Conjugation trumps cumulation: the global minimum on the C₃H₄O surface

Methylketene (CH3CHCO) and acrolein (CH2CHCHO) have received considerable attention from both theoreticians and experimentalists. In a number of computational studies, it has been identified that both isomers sit alongside their dissociation products C2H4+CO, at the bottom of the potential energy surface with energies very close to each another. The most striking feature from preliminary calculations is that the relative energy difference between methylketene and trans-acrolein (Delta E 0.8-3.4 kJ/mol) changes with respect to choice of reference geometry, correlation treatment, and basis set expansion. While the thermodynamic profile favours trans-acrolein as the lowest-lying isomer, it is difficult to estimate the quality of the currently available results, which are determined with a variety of methods. At higher levels of theory together with reducing basis set incompleteness methylketene might be the true minimum on the C3H4O potential energy surface. In addition to their being almost isoenergetic, much interest in these C3H4O isomers stems from an astrochemical standpoint. The discovery of both cis and trans-acrolein in the interstellar medium has led to speculation that methylketene may also be present. While thermodynamically methylketene proves to be a good candidate for extra-terrestrial observation, as yet there have been no reports of its detection. To this end, by applying the best correlation treatment that has been used for this species, using by far the most elaborate and accurate basis sets (up to sextuple-zeta quality), we establish the relative ordering and energetic difference between methylketene and trans-acrolein to within chemical accuracy (within 1 kJ/mol). We find the inclusion of zero-point energy, explicit core-correlation, and augmentation with diffuse functions have significant effects in determining the relative energy difference of the isomers, and assignment of the C3H4O global minimum. [GRAPHICS] .