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Some studies of hydrogen bonding and of some unstable positive ions by nuclear magnetic resonance Connor, Thomas Michael

Abstract

(i) Hydrogen Bonding Studies - The nature of hydrogen bonding in solutions of alcohols, ROH, in various solvents has been studied using nuclear resonance techniques. Data obtained from dilution-shift curves for the OH protons in alcohols have been combined with information derived from infra-red investigations of the OH stretching regions in these compounds. The information obtained has been interpreted in terms of three effects, (i) The electronic effects of the group R. (ii) The steric effects of the group R. (iii) Effects due to other forms of molecular association. On this basis, deductions have been made concerning the degree and type of association in these compounds. The relative hydrogen bonding strengths have been predicted in some instances. The importance of steric inhibition of hydrogen bonding by bulky substituent groups has also been demonstrated. Some dilution-shift studies of acrylic acid in various solvents have been carried out. Studies of the effect of temperature on the OH proton resonances of alcohols have led, amongst other things, to a value for the average hydrogen bond energy in a methanol / carbon tetrachloride solution. The temperature-shift curves for a variety of ortho-substituted phenols have also been obtained and discussed in the light of existing infra-red spectral evidence concerning the nature of hydrogen bonding in these substances. The relation between the association shifts and the integrated absorption intensities of alcohols has been discussed. A correlation between these two quantities was found for alcohols of a similar type. (ii) Studies of Unstable Positive Ions. (a) Triphenyl Carbonium Ions. The NMR spectra of a variety of substituted triphenyl carbonium ions in a trifluoroacetic acid trifluoroacetic anhydride solvent have been obtained at 40 Mc and 60 Mc. No unequivocal evidence as to the structures of these compounds has been obtained, i.e. no distinction between 'symmetrical propeller’ and assymetric forms was possible, due to the presence of exchange effects. The data have given information about the changes in electron density in the aromatic rings due to the various substituent groups. Partial assignments of the aromatic proton spectra have been given. The importance of hyperconjugative electron release by aliphatic substituents is indicated. Some preliminary investigations of the protonated form of 1,1-Di-p-anisylethylene have also been carried out. (b) The l⁺ Ion. The NMR spectra of solutions of iodine in oleum have been investigated to try and shed light on the possibility of the existence of the l⁺ ion in such systems. The measured broadenings and shifts of the oleum proton resonances at various iodine concentrations have been interpreted in terms of the presence of this species, which should be paramagnetic. A value for the magnetic moment of this ion has been obtained. Other evidence for the existence of the l⁺ ion has been fully discussed.

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