Enthalpies of formation (Delta H-f degrees) of syn-syn (C-2v), anti-syn (C-s), and the newly proposed C-2 isomer of carbonyl diazide, OC(N-3)(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 Delta H-f degrees 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 (C-2v), 471.5; anti-syn (C-s), 479.5; C-2, 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(N-3)(2) possessing C-2 symmetry. (C) 2013 Elsevier B.V. All rights reserved.