The torsional flexibility of aliphatic chain molecules

Abstract

The rotation of the CH 3 groups round the single C—C bond in ethane is associated with a threshold energy of about 3000 gcal./gmol. or 2 x 10 -13 erg/mol. (Schäfer 1938; Kistiakowsky, Lacher and Strutt 1939). In an aliphatic CH 2 chain where the carbon atoms are linked together by single bonds the corresponding energy must be of the same order and is most likely rather smaller. Supposing we consider any particular C—C bond in the chain and treat the two parts at each side of this bond as rigid rotators, then their kinetic energy would be 2 x 1/2 kT which at room temperature amounts to about one-fifth of the threshold energy. It seems very likely under these circumstances that a chain molecule of say ten to twenty carbon atoms should already at room temperature show signs of distortion due to internal rotation. If this is true, then the previously observed increase of the crystal symmetry at the melting-point of paraffins (Müller 1930, 1932) and the corresponding changes of the polarization of long-chain ketones (Müller 1937, 1938) can no longer be ascribed entirely to a rotation of the molecule in the field of the surrounding molecules but must at least partly be due to this internal distortion. It is clear that a distortion of this type tends to destroy the anisotropy of the molecule and to give an apparent isotropy to the crystal. The present experiments were made in order to obtain an estimate of the magnitude of the distortion effect. It is found to be surprisingly large.