The role of the second order Jahn–Teller effect in the thermal reactions of ethylene

The UV absorption spectrum of ethylene during the pulse heating of an ethylene–argon mixture to prepyrolysis temperatures in the range 950 < Т < 1285 K was studied by kinetic spectroscopy in a free-piston adiabatic compression unit. New intense light absorption bands were found at 210 < λ < 260 and 440 < λ < 490 nm. Ab initio quantum-chemical calculations of the mechanism of the thermal cis-trans isomerization of ethylene were performed. When this mixture is heated, the point symmetry group of the ethylene molecule in the ground state S 0 reduces to С 1 , which is characteristic for ethylene at the minimum of the S 1 state, due to the second order Jahn–Teller effect.