Sherman W. Rabideau

Oxygen‐17 Quadrupole Coupling Parameters for Water in Its Various Phases

Analysis of 17O NMR data in D217O ice yields a quadrupole coupling constant, e2qQ / h = 6.66 ± 0.10MHz, and an asymmetry parameter, η = 0.935 ± 0.01. Comparison with the data for liquid and gas indicates a temperature dependence of the 17O quadrupole coupling constant in the liquid and explains the unusual linewidth behavior for liquid D217O. Charge distributions in the water molecule in both the solid and gaseous states have been obtained by a valence‐shell atomic‐orbital population analysis based on the experimental data for the 17O quadrupole coupling parameters and the molecular dipole moment. The results were used to calculate the quadrupole coupling constant for the deuterium nucleus. Good agreement with experimental data was obtained for both the solid and gaseous states. Comparison of the calculations for the two phases suggests that the intermolecular effects in ice are mostly electrostatic.

Electronic and vibronic states of uranium hexafluoride in the gas and in the solid phase at very low temperatures

Complex vibronic structure has been observed in the electric‐dipole‐forbidden charge transfer absorption bands of solid and matrix‐isolated UF6 in the temperature range 8–14 K. These bands have their maximum intensity near 260 and 375 nm. Associated with the 260 nm band are four electronic transitions with origins at 30 331, 31 032, 32 120, and 32 821 cm−1, the first two being observed directly. Two more no‐phonon transitions are associated with the weak band at 375 nm, one at 24 564 cm−1 and another at 25 265 cm−1. These levels are assigned via a weak j‐j coupling scheme as excitations from the ligand t1uσ orbital to empty uranium 5f orbitals. Uranium spin–orbit coupling in UF6 charge transfer states strongly resembles that of UF6 −. Several progressions in the symmetric stretch frequency ν1=580–595 cm−1 are present which have as their origins various combinations of the above electronic levels with the bending modes ν4, ν5, and ν6 or their overtones 2ν4, 2ν5, and 2ν6. Temperature effects, which account ...

Oxygen‐17 NMR Linewidths as Influenced by Proton Exchange in Water

Linewidths for the 17O nuclear magnetic resonance have been obtained at 29.0°±0.2°C in 10% H217O as a function of acidity with the sideband technique at 8 MHz. Computer methods have been used to solve the set of coupled linear equations from which line shapes as functions of the residence time and 2δ, the proton—17O spin—spin coupling constant, have been calculated. It has been possible to obtain a linear relationship between k1, the specific rate constant for the proton catalyzed exchange process, and 2δ. The spin—spin coupling constant has been redetermined by the acetone‐dilution procedure, and a value of 82±1 Hz has been obtained. Because of quadrupole relaxation of the 17O, the true coupling constant will be somewhat larger than the value obtained under these conditions of slow exchange. With the assumption of a value of 2δ of 580 rad sec−1, the acid and base catalyzed specific rate constants at 29.0°C are k1=8.2×109 mole−1 liter sec−1 and k2=4.6×109 mole−1 liter sec−1, respectively. However, it appe...

Neutron Diffraction Study of Ice Polymorphs under Helium Pressure

Neutron diffraction studies of several of the D2O ice polymorphs were made with helium pressures to 3.5 kbar. Volume compressions (ΔV / V) were obtained for ices Ih, Ic, and IX at 2.1 to 2.8 kbar. The equilibrium phase boundaries between ices I–II and I–III in the presence of helium are shifted toward higher pressure values. The crystal structures of ices Ih, Ic, II, and IX at approximately 2.5 kbar are the same as the structures found for these ice forms at atmospheric pressure. A tetragonal structure is indicated for ice III with lattice parameters at − 23°C and 2.7 kbar of a = 6.674 ± 0.015 A and c = 6.97 ± 0.03 A. The c / a ratio for ice IX has been found to be 1.000 ± 0.003 both at 1 bar and at 2.8 kbar.

Deuteron Magnetic Resonance of Polycrystalline Deuteroammonia

Deuterium magnetic resonance spectra of polycrystalline ND3 have been observed at 75°, 159°, and 185°K. The measured values of e2qQ/h were 156±7 kc/sec at 75°K, and 72.5±0.9 kc/sec at 159° and 185°K. The decrease at the higher temperatures has been attributed to rotational averaging. Single‐crystal growth in the polycrystalline matrix was noted at the higher temperatures; it did not seriously interfere with the measurement of e2qQ/h. Proton second‐moment measurements were made on a polycrystalline sample of NH3 at 75°K. The moment obtained was intermediate between the moments expected for the cases of fast and slow C3 reorientation. The rotational‐transition temperature has been estimated to be 72°±2°K. A lower limit of 1.75 kcal/mole for the reorientational activation energy was estimated. An anomalous absorption peak was recorded in the center of the ND3 spectrum at 75°K. It may possibly be ascribed to a double‐quantum transition.

A study of the kinetics of the reaction between H2 and F2 by EPR methods

Abstract Attesting to its interest for laser-related studies, the reaction between hydrogen and fluorine has recently been studied by a variety of experimental approaches in laboratories throughout the world. An EPR superheterodyne X-band spectrometer was used in the present work in conjunction with a fast flow gas mixing system to obtain both hydrogen and fluorine atom concentrations. The reactant hydrogen atoms were produced by the dissociation of molecular hydrogen in a 2450 MHz microwave discharge unit; the product fluorine atoms were produced by the reaction of hydrogen atoms with molecular fluorine. The changes in both atom concentrations were followed as fluorine was added incrementally to the helium-diluted reactants, H + H 2 . Values of the specific rate constants for the H + F 2 and F + H 2 reactions at 300°K have been found to be 2.5 ± 0.2 × 10 12 cm 3 mole −1 sec −1 and 4 ± 1 × 10 12 cm 3 mole −1 sec 1 , respectively. Under the conditions of our experiments, i.e., at pressures of ca. 2 Torr and with quartz flow tube walls coated with boric acid, the contribution of a branched-chain step to the reaction mechanism appears to be minimal.

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...

Deuteron Magnetic Resonance in Deuterated Polycrystalline Ammonium Chloride and Sulfate

Values of the quadrupole coupling constants |eQq|/h obtained at the temperature of liquid nitrogen for powdered specimens of deuterated ammonium sulfate and ammonium chloride have been found to be 161.9±0.3 and 175.7±0.7 kc/sec, respectively. Absolute values of the asymmetry parameter were estimated to be less than 0.2 from the powder spectra of the two salts. To obtain the deuteron quadrupole split spectrum of ND4Cl at liquid‐nitrogen temperature, it was found necessary to use very low rf field levels and also to irradiate the sample with a 60Co source to reduce the spin—lattice relaxation time. Second‐moment measurements were made on the single‐line spectrum of ND4Cl between room temperature and −83°C, from which an activation energy for rotational motion of the ND4+ ion was calculated to be 5.0±0.5 kcal/mole. At the temperature of the ferroelectric phase transition in (ND4)2SO4, a significant alteration of the electric field gradient was revealed by the change in the separation of the peaks in the five...

17O Nuclear Magnetic Resonance of Single Crystals of D2O Ice

17O NMR spectra have been obtained for two orientations of a single crystal of D2O enriched to 22% in 17O. The spectra of only one orientation could be assigned. This assignment yields quadrupole coupling constants for 17O in ice as follows: | e2Qqzz/h | = 1.79±0.05 MHz, η=−1.01±0.04, where the z axis is the C2 axis of the D2O molecule The isotropic chemical shift is 612±37 ppm paramagnetic to liquid D2O.

Magnetic Susceptibilities of Praseodymium Oxides

The magnetic susceptibilities of the three oxides of praseodymium, Pr2O3, PrO2, and Pr6O11, have been measured with the faraday magnetic balance over a range of temperatures from 80° to 300°K. The sesquioxide and the dioxide have been found to have moments of 3.55 and 2.48 Bohr magnetons, respectively, and to obey the Weiss modification of the Curie law, χ(T+Δ)=C, over the range of temperatures studied with corresponding values of Δ equal to 55.0° and 104°. A curvature was noted in the plot of 1/χ vs T for the oxide Pr6O11. Thus the Weiss‐Curie law does not adequately express the relationship between the susceptibility and the temperature for this oxide. A moment of 2.8 Bohr magnetons was calculated for Pr6O11 from the slope of the above plot between 200° and 300°K.