Rotational spectrum of the 2-fluoroethanol---water complex:water tunneling dynamics and struture

Abstract

The hydrogen-bonded complexes between 2-fluoroethanol and water molecules have been investigated by using Fourier transform microwave spectroscopy and ab initio calculations. The computational and experimental theories are briefly described. The instrumentation and theoretical consideration about two primary spectrometers I used during the thesis studies, namely the cavity-based and the chirped-pulse Fourier transform microwave spectrometers, are discussed. The fluorinated ethanol and water cosolvent has been long studied due to its capability to alter the secondary and tertiary structures of the proteins and polypeptides. The mechanism is still mysterious but hydrogen bonds are believed to play an important role during this process. Thus it is worthwhile to develop a comprehensive molecular level understanding of the associated hydrogen-bonded complexes. The hydrogen-bonded topologies of the 2-fluoroethanol···water binary adduct have been probed from the rotational spectra in the frequency range of using the jet pulsed Fourier transform microwave spectrometers in combination with high level ab initio calculations. The assignments for the most stable 2-fluoroethanol···water complex along with its deuterium substituted isotopolugues have been achieved. The analyses of the spectroscopic date reveal the strong preference of the insertion geometry. Each rotational transition in the parent isotopologue is split into a doublet due to the tunneling motion of interchange in the hydrogen-bonded and non-bonded hydrogens of water. The tunneling assignment has been further proved by the intensity ratios as well as the isotopolugues. The tunneling has also been investigated through the transition state calculation and potential energy scan. The 2-fluoroethanol···(water)2 complexes have also been optimized and several true minima have been identified. The result of my studies confirms the insertion binding topology is favorable in the formation of monohydrate of 2-fluoroethanol. The same preference was reported for the 2,2,2-trifluoroethaonl···water.