Computational investigation of the photochemistry and spectroscopy of cyclic aromatic hydrocarbons in interstellar ice analogs
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This thesis describes the photochemistry and ultraviolet (UV) spectroscopy of cyclic aromatic hydrocarbons such as benzene and naphthalene, along with small water clusters and crystalline water ice clusters. Firstly, benzene and naphthalene interactions with small water hexamer (W6) clusters, and then benzene interactions with crystalline water ice clusters are investigated. This thesis primarily focuses on the applications of a range of computational chemistry techniques to investigate and characterize excited states of these complex systems, which are generated following one-photon absorption. Benzene and naphthalene, as prototypical polycyclic aromatic hydrocarbons (PAHs), and water and crystalline ice clusters, taken as representative of interstellar ices, could also be considered as useful model systems to replicate polycyclic aromatic hydrocarbons (PAHs) in interstellar ices, and to study their behaviour under UV processing. From coupled cluster (CC) benchmark studies on small water clusters up to water the pentamer, it is shown that that highly correlated linear-response coupled cluster methods such as CCSD and CC3 are important to consider while studying electronic excitations, as electron correlation effects play an important role in such systems, with double excitations playing a dominant role. However, triple excitations contributions calculated are negligible with CCSD and CC3 methods converging monotonically to similar results. The aggregation effect on water at CCSD level has shown a blue shift of ~ 0.7 eV in the central water molecule of water pentamer (C2v) relative to water monomer (C2v), and is in good agreement with the experimental blue shift of ~ 1 eV in condensed phase. For both benzene- and naphthalene-bound water W6 clusters, we have calculated interesting features of benzene- and naphthalene-mediated electronic excitations of the water W6 cluster at wavelengths where photon absorption cross section of water is negligible i.e., above 170 nm. These excitations were originally absent in the isolated water W6 cluster. Similar features are calculated for benzene-bound crystalline ice clusters, which also illustrate the effect of cyclic aromatic hydrocarbons on electronic excitations of ice clusters, and are also observed experimentally. The brightest 𝜋 → 𝜋∗ electronic transition of benzene and naphthalene is calculated to be red-shifted in wavelength and occurs with lower intensities after interacting with the water W6 and ice clusters. The degeneracy of this transition is also slightly broken in benzene. We have observed new electronic transition features such as charge transfer (CT), and locally diffuse Rydberg type excitation in these complexes. We have found a good performance of hybrid DFT functionals i.e. M06-2X and CAM-B3LYP in calculating vertical excitation energies of these complexes using time dependent density functional theory (TD-DFT). Further, diffusion studies of the deuterium (D) atom have shown the importance of surface morphology in generating different potential sites and hopping characteristics of the D atom on crystalline and amorphous ice surfaces. D2 formation is found to be efficient on the amorphous ice surface, with longer residence times of the D atom indicating a possibility of the deuterium atom getting trapped in such sites. There is then a further possibility of the diffusing D atom to recombine with the trapped D atom to form a D2 molecule. However, such D atom trapping is a rare possibility on crystalline surface, as hopping is fast and thus the recombination process is not efficient on crystalline ice surface.