Methods for critically assessing old and for estimating new organic gas-phase neutral and ion thermochemical data. A user's guide

A broad, general understanding of the relationships between chemical structures and their various energies is of primary importance when assessing the validity of an energy derived from experiment or computation. This review is intended to provide guidance and advice for scientists seeking data for standard enthalpies of formation (Delta H-f(0)) and ionisation energies (IE) for a wide variety of organic compounds and Delta H-f(0) values for their odd-and even-electron cations. The major reference sources are critically reviewed and methods are described for the accurate estimation of Delta H-f(0) of neutral compounds and correlation schemes for IE, proton affinities (PA) and Delta H-f(0) values for odd- and even-electron positive ions. Delta H-f(0) data for neutral organics are well reproduced by the additivity method and up-to-date tables of group values are provided, as well as directions for their application. Free radical Delta H-f(0) values are also reported. The situation for odd-electron cations (molecular ions) is not as simple, but reliable correlations exist between adiabatic ionisation energies (IEa) and molecular size, particularly for closely related species such as homologues, where IEa falls linearly with 1/n, n being the number of atoms. The same relationship obtains for PA. The Delta H-f(0) values for even-electron cations (ionised free radicals) also display useful correlations, chiefly based on the effects of substitution at a charge site, localised or delocalised. Here the relationship is exponential, with Delta H-f(0)(Ion) being a linear function of ln(n). A separate brief guide, intended for users of the NIST WebBook seeking molecular Delta H-f(0) values and IE, is appended.