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C Dybowski

Publications and source records attributed to C Dybowski.

8 recordsLinked to original sources
PubMed2001

A new mechanism for spin--lattice relaxation of heavy nuclei in the solid state: 207Pb relaxation in lead nitrate.

A detailed investigation of the spin-lattice relaxation time, T1, for 207Pb in solid lead nitrate has been undertaken in an effort to understand the mechanism of relaxation. The results show that the 207Pb T1 is independent of magnetic field strength and inversely proportional to the square of the temperature. These are signatures of relaxation by a spin-phonon Raman scattering mechanism. Nuclear spin-lattice relaxation in solid lead salts is more efficient for sites with smaller magnetic shielding anisotropy. A coupling mechanism is proposed whereby phonons create a local magnetic field by modulating the valence electron shell motion relative to the nuclear/electron core. Literature data suggest that spin-phonon scattering is a common relaxation pathway for other spin-1/2 heavy nuclei in solids.

Journal Article↗
PubMed1999

207Pb NMR of minium, Pb3O4: evidence for the [Pb2]4+ ion and possible relativistic effects in the Pb-Pb bond.

Solid Pb3O4 has been studied with 207Pb nuclear magnetic resonance (NMR) spectroscopy. The 207Pb NMR chemical-shift tensor of the Pb2+ site has principal values of delta11 = 1980 +/- 5 ppm, delta22 = 1540 +/- 5 ppm, and delta33 = -1108 +/- 10 ppm; delta(iso) = 804 +/- 10 ppm. The chemical-shift tensor of the Pb4+ site is axial, with principal values delta(parallel) = -1009 +/- 3 ppm and delta(perpendicular) = 1132 +/- 3 ppm; delta(iso) = -1091 +/- 3 ppm. The Pb4+-Pb2+ scalar coupling constant J(Pb-Pb) = 2.3 +/- 0.1 kHz. The main contribution to the Pb2- chemical-shift anisotropy is proposed to arise from an exchange interaction in the Pb2+-Pb2+ pairs, conventionally regarded as molecular [Pb2]4+ ions.

Cations↗
PubMed1996

Determination of 207Pb2+ chemical shift tensors from precise powder lineshape analysis.

207Pb solid state NMR powder spectra at 296 K are presented for PbSO4, PbMoO4, PbCrO4, PbCO3, PbTiO3, PbZrO3, Pb(NO3)2, Pb(SCN)2, and PbS. Analysis for principal values of the anisotropic chemical shift tensors of the generally very broad spectra included the frequency dependent excitation of the pulse sequence used. Commonly used solid and liquid secondary shift standards for lead were studied with high precision as a function of temperature between 295 K and 315 K to establish a clean 207Pb shift scale. Errors in the existing literature are discussed.

Chemical Phenomena↗
PubMed1994

Fitting of low-intensity wide-line spectra dominated by chemical shift anisotropy.

How experimental parameters affect the appearance, and consequently the fitting, of signals having very wide lines is discussed. As these spectra usually have very low intensities, pulse sequences that remove acoustic ringing and other experimental artefacts have to be used. These pulse sequences introduce further spectral distortions. A FORTRAN 77 program was developed that accounts for these effects. The fitting of these broad spectra is demonstrated by application to 207Pb solid-state nuclear magnetic resonance (NMR) spectra.

Anisotropy↗
PubMed1992

Nuclear magnetic resonance of hydrogen sorbed by powdered palladium metal and alumina-supported palladium.

Hydrogen in 0.2 micron beta-palladium powder and in an alumina-supported palladium catalyst was investigated by NMR spectroscopy. The dependence of the shift on hydrogen pressure and the hydrogen-to-palladium (H/Pd) ratio in beta-palladium hydride was determined for 273 K < T < 368 K. The activation energy for the process affecting the hydrogen shift is 9.4(+/- 0.5) kcal mol-1, similar to the enthalpy of transition from the alpha- to beta-phase. NMR measurements of hydrogen in alumina-supported palladium show a similar behavior, which may allow one to use NMR shifts as a barometer of the state of the hydrogen in the metal.

Adsorption↗