C-90 TEMPERATURE EFFECTS ON RELATIVE STABILITIES OF THE IPR ISOMERS

The complete set of 46 isolated-pentagon-rule (IPR) isomers of C-90 is treated by the SAM1 quantum-chemical method (Semi-Ab-Initio Model 1), and their energetics is also checked by ab initio SCF computations (H F/3-21G, HF/4-31G) and the AM1 and PM3 semiempirical methods. All the methods point out a C-2 species as the system ground state (with an ex ception at the HF/3-21G level). However, the energetics itself is not able to produce a good agreement with recent observations (one C-2 ups ilon, three C-2, and one C-1, separated into three HPLC fractions). Th e symmetries of the five SAM1 lowest-energy structures are: C-2, C-2 u psilon, C-s, D-5h, C-1. In order to respect temperature effects on rel ative stabilities, entropy contributions are also computed and signifi cant changes are found. The symmetries of the five structures most pop ulated in a high-temperature region are according to the SAM1 computat ions: two times C-2, C-s, C-2 upsilon, C-1. One of the recently report ed HPLC fractions shows 70 lines in the C-13 NMR spectrum, assigned to a C-2 species (45 lines) and C-2 upsilon species (25 lines, 5 weaker) . There is however an alternative interpretation of the spectrum: a C- s species (46 lines, 2 weaker) and a C-2 upsilon species (24 lines, 3 weaker). The last two structures are indeed present in the SAM1 high-t emperature stability set so that agreement between observations and ca lculations can be achieved. (C) 1997 Published by Elsevier Science B.V .