Electromagnetic moments of short lived β emitters 21F, 23Mg, 27Si and 39Ca

The β-NMR spectra of ²¹F, ²³Mg, ²⁷Si and ³⁹Ca which were produced in heavy ion collisions and implanted in various crystals have been observed. The magnetic moments of ²¹F and ²⁷Si were determined to be |μ(²¹F)| = 3.9194 ± 0.0012 μ_N and |μ(²⁷Si)| = 0.8653 ± 0.0003 μ_N, respectively. The electric quadrupole coupling constants were determined for the first time to be |eqQ(²¹F in MgF₂)/h|= 9.94 ± 0.09 MHz, |eqQ(²³Mg in MgF₂)/h|= 1.96 ± 0.06 MHz, |eqQ(²⁷Si in Al₂O₃)/h|= 1.90 ± 0.12 MHz, |eqQ(³⁹Ca in CaCO₃)/h|= 0.60 ± 0.04 MHz. From the present eqQ/h, the Q moments were deduced as |Q(²¹F)|= 110 ± 22 mb, |Q(²³Mg)|= 114 ± 3 mb, |Q(²⁷Si)|= 60 ± 13 mb and |Q(³⁹Ca)|= 36± 7 mb. The present data were compared with the theoretical values obtained by the OXBASH shell model code. This revised version was published online in August 2006 with corrections to the Cover Date.     

The triethylenediamine cation radical: An ESR study

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.     

Two-particle rapidity correlations in αα, αp,andp p interactions at the CERN intersecting storage rings

Two-particle rapidity correlations have been studied for αα, αp, andp p interactions at the CERN ISR using the Split-Field Magnet (SFM) detector. In order to isolate the true two-particle correlations, the analysis was performed at fixed charged multiplicity. In the framework of a simple cluster model, it is found that cluster widths as well as cluster multiplicities are the same for αα, αp, andp p interactions, and both decrease with increasing charged multiplicity.     

The lifetime of the 3907 keV state of 19F

The mean lifetime of the 3907 keV state of ¹⁹F has been measured by the Doppler shift attenuation method (DSAM). The value obtained was 9 ± 5 fs. Transition strengths derived from this lifetime and previously measured branching ratios are compared with the results obtained from shell model calculations. The importance of including particle-hole configurations is discussed and it is shown for example that the energy and lifetime of the state are consistent with a predominantly p^?2 sd⁵ structure.     

Electron spin resonance spectra of two naturally occurring paramagnetic centres in diamond

A centre, characterised by an isotropie singlet at g = 2.000 and satellite lines having the correct relative intensity for ²⁹Si hyperfine features, has been detected in natural diamond. The form of the ²⁹Si hyperfine tensor suggests that the electronic structure of this silicon centre is similar to that of the common nitrogen centre. Although silicon is an expected impurity in diamond, paramagnetic centres containing it have not previously been reported. We have also detected the paramagnetic nitrogen centre previously reported, characterised by a hyperfine triplet with $\text{A}{}_6\text{(}{}^{14}\text{N) = 7.5G}$ and $\text{A}{}_{\mathrm{\perp }}\text{(}{}^{14}\text{N) = 5.0 G}$. This centre was accompanied by the common nitrogen centre in only one case. Other naturally occurring centres, apparently comprising two weakly coupled electrons in triplet states, were too poorly defined to characterise fully.     

Optical and ESR studies of silver clusters

The optical and ESR spectra of Ag(O), Ag₂⁺ and Ag₄³⁺ centres formed in γ-irradiated aqueous and ethanol glasses have been monitored under identical conditions. The ESR spectra of the Ag(O) centres sugest that they initially retain the solvation of the parent Ag⁺ ions but that solvent molecules are gradually lost on annealing. This loss is reflected in marked high-frequency shifts in the electronic transitions. An intense band observed in the range 265–290 nm grew in simultaneously. This is assigned to Ag₂³⁺ ions. These have a well characterised ESR spectrum, but have not previously been studied optically.     

Radical anions of trimethyl and triphenyl lead chloride and of triphenyl lead bromide: an E.S.R. study

We report the gas phase microwave observation and subsequent analysis of the ring puckering level splitting of indoline and its N-D isotopomer. The spectra of the four lowest vibrational states are modelled through a two-dimensional vibrational Hamiltonian to furnish a coherent picture. According to this new picture, five-membered rings with a single double bond and five-membered rings in which the double bond is fused with a benzenic ring share a similar low ring puckering barrier. Discrepancies with previous work are ascribed to the use of a one-dimensional vibrational Hamiltonian. Ab initio calculations concur with the present analysis.     

Electron-spin resonance and theoretical studies of radicals formed from trimethylamine borane : the radicals Me2[Ndot]-BH3 and (Me3N-BH3)-

Exposure of trimethylamine borane to ⁶⁰Co γ-rays at 77 K gave, initially, a four-line E.S.R. spectrum assigned to the radical anion, Me₃[Ndot]-BH₃ ^-, having a large (58 G) hyperfine coupling to boron. Theoretical calculations suggest that the radical resembles a perturbed ^.BH₃ ^- in which the normally planar unit still has considerable pyramidal character.

After annealing above 77 K this species was lost and a multiline, isotropic spectrum grew in which, by comparison with spectra obtained from Me₃N-BD₃, is shown to belong to Me₂[Ndot]-BH₃ radicals. This species is of interest because of the large ¹H coupling to the -BH₃ protons (ca. 45 G). This coupling, qualitatively ascribed to hyperconjugation, is correctly reproduced by theoretical (ab initio UHF) calculations.     

Hyperfine coupling to β-muonium and the theory of hyperconjugation

The theory of hyperconjugation, orσ-π delocalization, which has been very successful in explaining the properties of carbocations, and the ESR parameters for a range of radicals, is invoked to explain two aspects of the results for muonated radicals. One is the commonly observed fact that the reduced muon hyperfine coupling constants in a range of radicals are greater by a factor of ca. 1.2 than the corresponding proton coupling constants. The other is the preference of C-Mu bonds in positionsβ to radical centers to occupy an eclipsed site which maximizesσ-π overlap. This theory, which has been largely dismissed by others, still seems to be an attractive and simple explanation of both phenomena. It is suggested that one alternative theory, which is termed the “zeropoint energy theory,” is closely linked to the hyper-conjugation theory proposed herein.