F¹⁹ Nuclear Spin Lattice Relaxation in Liquids

Author: Burke, Thomas Edmund

Year: 1969

Degree: Dissertation (Ph.D.)

Advisor: Chan, Sunney I.

Committee Member: Unknown, Unknown

Option: Chemistry

DOI: 10.7907/Q5X7-KP46

Abstract

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A magnetic nucleus located on an internal rotor is coupled not only to the rotational magnetic field generated by the end-over-end rotation of the molecule but also to the magnetic field produced by the internal rotation of the top relative to the frame. Although the importance of the nuclear-spin-internal-rotation coupling as a nuclear spin-lattice relaxation mechanism has been established, the nature of the stochastic processes which modulate the interaction and are thereby responsible for the coupling between the nuclear spin system and the lattice has yet to be determined.

In order to elucidate the nature of the dynamical processes that provide the coupling between the nuclear spins and the lattice, both the temperature and viscosity dependence of the fluorine spin-lattice relaxation times for benzotrifluoride have been examined. The viscosity dependence of the fluorine spin-lattice relaxation times for hexafluorobutyne-2 has also been investigated.

It is concluded that the rotational magnetic fields generated by overall and internal rotation are fluctuating independently and that each field is described by a distinct correlation time. It is also concluded that, contrary to previous proposals, the nuclear-spin-internal-rotation interaction contributes significantly to the spin-lattice relaxation of the methyl group protons in molecules such as toluene.

The effect of solvents on the relaxation of rigid molecules has also been investigated. The fluorine spin-lattice relaxation times of CF[subscript 3]CH[subscript 2]C1, CF[subscript 3]CH[subscript 2]Br, CF[subscript 3]CH[subscript 2]I, and CF[subscript 3]CC1[subscript 3] have been measured as a function of concentration in various solvents, both polar and non-polar. The intramolecular contributions to relaxation, arising from the intramolecular dipolar and spin--overall-rotation interactions, were analyzed in terms of various correlation time models. It is found that the Hill[superscript 1] model for molecular reorientation provides the only satisfactory explanation for all the results; it is also found that, in general, the efficacy of the Hill model is dependent upon the mutual viscosity.

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