George W. Eastland

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.

An ESR study of electron loss and electron capture by silver (I) ions in methyl cyanide

Abstract Exposure of solutions of silver (I) perchlorate in methyl cyanide to 60Co γ-rays at 77 K gave Ag(II) and Ag(O) centres in addition to radicals formed from the solvent. Their ESR spectra showed hyperfine features from 109Ag and 107Ag together with features from four equivalent 14N nuclei. The results are contrasted with those previously reported for solutions of silver (I) nitrate in methyl cyanide, which gave Ag(II) and Ag(O) centres whose ESR spectra were devoid of any 14N hyperfine features.

Effect of solvation on the electron spin resonance spectrum of the superoxide ion

The solvation of superoxide ions by protic solvents has been studied by e.s.r. spectroscopy. It is concluded that strong hydrogen bonds lift the degeneracy of the π-orbitals, with solvation being effectively confined to one of these orbitals. It is suggested that, when aqueous solutions are frozen, phase separation occurs so that the gz value (2.11) is not characteristic of the aquated ion. By extrapolation from data for aqueous methanolic glasses, we obtain a gz value of 2.065 for the aquated ion.Solutions in dry aprotic solvents generally give broad gz features at much lower fields. However, water in low concentrations is scavenged by the anions and this results in a shift of the gz feature into the 2.1 region. It is suggested that dry O–2 may add reversibly to dimethyl formamide to give a peroxy-radical adduct. Ion-pair formation with divalent cations has also been studied.

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.

Amine radical cations: an electron spin resonance study of cations generated by radiolysis

Amine radical cations have been generated by exposing dilute solutions of amines to 60Co γ-rays at 77 K. They are well characterised by their e.s.r. spectra, the method and interpretation being established by showing that the e.s.r. data for Me3Ṅ+ cations are identical with literature values. Results for the triethylammonium cation show that the β-protons do not occupy the most favourable sites for maximum hyperconjugation, in contrast with other radical cations. This is explained in terms of steric factors, and an equilibrium between two preferred conformations. 1,1-Diamines, R2NCH2NR2, give cations having localised structures, R2Ṅ[graphic omitted]H2NR2, rather than the delocalised structure adopted by acetal cations. Possible evidence for weak N–N bonding and delocalisation is adduced for NN′-dimethylpiperazine cation, but we confirm that the cation of triethylenediamine has a truly delocalised σ* structure. There is no tendency for R3N+˙ cations to react with neutral amines to give (R3N[graphic ommitted]NR3)+ dimer cations, in marked contrast with results for corresponding trialkylphosphine systems.

Structures of diazabenzene cations formed by radiolysis. An electron spin resonance study

Exposure of 1,2-, 1,3- and 1,4-diazabenzene in dilute solution in trichlorofluoromethane at 77 K to 60Co γ-rays gives the corresponding radical cations, characterised by their e.s.r. spectra. The 1,2 derivative has a SOMO (semi-occupied molecular orbital) largely confined to the two nitrogen n orbitals, with a 2s-orbital contribution of approximately twice that observed for pyridyl cations. This suggests that the extent of distortion for these radicals is far less than that for pyridyl cations. In contrast, the 1,4-diazabenzene cation has a considerably reduced 14N isotropic coupling to two equivalent nitrogen nuclei, and hence a reduced 2s-orbital contribution, suggesting more extensive flattening at both nitrogen atoms.In marked contrast, the pyrimidine cation has a SOMO largely localised on one of the nitrogen atoms in a non-bonding orbital closely resembling that for the pyridine cation. This is interpreted in terms of a preferred asymmetric distortion which lowers this SOMO below that of the expected symmetrical SOMO for this cation.

Radiation chemical formation of radical cations of halogenoalkanes and their electron spin resonance spectra and structure

Exposure of dilute solutions of chloro-, bromo- and iodo-alkanes in trichlorofluoromethane to 60Co γ-rays at 77 K gave the corresponding cations. However, for RCI+ and RBr+ the hole is shared with one chlorine atom of a solvent molecule via the formation of a σ bond, as in [RCI[graphic ommitted]CICFCI2]+. For alkyl chlorides the hyperfine coupling constants of the two chlorine nuclei are nearly equal. The (RBr)+ adduct shows e.s.r. spectra consisting of dominant quartet splittings from bromine and additional quartet splittings from chlorine. For the iodo-cations, no clear evidence for such bonding is obtained, and there are very large shifts in the g-values resulting from marked spin-orbit coupling. For more concentrated solutions, the e.s.r. spectra show septet hyperfine patterns arising from coupling to two equivalent halogen nuclei, which indicates the formation of dimer radical cations, (RX)2+, again having the unpaired electron in a σ* orbital. In the particular case of t-butyl chloride, neither the parent cation, nor its complex with a solvent molecule were detected. Instead, a multiline spectrum was obtained which we assign to (Me2CCH2)+ cations weakly complexed with HCl. Theoretical calculations on the parent cations, their dimers, and adducts with CFCl3 support these assignments.

Silver(I) imidazole perchlorate. An (Ag+)6 cluster and its radiolytically produced one-electron adduct studied by X-ray diffraction and electron spin resonance spectroscopy

The crystal structure of silver(I) imidazole perchlorate reveals the presence of a planar (Ag+)6, cluster, in which three radiating pairs of Ag+ ions 3.051(1)A apart are disposed on the corners of an equiliateral triangle, the inner Ag+ ions being 3.493(1)A apart. Each silver ion carries two linearly co-ordinated imidazole ligands, the whole unit has 32(D3) symmetry. Exposure to 60Co γ-rays at 77 K results in electron addition to a group of three equivalent silver atoms. The 109Ag, 107Ag, and 14N hyperfine coupling constants show that the total 5s character of the unpaired electron is only ca. 0.55, and delocalisation onto six equivalent nitrogen ligands accounts for ca. 0.25. Low g values suggest that the remaining spin density is in 5p orbitals on silver. There is no indication of delocalisation onto the remaining three Ag+ ions in the cluster. Possible reasons for this selectivity are discussed. The electron-loss centre appears to be a normal Ag2+ complex. We suggest that marked distortion results in the hole being trapped on one silver rather than being delocalised.

The triethylenediamine cation radical: An ESR study

Abstract Compared with normal and bridge-head aminium cations, the triethylenediamine cation is relatively stable. Its ESR spectrum shows that the two nitrogen atoms and all the protons are magnetically equivalent even at 77 K. This favours a symmetrical ground state rather than a dynamic equilibrium between classical, asymmetric structures.

Vinyl radical cations formed by radiolysis: an electron spin resonance study

Exposure of dilute solutions of a range of vinyl compounds, H2CCHX or H2CCMeX in trichlorofluoro-methane to 60Co γ-rays at 77 K generated the corresponding cations, most of which were well characterised by their e.s.r. spectra. Results for X = Me are discussed in terms of possible out-of-plane twisting for the cation. The 2-vinylpyridine cation favours a π SOMO similar to that of the styrene cation, whereas the 4-vinyl-derivative favours a SOMO which is largely the n(σ)-orbital on nitrogen. The cations of allyl chloride and bromide exhibit large hyperfine coupling to the halogen showing that these atoms favour the extreme out-of-plane site, with considerable delocalisation of the unpaired electron on to the halogen. Results for methyl acrylate and methyl methacrylate cations are difficult to interpret in terms of the expected π-structures. However, for X = CHO, the SOMO is clearly the n(O) orbital in the radical plane rather than the π-orbital. The cyano cations favour a π SOMO with significant spin density on nitrogen. For X = Br, large hyperfine coupling to bromine indicates ca. 39% delocalisation. For the vinyl ethers and acetate the SOMO is quite strongly localised on the H2Ċ-unit with α-proton couplings of ca. 19 G. Results for X = SiMe3 are of interest in that they provide clear evidence for out-of-plane twisting, as was recently established by Sakurai and co-workers. Also, on annealing to ca. 120 K these cations lost methyl radicals, which were detected by their characteristic e.s.r. spectrum.