Institut für Physikalische Chemie, Georg-August-Universität Göttingen, Tammannstr. 6, 37077 Goettingen, Germany.
School of Physical and Chemical Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand.
Molecules. 2021 Dec 24;27(1):101. doi: 10.3390/molecules27010101.
Rotational microwave jet spectroscopy studies of the monoterpenol α-fenchol have so far failed to identify its second most stable torsional conformer, despite computational predictions that it is only very slightly higher in energy than the global minimum. Vibrational FTIR and Raman jet spectroscopy investigations reveal unusually complex OH and OD stretching spectra compared to other alcohols. Via modeling of the torsional states, observed spectral splittings are explained by delocalization of the hydroxy hydrogen atom through quantum tunneling between the two non-equivalent but accidentally near-degenerate conformers separated by a low and narrow barrier. The energy differences between the torsional states are determined to be only 16(1) and 7(1) cm-1hc for the protiated and deuterated alcohol, respectively, which further shrink to 9(1) and 3(1) cm-1hc upon OH or OD stretch excitation. Comparisons are made with the more strongly asymmetric monoterpenols borneol and isopinocampheol as well as with the symmetric, rapidly tunneling propargyl alcohol. In addition, the third-in contrast localized-torsional conformer and the most stable dimer are assigned for α-fenchol, as well as the two most stable dimers for propargyl alcohol.
目前，尽管计算预测其能量仅略高于全局最小值，但旋转微波喷射光谱研究仍未能确定单萜醇 α-葑醇的第二个最稳定扭转构象体。与其他醇相比，振动 FTIR 和拉曼喷射光谱研究揭示了异常复杂的 OH 和 OD 伸缩光谱。通过对扭转态的建模，观察到的光谱分裂通过羟基氢原子通过量子隧穿在两个非等效但偶然接近简并构象体之间的能量转移来解释，这两个构象体由一个低而窄的势垒隔开。确定了扭转态之间的能量差仅为 16(1)和 7(1)cm-1hc 对于氘代和未氘代醇，分别在 OH 或 OD 伸缩激发时进一步缩小至 9(1)和 3(1)cm-1hc。与更不对称的单萜醇龙脑和异龙脑醇以及对称、快速隧穿炔丙醇进行了比较。此外，为 α-葑醇分配了第三个局部化扭转构象体和最稳定二聚体，以及炔丙醇的两个最稳定二聚体。
