Determination of enthalpies (‘Heats’) of formation

Abstract Determination of enthalpies of formation, now well into its second century, continues to be an active research field. Classical combustion thermochemistry, known by Lavoisier, is carried out with precision in several laboratories, though usually on the microscale, as appropriate to the small quantities of rare or unstable species preparative chemists are able to win and purify. Nonclassical methods such as differential scanning calorimetry and proton emission techniques are practiced. Enthalpy estimation based on additivity has been brought to an improved level of accuracy, and its basis in molecular structure has been examined with the goal of achieving maximum simplicity. Discrepancies between experimental results and additive estimates due to ‘special effects’ have brought about a considerable amount of causative speculation in the literature. Quantum mechanical methods have enjoyed increased proliferation through new methods of finding enthalpies of formation and other thermochemical and molecular properties such as heat capacity and entropy. Powerful basis set and configuration interaction software is available within the Gaussian© suites of programs. New levels of accuracy, in the kilojoules per mole range, have been achieved by Wn methods, and wider generality is enjoyed by methods based on density functional theory. New tabulation methods have been introduced that use computer error estimation procedures to root out flawed experimental results and increase overall reliability of the data one selects from the compilation. This article is categorized under: Structure and Mechanism > Molecular Structures