Thermochemistry of cyclopentadienylidene (c-C5H4, C2v, 3B1), cyclopentadienyl radical (c-C5H5•, C2v, 2B1) and 1,3-cyclopentadiene (c-C5H6, C2v, 1A1): A theoretical study by the G2M(RCC,MP2) method

Abstract

The thermochemistry of c-C5H4 (3B1), c-C5H5• and c-C5H6 was investigated at the G2M(RCC,MP2) level. The heat of formation (ΔH°f,298), ionization potential (IP), electron affinity (EA) and proton affinity (PA) are respectively, 125.1, 204.4, 42.5 and 230.7 kcal mol-1 for c-C5H4 (3B1), 63.5, 195.2, 44.9 and 197.2 kcal mol-1 for c-C5H5• and 32.1, 199.9, 26.0 and 195.2 kcal mol-1 for c-C5H6. The computed values for the last two molecules are in excellent agreement with available experimental data, with the errors of ±1 and ±2 kcal mol-1 for the heat of formation and the other parameters, respectively. In addition, the heat of formation of c-C5H4•- is evaluated as 82.0 kcal mol-1, about 10 kcal mol-1 higher than the experimental value. This leads to the suggestion that the experimental ΔH°f,298(c-C5H4•-) value needs to be re-examined. Copyright © 2001 John Wiley & Sons, Ltd.