An Automated Thermochemistry Protocol Based on Explicitly Correlated Coupled-Cluster Theory: The Methyl and Ethyl Peroxy Families

Abstract

An automated computational thermochemistry protocol based on explicitly correlated coupled-cluster theory was designed to produce highly accurate enthalpies of formation and atomization energies for small-to mediumsized molecular species (3−12 atoms). Each potential source of error was carefully examined, and the sizes of contributions to the total atomization enthalpies were used to generate uncertainty estimates. The protocol was first used to generate total atomization enthalpies for a family of four molecular species exhibiting a variety of charges, multiplicities, and electronic ground states. The new protocol was shown to be in good agreement with the Active Thermochemical Tables database for the four species: the methyl peroxy radical, methoxyoxoniumylidene (methyl peroxy cation), methyl peroxy anion, and methyl hydroperoxide. Updating the Active Thermochemical Tables to include those results yielded significantly improved accuracy for the formation enthalpies of those species. The derived protocol was then used to predict formation enthalpies for the larger ethyl peroxy family of species.

Active Thermochemical Tables: Enthalpies of Formation of Bromo- and Iodo-Methanes, Ethenes and Ethynes

“…For the explicitly correlated calculations, the CCSD and (T) correlation energies were extrapolated separately, as previously recommended, due to their different convergences. , A Schwenke style CBS extrapolation was carried out in which E corr CBS = ( E corr VQZ ‐ F 12 − E corr VTZ ‐ F 12 ) F + E corr VTZ ‐ normalF 12 where E corr VQZ ‑ F12 and E corr VTZ ‑ F12 are the correlation energies evaluated with those respective basis sets. F is the constant optimized for first and second row p -block element containing molecules by Hill et al with the CCSD correlation energy F = 1.363388 and with the (T) correlation energy F = 1.769474 A previous study on the thermochemistry of molecules containing 3d transition metals showed that the specific coefficients are not particularly sensitive to the system, despite being calibrated on systems containing only first and second row p -block elements.…”

ATOMIC-2 Protocol for Thermochemistry

“…Ab initio thermochemistry has emerged as a reliable resource for molecular enthalpies of formation and related atomization energies and so has been established as a viable alternative to traditional experimental techniques including combustion calorimetry. , This may be most visible in the expanded use of theoretical data in the Active Thermochemical Tables (ATcT), − which provide accurate enthalpies of formation for a still-limited, but growing, number of compounds through the solution of complex thermochemical networks. The W4-17 database is another popular source of reference data, and it is based exclusively on high-level theory, listing 200 atomization energies with a claimed 3σ confidence interval of only ±1 kJ/mol.…”

Mechanism, thermochemistry, and kinetics of the reversible reactions: C2H3 + H2 ⇌ C2H4 + H ⇌ C2H5

“…1.124 is a successor to ver. 1.122× that was used in a recent study of the C–H bond dissociation enthalpies (BDE) of aromatic aldehydes, 24 itself being a result of successive expansions of earlier versions, such as 1.122h, 25 1.122o, 26 1.122p, 27 1.122q, 28,29 1.122r, 30 and 1.122v. 31…”