Kirsch, Peter Alexander; (1991) Molecular equilibria determined by van der Waals' attraction. Doctoral thesis (Ph.D.), University College London.

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Abstract

This thesis is primarily concerned with a nuclear magnetic resonance study on the effect of intramolecular van der Waals' attractive forces between alkyl groups. This phenomenon is investigated by devising certain two-fold conformational equilibria, such that one side of the equilibrium has two alkyl groups positioned relatively close to one another and attracting one another, the other side has the alkyl groups further apart and experiencing a greatly reduced interaction. The valence bond isomerisation between 1,4 and 1,6-dialkylcyclooctatetraenes 7(R=Me), 8(R=Et), 9(R=ipr), 10(R='Bu) is observed to favour the more compact 1,6-isomers in solution. This preference is attributed to van der Waals' attraction between the pendant alkyl groups. Molecular mechanics agree as to the preferred valence isomer and confirm it has greater attractive steric interactions. The two-fold 1-(3-alkylphenyl)-group rotational equilibrium in 1 (e)-(3-alkylphenyl)-2(e),6(e)-dimethylcyclohexan-1 -ols 23(R=Me), 24(R='Bu) and 1 (e)(3-alkylphenyl)-2(e),6(e)-dimethylcyclohexanes 25(R=Me), 26(R='Bu) is observed to favour the more compact conformer for 23, 24 but the less compact conformer for 26, while no preference is seen for 25. A combination of buttressing and attractive steric effects are postulated to explain these preferences, the results being supplemented with molecular mechanics calculations. The l-(l-methylneopentyl) rotational equlibrium in 1-(1-methylneopentyl)-3-alkylbenzenes 33(R=Me), 34(R='Bu) and 3-(1-methylneopentyl)phenanthrene 35 favours the less compact conformers for 34 and 35, while no preference is seen for 33. Buttressing is invoked to explain the observed conformational preferences, the results being supplemented with molecular mechanics calculations. The rotational conformations of a 2-methylalkyl-group in 2-methylalkyl-4,4-dimethyl-1,3-dioxanes 39(R=ipr), 40(R=1-Ad), 41(R=Ph) are investigated via analysis of the three bond methyl 1H-NMR coupling constants. For 41, two unequally populated gauche conformations are indicated while for 39,40 no population difference is observed about the indicated bond. However, spin decoupling at the 2-H ring hydrogen in 39 suggests a preferred conformation about the ipr-CH2 bond in this compound. These results are interpreted in terms of buttressing and attractive steric effects. An assignment of NMR signals to rotamers for 23, 24, 25, 26 is performed via an NOE difference experiment on 1 (a)-(3-tBuphenyl)-2(e),6(e)-dimethylcyclohexane, 30. Signal assignment for meta-alkylbenzenes 33, 34, 35 is via an NOE difference experiment on 35.

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