Dynamic nuclear polarisation enhanced (14)N overtone MAS NMR spectroscopy.

Dynamic nuclear polarisation (DNP) has been used to obtain magic angle spinning (14)N(OT) (nitrogen-14 overtone) solid-state NMR spectra from several model amino acids, with both direct and indirect observation of the (14)N(OT) signal. The crystalline solids were impregnated with biradical solutions of organic liquids that do not dissolve the crystalline phase. The bulk phase was then polarized via(1)H spin diffusion from the highly-polarized surface (1)H nuclei, resulting in (1)H DNP signal enhancements of around two orders of magnitude. Cross polarisation from (1)H nuclei directly to the (14)N overtone transition is demonstrated under magic angle spinning, using a standard pulse sequence with a relatively short contact time (on the order of 100 μs). This method can be used to acquire (14)N overtone MAS powder patterns that match closely with simulated line shapes, allowing isotropic chemical shifts and quadrupolar parameters to be measured. DNP enhancement also allows the rapid acquisition of 2D (14)N(OT) heteronuclear correlation spectra from natural abundance powder samples. (1)H-(14)N(OT) HETCOR and (13)C-(14)N(OT) HMQC pulse sequences were used to observe all single-bond H-N and C-N correlations in histidine hydrochloride monohydrate, with the spectra obtained in a matter of hours. Due to the high natural abundance of the (14)N isotope (99.6%) and the advantages of observing the overtone transition, these methods provide an attractive route to the observation of C-N correlations from samples at natural isotopic abundance and enable the high resolution measurement of (14)N chemical shifts and quadrupolar interaction parameters.

Diagnosis of the redox levels of TCNQF4 compounds using vibrational spectroscopy

The vibrational spectroscopy of TCNQF4, TCNQF41- and TCNQF42- has been investigated by means of density functional theory. Band assignments in infrared and Raman spectra have been clarified and a series of diagnostics developed for redox level characterisation of TCNQF4 compounds. In the C£C stretching region (1460-1600 cm-1), TCNQF40 and TCNQF 41- show two bands, with the more energetic being at 1600 cm-1 in TCNQF40 and at approximately 1535 cm-1 in TCNQF41-; in TCNQF42- both modes absorb below 1500 cm-1, often merging to give a single band. In the C-F and endocyclic C-C stretching region (1290 and 1360 cm-1), TCNQF40 and TCNQF41- show strong bands, whereas TCNQF42- absorbs weakly or not at all. (Additional bands, e.g. from co-crystallised solvent molecules, may complicate this region.) In the nitrile stretching region (2000-2250 cm-1), modes are highly sensitive to nitrile coordination by metal cations. All three redox levels can produce bands above 2200 cm -1, however bands below 2150 cm-1 are usually due to TCNQF42-. This sensitivity to coordination is likely to affect the spectra of many organic molecular ions. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Solid-state NMR as a probe of anion binding: molecular dynamics and associations in a [5]polynorbornane bisurea host complexed with terephthalate

A range of solid-state NMR techniques is used to characterise a molecular host:guest complex consisting of a [5]polynorbornane bisurea host binding a terephthalate dianion guest. Detailed information is obtained on the molecular dynamics and associations from the point of view of both the host and guest molecules. The formation of the complex in the solid state is confirmed using (1)H 2D exchange NMR, and the 180° flipping of the (2)H-labelled terephthalate guest and its eventual expulsion from the complex at elevated temperatures are quantified using variable-temperature (2)H spin-echo experiments. Two-dimensional (1)H-(13)C HETCOR spectra obtained under fast magic angle spinning conditions (60 kHz) show a high resolution despite the poor crystallinity of the solid complex, and clearly reveal changes in the rigidity of the host molecule when complexed. Short-range intra- and intermolecular (1)H-(1)H proximities are also detected using 2D SQ-DQ correlation methods, providing insight into the molecular packing in the solid phase.

Elemental and molecular profiling of licit, illicit, and niche tobacco

The recognition of differences between regulated large-scale mass manufactured products and the uncontrolled cultivation of tobaccos for illicit purposes plays a significant role within identification of provenance. This research highlights X-ray fluorescence and Fourier transform infrared spectroscopy as useful analytical techniques for the rapid identification of tobacco samples of unknown provenance. Identification of key discriminative features within each technique allowed for the development of typical characteristic profiles for each type of tobacco. Analysis using X-ray fluorescence highlights chlorine, potassium, calcium and iron as key elemental indicators of tobacco provenance. Significant levels of chlorine seen within Snüs samples prompted attempts to visualise chlorine containing regions and structures within the sample. Scanning electron microscopy images showed crystalline structures visible within the Snüs tobacco, structures which Energy dispersive X-ray spectroscopy qualitatively confirmed to contain chlorine. Chloride levels within Snüs samples were quantified using ion chromatography with levels found to range between 0.87mgmL‾¹ and 1.28mg. Additionally, FTIR indicated that absorbances attributed to carbonyl stretching at 1050-1150cm‾¹, alkane bending at 1350-1480cm‾¹and amide I stretching at 1600-1700cm‾¹ highlighting a spectral fingerprint region that allowed for the clear differentiation between different types of tobaccos using PCA analysis, but was limited by differentiation between provenance of cigarettes and hand rolled tobacco. X-ray fluorescence and Fourier transform infrared spectroscopy yielded different information with regards tobacco discrimination and provenance, however both methods overall analysis time and cost reduced indicating usefulness as potential handheld analytical techniques in the field.