Structure of sodium tetradeuteroborate,NaBD4

The structure of NaBD₄ was determined from powder neutron data and refined using Rietveld's method. The compound has a sodium chloride-type structure, in space group $\text{F}\text{4}\text{3m, a = 6.137(7)}$, Å, BD, 1.160(7); angle DBD, 109.5(2)° at 295 K, with D atoms tetrahedrally oriented about B (at $\text{1}\text{2}\text{,0,0}$, etc.), and along all cube diagonals. This gives a random distribution of BD^?₄ tetrahedra in two different configurations.     

Direct absorption measurement of the spin—orbit splitting of the ground state in atomic fluorine

We have measured the three hyperfine components of the spin—orbit split (²P_{$\text{3}\text{2}$} → ²P_{$\text{1}\text{2}$}) ground state in atomic fluorine by diode laser absorption spectroscopy. The measurement improves the accuracy of the two lines previously reported (404.175 and 403.969 cm^?1), with the third line at 404.210 cm^?1. This confirms the spacing of the hyperfine components measured by EPR, and establishes diode laser absorption as a viable technique for determining F-atom concentrations.     

Electronic-to-vibrational energy transfer from Br (42P12 to HF

Room temperature experiments have measured the rate of electronic-to-vibrational energy transfer between spin—orbit excited Br(4²P_{$\text{1}\text{2}$}) and HF. The Br^* + HF quenching rate is very fast, (1.1 ± 0.2) × 10⁶ s^?1 torr^?1, due to a near resonance between the spin—orbit splitting and the vibrational spacing. The majority of the Br^* spin—orbit energy goes directly into HF vibration.     

ESR studies on the reactions of sulphite radical anion,SO3.̄−, with nitroalkane compounds in aqueous solutions

The reactions of the sulphite radical anion, SO₃^{$\text{.}\text{?}$?} (generated from the Ti³⁺-H₂O₂-Na₂SO₃ system at pH 9), in aqueous solutions with some nitroalkane compounds were investigated by using a rapid-mixing flow technique coupled with electron spin resonance (ESR) which can detect the radicals having a lifetime of 5–100 ms. The SO₃^{$\text{.}\text{?}$?} radical added to the double bond of CN in the nitroalkane aci-anions which are the main form of nitroalkanes in aqueous alkaline solutions. From the observed hyperfine splitting constants of the SO₃^{$\text{.}\text{?}$?} adducts of nitroalkane aci-anions, the preferred conformation of the adducts was deduced.     

Neutron powder diffraction and magnetic measurements on CsMnI3

Results of neutron powder diffraction and magnetic measurements on single crystals of CsMnI₃ are reported. Three-dimensional ordering takes place at T_c = 11.1(3) K. Above T_c very broad peaks occur in the neutron powder diffraction diagram, indicating one-dimensional correlations along the chain. Below T_c the Mn²⁺ ions are coupled antiferromagnetically along the chain. Interchain exchange leads to a 120° structure, slightly distorted due to anisotropy. One-third of the chains have their magnetic moment parallel to the c axis and the rest of the chains have magnetic moments making an angle of 50(2)° with the c axis. The magnetic moment as found from neutron diffraction extrapolated to 0 K is 3.7(1)μ_B, indicating a considerable zero-point spin reduction. The intrachain exchange $\text{J}\text{k}$ was found to be ?9.1(1)K, whereas the ratio of the inter- to intrachain interaction was determined as $\text{J′}\text{J}\text{= × 10}{}^{\mathrm{?3}}$. A spin flop occurs at H = 54 kOe on application of a magnetic field parallel to the c axis. When a field perpendicular to the c axis is applied a spin reorientation occurs at 1 kOe.     

An unusual example of a rapid spin-equilibrium iron complex

Bis(N-(1-benzoyl-2-propylidene)-2-pyridylmethylaminato)iron(III) hexafluorophosphate (Fe(bzpa)₂PF₆ is shown to be a spin-equilibrium compound between low- (S = $\text{1}\text{2}$) and high- (S = $\text{5}\text{2}$) spin states depending on temperature, and to have the unusual property that the relaxation time of the spin change is shorter than the Mössbauer lifetime (1 × 10^?7s) of iron- 57 (l = $\text{3}\text{2}$) in the solid state.     

C2O(X3Σ−): absolute reaction rates measured by laser induced fluoresence

Absolute rate constants are reported for reactions of C₂O($\text{X}\text{?}$³Σ^?) under pseudo-first-order decay conditions. C₂O is generated by laser photodissociation of C₃O₂ at 266 nm, and detected by dye-laser induced fluorescence on the $\text{A}\text{?}$³Π_i-$\text{X}\text{?}$³Σ^? transition. Rate constants of (433 ± 12), (3.30 ± 0.12) and (1.12 ± 0.05) × 10^?13 cm³ molecule^?1 s^?1 are reported for reactions with NO, O₂ and isobutene. The NO value is approximate due to an apparent dark reaction between NO and C₃O₂. Upper limits of 1 × 10^?14 cm³ molecule^?1 s^?1 are reported for reactions with H₂, CO₂, C₃H₂ and C₂H₄. The C₂O + C₃O₂ reaction does not follow pseudo-first-order decay kinetics. Two explanations are proposed to explain this observation. Results are compared with previous relative rate measurements and are discussed in terms of their relevance to combustion chemistry.     

Equilibrium and kinetics of reversible capture of solvated electron-sodium+ pairs by 1,1-diphenylethylene in tetrahydrofuran

Solvated electron-Na⁺ pairs, e^?,Na⁺, and 1,1-diphenylethylene reversibly recombine in THF, the capture constant = 3 × 10⁷ M^?1 s^?1 and the detachment constant = 46 s^?1; the e^?,Na⁺, formed by flash-photolysis of Na⁺,$\text{C}\text{?}$(Ph)₂CH₂CH₂$\text{C}\text{?}$(Ph₂, Na⁺ survive for 0.1 s in this solvent at ambient temperature without any detectable decay.     

Yield of I(2Psol:12) in the photodissociation of CH2I2

The production of I(²P_{$\text{1}\text{2}$}) in the photolysis of CH₂I₂ has been studied optoacoustically at excitation wavelengths between 365.5 and 247.5 nm. Bands found at 32200 and 47000 cm^?1 correlate with I(²P_{$\text{3}\text{2}$}) whilst those at 34700 and 40100 cm^?1, which correlate with I(²P_{$\text{1}\text{2}$}), give final ²P_{$\text{3}\text{2}$}/²P_{$\text{1}\text{2}$} ratios of 1.75 and 1.1, respectively, after curve crossing.     

Experimental and theoretical investigations of the nuclear quadrupole resonance of 127I in orthoperiodic acid,H5IO6

Pure nuclear quadrupole resonance of ¹²⁷I in H₅IO₆ has been reported to occur at 44.976 MHz (m = ±$\text{3}\text{2}$?m = ±$\text{1}\text{2}$ transition) and 83.960 MHz (m = ±$\text{5}\text{2}$? m = ±$\text{3}\text{2}$ transition) at 296 K (ref. 1). Using an externally quenched frequency modulated super regenerative spectrometer Zeeman studies have been performed at 296 K on single crystals of H₅IO₆ growm from aqueous solution by observation of the ±$\text{sol3}\text{2}$?±$\text{1}\text{2}$ transition. The zero-splitting locus method has been employed for the determination of the EFG parameters. The data show that there are two physically inequivalent sites, having average asymmetry parameters, η, of 0.25, located in the unit cell. The maximum field gradient is found to lie along the short bond I-O(2) as suggested by Rama Rao and Weiss (ref. 1). The EFG parameters obtained by detailed molecular orbital calculations (CNDO/2 and INDO method) are compared with the experimental results.     

Magnetic resonance experiments on the radical cation salt (fluoranthenyl)2+ PF6−. Correlation of proton-NMR,fluorine-NMR and ESR

The longitudinal relaxation rates of the protons and the fluorine were determined for the radical cation salt (fluoranthenyl)₂⁺PF₆^? as a function of the temperature. These data are compared with the temperature dependence of the pulsed ESR signal amplitude at the same Larmor frequency (44 MHz). Whereas the fluorines are relaxed mainly by the reorientational motion of the anions and by the interaction with fixed paramagnetic impurities, the protons are relaxed additionally above 150 K predominantly by highly mobile paramagnetic species, whose concentration could be determined directly via the signal amplitudes of the NMR and ESR signals in the same set up. The Pauli susceptibility χ_c^s = 3.1 ×10^?5 cm³/mole which is derived from this experiment and the Korringa relation (T_1H4^?)^?1 = constant of the proton relaxation leads to the assumption of a metal-like behavior of the salt above 183 K. The observation that the protons of the cation stacks, rather than the fluorines on the anions are relaxed by the mobile paramagnetic species favors the assumption of one-dimensional spin transport within the fluoranthenyl stacks, which is further supported by an ω^{$\text{?1}\text{2}$} dependence of T_1H^?1.     

The importance of spin relaxation in the delayed fluorescence of molecular crystals

Deactivation rate constants of spin-orbital excited Br atoms in the reactions Br(²P_{$\text{1}\text{2}$}) + O₂ → Br(²P_{$\text{3}\text{2}$}) + O₂ (k₁), and Br(²P_{$\text{1}\text{2}$}) + NO → Br(²P_{$\text{3}\text{2}$}) + NO (k₄) have been measured with a photodissociative IBr laser on the electronic transition ²P_{$\text{1}\text{2}$}?²P_{$\text{3}\text{2}$} in the Br atom (λ = 2.7 μm). The values obtained are (6.4 ± 1.8) × 10^?14 cm³ s^?1 and (1.9 ± 0.6) × 10^?12 cm³ s^?1, respectively. Comparison with published data leads to the conclusion that, contrary to a widely accepted point of view, the high rate constants for the quenching of excited halogen atoms are due to resonant energy transfer processes and not to the paramagnetic nature of the quencher.     

Un nouveau conducteur ionique (LaO)AgS

The compound (LaO)AgS is prepared by reaction of Ag₂S and La₂O₂S, in the presence of a small proportion of iodine, between 580 and 640°C. The cell is tetragonal, space group $\text{P4}\text{n}$ or $\text{P4}\text{nmm}$. The structure is not known, but is probably formed by alternating (LaO) and (AgS) sheets. The electric conductivity is determined by the complex impedance method, with blocking electrode, and by emf measurements; it is mainly ionic, with σ = 10^?3 to 10^?1 Ω^?1 cm^?1 between 25 and 250°C. The activation energy is 0.195 ± 0.005 eV.     

Magnetic susceptibility of Co4+(d5) in octahedral and tetrahedral environments

Measurements of magnetic susceptibility on compounds containing stoichiometric Co⁴⁺ are reported. The compound Ba₂CoO₄ has the Co⁴⁺(d⁵) ion at a tetrahedral site and displays a susceptibility of the expected magnitude for $\text{S =}\text{5}\text{2}$. The compounds Ba₃Co₂CO₉ and BaCoO₃ have the Co⁴⁺ at an octahedral site and show a susceptibility expected for low spin, $\text{S =}\text{1}\text{2}$. For the low spin case significant deviations from Kotani's calculated susceptibility were observed. Improvement of the theory was made through incorporation of the effects of distortion from perfect octahedral symmetry and the inclusion of higher electronic configurations above t⁵₂ in the ²T₂ ground state. A case of low spin Ni in octahedral environment is also reported.     

Reactions of triphenylarsine oxide with aqueous hydrogen fluoride: Crystal structure of bis(triphenylarsine oxide)hydrogen(I) tetrafluoroborate

Fluorination of triphenylarsine oxide by aqueous hydrogen fluoride (1–40%) in the absence of glass readily gives triphenylarsine difluoride. When the reaction with dilute (1%) aqueous hydrogen fluoride is carried out in borosilicate glass apparatus, the glass participates in the reaction resulting in the formation of the crystalline 2:1 adduct 2Ph₃AsO·HBF₄. Crystals of this compound are monoclinic, $\text{P}$2₁/$\text{c}$, $\text{a}$ = 12.926(4), $\text{b}$ = 17.819(6), $\text{c}$ = 14.994(4) Å, β = 98.97(3)°, Z = 4. The structure contains cations [(Ph₃AsO)₂H]⁺ in which $\text{O}\text{?}$O is 2.44(2)Å, and anions BF₄^?.     

Double quantum modulation NMR spectroscopy on spins with I = 1 in solids

NMR experiments with amplitude modulated rf pulses are introduced for the detection of double quantum coherences from spin I = 1 nuclei with large quadrupolar frequencies v_Q in solids. rf pulses of intensity v₁ and with modulation frequency v_m create effective irradiation fields of intensity $\text{1}\text{4}$v²₁/(v_Q-v_m) on the double quantum transition when v₁<¦v_Q-v_m¦?v_Q+v_m. ²D-NMR measurements on an oriented single crystal of deuterated malonic acid are presented.     

Low density spectrum of transverse NMR in molecular gases. Application to hydrogen

The low density behaviour of the spectral lineshape function associated with the relaxation of the transverse components of the nuclear magnetization of a homonuclear diatomic gas of spin $\text{1}\text{2}$ nuclei is studied from the point of view of a recent kinetic theory approach to NMR in molecular gases. It is shown that as a critical density is approached (from above), the relaxation (in time-domain) passes over from exponential to non-exponential behaviour. For the special case in which the spin—rotation relaxation mechanism predominates, an analytic solution to the problem has been given, while for molecular hydrogen which has, in addition to the aforementioned mechanism, dipolar relaxation, the behaviour of the lineshape function has been obtained numerically. The critical density at which the relaxation passes from exponential to nonexponential in H₂ is found to be of the order of 3 × 10^?3 amagats which lies, at present, still outside experimental accessibility. One important consequence of this result is that the traditional Abragam formula for the transverse relaxation time T₂ is clearly seen to be invalid below ≈ 5 × 10^?3 amagats.     

On the validity of using the TKR sum rule to place zero angle electron impact spectra on an absolute scale

The use of the Thomas—Kuhn—Reiche (TKR) sum rule to place electron impact spectra on an absolute scale is only rigorously correct if the observed intensities can be extrapolated to their first Born zero momentum transfer limit. This note investigates the error involved in using the TKR sum rule at zero scattering angle instead of zero momentum transfer. By considering an expansion of the generalized oscillator strength at zero angle it is shown that the first order correction to the TKR sum rule can be written as ($\text{1}\text{3}\text{k}{}^2$)[|E_o|?($\text{4}\text{5}$)$\text{V}$_ee] for target systems randomly oriented in space where k² is the incident electron energy and |E_o| and $\text{V}$_ee are the magnitude of the total electronic energy and the electron—electron repulsive energy of the ground state of the target system respectively. Estimates based on Hartree—Fock calculations indicate a 0.27% error in the use of the TKR sum rule for F and 0.79% for Ar at an incident electron energy of 25 keV.     

The rotational spectrum and molecular properties of a hydrogen-bonded dimer of phosphine and hydrogen fluoride

⁵⁷Fe Mössbauer spectra have been obtained for Fe(p-CH₃C₆H₄SO₃)₂ between 2.3 and 300 K in zero field, and at 2.3 and 4.2 K in longitudinal applied magnetic fields ranging from 1.1 to 5.6 T. The complex is a fast-relaxing paramagnet under all conditions studied and there is no evidence of antiferromagnetic exchange coupling. The FeO₆ chromophore is distorted by a trigonal elongation and the orbital ground state is the [($\text{2}\text{3}$)^{$\text{1}\text{2}$}|±2〉 ? ($\text{1}\text{3}$)^{$\text{1}\text{2}$}|?1〉] doublet. The temperature dependence of the quadrupole splitting has been analysed via a crystal-field model to provide estimates of the axial field splitting parameter Ds = -93 cm^-1, spin-orbit coupling constant λ = -70 cm^-1, and fine structure constant Dσ = -28 cm^-1. The magnetic properties of the complex are described by treating the ground state as a non-Kramers doublet with fictitious spin ? = $\text{1}\text{2}$. Five separate Mössbauer-Zeeman spectra can be fitted in this spin-hamiltonian approximation with identical values of the g- and A-tensor components, viz. g_⊥ = 1.0, g_u = 9.0; A_⊥ ≈ 2.0 mm s^-1. A_u = -1.79 mm s^-1. The trigonal z axis, the z axis of the electric field gradient tensor, and the easy axis of magnetisation are collinear, and the saturation value of the internal hyperfine field along this axis is +13.0 T.     

EPR Spectrum of Kr2F

An intense EPR spectrum observed in UV-irradiated solid Kr/CF₃OF solutions at 10 K shows anisotropic hyperfine interactions of a single unpaired electron with one nuclear spin of $\text{1}\text{2}$ and (using isotopically enriched ⁸³Kr) two equivalent spins of $\text{9}\text{2}$. The spectrum is attributed to the linear radical KrFKr having a ²Σ_u ground state.