Sub 20 cm computational prediction of the CH bond energy - a case of systematic error in computational thermochemistry.

The bond dissociation energy of methylidyne, (CH), is studied using an improved version of the High-Accuracy Extrapolated Thermochemistry (HEAT) approach as well as the Feller-Peterson-Dixon (FPD) model chemistry. These calculations, which include basis sets up to nonuple (aug-cc-pCV9Z) quality, are expected to be capable of providing results substantially more accurate than the 1 kJ mol level that is characteristic of standard high-accuracy protocols for computational thermochemistry. The calculated 0 K CH bond energy (27 954 ± 15 cm for HEAT and 27 956 ± 15 cm for FPD), along with equivalent treatments of the CH ionization energy and the CH dissociation energy (85 829 ± 15 cm and 32 946 ± 15 cm, respectively), were compared to the existing benchmarks from Active Thermochemical Tables (ATcT), uncovering an unexpected difference for (CH). This has prompted a detailed reexamination of the provenance of the corresponding ATcT benchmark, allowing the discovery and subsequent correction of a systematic error present in several published high-level calculations, ultimately yielding an amended ATcT benchmark for (CH). Finally, the current theoretical results were added to the ATcT Thermochemical Network, producing refined ATcT estimates of 27 957.3 ± 6.0 cm for (CH), 32 946.7 ± 0.6 cm for (CH), and 85 831.0 ± 6.0 cm for IE(CH).