Cecil Dybowski

Solid state NMR spectroscopy

Due to the development of new techniques and further increase of magnetic field strength available for commercial applications, solid-state NMR spectroscopy became a routine method for the characterization of zeolites. As an important advantage, solid-state NMR spectroscopy allows the investigation of the local structure of nuclei in the solids under study. The specific behavior of zeolites often depends on local effects, such as framework defects, the substitution of framework atoms, guest compounds etc. Therefore, solid-state NMR spectroscopy is a widely applied analytical method for delivering structure data, which are complementary to those of diffraction methods suitable for investigating the long-range order. While early solid-state NMR spectroscopic studies often focused on the characterization of the zeolite framework in the as-synthesized and hydrated state, an increasing number of recent works is dealing with the investigation of the framework of dehydrated and calcined zeolites and of surface sites, i.e., on the determination of their concentration, strength, and accessibility. In all these applications, the advantage of solid-state NMR spectroscopy to be a quantitative method is utilized. The present chapter demonstrates the fundamentals, various techniques, and most important applications of solid-state NMR spectroscopy making this method to an important tool of research in zeolite science.

Chemisorption and surfaces studied by nuclear magnetic resonance spectroscopy

Abstract This paper presents an introduction to the study of surfaces and chemically adsorbed species with nuclear magnetic resonance (NMR) spectroscopy. The analysis is based on nuclear magnetic interactions in the solid state: dipole-dipole couplings, chemical shift anisotropy, Knight shifts, and quadrupolar splitting. The physical origins and characteristics of each interaction, as well as relative intensities for different nuclei, are discussed. In particular, emphasis is placed on the relation of these interactions to quantities of interest to studies in adsorption and catalysis: motional properties of the adsorbate, the distribution of adsorption sites, the chemical state of atoms adsorbed at the surface, electrostatic field gradients, and the metallic character of surface atoms. Techniques to observe these interactions are described; subdivided by the type of nucleus: strongly coupled nuclei (e.g. 1H, 19F), weakly coupled nuclei (e.g. 13C, 15N, 29Si, 195Pt), and quadrupolar nuclei (e.g. 2H, 14N, 27Al). The techniques described to isolate and identify the overlapping effects in the spectra include multiple-pulse spin echoing and decoupling, double-resonance irradiation, multiple-quantum excitation, and mechanical sample spinning. A review of the recent application of these techniques to studies of adsorption and surfaces illustrates the potentials and limitations. Finally, a procedure for formulating a NMR study of surface samples is proposed, with respect to sample composition and character, and the type of information desired.

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.

NMR, ESR, and TPD study of H2 adsorption on RhTiO2 catalyst

Abstract The adsorption of H 2 on a Rh TiO 2 catalyst results in spectroscopic properties which depend on the method of pretreatment. An explanation consistent with the results is proposed, which invokes the presence of at least two distinct hydrogenic species.

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.

Density functional investigation of intermolecular effects on 13C NMR chemical-shielding tensors modeled with molecular clusters

A quantum-chemical method for modeling solid-state nuclear magnetic resonance chemical-shift tensors by calculations on large symmetry-adapted clusters of molecules is demonstrated. Four hundred sixty five principal components of the (13)C chemical-shielding tensors of 24 organic materials are analyzed. The comparison of calculations on isolated molecules with molecules in clusters demonstrates that intermolecular effects can be successfully modeled using a cluster that represents a local portion of the lattice structure, without the need to use periodic-boundary conditions (PBCs). The accuracy of calculations which model the solid state using a cluster rivals the accuracy of calculations which model the solid state using PBCs, provided the cluster preserves the symmetry properties of the crystalline space group. The size and symmetry conditions that the model cluster must satisfy to obtain significant agreement with experimental chemical-shift values are discussed. The symmetry constraints described in the paper provide a systematic approach for incorporating intermolecular effects into chemical-shielding calculations performed at a level of theory that is more advanced than the generalized gradient approximation. Specifically, NMR parameters are calculated using the hybrid exchange-correlation functional B3PW91, which is not available in periodic codes. Calculations on structures of four molecules refined with density plane waves yield chemical-shielding values that are essentially in agreement with calculations on clusters where only the hydrogen sites are optimized and are used to provide insight into the inherent sensitivity of chemical shielding to lattice structure, including the role of rovibrational effects.

1H nuclear magnetic resonance spin-lattice relaxation, 13C magic-angle-spinning nuclear magnetic resonance spectroscopy, differential scanning calorimetry, and x-ray diffraction of two polymorphs of 2,6-di-tert-butylnaphthalene

Polymorphism, the presence of structurally distinct solid phases of the same chemical species, affords a unique opportunity to evaluate the structural consequences of intermolecular forces. The study of two polymorphs of 2,6-di-tert-butylnaphthalene by single-crystal x-ray diffraction, differential scanning calorimetry (DSC), 13C magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, and 1H NMR spin-lattice relaxation provides a picture of the differences in structure and dynamics in these materials. The subtle differences in structure, observed with x-ray diffraction and chemical shifts, strikingly affect the dynamics, as reflected in the relaxation measurements. We analyze the dynamics in terms of both discrete sums and continuous distributions of Poisson processes.

Coke formation on ZSM-5 zeolites: Evidence from NMR spectrometry of sorbed xenon gas

Abstract NMR spectrometry of xenon-129 adsorbed in coked samples of a totally protonated H-ZSM-5 zeolite and a Na,H-ZSM-5 zeolite acidified at the surfaces of the particles shows variations attributable to differences in coke distribution.

Modification of photochemical reactivity by zeolites: arrested molecular rotation of polyenes by inclusion in zeolites

In this work we have utilized faujasite [2] and pentasil [3] zeolites as hosts to carry out phototransformations of several organic molecules. Although they possess completely interconnecting three dimensional pore structures these two types of zeolite have fundamentally different void space topologies (Fig. 1). While the former consists of relatively large and spherical cages (diameter about 13 A; entrance pore diameter about 8 A), the latter contains only interconnecting channels (diameter about 5.5 A).