Conformational adaptation and large amplitude motions of 1-phenyl-2,2,2-trifluoroethanol with two water molecules: a rotational spectroscopic and investigation

The 1 : 2 adduct of 1-phenyl-2,2,2-trifluoroethanol (PhTFE), a chiral fluoroalcohol, with two water molecules (PhTFE 2H 2 O) was investigated via chirped pulse Fourier-transform microwave (CP-FTMW) spectroscopy and theoretical calculations. A systematic search of the PhTFE 2H 2 O conformational landscape identified 38 stable minima at the B3LYP-D3BJ/def2-TZVPPD level of theory, 27 of which are within an energy window of 10 kJ mol −1 after applying zero-point energy corrections. Rotational spectra of a single PhTFE 2H 2 O conformer along with eight deuterated and three oxygen-18 isotopologues were assigned. Interestingly, the observed PhTFE 2H 2 O conformer contains PhTFE II , the second most stable monomer conformer, and the most stable PhTFE I dihydrate is ca. 4 kJ mol −1 higher in energy. In contrast, PhTFE I H 2 O was identified experimentally and theoretically as the most stable 1 : 1 conformer. Furthermore, the observed dihydrate structure experiences large amplitude motions connecting three theoretical minima which differ only in which water oxygen lone pairs are involved in the hydrogen-bonds, i.e. , the free OH pointing directions. Additionally, the ortho and para -H 2 O tunnelling splittings were detected and attributed to the interchange water hydrogen atoms which interact with the aromatic part of PhTFE but not for the water interacting with PhTFE hydroxy group. Extensive theoretical modelling was carried out to gain insight into the associated large amplitude motions including tunnelling, supported by the experimental isotopic and tunnelling splitting data. The conformational dynamics of 1-phenyl-2,2,2-trifluoroethanol water 2 was explored with rotational spectroscopy and extensive theoretical modelling which reveal the interplay of several large amplitude motions including water tunnelling.