Rotational energy barriers ethylene

Most of the ground states of complexes seem to have structure XXIV, but XXV reasonably could provide a mechanism for rotation about the metal-olefin bond axis with a low energy barrier. Cramer (II) found that 7r-cyclopentadienylbis(ethylene)rhodium(I), XXVI, gave two broad signals (r = 7.23, 8.88 ppm) for the ethylene protons at —25° and that... 

Before discussing structural effects on barrier heights, it is necessary to distinguish between the two processes planar nitrogen inversion and rotation about the C=N bond, which may both lead to interconversion of the isomers. Rotation about a non-activated C=N double bond is expected to be hindered by an energy barrier similar to the ethylene barrier, i.e. of the order of 50—60 kcal/mole 121.132) Thus, alkyl- and presumably also aryl-substituted imines, which show interconversion barriers below 30 kcal/mole (Table 6), undergo nitrogen inversion ). 

The catalysed isomerizations of ethylenes are usually ascribed to addition of positive, negative or radical species to the double bond (71). This lowers the barrier to rotation (around the central C=C bond) energy barrier. 

With the exception of poly (ethylene oxalate) [poly(ethylene glycol oxalate)], saturated aliphatic polyesters have a relatively low melting point because of the low energy barrier to rotation (see Section 4.2). For example, poly(ethylene adipate) melts at 54°C. 

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