H , F , Cl ] : basis set limit ab initio results and thermochemical analysis

The heats of formation of haloacetylenes are evaluated using the recent W1 and W2 ab initio computational thermochemistry methods. These calculations involve CCSD and CCSD(T) coupled cluster methods, basis sets of up to spdfgh quality, extrapolations to the one-particle basis set limit, and contributions of inner-shell correlation, scalar relativistic effects, and (where relevant) first-order spin-orbit coupling. The heats of formation determined using W2 theory are: ∆Hf 298(HCCH) = 54.48 kcal/mol, ∆Hf 298(HCCF) = 25.15 kcal/mol, ∆Hf 298(FCCF) = 1.38 kcal/mol, ∆Hf 298(HCCCl) = 54.83 kcal/mol, ∆Hf 298(ClCCCl) = 56.21 kcal/mol, and ∆Hf 298(FCCCl) = 28.47 kcal/mol. Enthalpies of hydrogenation and destabilization energies relative to acetylene were obtained at the W1 level of theory. So doing we find the following destabilization order for acetylenes: FCCF > ClCCF > HCCF > ClCCCl > HCCCl > HCCH. By a combination of W1 theory and isodesmic reactions,