D. N. Ramakrishna Rao

Radiation generation of radical cations of amides. An electron spin resonance study

Exposure of dilute solutions of a range of fully alkylated amides in dichlorofluoromethane (CFCl3) to 60Co γ-rays at 77 K gave good yields of the corresponding radical cations, characterised by their e.s.r. spectra. Dimethyl-formamide and -acetamide gave the parent cations, the spin density being largely localised on nitrogen. Proton hyperfine coupling to the two N-methyl protons was surprisingly high (ca. 33 G). These cations were stable up to ca. 160 K, in marked contrast with the corresponding ester cations such as methylacetate, which probably undergo internal rearrangement even at ca. 4 K.Two cyclic amides (N-methylpyrolidone and N-formylpiperidine) also gave the parent π-cations. The former exhibited the expected large hyperfine couplings to the two β-ring protons, whereas the latter gave relatively small coupling constants. This decrease is explained in terms of a switch in conformation on going from the five- to the six-membered ring systems. In contrast, N-methyldiacetamide, MeCO–N(Me)–CoMe, failed to give the expected π-cation with high spin density on nitrogen and the N-methyl group. However, a well defined spectrum was obtained for a species exhibiting relatively large g-shifts and a small coupling to 14N. The results are tentatively interpreted in terms of a SOMO confined to one, or probably, to both in-plane carbonyl oxygen atoms, which may become weakly σ-bonded as a result of electron loss. Tetramethylurea failed to give the expected π-cation with high spin densities on the two nitrogen atoms. The e.s.r. results established the presence of only one nitrogen and two strongly coupled methyl groups. The e.s.r. parameters are close to those for the dimethylamino radical, so we suggest that fragmentation to give ˙NMe2 and Me2NCO+ is facile. The latter diamagnetic product is isoelectronic with dimethylcyanamide and is expected to be reasonably stable. A similar reaction gave ˙NEt2 from the tetraethyl derivative.

Radical cations of organic nitro-derivatives

Abstract Attempts to generate the radical cations of nitroalkanes using the standard procedure of exposing dilute solutions in fluorotrichloromethane at 77 K to ionizing radiation gave low yields of the expected products, having a SOMO confined to oxygen as judged by their e.s.r. spectra, together with a species whose e.s.r. spectra closely resembles that of NO 2 radicals. It is suggested that these are rearranged cations, (ONOR) + . Similar treatment of nitrobenzene and a range of its ring-substituted derivatives gave initial electron loss from benzene π orbitals, but on annealing these rearranged into radicals similar to those for the nitroalkanes.

Conversion from a π to a σ structure on replacing oxygen by sulphur: an ESR study

The radical cations of 1,3-dioxacylohexane and 1,3-dioxacyclopentane have π-structures involving delocalisation within the [-OCH2O-]+ unit with high spin-density on the CH2 group, whilst, in complete contrast, the corresponding sulphur derivatives have a weak σ bond between the two sulphur atoms, the unpaired electron being in the σ* orbital with negligible spin-density on the CH2 group.

Radical cations of some amides, thioamides and dimethyl sulphoxide

Abstract Exposure of dilute solutions of N,N-dimethyl-formamide,-acetamide.-thioformamide and of dimethyl sulphoxide in fluorotrichloromethane to 60Co γ-rays at 77 K gave the corresponding radical cations, identified by ESR spectroscopy.

The radical cations of sulphur (S8˙+) and tetrasulphur tetranitride (S4N4˙+): a radiation–electron spin resonance study

Exposure of dilute solutions of S8 and S4N4 in trichlorofluoromethane to 60Co γ-rays at 77 K gave the corresponding radical cations. The first formed species from sulphur, (A), thought to be S8+, had gx= 2.069, gy= 2.057, and gz= 1.997. Enrichment (99%) with 33S gave greatly broadened x and y features, with A(33S)≈± 4 G. The z features showed a clear central line flanked by others with AZ≈ 28 G. The results suggest the presence of two equally coupled sulphur atoms. On annealing, species (A) changes irreversibly into species (B)(gx= 2.044, gy= 2.034, gz= 2.0035). It is suggested that (B) is also S8˙+ in a relaxed form in which two opposite atoms have formed a weak three-electron bond. The e.s.r. parameters for (B) are very similar to those previously assigned to S8+, formed from S82+ in oleum. A clear spectrum was produced from S4N4 which showed little g-value variation (gav= 2.004) and no evidence for 14N splitting. It is concluded that the S4N4˙+ cation has a relatively isolated semi-occupied molecular orbital, with low spin density on nitrogen.

Mechanisms for the photolysis and radiolysis of alkylcobaloximes and related compounds. An electron spin resonance study

Exposure of a range of alkyl(pyridine)cobaloximes and related compounds in dilute solutions in methanol or 2-methyltetrahydrofuran at 77 K to 60Co γ-rays resulted in electron capture. For the methyl derivative the pyridine ligand was lost leaving a novel CoII derivative whose e.s.r. parameters suggests a (dz2)1 configuration (z is normal to the oxime plane). The 13CH3 derivative gave well defined hyperfine coupling 13C in accord with the proposed structure. Other alkyl derivatives gave similar similar results, the pyridine ligand being lost in preference to the alkyl ligand. These results make an interesting contrast with previous results for alkylcobalamins, for which initial elctron addition was into a corrin π* orbital. The chloro derivative lost Cl– in preference to pyridine on electron addition. The chlorotriphenylphosphine derivative gave evidence for electron addition into the dz2 orbital, chloride ions being lost on annealing above 77 K. The hydridotriphenylphosphine derivative lost hydrogen on electron addition: possible mechanisms for this reaction are discussed.On photolysis at 77 K in CD3OD solution poorly defined e.s.r. features for CoII complexes were obtained together with well defined features for the alkyl radicals [·CH3, ·CH2CH3, ·CH(CH3)2 and ·C(CH3)3, the last being formed from (CH3)2CHĊH2 radicals]. At high microwave powers broad features tentatively assigned to CoII···R · pairs were detected, and the marked broadening of the CoII features is also interpreted in terms of such triplet-state pairs. Similar pair trapping was postulated for the photolysis of alkyl cobalamins. These e.s.r. results are compared with the e.s.r. doublet detected when reacting mixtures of substrates and the enzyme–coenzyme B12 are cooled to 77 K. These results suggest that the primary act of photolysis is homolysis of the Co—C bond. This conclusion is discussed in the light of other e.s.r. studies of these systems.

Radical cations of substituted pyridines formed by radiolysis. An electron spin resonance study

Exposure of dilute solutions of various substituted pyridines in trichlorofluoromethane to 60Co γ-rays at 77 K gave the corresponding cation radicals, characterised by their e.s.r. spectra. As was first established by Shida and coworkers, the pyridine cation has a σ-structure rather than π, the unpaired electron being in the in-plane s–p hybrid ‘lone-pair’ orbital on nitrogen. Photoelectron-spectroscopic studies and theory suggest that the orbital energies of the highest filled π-orbital and the N(σ)-orbital are very similar, so we have attempted to produce π-cations by using suitably substituted derivatives. Surprisingly mono- and most di-methylated derivatives gave only the N(σ)-cations, as did the o- and m-CHO derivatives. However, the 2,5-dimethyl derivative gave only the π-cation. Also the 2-chloro and 2-bromo derivatives gave π-cations, whilst the m-chloro derivative probably gave both N(σ)- and π-cations. The π-cation was also obtained from the o-amino derivative. Aspects of structure for these cations are discussed.On annealing more concentrated solutions, bimolecular reactions gave 2-pyridyl radicals rather than σ*-dimers.

Radical cations of organic sulphides and disulphides formed by radiolysis: an electron spin resonance study

Exposure of dilute solutions of various organic sulphides in fluorotrichloromethane at 77 K to 60Co γ-rays gave species identified by their e.s.r. spectra as the parent radical cations. The average β-proton coupling (R2CHSR)+ of ca. 20 G is about half that for the corresponding ether cations, indicating greatly reduced σ-π delocalisation. However, the spread of g-values (ca. 2.032, 2.015, 2.00) is much greater than that for the ether cations. These cations readily react with other disulphide molecules to form (R2S[graphic ommitted]S2)+ dimer cations. The spectrum for tetrahydrothiophene (tetramethylene sulphide) exhibits a large coupling to two axial protons (ca. 42 G) and two equatorial protons (ca 19 G), the coupling constants again being about half those for tetrahydrofuran cations. However, conformational inversion did not occur below the softening point of the solids (ca. 160 K) in marked contrast with the ether cations. The three-and four-membered ring cations (ethylene and trimethylene sulphide cations) gave very similar spectra with gX=gY, and four equivalent protons having an isotropic coupling of ca. 31G. Thus their structures are similar to the normal R2S·+ cations, the equivalence of the g-values being interpreted in terms of effective C–Ŝ–C bond angles (θ) close to 90°. This, in turn, implies that θ >90° for the unconstrained cations. It is noteworthy that the ethylene oxide (oxirane) cation exhibits a smaller coupling to its four protons (16 G). This implies a drastic change in structure for the oxirane cation which is clearly not the case for the sulphur analogue. Disulphides of structure R-S-CH2-S-R form cyclic σ* radicals with weak S-S bonding, in marked contrast with the oxygen analogues (acetal cations) which have π-structures conferring very high spin-density onto the CH2 protons. Other molecules containing two RSR units form similar cyclic σ* radicals. Persulphides of structure RS-SR readily give the π-cations (RS˙S)+, characterised by g-values in the region of 2.035, 2.018, 2.002. The smaller range of g values for (PhS[graphic ommitted]SPh)+ cations is interpreted in terms of ca. 30% spin-delocalisation into both the benzene rings.

Electron addition to alkyl cobalamins, coenzyme B12 and vitamin B12. A radiolytic and electron spin resonance study

Exposure of dilute solutions of methyl and ethyl cobalamins and coenzyme B12 in dilute solutions (D2O + CD3OD) to 60Co γ-rays at 77 K gave a single broad feature in the free-spin region assigned to electron-capture species with the excess electron largely confined to a π* corrin orbital. On warming above 77 K the methyl derivative gave a novel species with spectral features characteristic of an unpaired electron in the Co(dx2–y2) orbital. The other two substrates gave spectra due to CoII B12r, both on warming and after photolyses with visible light.The acetyl derivative gave an electron-capture species whose e.s.r. spectrum was characteristic of an electron in the Co(dz2) orbital, which on warming above 77 K changed to the normal CoII B12r spectrum. The cyano derivative (vitamin B12) gave electron addition into the Co(dz2) orbital, as evidenced by the large hyperfine coupling to 13C from 13CN ligands. On annealing, cyanide ions were lost irreversibly, B12r being detected by e.s.r. spectroscopy. In contrast, the dicyano derivative on electron addition at 77 K gave a species containing only one 13CN ligand. Hence in this case one CN– ligand was lost at 77 K, with no return of the dimethylbenzimidazole ligand.These results are discussed in terms of a new mechanism for electron-addition to alkyl cobalamins.

Radical cations of organic carbonates, trimethyl borate and methyl nitrate. A radiation–electron spin resonance study

Exposure of a range of dialkyl carbonates as dilute solutions in trichlorofluoromethane at 77 K gave rearranged cations in which hydrogen is transferred from carbon to the carbonyl oxygen. Similar species were obtained from the cyclic derivatives, ethylene and propylene carbonates. No evidence has been found for complex formation with solvent molecules such as occurs for methyl formate cations. For the dimethyl derivative the rearrangement appeared to occur even at ca. 4 K. For trimethyl borate a similar reaction occurred, giving either H2ĊOB(OMe)([graphic omitted])+or H2ĊOB(OMe)+ MeOH. However, methyl nitrate gave ˙NO2H+ radicals, thought to be derived from H2ĊONO2H+. These results are compared with those for ester and lactone cations and also with those for trimethyl phosphate and dimethyl sulphate cations.