Thermochemistry of key soot formation intermediates:: C3H3 isomers

Accurate standard enthalpies of formation for allene, propyne, and four C(3)H(3) isomers involved in soot formation mechanisms have been determined through systematic focal point extrapolations of ab initio energies. Auxiliary corrections have been applied for anharmonic zero-point vibrational energy, core electron correlation, the diagonal Born-Oppenheimer correction (DBOC), and scalar relativistic effects. Electron correlation has been accounted for via second-order Z-averaged perturbation theory (ZAPT2) and primarily through coupled-cluster theory, including single, double, and triple excitations, as well as a perturbative treatment of connected quadruple excitations [ROCCSD, ROCCSD( T), ROCCSDT, and UCCSDT( Q)]. The correlation-consistent hierarchy of basis sets, cc-pVXZ (X = D, T, Q, 5, 6), was employed. The CCSDT( Q) corrections do not exceed 0.12 kcal mol(-1) for the relative energies of the systems considered here, indicating a high degree of electron correlation convergence in the present results. Our recommended values for the enthalpies of formation are as follows: Delta(f)H(0)(o) (propargyl)) 84.76, Delta(f)H(0)(o) (1-propynyl)) 126.60, Delta(f)H(0)(o) (cycloprop-1-enyl) = 126.28, Delta(f)H(0)(o) (cycloprop-2-enyl) = 117.36, Delta(f)H(0)(o) ( allene) = 47.41, and Delta(f)H(0)(o) (propyne) = 46.33 kcal mol(-1), with estimated errors no larger than 0.3 kcal mol(-1). The corresponding C(3)H(3) isomerization energies are about 1 kcal mol(-1) larger than previous coupled-cluster results and several kcal mol(-1) below those previously obtained using density functional theory.