The theory of nuclear quadrupole resonance and quadrupolar relaxation is reviewed, and the chemical interpretation of n.q.r. data is discussed. The super-regenerative oscillator system as employed in the Decca spectrometer for the detection of n.q.r. is described. Details of the measurement of relaxation times using a pulsed r.f. spectrometer are also given. In a study of hydrogen-bonded anions containing conveniently situated chlorine atoms a combination of the relative value of the 35C1 n.q.r. frequency and the magnitude of its shift on deuterium substitution is used to distinguish between centrosymmetric and asymmetric hydrogen bonds. The effect is clearly observed in the n.q.r. spectra of eight salts of the hydrogen or deuterium dichloride ion, and in four series of hydrogen di-chlorocarboxylate anions. For caesium and tetramethylammonium hydrogen dichlorides the direction of the n.q.r. frequency shift suggests that the asymmetry of the bond in the anion is increased on deuteriation. The very small deuteriation shift observed for the symmetrically hydrogen- bonded anion in tetraethylammonium hydrogen dichloride and caesium chloride, 1/3 hydronium hydrogen dichloride, is consistent with a single broad minimum potential function. The assignment to a torsional mode of a line at 85 cm- l in the neutron inelastic scattering spectrum of the latter salt is consistent with the observed temperature dependence of the 35 Cl n.q.r. frequency. The temperature dependences of the 35Cl spin-lattice relaxation time in caesium and tetramethylammonium hydrogen dichlorides are not sensitive to deuteriation. It is proposed that below 250 K in the former salt and 300 K in the latter, relaxation occurs by field gradient fluctuations due to lattice vibrations, and that above these temperatures the severe decrease in TI with temperature may be explained by diffusion of hydrogen chloride molecules throughout the lattice. The 35C1 T1 temperature dependence of the symmetrically bonded hydrogen dichloride ion in the hydronium dichloride salt is however sensitive to deuteriation, and this is consistent with a relaxation mechanism arising from field gradient fluctuations due to reorientation of the hydronium ions. Activation energies for this motion are estimated to be 8.1 k cals/mole for H30 +and 10.5 k cals/mole for D30 + • The n.q.r. spectra of acid salts of mono, di, and trichloroacetic acids and ortho-, meta- and para-chlorobenzoic acids are discussed with relevance to the particular hydrogen bond structure of the anion. In a series of complexes of general formula trans-L2 0sC14 , the measurement of the 35 C1 n.q.r. frequency provides the following order of cis-influence of ligands: C1 <AsEt3 < PEt2Ph < PEt3 A phase transition in trans-(PEt3)20sC14 is indicated near 100 K. It is suggested that intramolecular non-bonding interactions for the earlier members of the series of complexes trans-LiMC14 and K 2 MC1 6 may have a Significant effect on the magnitude of the chlorine n.q.r. frequency and its temperature coefficient. The chlorine n.q.r. frequencies of some 3-chloroacetylacetonato-complexes are shown to vary considerably with the polarity or charge of the complex, and to depend very little on the central element. The differences can be explained in terms of linear intramolecular electric field shifts, the signs and magnitudes of which correlate with experimental values of the Stark coefficient. The n.q.r. spectra of a series of six addition compounds of phosphorus pentachloride favour the ionic structure containing the tetrachloro-phosphonium ion in each case. Values for the libration frequency of the PC1 4 + ion are predicted.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:594994 |
| Date | January 1972 |
| Creators | Lynch, Roderick John |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/74155/ |
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