Theoretical study on the pyrolysis of sulphonyl oximes in the gas phase

The reaction mechanism of the pyrolysis of sulphonyl oximes (CH3-C6H4-S(O)(2)O-N=C(H)-C6H4Y), in the gas phase is studied theoretically at HF/3-21G, ONIOM (B3LYP/6-3lG**:HF/3-21G) and ONIOM (MP2/6-31G**:HF/3-21G) levels. All the calculations show that the thermal decomposition of sulphonyl oximes is a concerted asynchronous process via a six-membered cyclic transition state. The activation energies (Ea) predicted by ONIOM (B3LYP/6-31G**: HF/3-21G) method are in good agreement with the experimental results for a series of tosyl arenecarboxaldoximes. Five para substituents, Y = OCH3, CH3, H, Cl, and NO2, are employed to investigate the substituent effect on the elimination reaction. Linear Hammett correlations are obtained in all calculations in contrast to the experimental finding.