Synopsis of the Contents of Thesis

 

The thesis consists of six chapters. In Chapter I, a brief review of the recent Overhauser effect studies of "free radicals in solution" is given and the scope of the present work is also discussed. 

Chapter II deals with the construction of an X-band Overhauser effect spectrometer for use at a magnetic field of 3300G in which a broad-band r.f. hybrid junction is employed for NMR detection. Due to difficulties in obtaining several associated equipments and components it has been necessary to make the fullest use of whatever equipment was available in our Institute for this study. Since a dual-purpose Varian EPR/ wide-line NMR spectrometer system was available we have built our Overhauser effect spectrometer around this unit. 

A coil-in-the cavity arrangement is used for the Overhauser effect probe. The cavity is connected to a Varian V-4502 EPR spectrometer system, which constitutes the ESR part of the Overhauser effect set-up. A Varian VF-4210 variable frequency r.f. transmitter-receiver unit in conjunction with the V-4270B output unit and the associated sweep units are used in the NMR part. The original crossed-coil NMR probe has been replaced by a single coil system using the broad-band r.f. hybrid junction for the NMR detection. Field modulation at 20 Hz and field scanning are used to record the NMR spectra. 

The mode of recording NMR spectra under Overhauser effect conditions, monitoring of microwave power and recording relaxation data are discussed in detail. It is pointed out here that the maximum microwave power available in our klystron was only 300 mW and the power reaching the sample cavity was even less. Nonetheless the enhancement factors extrapolated to infinite microwave power yielded U~ values in agreement with literature values for several proton containing_ systems. The effect of sti­mulated emission on these enhancement factors has been consider­ed and it was found that under these experimental conditions the correction factors were within the experimental error. In Chapter III some results of Overhauser effect studies on protons in solutions of free radicals at room Chapter III some results of Overhauser effect studies on protons in solutions of free radicals at room m temperature are reported. These studies were made using the set-up described in Chapter II. Tri-tertiary butyl phenoxyl (TBP) radical has been used as the solute to observe the signal enhancement for protons of cyclohexane, o-, m-, and p-xylenes, n-pentane and n-hexane. These results have not been so far reported in the literature. In all these cases a  dominant dipolar interaction  has been noticed. U_∞ values for these systems are reported. Results have been interpreted assuming a pure translation diffusion modulating the dipolar interaction. 

The construction of an Overhauser effect spectrometer with higher NMR resolution capabilities at 3300 G is described in Chapter IV. The spectrometer described in chapter II does not have the capability to resolve chemically shifted NMR peaks. It was felt desirable to have a better resolving power NMR spectrometer for the Overhauser effect studies. A V-4311 fixed frequency (25.1 MHz) r.f. transmitter-receiver unit for ¹³C studies using the high resolution HA-100 NMR spectrometer was available to us. We therefore designed our new system incorporating this unit. The ESR part and the Overhauser effect probe are the same as in the earlier set-up described in Chapter II. The Varian V-4311 fixed frequency (25.1 MHz) unit has r.f. phase sensitive detection capabilities. The output of the r.f. phase-sensitive detector is available for recording or presenting on the Oscilloscope. But at 3300G the operating frequency of NMR for ¹H is ~14 MHz. Moreover, slightly variable frequency for the NMR is required due to the variability of the ESR frequency depending on the nature of the solvents and free radicals. Since V-43l1 is only a fixed frequency unit at 25.1 MHz a mixing scheme with an external local oscillator has been adopted to suit the NMR frequency requirements. With this scheme it was possible to detect not only ¹H resonance but ¹⁹F resonances also by altering the L.O. frequency appropriately. A set of homogeneity coils mounted on the magnet pole faces were used to improve the magnetic field homogeneity at the sample. With this set-up we could achieve a moderate resolution for protons, which are chemically, shifted by ~ 10 Hz even without sample spinning. A field-frequency lock system suitable for incorporation in this spectrometer has also been constructed and tested. The sensitivity and performance of this set-up was significantly superior to our earlier set-up. 

In Chapter V some of the results of our Overhauser effect studies at room temperature under moderate resolution conditions are reported. Some of the systems which have been studied using the earlier set-up have been reinvestigated with the new spec­trometer and good agreement for U_∞ values was obtained. The studies on 0-, m- and p-xylenes show that the signal enhance­ment factors for the methyl protons and the ring protons are nearly equal. Studies made on mixtures of some proton containing solvents with TBP radical show that for these systems the variation in enhancement factors can be understood on the basis of the changes in the viscosity of the medium. Enhancement of the t-butyl protons of tri-tertiary butyl phenol could be observed when tritertiary butyl phenol was dissolved in carbon tetra chloride and a small fraction of this was reduced to tritertiary butyl phenoxyl radical. The interaction in this case has been reported earlier to be of the scalar type and this could be confirmed. ¹⁹F resonance could be observed using this set-up by changing the L.O. frequency. 1, 2, 2-Trifluoro trichloro ethane showed two chemically shifted ¹⁹F peaks in the intensity ratio 1:2. When Overhauser effect studies were carried out on this solvent with TBP radical as the solute a dominant dipolar inter­action was observed for both the peaks. The two peaks, however, enhanced to different extents. This has been attributed to the differences in the scalar interaction between the solvent ¹⁹F nuclei and TBP radical due to the presence of different number of chlorine atoms in the neighborhood of the two types of ¹⁹F nuclei. A model based on molecular orbital theory has been discussed in Chapter V to explain the trends observed in 1,2,2-­ Trifluoro trichloro ethane. In the last chapter of this thesis suggestions are made for further possible improvement of the present Overhauser effect spectrometer system.

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