A Hierarchy of Homodesmotic Reactions for Thermochemistry

Abstract

Chemical equations that balance bond types and atom hybridization to different degrees are often used in computational thermochemistry, for example, to increase accuracy when lower levels of theory are employed. We expose the widespread confusion over such classes of equations and demonstrate that the two most widely used definitions of “homodesmotic” reactions are not equivalent. New definitions are introduced and a consistent hierarchy of reaction classes (RC1 – RC5) for hydrocarbons is constructed: isogyric (RC1) ⊇ isodesmic (RC2) ⊇ hypohomodesmotic (RC3) ⊇ homodesmotic (RC4) ⊇ hyperhomodesmotic (RC5). Each of these successively conserves larger molecular fragments. The concept of isodesmic bond separation reactions is generalized to all classes in this hierarchy, providing a unique sectioning of a given molecule for each reaction type. Several ab initio and density functional methods are applied to the bond separation reactions of 38 hydrocarbons containing five or six carbon atoms. RC4 and RC5 reactions provide bond separation enthalpies with errors consistently less than 0.4 kcal mol−1 across a wide range of theoretical levels, performing significantly better than the other reaction types and far superior to atomization routes. Our recommended bond separation reactions were demonstrated by determining the enthalpies of formation (at 298 K) of 1,3,5-hexatriyne (163.7 ± 0.4 kcal mol−1), 1,3,5,7-octatetrayne (217.6 ± 0.6 kcal mol−1), the larger polyynes C10H2 through C26H2, and an infinite acetylenic carbon chain.

Enthalpy of Formation of the Cyclohexadienyl Radical and the C−H Bond Enthalpy of 1,4-Cyclohexadiene: An Experimental and Computational Re-Evaluation

“…Recently, Wheeler, Allen, and co-workers have examined these reaction schemes and defined a hierarchy of essentially convergent fragmentation reactions for closed-shell hydrocarbons. 55 While they suggest a practical scheme for homodesmotic and hyperhomodesmotic radical reactions, there may be many ways of constructing such reactions. Using conventions of closed-shell organic reactions, our predicted "homodesmotic" and "hyperhomodesmotic" ∆ f H 298.15°v alues for cyclohexadienyl differed from those of the isodesmic approaches by up to 17.6 kJ mol -1 !…”

“…33 Wheeler, Allen, and coauthors have found an average error of 2.5 kJ mol -1 relative to their post-CCSD(T) "focal point" model chemistry values for the BDEs of 1,3-cyclohexadiene and cyclopentadiene using an isodesmic scheme with CCSD(T)/cc-pVTZ computations. 55 Post-CCSD(T) electron correlation effects often cancel and quickly converge to a "full-CI" limit once connected quadruple excitations are incorporated into the wave function. This has been observed for isogyric reaction energies of smaller hydrocarbons, 57 as well as for atomization energies and proton affinities of small molecules.…”

Basis Set Effects in the Description of the Cl-O Bond in ClO and XClO Isomers (X=H,O,Cl) Using DFT and CCSD(T) Methods

“…In the case of direct calculations, high level molecular orbital methods (CCSD(T) usually) or density functional methods (DFT) are combined with isogiryc or isodesmic reactions [8] [9].…”

Thermochemical benchmarking of hydrocarbon bond separation reaction energies: Jacob's ladder is not reversed!

“…SCC try to separate inter-and intra-molecular dispersion by a double-damping formula and the inclusion of higher multipole-rank (R −8 , R −10 ) terms. In addition to vdW complexes it has been tested on four thermochemical benchmark sets: a modified version of IDHC77 [64] (the alkane folding reaction of C 30 H 62 is excluded and the dimerization of anthracene is added) and three test sets [65] consisting of isodesmic bond separation reactions (BSR) [66][67][68] of saturated hydrocarbons (corresponding to reaction class two according to the nomenclature of Schleyer and coworkers [69]). The latter three are subdivided into chains (H), rings (R) and cages (C) (for structures see Fig.…”