Enthalpy of Formation of C₂H₂O₄ (Oxalic Acid) from High-Level Calculations and the Active Thermochemical Tables Approach

Abstract

High-level coupled cluster calculations obtained with the Feller−Peterson−Dixon (FPD) approach and new data from the most recent version of the Active Thermochemical Tables (ATcT) are used to reassess the enthalpy of formation of gas-phase C 2 H 2 O 4 (oxalic acid). The theoretical value was further calibrated by comparing FPD and ATcT gas-phase enthalpies of formation for H 2 CO (formaldehyde) and the two low-lying conformations of C 2 H 4 O 2 (syn and anti acetic acid). The FPD approach produces a theoretical enthalpy of formation of gasphase oxalic acid of −732.2 ± 4.0 kJ/mol at 298.15 K (−721.8 ± 4.0 kJ/ mol at 0 K). An independently obtained ATcT value, based on reassessing the existent experimental determinations and expanding the resulting thermochemical network with select mid-level composite theoretical results, disagrees with several earlier recommendations that were based solely on experimental determinations but is in excellent accord with the current FPD value. The inclusion of the latter in the most recent ATcT thermochemical network produces a further refined value for the gasphase enthalpy of formation, −731.6 ± 1.2 kJ/mol at 298.15 K (−721.0 ± 1.2 kJ/mol at 0 K). The condensed-phase ATcT enthalpy of formation of oxalic acid is −829.7 ± 0.5 kJ/mol, and the resulting sublimation enthalpy is 98.1 ± 1.3 kJ/mol, both at 298.15 K.

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

“…TN version 1.122q ,, was used for the results discussed below, as it did not contain any of the present calculations. A review of the provenance of hydrocarbon species without halides is beyond the scope of the present work, and interested readers are encouraged to review sources detailing these. ,,,, …”

Sub 20 cm−1 computational prediction of the CH bond energy – a case of systematic error in computational thermochemistry

“…Namely, if one of the ATcT benchmark values differs from the corresponding computed values by more than the expected amount and thus effectively represents an outlier, baring the possibility that the particular chemical species in question happens to be a pathological case within the framework of the benchmarked procedure, such an occurrence may well imply that one or more determinations governing the provenance of the corresponding ATcT values may be in error. Earlier, there were at least three cases where highly accurate FPD-type computations helped demonstrate that the accepted thermochemical value, based on apparently accurate experimental data, is in error (spectroscopic D 0 (OH) and the resulting D 0 (H-OH), with new experimental data using photoionization mass spectrometry augmented with high-level theory; 64,65 the enthalpy of formation of gas-phase hydrazine 135 and the enthalpy of formation of gas-phase oxalic acid, 136 both of which were originally based on experimental combustion calorimetry of the condensed phase combined with the corresponding vaporization enthalpy). However, the present study is the first case where we were able to demonstrate that a substantial number of ostensibly highly accurate theoretical results had, in common, a hidden systematic error.…”

Active Thermochemical Tables: The Partition Function of Hydroxymethyl (CH2OH) Revisited

“…Since ver. 1.122, , the ATcT TN has gone through a number of additions and expansions, − and the current version (1.122r) encompasses >2000 chemical species, interconnected by >25 000 experimental and theoretical determinations. For the sake of completeness, we report here some of the relevant thermochemical quantities produced using the most current ATcT TN (see Table ).…”