The heats of formation of the haloacetylenes XCCY [X, Y = H, F, Cl]: basis set limitab initioresults and thermochemical analysis

Abstract

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: ∆H f 298 (HCCH) = 54.48 kcal/mol, ∆H f 298 (HCCF) = 25.15 kcal/mol, ∆H f 298 (FCCF) = 1.38 kcal/mol, ∆H f 298 (HCCCl) = 54.83 kcal/mol, ∆H f 298 (ClCCCl) = 56.21 kcal/mol, and ∆H f 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, we show that the generally accepted heat of formation of 1,2-dichloroethane should be revised to -31.8±0.6 kcal/mol, in excellent agreement with a very recent critically evaluated review. The performance of compound thermochemistry schemes such as G2, G3, G3X and CBS-QB3 theories has been analyzed.

Understanding the Complex Dissociation Dynamics of Energy Selected Dichloroethylene Ions: Neutral Isomerization Energies and Heats of Formation by Imaging Photoelectron−Photoion Coincidence

“…Taking the average 0 K onset energy of 12.083 eV (1165.8 kJ mol −1 ), average Δ f H 0K (C 2 H 2 Cl 2 ) = 2.1 kJ mol −1 , together with the 0 K heat of formation of Cl yields Δ f H 0K (C 2 H 2 Cl + ) = 1048.3 kJ mol −1 . Together with the 0 K heat of formation of HCCCl of 228.8 ± 2 kJ mol −1 , this yields a 0 K proton affinity for chloroacetylene of 708.6 ± 2.5 kJ mol −1 , in excellent agreement with the results of Rodriquez et al of 707 kJ mol −1 .…”

A Hierarchy of Homodesmotic Reactions for Thermochemistry

“…Bracketed numbers are the result of basis set extrapolations (using the fits denoted in the table), while unbracketed numbers were explicitly computed. All energies evaluated at frozen-core CCSD(T)/TZ(2d1f,2p1d) optimized geometries. b Reference enthalpies of formation (kcal mol −1 ) were as follows: ${\mathrm{normal\Delta }}_f{H}_0^{\circ }$ (acetylene) = 54.69 ± 0.07 (ref 16); ${\mathrm{normal\Delta }}_f{H}_{298}^{\circ }$ (acetylene) = 54.52 ± 0.07 (refs 16 and 165); ${\mathrm{normal\Delta }}_f{H}_0^{\circ }$ (diacetylene) = 109.4 ± 0.3 (ref 76); ${\mathrm{normal\Delta }}_f{H}_{298}^{\circ }$ (diacetylene) = 109.7 ± 0.3 (ref 76) …”

Interplay of Thermochemistry and Structural Chemistry, the Journal (Volume 14, 2003) and the Discipline

“…The experimental thermochemistry of all of these species is unreliable, as clarified by considering high-level post Hartree-Fock mathematical theory that has been useful for understanding the energetics of the fluoro and chloro compounds [34]. Even so, the related halogenated di and polyacetylenes' thermochemistry remains unstudied by experimentalists.…”