Accurate theoretical predictions for carbonyl diazide molecules: A coupled-cluster study of the potential energy surface and thermochemical properties

Enthalpies of formation ( Δ H f ° ) of syn-syn (C2v), anti-syn (Cs), and the newly proposed C2 isomer of carbonyl diazide, OC(N3)2, at 298 K have been predicted using CCSD(T) theory in conjunction with a systematic basis set expansion that has been extrapolated to the complete basis set (CBS) limit. The Δ H f ° has been calculated via three independent pathways and ‘chemical accuracy’ has been obtained. The predicted CCSD(T)/CBS values are (kJ mol−1): syn-syn (C2v), 471.5; anti-syn (Cs), 479.5; C2, 517.4. These values deviate by 14.5 kJ mol−1 compared to prior G4 approximations. CCSD(T) torsional potential energy scan reveals a previously undiscovered conformer of OC(N3)2 possessing C2 symmetry.