Darren H. Brouwer

NMR Crystallography of Zeolites: Refinement of an NMR-Solved Crystal Structure Using ab Initio Calculations of 29Si Chemical Shift Tensors

An NMR structure refinement method for the NMR crystallography of zeolites is presented and demonstrated to give an NMR-determined crystal structure for the zeolite Sigma-2 that is in very good agreement with the single-crystal X-ray diffraction structure. The Si coordinates of the zeolite framework were solved from 29Si double-quantum NMR data obtained at a low magnetic field strength (7.0 T) and the Si and O coordinates were subsequently refined using the principal components of 29Si chemical shift tensors experimentally measured at an ultrahigh-field (21.1 T) and calculated using ab initio quantum chemical methods.

Probing local structures of siliceous zeolite frameworks by solid-state NMR and first-principles calculations of 29Si–O–29Si scalar couplings

Subtle structural details of siliceous zeolites are probed by using two-bond scalar (J) coupling constants to characterize covalently bonded 29Si-O-29Si site pairs and local framework order. Solid-state two-dimensional (2D) 29Si{29Si} NMR measurements and first-principles calculations of 2J(29Si-O-29Si) couplings shed insights on both the local structures of siliceous zeolites Sigma-2 and ZSM-12, as well as the sensitivity of J couplings for detailed characterization analyses. DFT calculations on a model linear silicate dimer show that 2J(Si-O-Si) couplings have complicated multiple angular dependencies that make semi-empirical treatments impractical, but which are amenable to cluster approaches for accurate J-coupling calculations in zeolites. DFT calculations of 2J(29Si-O-29Si) couplings of the siliceous zeolite Sigma-2, whose framework structure is known to high accuracy from single-crystal X-ray diffraction studies, yield excellent agreement between calculated and experimentally measured 2J(Si-O-Si) couplings. For the siliceous zeolite ZSM-12, calculated 2J(29Si-O-29Si) couplings based on less-certain powder X-ray diffraction analyses deviate significantly from experimental values, while a refined structure based on 29Si chemical-shift-tensor analyses shows substantially improved agreement. 29Si J-coupling interactions can be used as sensitive probes of local structures of zeolitic frameworks and offer new opportunities for refining and solving complicated structures, in combination with complementary scattering, modeling, and other nuclear spin interactions.

NMR crystallography of p-tert-butylcalix[4]arene host-guest complexes using 1H complexation-induced chemical shifts.

(1)H magic-angle spinning (MAS) NMR spectra of p-tert-butylcalix[4]arene inclusion compounds with toluene and pyridine show large complexation-induced shifts of the guest proton resonances arising from additional magnetic shielding caused by the aromatic rings of the cavities of the host calixarene lattice. In combination with ab initio calculations, these observations can be employed for NMR crystallography of host-guest complexes, providing important spatial information about the location of the guest molecules in the host cavities.

Probing the Local Structure of Pure Ionic Liquid Salts with Solid‐ and Liquid‐State NMR

Room-temperature ionic liquids (RTILs) are gaining increasing interest and are considered part of the green chemistry paradigm due to their negligible vapour pressure and ease of recycling. Evidence of liquid-state order, observed by IR and Raman spectroscopy, diffraction studies, and simulated by ab initio methods, has been reported in the literature. Here, quadrupolar nuclei are used as NMR probes to extract information about the solid and possible residual order in the liquid state of RTILs. To this end, the anisotropic nature and field dependence of quadrupolar and chemical shift interactions are exploited. Relaxation time measurements and a search for residual second-order quadrupolar coupling were employed to investigate the molecular motions present in the liquid state and infer what kind of order is present. The results obtained indicate that on a timescale of approximately 10(-8) sec or longer, RTILs behave as isotropic liquids without residual order.

35Cl Solid-State NMR of Halide Ionic Liquids at Ultrahigh Fields

This Letter describes recent work investigating the solid-state NMR spectra of (35)Cl nuclei in an assortment of ionic liquids under static and MAS conditions at field strengths of 9.4 and 21.1 T. At high field it was possible to resolve and extract information from multiple unique crystallographic sites and to resolve otherwise complex spectra that were analyzed to extract information regarding the electric field gradient (EFG) and chemical shift tensors, including their relative orientation. The NMR parameters were found to be typical of organic salts in general.

Some New Halogen-containing Hydrate-formers for Structure I and II Clathrate Hydrates1

Abstract The clathrate hydrate-forming capabilities of 15 hydrofluorocarbon (HFC), fluorocarbon (FC), hydrochlorofluorocarbon (HCFC) and chlorofluorocarbon (CFC) guests are reported. These molecules form stable clathrate hydrates at T > 0°C and have been characterized in detail by X-ray powder diffraction and the measurement of the decomposition temperature, T decomp. The clathrate hydrate structure type depends on the size of the guest molecule and is consistent with the known trend: guest r VDW = 4.1–5.9 A gives structure I, guest d VDW < 4.1 A or = 5.6–6.9 A gives structure II. In addition, the phase diagram of the 1,1-dichloro-1-fluoroethane (R-141b) clathrate hydrate system is reported with a quadruple point of T = 8.4°C and P = 415 mbar. A number of guests give hydrates with relatively high decomposition temperatures (T ∼ 10°C) at low vapour pressures (P < 1 bar) that may be suitable for industrial applications.

A double quantum 129Xe NMR experiment for probing xenon in multiply-occupied cavities of solid-state inclusion compounds

A method is presented for detecting multiple xenon atoms in cavities of solid-state inclusion compounds using (129)Xe double quantum NMR spectroscopy. Double quantum filtered (129)Xe NMR spectra, performed on the xenon clathrate of Dianins compound were obtained under high-resolution Magic-Angle Spinning (MAS) conditions, by recoupling the weak (129)Xe-(129)Xe dipole-dipole couplings that exist between xenon atoms in close spatial proximity. Because the (129)Xe-(129)Xe dipole-dipole couplings are generally weak due to dynamics of the atoms and to large internuclear separations, and since the (129)Xe Chemical Shift Anisotropy (CSA) tends to be relatively large, a very robust dipolar recoupling sequence was necessary, with the symmetry-based SR26 dipolar recoupling sequence proving appropriate. We have also attempted to measure the (129)Xe-(129)Xe dipole-dipole coupling constant between xenon atoms in the cavities of the xenon-Dianins compound clathrate and have found that the dynamics of the xenon atoms (as investigated with molecular dynamics simulations) as well as (129)Xe multiple spin effects complicate the analysis. The double quantum NMR method is useful for peak assignment in (129)Xe NMR spectra because peaks arising from different types of absorption/inclusion sites or from different levels of occupancy of single sites can be distinguished. The method can also help resolve ambiguities in diffraction experiments concerning the order/disorder in a material.

High Field Solid-State NMR Spectroscopy Investigation of 15N-Labeled Rosette Nanotubes: Hydrogen Bond Network and Channel-Bound Water

(15)N-labeled rosette nanotubes were synthesized and investigated using high-field solid-state NMR spectroscopy, X-ray diffraction, atomic force microscopy, and electron microscopy. The results established the H-bond network involved in the self-assembly of the nanostructure as well as bound water molecules in the nanotubes channel.

NMR crystallography strategies for structure determination of zeolites and layered silicates

Solid-state nuclear magnetic resonance (SSNMR) spectroscopy has emerged as an important technique for structural characterization of solids. Due to the fact that it provides local structural information about the environments of NMR-active nuclei, SSNMR is highly complementary to diffraction techniques whose strength lies in providing information about the long-range periodic structure of a material. By combining solid-state NMR and diffraction techniques with various computational methods (modeling, density functional theory, etc), powerful approaches to structure determination of materials are being developed. These integrated structure determination strategies in which SSNMR spectroscopy plays a crucial role is broadly referred to as NMR crystallography. This talk will provide an overview of our NMR crystallography strategies for solving and refining zeolite crystal structures. Additionally, the application of this NMR crystallography approach to the structure determination of layered silicates that lack full 3D crystallinity, a considerably more challenging problem, will also be described.