Abstract
The large molecules 1โ3 (69, 90, and 102 atoms, respectively), prepared by cyclotrimerization of enantiomerically pure derivatives of (โ)โbornyl acetate, show intense ECD spectra, high optical rotation (OR) values (200โ1300, in absolute value) dominated in sign and order of magnitude by the lowestโenergy Cotton effects, that is, they are the ideal candidates to test the reliability of our โapproximateโ (TDDFT/B3LYP/6โ31G* or smaller basis set) approach to the calculation of chiroptical properties. As a matter of fact, a correct simulation of the OR values and ECD spectra of 1 and 2 can be obtained even using STOโ3G basis set and semiempirical or molecular mechanics input geometries: for 1, at the TDDFT/B3LYP/STOโ3G level, the OR values are of the order of 500โ550, versus an experimental value ranging between 660 and 690, depending on the solvent. On the contrary, the case of 3 (exp. OR between โ1330 and โ1500) is really complex (for instance, the OR values range between โ3216 and โ729 (TDDFT/B3LYP/6โ31G* calculations) or โ1824 and โ444 (TDDFT/B3LYP/STOโ3G calculations)), making the comparison between calculated and experimental values more difficult. The behavior of 3 is due to its molecular flexibility, whereas 1 is a really rigid molecules and 2 behaves (vide infra) as it were a rigid system. These observations strongly indicate that the conformational freedom constitutes one of the major difficulties for a correct but simple simulation of the chiroptical properties. Chirality 21:E86โE97, 2009. ยฉ 2009 WileyโLiss, Inc.