The Jahn-teller Eeect Experimental and Theoretical Studies of Vcl 4

A survey of ligand eld theory is given and the possibility of ob serving the Jahn Teller e ect in molecules by means of gas phase elec tron di raction is discussed The theory of the dynamic Jahn Teller e ect is outlined and theoretical and experimental results for VCl are reviewed In H A Jahn and E Teller predicted that a nonlinear molecule which is in a degenerate state will seek to change its structure in order to remove as much of the degeneracy as possible It has however proven to be very di cult to demonstrate this e ect experimentally Van Vleck expressed it this way It is a great merit of the J T e ect that it disappears when not needed But as Ballhausen states in his book the e ect is very di cult to nd when one does wish to demonstrate it As we shall see Jahn Teller e ects are expected in a series of octahedral and tetrahedral complexes these complexes should therefore not have a regular octahedral or tetrahedral structure It is indeed found from x ray di raction studies that such molecules are distorted It cannot however be immediately concluded that these distortions are caused by the Jahn Teller e ect In a crystal lattice one must take account of the intermolecular forces and van Vleck as long ago as pointed out that such forces can lead to completely equivalent distortions It is therefore tempting to study molecules in the gas phase by means of for example electron di raction Structural Studies by Electron Di raction Electron di raction methods are described in for example ref The experiment consists of a beam of electrons colliding with a beam of gas molecules and the scattering pattern being recorded on a photographic plate An experimental intensity curve is obtained which can be compared with the theoretical expression