C90 temperature effects on relative stabilities of the IPR isomers Original Research Article

The complete set of 46 isolated-pentagon-rule (IPR) isomers of C90 is treated by the SAMI quantum-chemical method (Semi-Ab-Initio Model 1), and their energetics is also checked by ab initio SCF computations (HF/3-21G, HF/4-31G) and the AM1 and PM3 semiempirical methods. All the methods point out a C2 species as the system state (with an exception at the HF/3-21G level). However, the energetics itself is not able to produce a good agreement with recent observations (one C2v, three C2, and one C1, separeted into three HPLC fractions). The symmetries of the five SAM1 lowest-energy structures are: C2, C2v, Cs, D5h, C1. In order to respect temperature effects on relative stabilities, entropy contributions are also computed and significant changes are found. The symmetries of the five structures most populated in a high-temperature region are according to the SAM1 computations: two times C2, Cs, C2v, C1. One of the recently reported HPLC fractions shows 70 lines in the 13C NMR spectrum, assigned to a C2 species (45 lines) and C2v species (25 lines, 5 weaker). There is however an alternative interpretation of the spectrum: a Cs (46 lines, 2 weaker) and a C2v species (24 lines, 3 weaker). The last two structures are indeed present in the SAM1 high-temperature stability set so that agreement between observations and calculations can be achieved.