Chapter Three - Recent advances in chlorine, bromine, and iodine solid-state NMR spectroscopy
Introduction
Chlorine-35/37, bromine-79/81, and iodine-127 are all quadrupolar nuclei with good natural abundances (N.A.) and reasonably high magnetogyric ratios (Table 1). Due to their quadrupolar nature (I = 3/2 for 35Cl, 37Cl, 79Br, and 81Br; I = 5/2 for 127I), broad NMR powder patterns are expected in the solid state for all but the most symmetric of coordination environments. In this Annual Report, we provide a survey of the literature from the period covering August 2008 to July 2014, with a new section added in January 2020 to bring the coverage up to date. Our previous reviews [1], [2], [3], in particular a previous Annual Report published in 2009 [2], offer a broader review of the literature covering prior years. An Encyclopedia of Magnetic Resonance article offers a more pedagogical perspective [4]. Butler et al. have also provided a review of literature covering both solid-state and solution phase data [5].
Given this previous coverage, here we aim to keep our focus on the reporting period, and largely avoid repeating previous information and references. The NMR properties of the quadrupolar halogens are shown in Table 1, and the primary and secondary chemical shift references are presented in Table 2. Further information on chemical shift referencing may be found in Refs. [2], [4].
Following a short section describing some basic parameter definitions and information on spectral line shape modelling, the article is broken down into three additional sections describing new chlorine, bromine, and iodine data and results. Each of these sections also presents in tabular form the new chemical shift and quadrupolar coupling data published during the reporting period. This is followed by some brief concluding remarks.
Section snippets
Theory and modelling
The theory relevant to the description of the NMR spectra of the quadrupolar halogens has been given previously [2], [3]. We present here only some of the useful definitions and key equations, along with a brief discussion on some aspects of the modelling of spectra of powdered samples.
Solid-state chlorine-35/37 nuclear magnetic resonance
Over the course of the past 6 years, several new developments and applications of 35/37Cl solid-state NMR have been reported. Most of these studies have been carried out on powdered samples, and line shape analyses most often yielded information such as the isotropic chemical shift, quadrupolar coupling constant, and quadrupolar asymmetry parameter. In favourable cases, information on the chlorine chemical shift tensor and the relative orientation of the CS and EFG tensors is also obtained. To
Solid-state bromine-79/81 nuclear magnetic resonance
Bromine-79 and bromine-81 have excellent natural abundances and resonance frequencies; however, their moderately large quadrupole moments and Sternheimer antishielding factors result in much broader NMR powder patterns when compared to 35/37Cl [3]. For this reason, there are fewer studies in the literature. We remind the reader to consult our previous reviews for data published prior to 2009 [1], [2], [4]. Recent data for 79/81Br are summarized in Table 18, Table 19, Table 20, Table 21, Table 22
Solid-state iodine-127 nuclear magnetic resonance
Iodine-127 has 100% natural abundance and a relatively high magnetogyric ratio (see Table 1). Due to its large quadrupole moment and Sternheimer antishielding factor, broad powder patterns are typically obtained for 127I in non-cubic solids [3]. For a given chemical environment, the lines are much broader than those for 79/81Br, which are already much broader than those for 35/37Cl. For this reason, 127I SSNMR spectra are often challenging to acquire. They can also be challenging to interpret
January 2020 update
We have revisited the field and surveyed the recent progress up to January 2020. In this section, we present the latest research in the field of chlorine, bromine, and iodine solid-state NMR. As observed in the past, 35Cl remained the most exploited nucleus, with fewer applications of 127I and 79/81Br SSNMR having been published. Interestingly, there has been more recent work on the 127I nucleus than on 79/81Br despite the larger quadrupole moment of the former.
As mentioned in the previous 35/37
Concluding remarks
Numerous advances have been reported in the past several years in the field of chlorine, bromine, and iodine solid-state NMR spectroscopy. As in the past, an increasing availability of high-field NMR instrumentation and new experimental methods have contributed to these advances. Unsurprisingly, most of the new experimental data are for the 35Cl and 37Cl isotopes, due to the manageable size of their quadrupole moments. Nevertheless, the measurement of new bromine and iodine data have been
Acknowledgement
We are grateful to the Natural Sciences and Engineering Research Council (NSERC) for funding.
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