A. B. Denison

Association reactions at low pressure. IV, The HC3N+/HC3N system

The reactions between HC3N+ and HC3N, and between HC5N+ and HC3N have been examined at pressures from 1×10−7 to 1×10−3 Torr by ion cyclotron resonance mass spectrometry. The reaction between HC3N+ and HC3N has both a bimolecular reaction path and a termolecular reaction path. The overall bimolecular reaction rate coefficient was found to be 1.3×10−9 cm3 s−1. The primary product, HC5N+, represents 90% of the product ions, while the minor products HC6N+2 and H2C6N+2 each represent 5%. The termolecular association rate coefficient was 3.7×10−24 cm6 s−1 with He as the third body. From double resonance experiments the mean lifetime of the collision complex was determined to be 180 μs. HC5N+ was found to react with HC3N and form the adduct ion H2C8N+2 through both bimolecular and termolecular channels. The bimolecular rate coefficient was 5.0×10−10 cm3 s−1 and the termolecular rate coefficient was observed to be 1.2×10−22 cm6 s−1 with HC3N as the third body. With He as the stabilizing molecule, the termolecular...

Proton and Deuteron NMR of Ice Polymorphs

Proton and deuteron nuclear magnetic resonances in the H2O and D2O ice polymorphs have been recorded at 60 MHz and at 8 MHz, respectively. Proton rigid‐lattice second moments have been obtained at 75°K for ices Ih, Ic, II, V, VI, and IX. The modulation‐corrected experimental values of the second moments ranged from 32.2 G2 in ice Ic to 37.9 G2 in ice VI. Theoretical inter‐ and intramolecular contributions to the second moments of these ices have been made, with corrections applied to the latter for librational and vibrational motions after the manner of Pederson. These theoretical results when compared with experimental values strongly support the bent hydrogen‐bond model with an H–O–H angle near 104°, in agreement with conclusions reached from our recent neutron‐diffraction pattern analyses of ices Ic, II, and IX. Deuteron quadrupole coupling constants for ices Ic, II, V, and IX have been estimated from derivative of dispersion traces by comparison with the ice Ih signal obtained under similar experiment...

Magnetic Resonance Studies on Copper (II) Complex Ions in Solution. III. NMR and EPR in Concentrated Ethylenediamine Solutions

Measurements of the proton and 14N NMR linewidths in ethylenediamine—water solutions of Cu(II) combined with EPR studies of Cu(II) in these media demonstrate that the first sphere relaxation and residence times both make singificant contributions to the over‐all relaxation of protons and 14N by Cu(II). Moreover, the results are consistent with the view that protons and 14N both experience the Cu(II) evironment through exchange of the ethylenediamine molecule as a whole between the bulk solvent and the Cu(II) first sphere. From the temperature dependence of the Cu(II) EPR linewidth it is further concluded that the relaxation of the electron spin occurs predominantly via the spin—rotational process at higher temperatures. At lower temperatures the EPR linewidth is broadened both by tumbling of a tetragonally distorted complex with an anisotropic g factor and hyperfine coupling constant and by the same fast chemical exchange process which provides 14N and proton relaxation in ethylenediamine.

Muon spin rotation in magnetic oxides (invited)

Systematic measurements have been made using the technique of muon spin rotation (μSR) for a series of magnetic oxides. These measurements have been made as a function of temperature and externally applied field on α‐Fe2O3, Cr2O3, magnetite (Fe3O4) and a set of rare‐earth orthoferrites RFeO3 (R=Sm, Eu, Dy, Ho, Y, and Er). The muon behavior is found to exhibit common features for these materials with the muon forming a weak bond near the oxygen. Local motion or delocalization may occur at lower temperatures while global diffusion or hopping to similar sites occurs at higher temperatures (>400 K). As the behavior of the muon in its chemical environment becomes understood the μSR technique may be used to study other physical effects of interest.

Muon bonding sites in rare earth orthoferrites

A muon site search has been performed for the RFeO3 series based on a calculation of dipole fields and assumptions that a μ-0 bond is formed at identical sites in each sample. The site previously identified by Holzschuh et al. /1/ has been verified and additional sites located which together explain all the observed μSR frequencies in the orthoferrites. Effects due to muon motion and covalent contributions to the internal fields are briefly discussed.

Muon-oxygen bonding in V2O3

A muon site search using calculated internal fields has been performed for V2O3, where purely dipolar fields allow a site determination free from covalent complications. The obtained sites are a subset of the Rodriguez and Bates sites found in α-Fe2O3 and indicate muon oxygen bond formation. The sites missing at low temperatures are consistent with the vanadium pairing mechanism for the metal-to-insulator (corundum-to-monoclinic) phase transition.

Muon spin rotation study of V2O3

Muon spin rotation (μSR) measurements have been made on V2O3 as a function of temperature and externally applied magnetic field. V2O3 is an insulating antiferromagnet below about 155 K, while above this temperature it is a high‐resistivity metal. The conduction arises from the band mechanism where the density of states is quite sensitive to temperature and doping. The local magnetic field, sensed by the positive muon, has been measured from room temperature down to 10 K, with emphasis on the transition region. Clear evidence of the onset of magnetization is provided by a jump in the muon precession frequency. From experiments using an externally applied magnetic field the directions of the internal fields at the muon stop sites are found. This information can be used to determine the muon stop site since the field at this site should be determined primarily by the magnetic dipoles of the V3+ ions. Dynamic effects are observed through the muon spin depolarization. The results obtained for V2O3 are discusse...

A new polaronic order-disorder phase transition in magnetite as observed through μSR

Recent μSR measurements on the Mott-Wigner glass, magnetite, as a function of temperature and external magnetic field have shown the existence of two inequivalent magnetic sites below TA=247K. These data are being interpreted in terms of the onset or destruction of local order manifested as local atomic correlations (molecular polarons).

The pseudo‐brookite spin‐glass system studied by means of muon spin relaxation

Zero‐field muon spin relaxation (μSR) experiments have been performed on the spin glass Fe1.75Ti1.25O5. Above the spin‐glass temperature of 44 K a distinct exponential μSR rate (λ) is observed, while below T g a square‐root exponential decay occurs, indicating fast spin fluctuations. Near 8 K, a maximum in λ is indicative of transverse spin ordering. The low λ values and the sharp λ peak at T g are very promising for the study of spin freezing models like the Vogel–Fulcher law or the power law.

Muon bonding versus muonium formation: Muon-Spin-Relaxation in α-Al2O3

Results of Muon-Spin-Relaxation (μSR) experiments on well-defined single crystals of corundum (α-Al2O3) are reported. Major issue in this study is the controversy of muon bonding or muonium formation in insulators. Transverse, zero-field and longitudinal field measurements were performed as a function of temperature or applied field. The obtained results indicate that muon-oxygen bonding occurs in α-Al2O3, having (at least) two (μO) states as found earlier for α-Fe2O3. The occurrence of a 10% missing fraction suggests Mu states, although Mu was not observed in our measurements. These results and their implications are discussed in light of other experimental μSR studies on insulating oxides and the above-mentioned controversy.