ON THE RELATIVE STABILITY OF CUMULENONE AND ALDEHYDE ISOMERS: WHEN WE HEAT345(Q) THINGS UP

Isomers of chem{H_2C_{2n+1}O} are examples of complex organic molecules that are either known or proposed to exist in the interstellar medium. For the smallest of these chains (chem{H_2C_3O}) only two of three isomers are observed in space: propynal (chem{HC(O)CCH}) and cyclopropenone (chem{c-C_3H_2O}), while evidence for the remaining isomer propadienone (chem{H_2C_3O}) is currently lacking. Potentially, this behaviour may be rationalised by a thermodynamic argument: several studies have provided quantum chemical calculations in an effort to determine the relative thermodynamic stability between these three isomers. An early study by Radom, at the SCF/6-31G** level ranked chem{HC(O)CCH} as the thermodynamic minimum, followed by chem{H_2C_3O}, and chem{c-C_3H_2O}. The most recent determination by Karton and Talbi, using W2-F12 theory, places chem{H_2C_3O} as the lowest energy isomer; 2.5 kJ mol$^{-1}$ lower than the chem{HC(O)CCH} form. In an attempt to resolve this long-standing ambiguity, we were motivated to provide high level calculations based on the HEAT protocol. In this talk, we will discuss the relative stability of chem{H_2C_3O} and chem{H_2C_5O} isomers, along with their sulfur analogues, as revealed by HEAT345(Q) theory