D.H. Whiffen

Determination of the relative signs of proton spin coupling constants by double irradiation

The method of Evans [1] for determining relative signs of nuclear spin coupling constants by double irradiation experiments has been extended to the case where the three coupled nuclei are all protons. It is thus possible to demonstrate from the high resolution proton magnetic resonance spectrum of the ring protons of 2-Furoic acid that the three spin coupling constants are all of like sign.

The optical and electron resonance spectra of SO3

The ionic radical SO3 - has been identified in γ-irradiated single crystals of sodium dithionate, sulphamic acid, potassium sulphamate, potassium amine disulphonate and potassium methane disulphonate. The electron resonance spectrum is a single line with an isotropic g factor of 2·004. The principal values of the 33S hyperfine coupling tensor are 428, 314 and 316 Mc/s, from which the percentages of 3s and 3p character of the unpaired electron orbital are obtained, enabling a value of 111° for the O-S-O angle to be deduced. Ultra-violet transitions at 2700 a and 2400 a were found for SO3 -. A value of 104 for the extinction coefficient of the 2400 a band was measured. A comparison of spin populations between the isoelectronic species PO3 =, SO3 - and ClO3 is included.

The electron spin resonance spectrum of CH3ĊHCOOH at 77°K in l-α-alanine

The change in the electron resonance spectrum of a γ-irradiated single crystal of l-α-alanine on cooling to 77°k has been interpreted in terms of the cessation of rotation of the methyl group of the trapped radical, CH3ĊHCOOH. The isotropic hyperfine couplings of the three hydrogens of the methyl group are found to be 120, 76 and 14 Mc/s respectively compared to the value of 70 Mc/s each at 300°k. The geometry of the free radical at 77°k is such that, in projection on a plane perpendicular to the Ċ-CH3 bond, one of the C-H bonds makes an angle of about 12° to the plane of the free radical centre and this hydrogen is probably adjacent to the carboxyl group.

Electron spin resonance and structure of the ionic radical, ·PO3 =

It is shown that the ionic radical, ·PO3 =, is formed by the action of γ-rays on disodium ortho-phosphite pentahydrate. The hyperfine coupling to the 31P nucleus has principal values of 1967, 1514 and 1513 Mc/s and is consistent with a pyramidal ion having OPO angles of 110°.

Electron spin resonance of (CO2H)CH2-ĊH(CO2H) in succinic acid

Electron spin resonance investigations on a γ-irradiated single crystal of succinic acid have confirmed and extended the findings of Heller and McConnell on the hyperfine couplings in the radical (CO2H)ĊH-CH2 (CO2H). Complete coupling tensors, and the g tensor, are given in the table. These confirm that the radical has almost the same orientation as the unchanged molecules. The plane of the free radical carbon is twisted 5° from the original carbon skeleton plane and this leads to appreciable non-equivalence of the hydrogen atoms of the CH2 group.

The structure, electron resonance and optical spectra of trapped CO3 - and NO3

Examination of irradiated potassium bicarbonate shows the crystal to contain trapped CO3 - radicals, as well as CO2(H) radicals, and examination of irradiated urea nitrate shows this crystal to hold trapped NO3 radicals. Electron resonance spectra and optical absorption spectra suggest that the radicals CO3 - and NO3 are planar, but do not possess a threefold axis. An examination of the expected wavefunction and energies discloses features which favour distortion of these radicals; these features are absent for CO3 = and NO3 -.

The electron spin resonance spectrum of CH3ĊH(CO2H) between 100°K and 200°K

The electron spin resonance spectra of CH3ĊH(CO2H) in γ-irradiated α-alanine between 100°k and 200°k differ from the spectra obtained above and below this range. The ‘intermediate temperature’ spectra are interpreted in terms of a rather critical rate of rotation of the methyl group. From the line-width change near 100°k and 200°k an estimate of approximately 4 kcals has been made for the activation energy hindering the rotation of the methyl group.

Electron spin resonance spectra of irradiated monofluoracetamide

The radical trapped in x-irradiated monofluoracetamide is shown to be CHF CONH2 by electron spin resonance studies. The principal values of the hydrogen hyperfine coupling are -96, -63 and -31 Mc/s and of the fluorine coupling are +530, -11 and -45 Mc/s. The relative signs of the principal couplings were established, using a spectrometer operating at 35 000 Mc/s which is briefly described. The differences in spectra between measurements at 9000 Mc/s and at 35 000 Mc/s are indicated. Finally the couplings are interpreted and shown to be consistent with Huckel parameters for fluorine with (αF-αC) = 2·4βC-F.

The electron spin resonance of irradiated glycine at 77°K

The electron spin resonance spectrum of the radical ĊH(NH3 +)CO2 -, I, trapped in irradiated glycine has been re-examined at 77°k and the hyperfine couplings obtained when the -NH3 + ceases to rotate. The individual hydrogens are clearly identified by reference to the x-ray structure. Improved values are obtained for the room temperature couplings of this radical. There is also, after irradiation at 77°k, a spectrum consisting of two broad lines tentatively attributed to one of the molecular ions; the species giving rise to this spectrum changes irreversibly into I on warming above 77°k.

Electron spin resonance of γ-irradiated malonic acid

The electron spin resonance spectrum of a γ-irradiated single crystal of malonic acid indicates the presence of at least two chemically distinct radicals. The most stable radical is ĊH(CO2H)2, but the present paper primarily concerns the structure and orientation of the radical ĊH2(CO2H). The anisotropy of the spectra of this radical with respect to the direction of the main magnetic field H shows that the free radical carbon atom has gone over to planar sp 2 hybridization, the HĊH angle being 116°±5°. Since the two ends of the undamaged malonic acid molecule are distinguishable in the crystal, two sites were expected for the radical ĊH2(CO2H). It appears, however, that approximately 80 per cent of the radicals occupy one site, and only about 20 per cent the other. Comparison of the orientation of the main ĊH2(CO2H) radical with that of the undamaged molecule indicates that in the radical the plane of the ĊH2 group is approximately perpendicular to the plane of the (CO2H) group. The spectra of the less pr...