Quantum mechanical study of the syn-anti isomerisation of 2-tellurophenecarboaldehyde: vive la différence.

The syn- and anti-conformers of 2-tellurophenecarboaldehyde are studied in the gas phase. A transition state is also modelled for the syn-anti isomerisation. Computations are done using different methods namely HF, DFT/B3LYP, MP2 and CCSD(T). The basis set used for all atoms is 6-311++G(d,p) except that LanL2DZ ECP is used for tellurium atom only. The optimised molecular structures and related structural parameters of these conformers are reported. The energy differences between the syn- and anti-conformers, associated rotational barriers and thermodynamic parameters are derived from the computations. The infrared frequencies of these conformers are also reported with appropriate assignments. This study is extended to include solvent effect and the conformers are fully optimised (DFT/B3LYP) using the integral equation formalism in the Polarisable Continuum model. In the gas phase, the theoretical rate constant for the unimolecular conversion, anti conformer to transition state, is reported for the first time; DFT/B3LYP (4.82×10(30) s(-1)) and MP2 (7.81×10(30) s(-1)). It is found that the structures are not much affected by the solvents but energy difference increases and rotational barrier decreases. The results indicate that there is a close agreement with the predictions from the different theoretical methods. The results obtained are critically analysed and compared with the furan, thiophene and selenophene analogues. A major factor affecting conformational preference and the mole fraction is the charge on the chalcogen heteroatom in the ring. An interesting outcome of this work is that in both the gas phase and solutions, the syn conformer is more stable and exists almost exclusively.