Molecular conformations of jet-cooled 2-methylindan and 2-phenylindan

Abstract

The molecular conformations of jet-cooled 2-methylindan (2MI) and 2-phenylindan (2PI) have been studied using resonant-enhanced two-photon ionization spectroscopy in combination with ab initio calculations. Both axial (2MIax) and equatorial (2MIeq) conformers of 2MI have been observed. A 2MIeq/2MIax conformer ratio of 2.3 was estimated at 298 K, leading to the energy difference, $$ \Updelta E = E_{{ 2 {\text{MI}}_{\text{ax}} }} - E_{{ 2 {\text{MI}}_{\text{eq}} }} $$ , of 0.49 kcal/mol. Ab initio calculations predicted three stable conformers of 2PI: two equatorial conformers (2PIeq0 and 2PIeq90), and one axial conformer (2PIax). Only the axial conformer of 2PI (2PIax) was experimentally observed. The indan ring of 2PIax is slightly more planar than the indan rings of the two equatorial conformers of 2PI because of the intramolecular Csp2–H/π interactions in 2PIax. The equatorial conformers of 2PI relax to the more stable axial conformer because of the high pre-expansion temperature (383 K), and relatively low barrier (1.68 kcal/mol) to axial–equatorial interconversion. The barrier (2.33 kcal/mol) to axial–equatorial interconversion in 2MI is high enough to prevent conformational relaxation at the pre-expansion temperature of 298 K. Intramolecular C–H/π interactions are found to be more important in determining the conformational preference of 2PI than 2MI; this can be attributed to the higher acidity of the Csp2–H bond than that of Csp3–H bond.