No abstract

This is the first part of a series of articles reporting critically evaluated thermochemical properties of selected free radicals. The present article contains datasheets for 11 radicals: CH, CH 2 (triplet), CH 2 (singlet), CH 3 , CH 2 OH, CH 3 O, CH 3 CO, C 2 H 5 O, C 6 H 5 CH 2 , OH, and NH 2. The thermochemical properties discussed are the enthalpy of formation, as well as the heat capacity, integrated heat capacity, and entropy of the radicals. One distinguishing feature of the present evaluation is the systematic utilization of available kinetic, spectroscopic and ion thermochemical data as well as high-level theoretical results.

The ab initio harmonic and cubic force field of methane has been determined, both from Hartree–Fock and from highly correlated CEPA wavefunctions. The effect of correlation is small on the force field, particularly on the cubic constants. The ab initio cubic force constants were used to calculate the equilibrium CH bond length from the electron diffraction rg and from the spectroscopic r0 values. The weighted mean of the results is re=1.0862 ±0.0005 Å. There remains a small but disturbing discrepancy between the best ab initio and the present corrected experimental results, both for the bond length and for the CH stretching force constants.