Unimolecular decomposition of tetrahydrofuran: Carbene vs. diradical pathways

The unimolecular decomposition of tetrahydrofuran (THF), a building block of possible second-generation biofuels, was studied at the CBS-QB3 level of theory. As its initial initiation routes remain unknown, a large number of pathways were explored involving three types of fundamental elementary mechanisms: diradical, carbenic and pericyclic. Based on the computed potential energy surfaces, thermochemical data and pressure dependant rate parameters were determined and included in a detailed chemical kinetic mechanism for THF pyrolysis. Simulations were performed and compared to pyrolysis products of the literature, measured behind reflected shock waves. The kinetic analyses showed that THF unimolecular decomposition is initiated by reactions involving diradicals and carbenes. A pericyclic rearrangement was also shown to be crucial in its thermal decomposition. Acyclic isomers of THF are major products of its unimolecular initiation and are predominantly formed from diradical intermediates isomerization and from the pericyclic reaction. The major pathway in the decomposition of carbenes ultimately yields two molecular products (CH2O and propene). Initial CC and CO bond fissions in the acyclic THF isomers lead to the formation of the initial radicals (ranked by importance): ethyl ≈ vinyloxy (CH2CHO) > allyl > methoxy ≈ hydroxymethyl. This study brings a new understanding on the nature of initial radicals and molecules formed in the unimolecular initiation of the THF ring.