Variable temperature solid-state NMR

Figure 30 Variable-temperature solid-state NMR spectra (relative intensities distorted due to different relaxation rates) (a) 29Si (referenced to Q8M8 (Bruker) (b) 2H (isotropic peak truncated to show hem and crystalline fractions).285 Reprinted with permission from Mueller, C. Schmidt, C. Frey, H. Macromolecules 1996, 29, 3320-3322. 1996 American Chemical Society.

Molecular mobility can be studied by solid-state NMR and X-ray crystallography. Solid-state NMR offers several approaches to studying the molecular mobility of solids. These include (a) the study of processes which result in peak coalescence of solid-state NMR resonances using variable temperature solid-state NMR, (b) determination of the 7 , relaxation of individual carbon atoms using variable temperature solid-state NMR, (c) use of interrupted decoupling to detect methylene and possible methine groups with unusual mobility, and (d) comparison of solid-state MAS spectra measured with and without cross-polarization. 

This chapter does not cover the most common aspects of the solid-state NMR techniques employed in the study of heterogeneous catalysts such techniques are described in Chapter 4. Since this chapter emphasizes the surface characterization of silica and alumina systems and silica aluminas by NMR methods, only those technical aspects highly relevant to surface characterization and not otherwise emphasized in this volume are explicitly discussed here. NMR studies of zeolites and clays are treated in separate chapters, and the bulk structures of silica and alumina systems are covered by Eckert. Unavoidably this chapter is also concerned with dynamics at the surface, although the amount of detailed work on that subject to date is limited. With the increasing availability of variable-temperature solid-state NMR equipment, however, one can expect that attention devoted to dynamics at surfaces will increase markedly during the next few years. 

Hexafluoroarseniate salts show phase transition behavior. [Fe( 7 -C6Fl6)(77 -CsHs)][AsF6], for instance, undergoes phase transition between three different crystal forms.Variable-temperature solid-state NMR measurements have shown that rotation of the entire cation takes place in a cubic phase above 310 K, while in the intermediate / -phase, the rotational motion is restricted to a 90° in-plane reorientation. Below 270 K, the crystal is in a low-symmetry phase, in which whole-body rotation does not take place though the rings execute jumping motion that persists down to 200 K. Transition from a rotational jumping state to a whole-body reorientation has also been detected from the Mossbauer spectra of the [PF6] salt of the same complex. 

The Tg values of polyferrocenylsilanes, which can be as low as ca. -50 °C, are remarkable for a polymer structure with a bulky unit such as ferrocene in the main chain. Even when the influence of the side groups is small, the Tg values are still close to ambient temperature (e.g., for 3.22 (R=R = H), Tg=16°C). By contrast, the Tg of poly(vinylferrocene) is reported to be high (185 or 233 °C Chapter 2, Section 2.2.1.1). The ability of the iron atom in each ferrocene unit to act as a freely rotating molecular ball-bearing [106] probably plays a key role in generating the observed conformational flexibility in polyferrocenylsilanes. This aspect has been explored by means of variable-temperature solid-state NMR studies on PFSs 3.22, which were specifically deuterated either on the Cp rings or in the side groups [107]. 

D.C. Apperley, A.H. Forster, R. Fournier, R.K. Harris, P. Hodgkinson, R.W. Lancaster, et al.. Characterisation of indomethacin and nifedipine using variable-temperature solid-state NMR, Magn. Reson. Chem. 43 (11) (2005) 881—892. 

Beshah, K. Mark, J. E. Ackerman, J. L., Topology of PDMS Endlinked Elastomeric Networks Studied by Variable Temperature Solid State NMR. Macromolecules 1986,19, 2194-2196. 

M. M. Guo. Variable temperature solid-state NMR studies of conformation change and order-disorder transition of poly(3-alkylthiophene). Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 43(2), 487-488 (2002). 

Ford JL, Mann TE (2012) Fast-scan DSC and its role in pharmaceuticed physical form chtiracteri-sation and selection. Adv Drug DeUv Rev 64 422-430 Forster A, Apperley D, HempenstaU J, Lancaster R, Rades T (2003) Investigation of the physical stability of amorphous drug and dmg/polymer melts using variable temperature solid state NMR. 

Petrov et al. have presented a variable temperature solid-state NMR investigation of cryptophane-Exhloroform and cryptophane-Erdichlorometh-ane inclusion complexes.The line shapes and nuclear spin relaxation rates were analysed in terms of the distribution of C-D bond orientations and the time scale of the guest dynamics. It was found that encaged chloroform produces broad spectra, and that its reorientation is relatively slow with a correlation time of 0.17 ps at 292 K. In contrast, the line shapes of encaged dichloromethane were narrow and the motion of this guest molecule was fast with a correlation time of 1.4 ps at 283 K. The NMR data were complemented by an X-ray diffraction study of the cryptophane-E dichloro-methane structure, which was utihsed in the analysis of the NMR parameters. 

Solid-state NMR H and T p relaxation times were used to evaluate the miscibility of amorphous solid dispersions of nifedipine (NIF) and polyvinylpyrrolidone (PVP) prepared by three different methods melt quenching in the typical lab setting, spray drying and melt quenching in the NMR rotor while spinning. Variable-temperature solid-state NMR H Tip relaxation measurements revealed a change in relaxation time at 20 °C below Tg, suggesting increased molecular mobility above that temperature. ... 

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