Multireaction Approach to Quantum Thermochemistry

Gas-phase enthalpies of formation are frequently obtained from electronic-structure calculations. For a compound containing elements that are not permanent gases, experimental data are needed to connect the ab initio energies with the elements in their standard states. Usually only one chemical reaction, such as atomization, is used to make this connection. Using a single reaction has the supposed advantage of producing a single value, but it remains challenging to estimate the associated uncertainty. Here, we propose using several chemical reactions instead of one. This yields several values for the target enthalpy of formation. The relative reliability of each reaction is estimated and used for weighted averaging of the corresponding enthalpies of formation. The resulting weighted mean is an estimate for the target quantity, and the standard deviation of the weighted mean is a reasonable estimate for the associated standard uncertainty. Two implementations are explored here and applied predictively to dihydrolevoglucosenone (cyrene), which has been proposed as a "green" polar aprotic solvent.