High-Accuracy Extrapolated Ab Initio Thermochemistry of C$_{3}$O, HC$_{3}$O$^{+}$, C$_{3}$S, and HC$_{3}$S$^{+}$

Enthalpies of formation for C$_3$O, HC$_3$O$^+$, C$_3$S, and HC$_3$S$^+$ 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$_f$$H$$_{298.15}$°(C$_3$O) = 337.8 ± 1.6 kJ mol$^{−1}$, Δ$_f$H$_{298.15}$°(HC$_3$O$^+$) = 1004.0 ± 1.3 kJ mol$^{−1}$, Δ$_f$H$_{298.15}$°(C$_3$S) = 583.6 ±3.1 kJ mol$^{−1}$, and Δ$_f$H$_{298.15}$°(HC$_3$S$^+$) = 1192.2 ± 2.6 kJ mol$^{−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 C$_3$O and C$_3$S and at T = 298.15 K to 864.0 ± 0.6 and 921.9 ± 0.8 kJ mol$^{–1}$ for C$_3$O and C$_3$S, 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.