NQR spectroscopy

Of the NMR-active nuclei around tluee-quarters have / > 1 so that the quadnipole interaction can affect their spectra. The quadnipole inter action can be significant relative to the Zeeman splitting. The splitting of the energy levels by the quadnipole interaction alone gives rise to pure nuclear quadnipole resonance (NQR) spectroscopy. This chapter will only deal with the case when the quadnipole interaction can be regarded as simply a perturbation of the Zeeman levels. 

Overall NMR and NQR spectroscopy data thus indicate that the diketo structure predominates for uracil, thymine, and their nucleosides or nucleotides. These studies have failed to detect other tautomeric forms of these compounds. 

In Sections II and IV we have described the application of NMR and NQR spectroscopy to the determination of the localization of mobile protons of the pyrimidine bases and for the prediction of the tautomeric structures of these molecules. Here, we shall describe the application of these spectroscopies to the elucidation of the electronic structures of the most stable tautomeric forms of the bases. 

A considerable number of NQR studies have been made on zinc and cadmium complexes, for example, the 35C1, 81Br and 127I NQR spectra of a number of CdX2-polyether complexes have been reported and indicate that the compounds are dimeric in solution, with symmetrical halogen bridges between the metal atoms.69 A number of amino acids and peptide complexes of cadmium(II) have been investigated by 14N NQR spectroscopy.70... 

The complexes [AuCl(CNR)] (R = Ph, p-MeOC6H4, or C6Hn), among others, have been studied by 35C1-NQR spectroscopy (135), and it was concluded that the 35C1-NQR frequencies were sensitive to the nature of the trans ligand and that cr-bonding effects were dominant. 

First and foremost, NMR spectroscopy is transferable from the solid state to solution phase this is not the case for NQR spectroscopy as it is... 

G. P. Wulfsberg, Nuclear Quadrupole Resonance (NQR) Spectroscopy, in Encyclopedia of Inorganic Chemistry, 2nd edn, (R. B. King ed.), John Wiley Sons, Ltd., Chichester, UK, 2008. 

Attempts to use 35C1 NQR spectroscopy to assign the structures of a number of polychlorinated polycylic hydrocarbons have also been reported90,91. 

The basic parameters which can be extracted from the spectrum are the line positions, their widths and intensities. Each spectrum consists of eight (sometimes twelve) lines, the positions of which are determined by two (three) parameters, the isomer shift and the quadrupole coupling constant (and the asymmetry parameter). The quadrupole coupling constant and asymmetry parameter have exactly the same significance as in NQR spectroscopy (see Chapter 7). 

Besides NQR spectroscopy and the study of nuclear quadrupole interaction effects in broad-line NMR spectroscopy, paramagnetic electron resonance 6°1, Mossbauer spectroscopy, and the study of perturbed angular correlation of y-rays, are suitable methods for studying nuclear quadrupole interactions in solids. Indirect methods are also available for acquiring information about the nuclear quadrupole couplinjg constant from the liquid state (particularly NMR spectroscopy in liquids and in liquid crystals in some cases gives information about this constant). By microwave spectroscopy, the nuclear quadrupole interaction may be studied in the gaseous phase (see the paper by Zeil). We shall deal here only with the aspect of NQR spectroscopy in solids since this method has the broadest applicability to chemical problems in comparison with the other methods mentioned. 

The interaction energy of the nucleus with its surroundings, as observed by NQR spectroscopy, generally contains two important terms... 

Before discussing NQR results, first we must realise that the experimental accuracy is very high. The transition frequencies may be determined to 5 kHz provided the line width is < 5-15 kHz. In most NQR spectra the line width is below 15 kHz. The NQR frequencies range between a few kHz and 1000 MHz Below 1 MHz the experimental measurement of NQR frequencies by pure quadrupole resonance is not successful. Therefore the main frequency range in NQR spectroscopy is 1 MHz < v < 1000 MHz and the accuracy is... 

Summarizing the use of NQR spectroscopy in the determination of chemical bonding and of charge distribution in ionic solids one can say ... 

NQR spectroscopy also gives some hint about the numer of molecules in the asymmetric unit of the crystal. Let us consider 2,4-dichloro-3,5-dimethyl-phenetole (see Table III.2). At 77 °K six 3SC1 NQR lines have been found 126) and it therefore follows that there are three formula units in the asymmetric crystallographic unit. 

Quite a different situation is experienced in NQR spectroscopy. Dehmelt and Kriiger (1950/1951) 3-6> found that the temperature dependence of NQR frequencies in organic materials is considerable. For instance, the temperature coefficient of i (35Cl) for chlorine bonded to carbon atoms is found to be in the range... 

The spin-lattice relaxation of nuclei with /> 1 /2 is mainly due to nuclear quadrupole relaxation and the relaxation time at a temperature of 77 °K is still fairly short (of the order of milliseconds for many nuclei of interest to chemists). Therefore the experimental methods usually applied do not involve relaxation problems. The full RF fields available — for instance, with a superregenerative NQR spectrometer — may be applied successfully, resulting in an improved signal-to-noise ratio for the resonance at 77 °K as compared with room temperature. There are exceptions to this rule (e.g. 14N NQR spectroscopy where saturation problems come into play at 77 °K). 

Some workers in the field of NQR spectroscopy have preferred a modification of Eq. (IV.3), considering the constant b in the term v0b- T) as a function of temperature. The reasoning for this assumption is the volume dependence of the different co . In molecular crystals the temperature dependence of the modes con,... 

The determination of the asymmetry parameters and the direction cosines, together with x-ray investigations of the crystal structure, will shed some light on the crystal field effect in solids. NQR powder spectra can only permit very rough and qualitative conclusions on the intermolecular forces. The results on the Mentschukin complexes with AsCl3 show that the metal-chlorine bond is virtually unaffected by the formation of molecular compounds. From singlecrystal NQR spectroscopy, particularly on the As nucleus, some geometrical information about the molecular compounds can be expected. 

Usually pure compounds are used in NQR spectroscopy. The matrix around a molecule is composed of molecules of the same kind. It appears quite interesting to change the matrix by incorporating the molecule studied into a matrix which can be varied. The crystal field effect can thereby be changed in a continuous way. An experimental method is to measure the NQR spectrum of a molecule B in mixed crystals in the range 0 [Pg.69]

With the help of 35C1 Nuclear Quadrupole Resonance (NQR) spectroscopy and AMI, MNDO h PM3 calculation data of tautomers of 2-trichloromethyl-5(6)-nitrobenzimidazole it is established that 5-nitro tautomer is more preferable than its 6-isomer (Scheme 3.76) [1407],... 

Thermodynamic stability of indazole has been studied by 14N NMR-NQR spectroscopy and ab initio calculations [1414],... 

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