A computational treatment of 35 IPR isomers of C-88

The complete sets of 35 isolated-pentagon-rule (IPR) isomers of Css is desc ribed by the SAM1 (Semi-Ab-Initio Model 1) quantum-chemical method. The sep aration energetics is also computed at the HF/STO-3G, HF/3-21G, and HF/4-31 G levels. The SAM1 and HF/4-31G data mostly agree within a few kJ/mol. As t he SAM1 energetics does not reproduce the recent NMR observations, entropy contributions are included, too, being based on the harmonic-oscillator and rigid-rotator model. Considerable temperature effects on the relative stab ilities in the system are found. The ground-state structure of C-88 is a C- s isomer, however, with an increase of temperature a C-2 structure becomes important. At still higher temperatures a near C-2 species is dominant. The results can be viewed as a good agreement with the available observations, and they further expand the family of the IPR sets where the thermodynamic equilibrium treatment allows for a satisfactory support of observations.