Effect of Co-Ordination Chemistry and Oxidation State on the (207)Pb Magnetic-Shielding Tensor: A DFT/ZORA Investigation.

The magnetic shielding tensor of (207)Pb is calculated for various solids exhibiting (1) a holodirected lead(II) center containing a stereochemically inactive lone pair, (2) a hemidirected lead(II) center with a stereochemically active lone-pair, or (3) a lead(IV) center. Tensors investigated at the scalar relativistic level are compared with those calculated with the full ZORA/spin-orbit Hamiltonian. The effect of using GGA density functionals is compared to the use of hybrid density functionals.

Chemical-shift tensors of heavy nuclei in network solids: a DFT/ZORA investigation of (207)Pb chemical-shift tensors using the bond-valence method.

Cluster models are used in calculation of (207)Pb NMR magnetic-shielding parameters of α-PbO, ß-PbO, Pb3O4, Pb2SnO4, PbF2, PbCl2, PbBr2, PbClOH, PbBrOH, PbIOH, PbSiO3, and Pb3(PO4)2. We examine the effects of cluster size, method of termination of the cluster, charge on the cluster, introduction of exact exchange, and relativistic effects on calculation of magnetic-shielding tensors with density functional theory. Proper termination of the cluster for a network solid, including approximations such as compensation of charge by the bond-valence (BV) method, is essential to provide results that agree with experiment. The inclusion of relativistic effects at the spin-orbit level for such heavy nuclei is an essential factor in achieving agreement with experiment.

Calculation of chemical-shift tensors of heavy nuclei: a DFT/ZORA investigation of ¹99Hg chemical-shift tensors in solids, and the effects of cluster size and electronic-state approximations.

Calculations of the nuclear magnetic resonance chemical-shielding tensors of a suite of mercury-containing materials using various cluster models for the structures provide a stringent test of the procedures for forming models and for calculation with various methods. The inclusion of higher co-ordination shells in the molecular clusters permits quantum chemical calculations of (199)Hg chemical-shielding tensor elements within 3% of the experimental values. We show that it is possible to reduce the size of computationally expensive molecular-cluster calculations with limited effect on calculated NMR parameters by carefully introducing the frozen core approximation. The importance of the relativistic Hamiltonian for accurate predictions of chemical-shielding values is demonstrated within the molecular cluster approach. The results demonstrate that careful design of a cluster to represent the solid-state structure, inclusion of relativistic components in the Hamiltonian at least at the spin-orbit level, and judicious use of approximations are essential to obtain good agreement with experimental results.

Prediction of (207)Pb NMR parameters for the solid ionic lead(II) halides using the relativistic ZORA-DFT formalism: Comparison with the lead-containing molecular systems.

Density functional calculations of (207)Pb NMR shielding in PbX(2) (X=F, Br, Cl and I) anionic fragments suggest that in solid PbX(2), the observed variation of chemical shift with halide is dominated by the paramagnetic contribution to the chemical shielding, with a lesser effect by the spin-orbit contribution. The calculations include relativistic effects at the level of the zero-order regular approximation (ZORA). The present observation contrasts with previous calculations for the molecular system, PbX(4), in which the spin-orbit contribution has been shown to be the major factor in the variation of the NMR chemical shift.

The relationship between 207Pb NMR chemical shift and solid-state structure in Pb(II) compounds.

The analysis of heavy-metal solids with NMR spectroscopy provides a means of investigating the electronic environment through the dependence of the chemical shift on structure. We have investigated the relation of the 207Pb NMR isotropic chemical shift, span, and skew of a series of solid Pb(II) compounds to lattice parameters. Complementary relativistic spin-orbit density functional calculations on clusters such as PbI64- that model the local environment in the dihalides show a dependence of NMR properties on the local structure in good agreement with experimental results.

Solid-state (207)Pb NMR studies of lead-group 16 and mixed transition-metal/lead-group 16 element-containing materials.

(207)Pb solid-state NMR studies have been conducted on binary lead-group 16 and mixed transition-metal/lead group 16 materials, correlating the NMR chemical shifts of the materials with their structures. The experimental results show that the (207)Pb chemical shifts are strongly influenced by the local electronic structure. Data are reported for lead selenide, lead selenate, calcium plumbate, strontium plumbite, barium plumbite, lead borate, lead zirconate, lead tungstate, lead meta-tantalate, lead niobate, lead molybdate, lead meta-vanadate, lead sulfite, and lead sulfate.

Kinetics of NMR spin-lock polarization transfer in crystalline glycine and spin-lattice relaxation of amino acids.

We report data determined from proton-carbon polarization-transfer kinetics at 23 degrees C for six common solid amino acids. Proton spin-lattice relaxation times in the rotating frame, T(1rhoH), for alpha-glycine, alanine, cysteine, leucine, isoleucine, and valine determined from the long-time decay of the carbon magnetization indicate that the presence of a mobile entity such as a methyl group shortens T(1rhoH) to a few milliseconds. Polarization transfer between protons and carbons in polycrystalline alpha-glycine is analyzed and compared to theoretical models, two of which account for the variation of polarization-transfer rate with orientation of the dipole-dipole vector in the magnetic field. A generalization of a model proposed by Mueller et al. (Phys. Rev. Lett. 32 (1974) 1402) reproduces the observed polarization transfer in alpha-glycine with reasonable accuracy, showing that the early time development reflects orientational variation of dipolar oscillations.

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.

A thermometer for nonspinning solid-state NMR spectroscopy

We discuss a method to determine temperature in a static NMR experiment from the temperature variation of the lead nitrate peak shift. Copyright 2000 Academic Press.

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.

Determining temperature in a magic-angle spinning probe using the temperature dependence of the isotropic chemical shift of lead nitrate.

The calibration of temperature in a magic-angle spinning probe with lead nitrate is discussed. The effects of rotation frequency on temperature are demonstrated.

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.

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.

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.