Alternating 1‑Phenyl-2,2,2-Trifluoroethanol Conformational Landscape With the Addition of One Water: Conformations and Large Amplitude Motions

The 1:1 adduct of 1-phenyl-2,2,2-trifluoroethanol (PhTFE),
a chiral
fluoroalcohol, with water was investigated using chirped pulse Fourier
transform microwave spectroscopy and computational methods. While
PhTFE itself was predicted to have three minima, I (gauche+), II (trans), and III (gauche−), only I and II were stable and only I was observed
experimentally. A systematic search of the PhTFE···H2O conformational landscape identified 110 stable minima, 14
of which are within a 15 kJ mol–1 energy window.
Rotational spectra of the two PhTFE···H2O conformers along with several deuterium and 18O isotopologues
were assigned, and the isotopic data were used to verify the corresponding
structures. In the two observed monohydrate conformers, one contains
PhTFE I where the water subunit is inserted into the
existing intramolecular OH···F contact of I, and the binary adduct is stabilized by two intermolecular contacts:
OH···OW and HW···F,
whereas the other contains PhTFE II where the water subunit
interacts with both the alcohol hydrogen and phenyl ring of II, demonstrating that interaction with water sufficiently
stabilizes II for its observation in a jet expansion.
Interestingly, the predicted electric dipole moment components at
the identified minima deviate considerably from the experimental ones.
Such deviations were analyzed in terms of dynamic effects associated
with the large amplitude motions of the unbound HW. In
addition, tunnelling effects associated with the exchange of the bonded
and nonbonded HW were also discussed.

本研究采用啁啾脉冲傅里叶变换微波光谱学和计算方法，对1-苯基-2,2,2-三氟乙醇（PhTFE）这一手性氟醇与水形成的1:1加合物进行了系统研究。尽管预测PhTFE本身存在三个极小值（I（顺式+）、II（反式）和III（顺式−）），但仅I和II是稳定的，且仅I在实验中被观察到。对PhTFE···H2O构象空间的系统搜索识别出110个稳定极小值，其中14个位于15 kJ mol–1的能量窗口内。对两种PhTFE···H2O构象体及其多个氘代和18O同位素异构体的转动光谱进行了指认，并利用同位素数据验证了相应的结构。在观察到的两种单水合物构象体中，一种包含PhTFE I，其中水分子插入到I中已存在的分子内OH···F接触，二元加合物通过两个分子间接触（OH···OW和HW···F）得到稳定；另一种包含PhTFE II，其中水分子与II中的醇氢和苯环相互作用，证明了与水的相互作用足以稳定II，使其在喷射膨胀中可被观察到。有趣的是，在确定的极小值处的预测偶极矩分量与实验值存在显著偏差。这些偏差从与未结合HW的大幅度运动相关的动态效应进行了分析。此外，还讨论了与键合和非键合HW交换相关的隧穿效应。