| dc.description.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. | en_US |
