Development and Application of Chlorine, Bromine, and Iodine Solid-State NMR Spectroscopy.

Chlorine, bromine, and iodine are important elements in a variety of materials, catalysts, pharmaceuticals, and can partake in biologically important processes (e.g., ion transport channels).

There exist quadrupolar NMR-active isotopes of all of these elements: ³⁵Cl, ³⁷Cl, ⁷⁹Br, ⁸¹Br, and ¹²⁷I.  Historically, studies of these nuclei have been challenging due to their large nuclear electric quadrupole moments (Br, I) and low resonance frequencies (Cl).  However, the availability of high field NMR spectrometers (i.e., > 18.8 T), the development of signal-enhancement techniques (RAPT, DFS, hyperbolic secants, QCPMG, WURST), and the use of stepped-frequency data acquisition methods have made the observation of these quadrupolar halogen nuclides more feasible (see representative publications below). 

We are pursuing the study of these nuclides in a variety of compounds.  For example, after pioneering studies which related the chlorine quadrupolar and chemical shift tensors to the hydrogen bonding environment in amino acid hydrochlorides, we have since probed the chlorine environment in a variety of group 13 chloride catalysts, and demonstrated the sensitivity of the halogen NMR parameters to hydration in alkaline earth halides.  We have also proposed a modified structure for MgBr₂ using ‘NMR crystallography’ methods coupled with advanced quantum chemical software.  Recent extensions of these methods include studies of triphenylphosphonium bromides (high-temperature ionic liquids), and covently-bound chlorine materials.

C. M. Widdifield and D. L. Bryce
Solid-State ¹²⁷I NMR and GIPAW DFT Study of Metal Iodides and their Hydrates: Structure, Symmetry, and Higher-Order Quadrupole-Induced Effects.
J. Phys. Chem. A, 114, 10810-10823 (2010).

Characterization of High-Order Quadrupole-Induced Effects in Solid-State Nuclear Magnetic Resonance.

Halogen Bonding.

The resurgence of halogen bonding (XB), the non-covalent interaction between an electron donor, such as a Lewis base or π electrons, and a covalently bound halogen, has had an impact on multiple fields of chemistry. It has been observed in biological processes such as molecular recognition, and is being exploited as having a key role in drug design. Also, since XB is a strong and highly directional interaction, it is used in the architecture of new materials. Usually, this interaction is characterized using X-ray crystallography. However, we are interested in characterizing the XB interaction by solid-state NMR, since it delivers information at the atomic level, permitting us to elucidate the molecular and electronic structure. Using signal enhancement methods and sometimes an ultrahigh magnetic field (21.1 T), we can determine the NMR parameters of the nuclei directly involved in XB. The systems we study are synthesized in our laboratory and we have been able to observe a direct correlation between the NMR parameters to its halogen bonding environment, and for certain groups of XB compounds the NMR data are indicative of when such compounds pack in the same space group. Our main objective is to determine trends in the NMR parameters which could be related to their halogen bonding environment. Also, we use density functional theory calculations of the NMR parameters to compare with the experimental results in order to validate trends.

Representative publications:

The vast majority of elements in the periodic table are amenable to some kind of NMR study, which makes solid-state NMR an obvious choice when probing the local electronic environment surrounding the nuclei under investigation in crystalline solids.  It is often perceived that, for quadrupolar nuclei (i.e., spin quantum number, I > ½), the NMR line shape is dominated by the quadrupolar interaction.  Our work focuses on the quantitative observation of the subtle effects of chemical shift anisotropy (CSA) in quadrupolar nuclei.  The ⁴³Ca solid-state NMR study of CaCO₃ has led us to show that the calcite, vaterite, and aragonite polymorphs each depend differently upon the chemical shift span.  It was also shown that the largest component of the ^79/81Br chemical shift tensor in a series of triphenylphosphonium bromides was dependent upon the bromide-phosphorus distance in the crystal structure.  These examples exemplify that minute changes in the crystal structure can be detected with large differences in the chemical shift tensor, which has great implication in the emerging field of NMR crystallography.  The use of National Ultrahigh Field NMR Facility (i.e., B₀ = 21.1 T), which is in close proximity to the University of Ottawa, is essential for current and future studies such as these as the effects of CSA are more apparent in the line shapes obtained at higher magnetic fields.

Representative publications

D. L. Bryce, E. B. Bultz, and D. Aebi
Calcium-43 Chemical Shift Tensors as Probes of Calcium Binding Environments.  Insight into the Structure of the Vaterite CaCO₃ Polymorph by ⁴³Ca Solid-State NMR Spectroscopy.
J. Am. Chem. Soc., 130, 9282-9292 (2008).

D. L. Bryce
Calcium Binding Environments Probed by ⁴³Ca NMR Spectroscopy
Dalton Trans., 39, 8593-8602 (2010).

K. M. N. Burgess, I. Korobkov, and D. L. Bryce
A Combined Solid-State NMR and X-ray Crystallography Study of the Bromide Anion Environments in Triphenylphosphonium Bromides
Chem. Eur. J. (2012), in press.

Biomolecular NMR Applications.

Characterization of biomolecular structure is necessary to gain information on their functionality. Using methods such as liquid-crystal NMR spectroscopy, we were able to develop novel applications of residual dipolar couplings as a means of structure refinement for nucleic acids. Also, hydrogen bonds are shown to be a common and functionally important feature in structured proteins. Through the use of scalar (J) coupling, we were able to unambiguously identify experimentally for the 1^st time, the donor and acceptor groups in the weak hydrogen bond in CH/π interactions in proteins. This line of research involves collaboration with Dr. Jerome Boisbouvier, Institut de Biologie Structurale- LNMR, CNRS Grenoble.

Representative publications:

Quantum chemical calculations of NMR parameters.

Development of DOuble-Rotation (DOR) NMR methodology and applications.

Cation-pi interactions have been found to play a key structural role in proteins, molecular receptors, zeolites, as well as more simple chemical compounds. Unfortunately, crystallographic studies of proteins do not always provide conclusive details on the location or identity of the bound ions.  We are carrying out a systematic investigation of the cation-pi interaction, and its effects on the electronic structure about the nuclei involved in the interaction, by applying solid-state NMR  spectroscopic methods.  Ultimately, it is our hope that ²³Na and ³⁹K NMR signatures may be developed as "fingerprints" for the presence or absence of cation-pi interactions.