Thermodynamic Properties of Ketenes: Group Additivity Values from Quantum Chemical Calculations
journal contributionposted on 30.07.2002, 00:00 by Raman Sumathi, William H. Green
The geometries and thermodynamic properties (ΔHf298, S298, and Cp(T) (300−1500 K)) of 45 ketenes of four different types namely alkylketenes (set I), oxygenated alkylketenes (set II), 1,2- and 1,3-bisketenes (set III), and ketene-substituted alcohols (set IV) were determined by means of the CBS-Q procedure. The heats of formation of oxygenated ketenes were reevaluated using isodesmic reactions, and a systematic and consistent (∼1.5 kcal/mol) over prediction in the stability of tert-butyl substituted systems has been identified at the CBS-Q level. Nearly 20 new thermochemical groups for the ketene molecular family and 3 nonnext neighbor interaction terms were evaluated from the computed thermodynamic functions by simultaneous parameter estimation. The nonnext neighbor corrections account for an eclipsing interaction between the >CCO group and its β bonds. Statistical analysis of the ab initio data demonstrates the validity of the group additivity approximation. The derived group values describe the ab initio ΔHf298, S298, and Cp(300 K) with an average deviation of less than 0.06 kcal/mol, 0.15 cal/(mol K), and 0.18 cal/(mol K), respectively. In most cases, these quantum-calibrated group additivity estimates are expected to be more accurate than the limited experimental data and empirical estimates; the only significant discrepancy (∼1.5 kcal/mol) is for substituted ketenes containing a tert-butyl group.