High Resolution Rotational Spectroscopic Studies of the Dynamics and Conformations of Dispersion and Hydrogen Bonded Molecular Complexes
Creators: Wu, Bowei 
Publisher: name: University of Alberta Library
Year: 2022

<p>High resolution rotational spectra of non-covalently bonded molecular clusters produced in a supersonic jet expansion can provide detailed and rich information about their structure-energy relationship and dynamical behaviors. In this thesis, several non-covalently bonded molecular clusters which contain water and/or fluoroalcohols, two important types of solvents, were investigated using a cavity based and two chirped pulse (CP) Fourier transform microwave (FTMW) spectrometers. The experimental spectroscopic studies were aided by high level density-functional theory (DFT) calculations to reveal the complex conformational landscapes of the systems and to understand the effects of large amplitude motions in these systems. The specific results and conclusions of these systems are summarized below. <br>
For pyrrole···water, we applied a recently developed semiempirical tight-binding quantum chemistry conformational searching program and DFT calculations to identify possible low energy conformers of the pyrrole-water complex which feature σ- and π-type hydrogen-bonding interactions. Rotational spectra of all monosubstituted 13C isotopologues of pyrrole-water in their natural abundances were measured and assigned, including the 14N nuclear quadrupole hyperfine splittings and water tunneling doublets. Moreover, the potential energy surface (PES) scans along the water-centered large amplitude motions were also carried out and the interconversion paths among the four previously proposed pyrrole-water geometries identified. The theoretical investigation together with the additional 13C experimental data allow one to confidently identify the ground state global minimum geometry and also estimate the water tunneling barrier. <br>
Two other water-containing complexes, i.e., 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP)···water 2,2,3,3,3-pentafluoro-1-propanol (PFP)···water were also investigated using rotational spectroscopy. In the first case, a previously unidentified, higher energy conformer, namely gauche HFIP (HFIPg)···water was recognized. To confirm the identity of this new isomer, rotational spectra of seven of its deuterated species were also measured and analyzed. Both the experimental and theoretical pieces of evidence indicate that the intermolecular interaction with water preferentially stabilizes the HFIPg monomer configuration over the global minimum configuration, HFIPt. In the second case, rotational spectra of two PFP∙∙∙water conformers and seven deuterated isotopologues of each, were assigned. Tunneling splittings were observed for both conformers and are attributed to the exchange of the bonded and non-bonded hydrogen atoms of water. Overall, these fluoroalcohol monohydrates exhibit even more complicated conformational landscapes and dynamics of water-centered large amplitude motions than pyrrole···water. The large amplitude motions associated with the water subunits were examined in detail to explain the very different magnitudes of the experimental and theoretical permanent electric dipole moment components. The studies highlight the challenge in correctly identifying the conformers observed when large amplitude motions are involved. <br>
The complexity of the conformational landscape increases noticeably when moving onto the PFP dimer. Nearly 80 stable, binary PFP geometries were identified in the theoretical search, highlighting the structural diversity of the system. For the corresponding PFP monomer, the two most stable monomeric PFP mirror imaged pairs, were observed and the associated large tunneling splittings in the rotational spectrum were analyzed in detail. The five most stable binary conformers predicted were also the ones observed experimentally. A combined kinetic and thermodynamic model was proposed to explain the observation or non-observation of low energy conformers, and the analysis was further verified by the ‘argon test’. The non-covalent intermolecular interactions of PFP and its binary conformers are also discussed with the aid of quantum theory of atoms in molecule (QTAIM) and non-covalent interaction (NCI) analyses, as well as the effects of fluorination by comparing to 1-propanol and its dimers.<br>
While the above molecular systems are mainly bound by hydrogen bonding interactions, we also investigated the HFIP···Ne and Ar binary complexes where the non-covalent intermolecular interactions are dominated by dispersion interactions. Rotational spectra of these two complexes including several isotopic species containing 20Ne, 22Ne, 40Ar, 13C, and hydroxyl D, were assigned. While multiple conformers were often detected in the hydrogen bonded systems reported above, only one isomer was detected for each species here, corresponding to the most stable one predicted. The QTAIM and electrostatic potential (ESP) calculations were employed to examine different rare gas binding sites at HFIP and the influence of fluorination since isopropanol···Ne and ···Ar, the non-fluorinated counter parts, exhibit much different binding topologies.</p>