High-Accuracy Extrapolated Ab Initio Thermochemistry of C₃O, HC₃O⁺, C₃S, and HC₃S

Enthalpies of formation for C₃O, HC₃O⁺, C₃S, and HC₃S⁺ have been determined using an extended HEAT (High-accuracy Extrapolated Ab initio Thermochemistry) protocol. Special attention has been paid to estimating conservative 95% confidence intervals and the uncertainties in HEAT estimates for molecules containing a second-row element. The recommended values (gas phase, 298.15 K, 1 bar) are as follows: Δ_fH_298.15^°(C₃O) = 337.8 ± 1.6 kJ mol^-1, Δ_fH_298.15^°(HC₃O⁺) = 1004.0 ± 1.3 kJ mol^-1, Δ_fH_298.15^°(C₃S) = 583.6 ±3.1 kJ mol^-1, and Δ_fH_298.15^°(HC₃S⁺) = 1192.2 ± 2.6 kJ mol^-1. The corresponding proton affinities (gas phase, 1 bar) at T = 0 K amount to 857.9 ± 0.6 and 916.0 ± 0.8 kJ mol^-1 for C₃O and C₃S and at T = 298.15 K to 864.0 ± 0.6 and 921.9 ± 0.8 kJ mol^-1 for C₃O and C₃S, respectively. The calculated values align with the available data from the ATcT (Active Thermochemical Tables) analysis and some previous experimental and computational results. However, the uncertainties associated with the extended HEAT values presented here are significantly smaller.