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Attractive Steric Interactions

Augustus, Adebayo Samuel; (1999) Attractive Steric Interactions. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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In majority of organic molecules a few pairs of atoms would be so close to each other to repel but most will be separated by more than the sum of their van der Waals radii as a result of which they will attract, albeit weakly, and help lower the enthalpy of formation of the molecule. This attractive steric interaction is one of the causes of folding in molecules and this thesis is devoted to the study of such interactions by NMR spectroscopy and force field calculations. We synthesised and investigated attractive steric interactions, principally, in hexasubstituted-benzenes and with some attention to monosubstituted cyclohexanes. The three types of hexasubstituted-benzene systems investigated are:- (i) The 1,3,5-trisustituted-2,4,6-trimethylbenzenes, where the groups in the 1, 3 and 5 positions are CH2X (X = Cl, Br, I and Me). (ii) The 1,3-disubstituted-2,4,5,6-tetramethylbenzenes, where the groups in the 1 and 3 positions are CH2X or CH2Z. Here X and Z ranged from a small atom or group such as -Cl, -CH3, to bulky groups such as 1-adamantyl, tert-butyl, naphthyl, xylyl, phenyl, iodine etc. In some of the compounds, X = Z and for others, X Z. (iii) The 1,4-disubstituted 2,3,5,6-tetramethylbenzenes, where the groups in the 1 and 4 positions are CH2X (X =tert-butyl, chlorine, bromine and iodine). Each one of these compounds has two major conformations, the syn and the anti. In the syn conformation all or both the X groups as in (i) and (iii), and both the X and Z groups as in (ii), lie on the same side of the ring plane and a 180 ° rotation about a Ar-CH2X or Ar-CH2Z bond should give the anti form. In each of these compounds we were able to slow down rotation about both bonds on the NMR time scale and determine whether or not the more compact syn form is the more populated and therefore more stable relative to the anti. The barrier to rotation about the Ar-CH2X and Ar-CH2Z bonds is also discussed. In majority of the compounds, with the exception of a few such as the 1-(halomethyl)-3- neopentyl-2,4,5,6-tetramethylbenzenes and the 1,4-disubstituted-2,3,5,6 tetramethylbenzenes, dynamic NMR spectroscopy show that the more compact syn conformations is more stable relative to the anti. The best results were obtained for the 1-neopentyl-2,4,5,6-tetramethylbenzenes {type (ii)}, where X =tert-butyl and Z = α-naphthy1, β-naphthy1 or phenyl. In each of the three compounds, molecular mechanic calculations also show that the more compact syn form benefits substantially, relative to the anti, from the pair wise atom-atom van der Waals interactions between the X and Z groups. In most of the compounds investigated calculation shows that the interlocking arrangement of the methyl and methylene groups on the main benzene ring, a process referred to as "gearing", favours one conformation over the other. Thus, in compounds where experiments indicate that the syn is the more stable, one has to be cautious in attributing the difference in enthalpy in its entirety to attractive steric interactions between the X and Z groups. In the cyclohexylethynyl derivatives, the less compact equatorial conformation was found to be the more stable relative to the axial form, presumably, because the total of the attractive steric interactions between the methylene groups from the ring and the substituent is not high enough to offset the equatorial advantage from 1,3-diaxial interaction.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Attractive Steric Interactions
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
URI: https://discovery.ucl.ac.uk/id/eprint/10123329
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