A theoretical re-evaluation of the heat of formation of phenylcarbene

Singlet-triplet separation and heat of formation of phenylcarbene were calculated by using the B3LYP, RCCSD(T), G2M, G3, CASSCF, CASPT2 and MRCI methods. Our calculated values are 2.5±1 kcal/mol for the singlet-triplet splitting energy, 113.5±1 kcal/mol for the standard heat of formation (ΔH_f298⁰) of singlet phenylcarbene, and 111.0±2 kcal/mol for the standard heat of formation of triplet phenylcarbene. While the singlet-triplet separation is in good agreement with previous theoretical and experimental data, the standard heat of formation of triplet phenylcarbene is about 7-8 kcal/mol higher than the experimental value (103.8±2.2 kcal/mol by Squires et al. [J. Chem. Soc., Perkin Trans. 2 (1999) 2249]).