George W. Neilson

Neutron diffraction study of aqueous transition metal salt solutions by isomorphic substitution

Experiments were initially carried out on 4.3 mol dm–3 MCl2(M = Ni or Mn) and 3.8 mol dm–3 M(ClO4)2(M = Ni or Cu) heavy-water solutions to test the method of isomorphic substitution and see whether Ni, Mn, Cu are suitable isomorphs. The results obtained for chloride solutions are compared with previous results obtained by the technique of isotopic substitution. Good agreement is found between the two techniques; the cation–oxygen and cation–deuterium distances are 2.09±0.02 and 2.69±0.02 A, respectively. The influence of the different-sized Mn2+ and Ni2+ ions on the results is discussed.The isomorphs used in the perchlorate solutions were copper and an isotopic mixture of nickel (ϕNi), which does not contribute to the coherent neutron scattering pattern. Again the results show good agreement in the region of the first hydration shell with those obtained by isotopic substitution on Ni(ClO4)2 solutions. The use of ϕNi as an isomorph with zero scattering length shows that the isomorphic technique gives precise information concerning the Cu2+–water conformation. The copper–oxygen and copper–deuterium distances are 1.97 and 2.60 A, respectively, and the angle of tilt between the copper–oxygen axis and the plane of the water was 39±10 °. There is no evidence for two water sites around copper.The isomorphic substitution technique was then applied to a 2.6 mol dm–3 MSO4 solution (M = Ni or Mn). The results were similar to those obtained for the chloride case. The angle of tilt between the cation–oxygen axis and the plane of the water molecules is calculated to be 20±15 ° for Mn2+.

The structure of concentrated aqueous ammonium nitrate solutions

Neutron diffraction studies have been carried out on heavy water solutions of ammonium nitrate at two salt concentrations, 18 and 12 mol/kg–1. The first-order difference method of isotopic substitution was applied to both ND+4 and NO–3, thus enabling the determination of detailed structural information regarding the hydration of these ions. Both ions show relatively weak interactions with water. At the highest concentration, ca. 18 mol kg–1, there is evidence which demonstrates a direct interaction between the ND+4 and NO–3 ions.

Unstable intermediates. Part 133.—Electron spin resonance studies of 1-monochloro and 1,1-dichloro radicals formed from irradiated alkyl chlorides and α-chlorocarboxylic acids

Exposure of various organic chlorides to 60Co γ-rays gave a range of radical products detected by e.s.r., prominent amonst which were 1-chlororadicals. Powder spectra for these radicals at S-, X- and Q-band frequencies are exemplified and interpreted with the aid of results from single crystals. The influence of the quadrupole moment of chlorine on these spectra is discussed.Methyl chloride gave mainly H2ĊCl radicals at 77 K, whilst methylene chloride gave H2ĊCl and HĊCl2 radicals. Monochloroacetic acid gave HĊCl(CO2H) radicals at room temperature. Dichloroacetic acid gave Cl2ĊCO2H radicals, and the amide gave corresponding radicals. In aqueous or methanolic solutions dichloroacetate ions gave both Cl2ĊCO–2 and ClĊHCO–2 radicals, but trichloroacetate ions gave only Cl2ĊCO–2 radicals.

Unstable intermediates. Part 137.—α-Bromoalkyl radicals in irradiated organic bromides: an electron spin resonance study

Exposure of a range of organic bromides to 60Co γ-rays at 77 K, either pure or in perdeuteromethanol gave, amongst others, radicals having e.s.r. features thought to be characteristic of α-bromo radicals, R2ĊBr. Analyses of such spectra are given, including the use of selected single crystal spectra and measurements at different microwave frequencies. The effect of the large bromine quadrupole interaction is indicated. It is concluded that the unpaired electron is delocalised onto bromine to the extent of ca. 19 %, this being relatively insensitive to the nature of the other substituents. Exposure of bromomaleic acid to hydrogen atoms gave an α-bromo radical exhibiting a large hyperfine coupling (38 G) to as single hydrogen, which was shown to be the added atom by use of deuterium in place of hydrogen. For the radical H2CCHĊHBr structure H2Ċ—CHCHBr makes only a minor contribution. Possible radiation mechanisms are discussed.

Hyperfine coupling to iodine in α-iodo alkyl radicals

One of the radicals formed in irradiated 5-iododeoxyuridine is shown unambiguously to be the α-iodo radical RCH2-Ċ(I)R′ formed by hydrogen atom addition to C6. The 127I hyperfine tensor components, Ax = + 90 G, Ay = (-) 50 G, Az = (-)40 G are proposed as being characteristic of the coupling to be expected for α-iodo alkyl radicals. Hence a radical recently detected in irradiated iodoacetamide with a maximum hyperfine coupling of 250 G cannot have this structure. Possible alternative structures are discussed. The way in which the E.S.R. spectra for the α-iodo radica in 5-iododeoxyuridine are modified by the quadrupole interaction from 127I is described and hence an estimate of the quadrupole coupling is obtained.

Hyperfine coupling to iodine in α-iodoalkyl radicals: the radical HĊ(I)CONH2 in γ-irradiated α-iodoacetamide

Reasons for supposing that a radical exhibiting well defined hyperfine coupling to 127I, formed in γ-irradiated α-iodoacetamide, is HĊ(I)CONH2 are summarised.

Reaction of metal carbonyls with oxygen

It is suggested that a complex previously identified as ·Mn(CO)5 is, in fact, the peroxo complex ·O2Mn(CO)5. The analogous cobalt complex ·O2Co(CO)4 has also been identified.

Unstable intermediates. Part 113.—E.S.R. data for R2ĊXR3 radicals, when X is from group IV or V. Evidence against significant d(π)—p(π) bonding

A wide range of R2ĊXR3 radicals has been prepared, generally in the solid-state, by γ-irradiation of the corresponding alkyl derivatives such as R4Sn, R4Pb, R4P+ hal– and R4As+ hal–. From the magnitudes of the hyperfine coupling to α-protons, methyl protons, and X nuclei, it is concluded that for all these radicals, delocalization onto X is undetectably small, and therefore < ∼1 %.Arguments in favour of π-bonding, previously adduced from trends in the g-values for radicals containing Group IV elements, are shown to be unconvincing, since for Group V elements the trend is reversed.

Unstable intermediates. Part 146.—Electron spin resonance spectra for linear BrCN– and Br2CN radicals in irradiated cyanogen bromide crystals

On exposure to 60Coγ-rays at 77 K, two well defined paramagnetic centres are for med in crystalline cyanogen bromide. One, identified as BrCN– had 81Br hyperfine and g-tensors parallel to the molecular axis and for this and other reasons is thought to be linear, with the unpaired electron in a σ* orbital largely confined to bromine and carbon. This makes an interesting contrast with the isostructural radicals FĊO and ClĊO which are reported to be strongly bent.The other well defined radical, containing two equivalent bromine atoms, is identified as Br2CN. A third, poorly defined radical containing two strongly coupled bromine atoms and exhibiting a large positive g-shift is tentatively identified as the cation dimer, (BrCN)+2. A fourth radical, formed on exposure of solutions of cyanogen bromide in dilute solution in acetic acid to γ-rays at 77 K is tentatively identified as H(Br)CN. The structures of these new species are discussed together with possible damage mechanisms.

Electron spin resonance studies of the effect of γ-radiation and of hydrogen atoms upon various olefinic carboxylic acids and their amides

Olefinic carboxylic acids and amides have been studied at 77 K and room temperature after exposure to H atoms and γ-rays. At 77 K certain radicals (e.g., Me2Ċ·CO2H, from α-methylacrylic acid and Me2Ċ·CH2·CO2H from ββ-dimethylacrylic acid) were observed, some of which were converted into polymer radicals on annealing to room temperature [e.g., RCH2·ĊMe(CO2H) from α-methylacrylic acid]. Several differences between H atom and γ-ray bombardment were noticed, especially the tendency of H atoms to add to olefinic bonds in cases where γ-rays gave allylic radicals. Results are discussed in terms of reaction mechanisms.