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, $$ \Updelta E = E_ 2 \textMI}}_\textax}} }} - E_ 2 \textMI}}_\texteq}} }} $$ , 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.
使用共振增强双光子电离光谱结合从头计算,研究了喷射冷却的
2-甲基茚满 (2MI) 和
2-苯基茚满 (2
PI) 的分子构象。已观察到 2MI 的轴向 (2MIax) 和赤道 (2MIeq) 构象异构体。在 298 K 时估计 2MIeq/2MIax 构象异构体比率为 2.3,导致能量差异 $$ \Updelta E = E_ 2 \textMI}}_\textax}} }} - E_ 2 \textMI}}_\texteq}} }} $$ ,0.49 kcal/mol。从头计算预测了 2
PI 的三种稳定构象异构体:两种赤道构象异构体(2
PIeq0 和 2
PIeq90)和一种轴向构象异构体(2
PIax)。实验仅观察到 2
PI (2
PIax) 的轴向构象异构体。由于 2
PIax 中的分子内 Csp2-H/π 相互作用,2
PIax 的
茚满环比 2
PI 的两个赤道构象异构体的
茚满环稍微更平坦。由于较高的预膨胀温度(383 K)和相对较低的轴-赤道互变势垒(1.68 kcal/mol),2
PI 的赤道构象异构体松弛为更稳定的轴向构象异构体。 2MI 中轴向-赤道互变的势垒 (2.33 kcal/mol) 足够高,足以防止在 298 K 预膨胀温度下发生构象松弛。发现分子内 C-H/π 相互作用对于确定构象偏好更为重要2
PI 优于 2MI;这可以归因于Csp2-H键的酸性高于Csp3-H键的酸性。