constants for the dimerisation equilibrium were determined, and the results indicate that dimerisation involves desolvation, suggesting that in the dimer the radical cations are arranged in a plane to plane configuration, the two monomer molecules being linked together by a π–π′ bond. For (1; RCH3), i.e. for the radical cation of paraquat, reduction of the temperature is also accompanied by the production of a second paramagnetic species, the structure of which has not yet been determined.

Bipyridyl radical cations. Part I. Electron spin resonance study of the dimerisation equilibrium of morphamquat radical cation in methanol

We have studied the e.s.r. spectra of methanol solutions of the morphamquat {bis-1,1′-[(2,6-dimethylmorpholin-4-yl)carbonylmethyl]bipyridyl} radical cation (I) from +40 to –80°. The value of the splitting constants obtained are A(pyridyl N) 0.400, A[pyridyl H(3)] 0.142, A[pyridyl H(2)] 0.048, A(NCH2) 0.245, and A(morpholine N) 0.024 mT. It is found that as the temperature decreases the concentration of the radical cation decreases until at –80° there is no e.s.r. spectrum. This process is reversible. Concentration experiments show that the morphamquat radical cation is in equilibrium with a dimeric diamagnetic species. The ΔH° value for the equilibrium is found to be –45.05 ± 0.3 kJ mol–1. The ΔG° and ΔS° values at 25 °C are –10.6 kJ mol–1 and –115.6 J mol–1 K–1 respectively.

Reactions of radical anions. Part X. Electron spin resonance study of the radical anions of azobenzene, naphthalene-1-azobenzene, 1,1′-azonaphthalene, and 2,2′-azonaphthalene including the measurements of tight ion-pair–loose ion-pair equilibria

The effect of temperature on the disproportionation equilibria of the radical anions of azobenzene, naphthalene-1-azobenzene, 1,1′-azonaphthalene, and 2,2′-azonaphthalene in tetrahydrofuran with various alkali metals as gegenions has been determined by measurement of the radical-anion concentration at different temperatures by e.s.r. Equilibrium constants, ΔH° values, and ΔS° values have been determined for these disproportionation equilibria.Well resolved e.s.r. spectra of these radical anions were obtained and analysed and the analysis was compared with the splitting constants obtained from simple Huckel and McLachlan theoretical treatment. The temperature dependence of the metal splittings were measured for azobenzene and naphthalene-1-azobenzene. In all cases except sodium, practically no temperature effect was observed. In the case of sodium, however, a marked temperature dependence of metal splitting and hyperfine splitting line widths were found which have been interpreted in terms of tight ion-pair–loose ion-pair equilibria. Equilibrium constants, ΔH° values, ΔS° values, rate constants, ΔH‡, and ΔS‡ values for these equilibria have been determined.

Reactions of radical anions. Part XIV. An electron spin resonance study of the radical anions of 9,9′-azophenanthrene and 9,9′-azoanthracene

The effect of temperature on the disproportionation equilibrium of the radical anions of 9,9′-azophenanthrene and 9,9′-azoanthracene in tetrahydrofuran, with various alkali metals as gegenions, has been determined by measurement of the radical-anion concentration at different temperatures, using electron spin resonance spectroscopy. The e.s.r. spectra were also obtained and analysed for these radical anions and the splitting constants assigned, using simple Huckel and McLachlan molecular orbital theory.

Reactions of radical anions. Part IX. The radical anion of 1-phenyl-2-trimethylsilylacetylene

The e.s.r. spectra of the radical anion of 1-phenyl-2-trimethylsilylacetylene with sodium, potassium, and caesium as gegenions in tetrahydrofuran have been obtained and analysed. Simple Huckel molecular orbital calculations were made, and coulomb and resonance integrals found which are consistent with the correct spin densities. For the carbon–silicon bond of the neutral molecule the calculated π-bond order is ca. 0·3. The radical anions are stable at –80 °C but when the temperature is raised they dimerize and the rate of dimerization has been followed in tetrahydrofuran over the temperature range –70 to –40 °C. The following values were obtained for the reaction at –60 °C: Na+ as gegenion, k= 41·1 l mol–1 s–1, ΔG‡= 10·8 kcal mol–1, ΔH‡= 9·3 kcal mol–1, ΔS‡=–6·9 cal mol–1 K–1; K+ as gegenion, k= 0·59 l mol–1 s–1, ΔG‡= 13·5 kcal mol–1, ΔH‡= 6·3 kcal mol–1, ΔS‡=–33·8 cal mol–1 K–1; Cs+ as gegenion, k= 0·26 l mol–1 s–1ΔG‡= 13·9 kcal mol–1, ΔH‡= 4·5 kcal mol–1, ΔS‡=– 44 cal mol–1 K–1.

Reactions of radical anions. Part XV. An electron spin resonance study of the radical anions derived from cis- and trans-1,2-bis(diphenylphosphinyl)ethylene

The radical anions produced by the passage of tetrahydrofuran or 2-methyltetrahydrofuran solutions of cis- or trans-1,2-bis(diphenylphosphinyl)ethylene (DPPE) over various alkali metal films at –80° have been studied. Their production occurs by a two-stage process (i) and (ii). Computer simulation of the e.s.r. spectra together Ph2P–CHCH–PPh2 [graphic omitted] –CHCH–[graphic omitted] + 2PhM (i), [graphic omitted]–CHCH–[graphic omitted][[graphic omitted]CHCH[graphic omitted]]–˙M+(ii) with chemical evidence show that the radical anion produced is as in equation (ii). The e.s.r. spectra also show that the unpaired electron is delocalised over the whole molecule. The nature of the e.s.r. spectra was dependent on the metal used. When Cs+ was the gegenion, metal splitting due to two caesium ions was observed. We find no metal splitting for the third Cs+ ion. The radical anion produced from cis-DPPE was the same as that from trans-DPPE.

Reactions of radical anions. Part XIII. Electron spin resonance study of the radical anions of 1,4-bistrimethylsilyl- and 1,4-di-t-butylbuta-1,3-diyne

We have studied the radical anions of the diacetylenes (CH3)3SiCC–CCSi(CH3)3 and (CH3)3CCC–CCC(CH3)3 in tetrahydrofuran by e.s.r. spectroscopy. These radical anions have only one type of proton present, the 18 methyl protons. E.s.r. experiments show clearly that with the silyl radical anion, unpaired electron density occurs not only on the silicon atoms but also on the methyl groups, and this we attribute to dπ–pπ interaction. In the radical anion of 1,4-di-t-butylbuta-1,3-diyne, however, no delocalisation of the electron through to the methyl groups occurs. There is a marked difference in reactivity between the radical anions of the two diacetylenes. The silyl radical anion is very stable whereas the di-t-butyl radical anion is very reactive. This difference in reactivity is discussed in terms of electron distribution and steric hindrance.

Photochemical reactions of N-(diphenylmethylene)methylthiomethylamine N-oxide. The diphenylmethylene(methylene)amine N-oxyl radical

U.v. irradiation of the title compound in carbon tetrachloride and deuteriochloroform solutions cleanly gave benzophenone and eventually, but less cleanly, dimethyl disulphide. Initial homolysis gave the diphenylmethylene(methylene)amine N-oxyl radical which was detected and identified by e.s.r. spectroscopy. A second radical, possibly the methylthiyl radical, was observed. Subsequent reactions gave benzophenone and N-methylthiomethyleneamine, which in turn, photochemically and thermally, gave dimethyl disulphide and polymeric materials that in part involved chlorine abstraction from the solvent. The well-resolved e.s.r. spectra of the nitroxide radical have been analysed, and splitting constants have been assigned, by comparison with the spectra of partially and fully deuteriated nitroxides.