Conformational Analysis by Nuclear Magnetic Resonance Spectroscopy

Nuclear magnetic resonance spectroscopy is a powerful and versatile spectroscopic technique and novel applications are continually being devised for it. This thesis concerns the application of nmr spectroscopy to the study and definition of both the time dependent and the time independent three dimensional geometries of organic molecules in solution. The purposes of this thesis are: (a) to establish the conceptual background to both nmr spectroscopy and conformational analysis and hence to relate the two fields, (b) to review in varying depth the published results in those areas of chemistry which have been ventured upon in the current research and (c) to present the results and conclus: :ions of the current studies. To these ends, nmr spectroscopy is first briefly reviewed and the areas of major topical interest touched upon. As nmr spectroscopy is a precisely mathematical subject, the theoretical applications later employed are briefly outlined. The thermodynamic considerations relevent to conformational analysis are then reviewed and this is followed by a discussion of the application of nmr spectroscopy to detailed conformational analysis. The current studies involve reviewing two discrete fields of conformational analysis. Firstly, nmr studies of conformations in medium ring compounds are reviewed in depth. The results of a study of syn-3,7-dibromo-cis,cis-cycloocta-1,5-diene are recorded in which a definitive statement of the solution conformation (in chloroform) of this molecule is possible. The consistency of the data from other techniques relating to this conformation is remarked upon and the similarity to the solid state conformation determined (F. B. Wilson, Ph. D. thesis, University of Glasgow, September 1971) by an X-ray study is deduced. Secondly, the vast field of nmr studies in conjugated systems is sketched relatively briefly before the few results relating to aromatic systems are detailed. Work on 9-nitroso, and 9-formyljulolidine rotation; :al barriers for the '9' group are then presented. Extensive electron delocalisation is observed from the nitrogen atom of the quinolizidine unit in these compounds to the '9' group and this is comparable to, or in excess of, that observed in para-substituted dimethylanilines. The rotational barriers for the dimethylamino function in three para-substituted N,N-dimethylanilines are then reported and a Hammett correlation between them is suggested. In this manner, an estimate of the rotational barrier for the dimethylamino function in N,N-dimethylaniline itself is presented. Upon this basis and upon results published for studies of aziridine inversion barriers, an estimate of the resonance energy in dimethylaniline is suggested at 10+/-1 kcal./mole. The necessary practical details of this work are then outlined. The importance of computing facilities in this work is made in the appendices and an original coding of a least squares method due to W. E. Wentworth (J. Chem. Ed. , 1965,42,96) is included. Published papers on this work are then appended