Orientation nuclear magnetic resonance spectroscopy

R301 S. S. D. Buechler, G. Kummerloewe and B. Luy, Naturally Occurring Biodegradable Polymers as the Basis of Chiral Gels for the Distinction of Enantiomers by Partially Oriented Nuclear Magnetic Resonance Spectroscopy , Int. J. Artif. Organs, 2011, 34, 134. 

Nuclear magnetic resonance spectroscopy of the solutes in clathrates and low temperature specific heat measurements are thought to be particularly promising methods for providing more detailed information on the rotational freedom of the solute molecules and their interaction with the host lattice. The absence of electron paramagnetic resonance of the oxygen molecule in a hydroquinone clathrate has already been explained on the basis of weak orientational effects by Meyer, O Brien, and van Vleck.18... 

Nuclear magnetic resonance spectroscopy is a form of absorption spectroscopy and concerns radio frequency (rf)-induced transitions between quantized energy states of nuclei that have been oriented by magnetic fields. Several nonmathemati-cal introductions to NMR are recommended to supplement the material here [1-9]. For greater mathematical depth, a number of excellent texts are available [10-26]. 

The techniques considered in this chapter are infrared spectroscopy (or vibrational spectroscopy), nuclear magnetic resonance spectroscopy, ultraviolet-visible spectroscopy (or electronic spectroscopy) and mass spectrometry. Absorption of infrared radiation is associated with the energy differences between vibrational states of molecules nuclear magnetic resonance absorption is associated with changes in the orientation of atomic nuclei in an applied magnetic field absorption of ultraviolet and visible radiation is associated with changes in the energy states of the valence electrons of molecules and mass spectrometry is concerned... 

Bechinger, B. Zasloff, M. Opella, S.J. "Structure and orientation of the antibiotic peptide magainin in membranes by solid-state nuclear magnetic resonance spectroscopy", Protein Sci., 1993,2,2077-2984. 

The potential of nuclear magnetic resonance spectroscopy for studying liquid crystalline systems is discussed. Typical spectra of nematic, smectic, and cholesteric mesophases were obtained under high resolution conditions. The observed line shape in the cholesteric phase agrees with that proposed on the basis of the preferred orientation of this phase in the magnetic field. The line shapes observed in lyotropic smectic phases appear to be the result of a distribution of correlation times in the hydrocarbon portions of the surfactant molecules. Thermotropic and lyotropic phase transitions are easily detected by NMR and agree well with those reported by other methods. The NMR parameters of the neat and middle lyotropic phases allow these phases to be distinguished and are consistent ivith their proposed structures. 

Nuclear magnetic resonance spectroscopy has been widely employed in the study of rotational isomer phenomena produced by substituents lacking rotational symmetry. The preferred conformations in solution have been determined by various NMR techniques, essentially (i) analysis of H and C chemical shifts regarding the orientation of the substituent (ii) analysis of stereospecific long-range coupling constants Vh,h and Vc,h (iii) relaxation parameters such as NOE and spin-lattice relaxation time (T,) (iv) LIS experiments at H and C frequencies. The results obtained... 

Solid state nuclear magnetic resonance spectroscopy (NMR), e.g. [107-109]. This technique is sensitive to the local environment of certain nuclei, their mobility and orientation [108]. It provides information about the heterogeneity of polymer blends to c. 5 nm or less (spin diffusion experiments) or c. 0.3 nm in cross-polarization experiments, from which the direct (averaged) distance between two types of nuclei in a sample can be determined [107,108]. Motions of moleuclar groups in a polymer chain can be analyzed and correlations with dispersion areas in the mechanical spectra may be possible [109]. Solid state NMR is not a standard technique at the present time but it is becoming increasingly important. 

Walker, T. E., R. E. London, T. W. Whaley, R. Barker, and N. A. Matwiyoff Carbon-13 Nuclear Magnetic Resonance Spectroscopy of (1- C) Enriched monosaccharides. Signal Assignments and orientation dependance of geminal and vicinal Carbon-Carbon and Carbon-Hydrogen Spin-Spin coupling constants. J. Amer. Chem. Soc. 98,5807 (1976) and references cited therein. 

Besides infrared spectroscopy the most convenient method for deducing the molecular structure is nuclear magnetic resonance spectroscopy (nmr). The basic principle of this method is the detection of changes in the orientation of the atomic nuclear spin of hydrogen (the spin of a proton) and carbon (the spin of the nucleus of the isotope). 

P.J. CoUings, T.J. McKee, J.R. McCoU, Nuclear magnetic resonance spectroscopy in cholesteric hquid crystals. I. Orientational order parameter measurements, J. Chem. Phys. 65 (1976) 3520. 

This anisotropy also poses a challenge to the analysis of crystals by nuclear magnetic resonance spectroscopy, which is covered in Section 5.5. Each change in orientation of a crystal in the spectrometer corresponds to a different angle between the external magnetic field and the chemical bond axes, and a different spectrum. The rapid and random motions of molecules in a liquid average over these differences to yield narrow lines in the spectrum. To duplicate the same effect in crystals, the crystal is powdered and the sample is then rotated at rates of 10 kHz or faster. 

As a prelude to the discussion it is necessary to consider the definition of orientation in terms of the Euler angles, and the definition ofan orientation distribution function in terms ofan expansion ofLegendre functions. These definitions set the scene for examining the information which can be obtained from different spectroscopic techniques. In this review, infra-red and Raman spectroscopy and nuclear magnetic resonance, will be considered. 

In this review the definition of orientation and orientation functions or orientation averages will be considered in detail. This will be followed by a comprehensive account of the information which can be obtained by three spectroscopic techniques, infra-red and Raman spectroscopy and broad line nuclear magnetic resonance. The use of polarized fluorescence will not be discussed here, but is the subject of a contemporary review article by the author and J. H. Nobbs 1. The present review will be completed by consideration of the information which has been obtained on the development of molecular orientation in polyethylene terephthalate and poly(tetramethylene terephthalate) where there are also clearly defined changes in the conformation of the molecule. In this paper, particular attention will be given to the characterization of biaxially oriented films. Previous reviews of this subject have been given by the author and his colleagues, but have been concerned with discussion of results for uniaxially oriented systems only2,3). 

In this review recent theoretical developments which enable quantitative measures of molecular orientation in polymers to be obtained from infra-red and Raman spectroscopy and nuclear magnetic resonance have been discussed in some detail. Although this is clearly a subject of some complexity, it has been possible to show that the systematic application of these techniques to polyethylene terephthalate and polytetramethylene terephthalate can provide unique information of considerable value. This information can be used on the one hand to gain an understanding of the mechanisms of deformation, and on the other to provide a structural understanding of physical properties, especially mechanical properties. 

Other optical and spectroscopic techniques are also important, particularly with regard to segmental orientation. Some examples are fluorescence polarization, deuterium nuclear magnetic resonance (NMR), and polarized IR spectroscopy [4,246,251]. Also relevant here is some work indicating that microwave techniques can be used to image elastomeric materials, for example, with regard to internal damage [252,253]. 

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