Geometric isomerization barrier

Geometrical Isomerism. Rotation about a carbon-carbon double bond is restricted because of interaction between the p orbitals which make up the pi bond. Isomerism due to such restricted rotation about a bond is known as geometric isomerism. Parallel overlap of the p orbitals of each carbon atom of the double bond forms the molecular orbital of the pi bond. The relatively large barrier to rotation about the pi bond is estimated to be nearly 63 kcal mol (263 kJ mol-i). 

With the availability of stable geometric isomers of doubly bonded germanium compounds, experimental determinations of the 7r-bond strength can be made. The enthalpy of activation for double bond isomerization in Mes(Tip)Ge=Ge(Tip)Mes (Tip = 2,4,6-triisopropylphenyl) has been determined for the Z-E conversion, 22.2 . 3 kcal/mol and for the E-Z conversion, 20.0 0.3 kcal/mol.15 These values agree well with recent theoretical estimations.7 The isomerization barrier in germaphos-... 

With a double bond, rotation would destroy the tt bond that arises from overlap of p orbitals consequently, there is a very large barrier to rotation. It is of the order of 263 kJmol , which is very much higher than any of the barriers to rotation about single bonds that we have seen for conformational isomerism. Accordingly, cis and trans isomers do not interconvert under normal conditions. Ring systems can also lead to geometric isomerism, and cis and trans isomers... 

A detailed account of the use of H-NMR for the measurement of A(f, the barrier of inversion at the N-center, etc., have been described in GHEC-II(1996) <1996CHEC-II(1B)911>. Only a few salient features have been covered in this. Proton NMR has been used for the structural elucidation and to study the geometrical isomerism of a number of diazetidines and their derivatives. For example, H-NMR has been used to study the inversion of (Z,Z)-isomers and (A, )-isomcrs of 1,2-diazetidines 46. 

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