Quadrupolar resonance spectroscopy

Starting from pentoses and hexoses having varied conformations, we can prepare two collections of pyranosyl chlorides, with axial or equatorial chlorine atoms, the analogues of 2.25 and 2.26, respectively. As with 2-chlorooxane, the axial orientation of the chlorine in 2.25 corresponds to the stable conformation of methyl chloromethyl ether. A comparison of the geometrical data in solid state, when available, shows that the axial carbon-chlorine bond is invariably longer than the same bond in an equatorial position. Finally, Fig. 2.6 shows that, according to quadrupolar resonance spectroscopy, these compounds are divided into two groups. 

Another important nuclear characteristic is the nuclear quadrupole moment which, possessed by nuclei for which 7 1, has given rise to the important field of nuclear quadrupole resonance spectroscopy. A major importance of the quadrupole moment with respect to NMR absorption resides in the effects of quadrupole coupling constants on nuclear relaxation times and, therefore, on the line widths and saturation characteristics of NMR absorption (9). In addition, in favorable situations, quadrupole coupling constants can be derived from the characteristics of nuclear resonance of quadrupolar nuclei 127). Some examples of these effects will be described in Sections III, IV and VI of this chapter. 

Whilst diffraction techniques immediately spring to mind for solid samples, the quadrupolar nature of the heavier halogen nuclei means that a principle technique related to SS NMR spectroscopy is nuclear quadruple resonance spectroscopy.6 For systems with a quadrupole moment, an EFG generates two energy states in the absence of an applied magnetic field.3/7 It is appropriate to start by considering illustrative cases in which SS NMR spectroscopy provides information that the closely related nuclear quadrupolar resonance (NQR) technique does not. 

In solid state. Nuclear Quadrupolar Resonance (NQR) [41] is the standard technique to measure quadrupolar coupling constants, while in gas phase Microwave (MW) spectroscopy [42] can be used for small molecules. The MW quantities, however, are obtained in the inertial frame of the molecule. In liquid state, there is no direct method to obtain QCCs. In some favorable cases, QCC can be obtained from simultaneous studies of non-deuterated compounds using NMR (Ti minima, partially oriented systems, etc). 

Abstract Recent advances in nuclear magnetic resonance spectroscopy of quadrupolar (/> 1/2) nuclei with half-integer spins in solids have been reviewed. The advent of multiple-quantum (MQ) magic-angle spinning (MAS) spectroscopy gave new momentum to the study of quadrupolar nuclei in materials of academic and industrial interest such as min-... 

H. Davis, K.R. effrey, M. Bloom, M.I. Valic, T.P. Higgs, Quadrupolar echo deu-teron magnetic resonance spectroscopy in ordered hydrocarbon chains, Chem. Phys. Lett. 42 (1976) 390-394. 

Attrell et al. looked at a series of monohaloanilinium halides exhibiting weak halogen bonding has been prepared and characterized by Cl, Br, and solid-state nuclear magnetic resonance spectroscopy in magnetic fields of up to 21.1 T. The quadrupolar and CSA tensor parameters were determined. ... 

Nuclear quadrupole resonance spectroscopy is a very promising novel technique for the study of coordination compounds. Its applicability includes observation of quadrupolar nuclei of cenual metal atoms (e.g. A1, Co, Cu, Cu, Ir, Au, etc.) as well as of ligand nuclei... 

R. Bhattacharyya, L. Frydman, Quadrupolar nuclear magnetic resonance spectroscopy in solids using frequency-swept echoing pulses, J. Chem. Phys. 127 (2007) 194503(1)-194503(8). 

Nuclear quadrupole resonance spectroscopy (N.Q.R.) utilizes the coupling energy between the nuclear quadrupole moment Q and the electric field gradient produced by all electrons and nuclei surrounding the quadrupolar nucleus in the molecule. 

JM Robert, RF Evilia. High-resolution nuclear magnetic resonance spectroscopy of quadrupolar nuclei nitrogen-14 and oxygen-17 examples. J Am Chem Soc 107 3733-3735, 1985. 

Antonijevic S, Wimperis S (2005) Separation of quadrupolar and chemical/paramagnetic shift interactions in two-dimensional H (I = 1) nuclear magnetic resonance spectroscopy. J Chem Phys 122 044312... 

The only sulfur isotope with a nuclear spin is which is quadrupolar (/ = 3/2) and of low natural abundance (0.76%). In view of these inherent difficulties and the low symmetry around the sulfur nuclei in most S-N compounds, S NMR spectroscopy has found very limited application in S-N chemistry. However, it is likely that reasonably narrow resonances could be obtained for sulfur in a tetrahedral environment, e.g. [S(N Bu)4], cf. [S04] . On the other hand both selenium and tellurium have isotopes with I = Vi with significant natural abundances ( Se, 7.6% and Te, 7.0%). Consequently, NMR studies using these nuclei can provide useful information for Se-N and Te-N systems. 

It is useful to emphasise from the outset that alternative techniques for investigating solid samples, other than NMR spectroscopy of the quadrupolar halogen nuclei, tend to be considered first this is likely due to the (perceived) difficulty of the technique and may be related to the issues of sensitivity and line broadening mentioned above. However, the information gained is often complementary, and hence solid state nuclear magnetic resonance (SS NMR) spectroscopy is primarily used when it can provide unique insight unavailable with other techniques. 

Correct setting of the magic angle is essential in solid-state NMR spectroscopy. The most commonly used method relies on the Br resonance in solid KBr. Br is a quadrupolar nucleus (spin 3/2) ensuring a short relaxation time. The Br first-order quadmpole coupling on the satellite transition ( 3 /2 1 /2) produces a set of distinct spinning... 

It is this absorption of electromagnetic energy that is measured in NQR spectroscopy. Radiation in the radio-frequency region is actually employed to effect transitions among the various orientations of a quadru-polar nucleus in a nonspherical field. In the experiment, which is generally carried out on a powder sample, one set of resonances is exposed for each chemically or crystallographically inequivalent quadrupolar nucleus... 

In the previous section we have outlined how SCFs are excellent solvents for extraction and chromatography, how they provide an environmentally acceptable alternative to conventional organic solvents, and how they can be superior media for many reaction types. These factors alone would be reason enough to study these fluids by NMR spectroscopy, the premier technique for studying structure in solution, but there are also other factors which make these media attractive to the NMR spectroscopist. Perhaps most pertinent in the context of inorganic chemistry is the low viscosity of SCFs, which leads to narrowing of resonances for quadrupolar nuclei [6,7]. Traditionally NMR spectroscopy has been restricted to a handful of spin-1/2 nuclei, while quadrupolar nuclei have been relatively imfashionable because of the low resolution attainable. However these nuclei make up three-quarters of the NMR active nuclei and include most metals, so increased resolution for quadrupolar nuclei would make the whole of the periodic table amenable to study by NMR spectroscopy. 

Serious complications arise, however, if the spins are subject to strong nuclear electric quadrupolar interactions, which tend to modify the echo amplitudes measured. In such cases, it is still possible to extract dipole-dipole coupling information from spin echo decay spectroscopy, if the 71-pulses are applied entirely selectively to the central l/2>o -l/2> transition [6]. If the resonance frequencies between the coupled nuclei are sufficiently similar to allow for spin-exchange via the flip-flop mechanism, Eq. (5) turns into... 

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