First Principles Study of Role of Nuclear Quantum Effects on Properties of HIV/Cancer Inhibitor Ellipticine in protic solvents and Descriptors for Predicting Efficient Dye-sensitized Semiconductor Photocatalysts for Hydrogen Evolution Reaction

Abstract

The thesis is divided into two parts. In the first part of the thesis (Chapter 3), we have studied the effect of the quantum nature of the nuclei on the structural, dynamical and electronic properties of ellipticine, a plausible anti-HIV/cancer drug solvated in water. In Chapter 4 we have extended the study to two more protic solvents, namely methanol and ethylene glycol and investigated the role of nuclear quantum effects (NQEs) on the solvent-solute hydrogen bond (H-bond) and how these affect the H-bonding network of the solvent. Further, we have also discussed how the structural fluctuations induced by NQEs strongly affect the %it is to incorporate NQEs in the simulations of H bonded systems even when %we are interested in the optical properties of such H-bonded systems. To achieve this we have used the state-of-the-art path integral molecular dynamics simulations coupled with a recently proposed coloured noise thermostat based on the generalized Langevin equations. In the second part (Chapter 5), using density functional theory based calculations, we have proposed novel descriptors that can be used to screen materials for the design of more efficient dye-sensitized semiconductor photocatalyst for photocatalytic hydrogen evolution reaction.

The thesis is divided into six chapters.

In Chapter 1, we have discussed the importance of nuclear quantum effects and explained how path integral molecular dynamics (PIMD) simulations could incorporate nuclear quantum effects. We have also summarized the challenges of performing a straight forward PIMD simulation and discussed the recent developments of the computational techniques that can overcome some of these challenges. Moreover, we have also motivated the importance of photocatalytic water splitting in the context of the present day energy challenges. Further, the remaining chapters of the thesis have also been summarized.

The computational techniques, which are used in the thesis, are discussed in Chapter 2. We briefly discuss the basic principles of both classical and Born-Oppenheimer Molecular dynamics formalism (BOMD). The BOMD surfaces on which the ions move were derived using density functional theory (DFT). Hence in this chapter, we have provided a short overview of the theory and practical aspects of DFT. Further to incorporate the NQEs we have used the PIMD simulations with the coloured noise thermostat. Hence we have also described both of these. Additionally, the excited states to study optical properties are computed using time-dependent-DFT (TDDFT). This is also described in a nutshell in this chapter.

Chapter 3, we present the results of the study of NQEs influence the structural and electronic properties of ellipticine, a plausible anti-cancer/HIV inhibiting drug in a common biological solvent water. From the calculations, it has been shown that quantum effects collectively enhance the fluctuations of both light and heavy nuclei of the covalent and hydrogen bonds in ellipticine. In particular, for the ellipticine water system, where the proton donor and acceptor have different proton affinities, we find that nuclear quantum effects (NQEs) strengthen both the strong and the weak H bonds. This is in contrast to what is observed for the cases where the proton affinity of the donors and acceptors is same. These structural fluctuations cause a significant red shift in the absorption spectra and an increase in the broadening, bringing it into closer agreement with the experiments. This chapter shows that nuclear quantum effects alter both qualitatively and quantitatively the optical properties of this biologically relevant system and highlights the importance of the inclusion of these effects in the microscopic understanding of their optical properties.

Chapter 4: In this chapter, we extend the study of the importance of NQEs for the H-bonding of ellipticine to two more protic solvents (methanol and ethylene glycol) and investigate the generality of conclusions in the previous chapter. Additionally, we have also investigated the influence of the ellipticine-solvent H-bonding on the H-bonding network amongst the solvent molecules. We find that as we move from water to methanol to ethylene glycol, the strong H bond between pyridine N of ellipticine and solvent molecule weakens. In contrast with water, the simulations show that NQE weakens this H bond for methanol while for ethylene glycol NQEs do not significantly change the H bond strength. For the weak H bond between pyrrole N and solvent molecules, NQE strengthens the H-bond for methanol (similar to what was observed for water) and weakens for ethylene glycol. Additionally, we show that if solute-solvent H bond is strong, the H-bond between solvent molecules that are close to the solute is further strengthened. Finally, we also show that for H-bonded systems, it is crucial to incorporate the NQEs even for qualitative agreements between computed and experimentally measured absorption spectra.

Chapter 5: In this chapter, we have proposed descriptors that can be used for screening dyes-semiconductor composites, which can be used to design novel photocatalysts for hydrogen evolution reaction (HER). Specifically, we have shown that for ZnO nanoparticles sensitized with azo and thiophene-based dyes, the electron-hole separation and charge injection capacity together correlates well with the turnover number for hydrogen production from photocatalytic water splitting. Further based on a recipe proposed by Bahers et al. and Luca et al. We show that reasonably good estimates of these two properties can be obtained from simple ground state calculations.

Finally, we summarize the results presented in this thesis in Chapter6, provide a broad outlook of the same and discuss the future directions in which further work can be pursued.