Jane L. Wyatt

A copper–copper bond by intent

The ESR spectra for a series of binuclear trigonal bipyramidal copper complexes show that the SOMO is largely composed of the two equivalent dz2 orbitals on copper; the temperature independence of the ESR parameters shows that this is a relatively strong bond, and since the unpaired electron is antibonding, the σ-bond must be even stronger; these results are contrasted with the more common situation that arises when the electron is shared between two dx2–y2 type orbitals.

Radical cations and anions of pyrazines: an electron paramagnetic resonance study

Dilute solutions of a series of pyrazines (1–13) in CFCl3 at 77 K were exposed to 60Co γ-rays, generating the corresponding radical cations which were identified by their EPR spectra. Two distinct types of spectra were observed. Those obtained from mono- and di-methyl derivatives showed large hyperfine coupling to two 14N nuclei similar to that of the parent pyrazine cation and are assigned to n(σ)-radical cations. The other spectra observed for the tetramethyl, OCH3, SCH3 and NR2 derivatives are assigned to π-radical cations. These findings confirm that the SOMO has switched from n(σ) to π between the dimethyl derivatives 4, 6 and 8 and the tetramethyl compound 9, respectively. The NR2-substituted radical cations 12˙+ and 13˙+ were also observed in fluid solution. The corresponding radical anions 1˙––13˙–, studied in solid solution, show major hyperfine coupling to the two ring 14N nuclei. Studies in fluid solution indicate that strong electron donor substituents (OCH3, SCH3, NR2) convert the S-type π-SOMO of the pyrazine radical anion to a π-SOMO with predominant A-type character.

Electron paramagnetic resonance studies of irradiated D-glucose-6-phosphate ions: relevance to DNA

Exposure of disodium D-glucose-6-phosphate to 60Co γ-rays at 77 K gave a major centre identified by its EPR spectrum as the parent phosphoranyl radical, formed by electron addition to phosphorus. Several carbon-centred radicals were also detected and their structures discussed. The expected primary electron-loss centre, (RO)PO˙–3 was not detected, and probably reacted by hydrogen-atom abstraction with a neighbouring sugar unit. On annealing, the phosphoranyl radicals were converted into two other phosphorus-centred radicals, one of which is identified as the ˙PO2–3 radical formed by loss of RO–. The other species, formed in comparable yields is probably also a phosphoryl radical centre.Dilute solutions in methanol or CD3OD gave only dissociative electron capture, giving R˙ radicals, as detected by EPR spectroscopy, plus phosphate ions. There was no evidence for phosphoranyl or phosphoryl radical formation. This switch in mechanism of electron capture between the disodium salt and methanolic solutions illustrates the major control exerted by the medium on reaction mechanism in such systems. These results are of significance with respect to processes of radiation damage to DNA. In particular, they confirm that electron addition to phosphate does not occur as a process of significance in irradiated DNA. Reasons for this are discussed.

The acetone cation — A 13C ESR and endor study

Abstract Exposure of dilute solutions of acetone in trichlorofluoromethane to 60 Co γ-rays at 77 K gave the radical cation, characterised only by its g

Formation of the radical cation N2H6˙+ in irradiated hydrazinium sulphate (N2H62+SO42–): an electron spin resonance study

Exposure of [N2H62+SO42–] at 77 K to 60Co γ-rays gave SO4˙– radicals and a novel species with two equivalent nitrogen nuclei and six equivalent protons, thought to be N2H6˙+ formed by electron capture; these species readily gave NH3˙+ radicals on annealing.

Electron paramagnetic resonance spectroscopic studies of organic radicals formed in bone during fracture

Mechanical damage to bone generates organic free radicals at the fracture surfaces which are readily converted into peroxy radicals. These radicals, characterised by EPR spectroscopy, are formed by homolytic breakage of collagen strands.

Alternative structures for the oxirane cation

Abstract The ESR spectrum for 13 C-labelled oxirane cations formed in CFCl 3 by exposure to ionizing radiation is discussed in terms of the 2 A 1 ring-closed structure and the oxallyl ring-opened structure.

Paramagnetic transition-metal carbonyls and cyanides. Part 10. Electron addition to various carbonyl(η5-cyclopentadienyl)metal halides; an electron spin resonance study

Exposure of [Mo(cp)(CO)3I], [W(cp)(CO)3I], [Fe(cp)(CO)2Cl], [Fe(cp)(CO)21], and [Mn(cp)(CO)3](cp =η5-cyclopentadienyl) either as the pure compounds or as dilute solutions in solvents that facilitate electron capture, to 60Co g;-rays at 77 K, gave the corresponding anions. All the halides had e.s.r. spectra dominated by large hyperfine coupling to the halogen nuclei, characteristic of electron capture into the metal-halogen σ* orbitals. This is expected for the iron complexes, which are formally 3d6, but surprising for the Mo and W complexes which are formally 3d4. No hyperfine features from coupling to metal nuclei were observed for the iron or tungsten complexes, but well defined features from 95/97Mo nuclei were observed in spectra of irradiated single crystals of [Mo(cp)(CO)31]. Analysis of this coupling together with the g-tensor components supports the σ* assignment, suggesting that the 3dz2 orbital is the major metal orbital involved. On annealing, the molybdenum complex anion was converted into another complex, exhibiting smaller hyperfine coupling to iodine which is thought to be the relaxed structure for [Mo(cp)(CO)3l]– with a π* semi-occupied molecular orbital (s.o.m.o.) rather than the σ*(s.o.m.o.). In contrast, [Fe(cp)(CO)2C1]– and the iodide gave an axially symmetric centre with no halogen hyperfine coupling, thought to be [Fe(cp)(CO)2]. This was formed together with a more stable protonated species, possibly [FeH (cp)(CO)2]+, which exhibited well defined proton hyperfine coupling (2H coupling when CD3OD was the solvent). Possible structures for this species are considered. The manganese carbonyl complexes, studied for comparative purposes, gave well defined anions, but we were unable to obtain clear-cut results for the species [Mn(cp)(CO)2]–, isoelectronic with the corresponding iron derivative.

Dichotomy in dissociative electron capture: electron paramagnetic resonance detection of iodine atoms in methanol glasses

Exposure of dilute solutions of N-iodosuccinimide and other iodo-derivatives (RI) in methanol to 60Co γ-rays at 77 K gives features assigned to σ* radical anions, in which the excess electron is accommodated in the R–I σ* orbital. On annealing, these features gave way to a new set of features exhibiting large hyperfine coupling to iodine and large g-shifts which were the same for all substrates. It is postulated that this centre is an iodine atom forming a weak three-electron bond to methanol. This was strongly supported by the observation that UV photolysis of solutions of potassium iodide in methanol gave the same electron paramagnetic resonance (EPR) features.The results are discussed in terms of the polarity of the R–I bonds, and the effects of solvation. Attempts to prepare a similar bromine atom centre using bromo-analogues have not yet been successful.

The structure of diphosphine radical cations

Abstract When R 2 NNR 2 molecules lose an electron to give (R 2 NNR 2 ) +· radical cations, the whole unit becomes planar, with a(π 1 ) 2 (π 2 ) 1 configuration. However, because R 3 P molecules are far more strongly pyramidal than R 3 N molecules, this flattening on electron loss is less, and phosphorous centred radical cations do not achieve planarity. This is clearly so for (R 2 PPR 2 ) + centres, whose liquid and solid state spectra analysed herein in terms of two equivalent 31 P hyperfine couplings, show ca. 9% 3s character. This indicates considerable bending at each phosphorous centre. Furthermore, the form of the spectra, with no x — y splitting of the ‘perpendicular’ lines, suggests that each 31 P coupling shares a common axis. This means that a trans conformation is required, as expected because this relieves steric strain and favours “π” type orbital overlap.