Investigations in nuclear magnetic resonance.

Abstract

Itoh and Kamiya?^ in their work on K?Cu?Cl?·2H?O have estimated that some of the unpaired electron spin density may lie on the protons of the water molecules. In the recent ENDOR study by van Ormondt et?al.?^ already referred to in Ch. V, the magnitude of the isotropic hyperfine interaction with the two water protons has been determined to be +0.890?MHz. A possible dipolar contribution from this transferred electron spin density as well as the g?tensor anisotropy may be expected to have appreciable effect on the angular variation of the shifts due to the paramagnetic interaction. Nevertheless, we have been able to demonstrate in our study a method for investigating the Fake interaction in the presence of paramagnetic shift; the paramagnetic nature of the interaction causing the asymmetry in the NMR spectra has been verified by the experiments at different fields and temperatures. The results obtained regarding the angular variation of the paramagnetic shifts are generally consistent with a dipolar model for this interaction. The Fake variation as well as the H?bonding picture in this case are closely similar to the situation in the isomorphous diamagnetic magnesium acetate. As will be seen in the next chapter, this seems to be largely true of nickel acetate also. The investigations that have been described in Chapters V, VI and VII clearly establish that in the case of all the isomorphous acetates of magnesium, cobalt and nickel, the angular variation of the Fake splitting and the hydrogen bonding are closely similar. The presence of the free water protons as inferred from the IR investigations has been confirmed by the NMR investigations described in this thesis, by means of a bifurcated bond for the water protons. The X?ray investigations for nickel and cobalt acetate tetrahydrates yield improbable values for 2?, while in the case of the magnesium salt the proton coordinates are not known. The water proton positions in nickel and cobalt acetate tetrahydrates have been determined in our investigation more accurately. In the case of magnesium acetate, the ambiguities in the X?ray structural data have not permitted a similar accurate location of the protons of H?O(2). The paramagnetic shift in the case of cobalt and nickel acetate tetrahydrate has been studied in the ac plane and their angular variation has been shown to correspond with the known magnetic data in the two cases.