Nuclear quantum effects in gas-phase systems with large amplitude motions: A study of 2-fluoroethanol, ethylene glycol, and H3O2

Abstract

This thesis explores the role of nuclear quantum effects in selected systems containing large amplitude motion through path integral simulations. Recent works have explored molecules with such floppy modes and examined how NQEs modulate the intramolecular interactions as well as their conformational free energy landscape. For instance, Sauceda et al. have examined how NQEs affect the torsional motions of toluene, aspirin, etc. and their structural and dynamical manifestations [5]. Bajaj et al. have analysed how NQEs can decrease the barrier of the bifurcation pathway in the iodide dihydrate complex and its H/D isotopomers, where the dangling and H-bonded hydrogens exchange places [6]. Mendez at al. have made similar observations in their simulations of the bifurcation pathway in H-bonded dimers, viz. water-water, water-ammonia, and water-methanol dimers [7-8]. Motivated by such studies, this thesis explores the manifestation of nuclear quantum effects in gas-phase 2-fluoroethanol (2FE), ethylene glycol (EG) and hydroxide hydrate, H3O−2. The first two of these contain weak intramolecular interactions whereas the third involves strong intramolecular H-bonding. Through the study of these systems with path integral simulations, we wish to explore how their structures and free energy landscapes in their large amplitude degrees of the freedom are affected by the NQEs. Chapter 1 introduces NQEs and motivates the importance of studying them. Chapter 2 provides an overview of the path integral simulation methodologies used in the subsequent chapters. Chapters 3 and 4 present a detailed investigation of the role of NQEs and tempera ture on the gas-phase structure and conformational dynamics of 2-fluoroethanol (2FE) and ethylene glycol (EG). These molecules belong to the set of 2-X-ethanols (X = F, Cl, OH, NH2) that have been extensively investigated with regards to their conformation populations and intramolecular interactions that stabilize the preferred XCCO gauche conformation [9-16]. The latter includes studies of the possibility of intramolecular X· · · H-O hydrogen bonding. In the present study, we focus on the NQEs and tempera ture effects in these molecules. Both have several stable conformers as a function of the XCCO and CCOH dihedrals, of which the G+g − and tG+g − are the most stable for 2FE and EG, respectively. The barriers to rotation are larger for the XCCO dihedrals than for the HOCC dihedrals in both molecules. We have attempted to understand how and why NQEs affect the free energy landscape in the space of dihedrals at different temper atures, the attendant structural changes and molecular spectra incorporating quantum effects. We have developed a full-dimensional anharmonic potential energy surface (FES) for both 2FE and EG at the MP2/aug-cc-pVTZ level of theory using the reaction surface Hamiltonian approach. Through PIMD simulation, we have analysed how the structural properties are modified vis-`a-vis classical simulations. At 300K, quantum and classical simulations indicate extensive confirmational sampling, while at 50 K the sampling is limited despite quantisation. We have also performed umbrella sampling using PIMD and classical MD to compute 1D free energy surface (FES) along the XCCO dihedral. Additionally for 2FE, we have obtained 2D FESs in the FCCO and CCOH space using well-tempered metadynamics simulations. We find that the barriers close to XCCO angles close to 0 are significantly reduced compared to other regions. Attempting to understand this, we have analysed the radius of gyration of the ring polymer as a function of the XCCO angle. We find the observed effect to be due to the increase in delocalization of the CH hydrogens and also the C and O atoms in this region, while the OH hydrogen plays only a small role. PIMD shows for EG at 50 K that there is an interesting role exchange for the two OH groups due to concerted dihedral flipping. We have also computed infrared and power spectra of 2FE and EG from quantum simulations and contrasted them with experimental spectra. In Chapter 5, we study some new aspects of NQEs in H3O − 2 . Like its counterpart H5O + 2 , a detailed study of this anion is important towards the understanding of the structure and dynamics in charged water clusters. On this account, several experimental and theoretical studies on H3O − 2 have been carried out over the decades, mostly focussing on the strong H-bonding, shared nature of the H-bonded H atom, and resulting spectroscopy of the associated red-shifted OH frequency [17-15]. Tuckerman et al. showed that although the minimum energy proton transfer path has a shallow double well and the classically simulated proton exhibits a double-well free energy profile, the ZPE along the shared proton coordinate washes out this barrier resulting in an average structure where proton is equally shared between the two O atoms [17]. Path integral studies by Tachikawa and coworkers have also observed this along with geometrical isotope effects on the equilibrium structure and the effect of temperature [22,24,25]. In this present work, we wish to understand the role of NQEs, temperature and isotope effects on the H-bond bifurcation pathway of H3O − 2 , where the shared proton and the dangling H atom exchange their positions. This process along with the proton transfer dynamics can lead to complete proton scrambling in this system. Towards this end, we have developed a potential energy surface using the sGDML (symmetric Gradient Domain Machine Learning) method [26-28] at the CCSD(T)/aug-cc-pVTZ level of theory. We have computed the free energy barrier along the bifurcation pathway using both PIMD and classical MD simulations. At 300K, we have observed that NQEs disfavour the bifurcation process due to the higher ZPE at the TS of the bifurcation path compared to the reactant and product states. However, at low temperatures of 50K and 30K, tunneling becomes the dominant pathway. The effective barrier is significantly reduced compared to the corresponding classical barriers as the anion enters a deep tunneling regime. In order to understand the role of the O-O motion on the bifurcation pathway, we have also computed 2D FES along ROO and bifurcation coordinates at low and high temperatures, where we observe the onset of corner cutting at low temperatures.