Towards Electronic Scale Dynamics of Chemical Bonding in Isolated Molecules and Solvated Species
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The chemical bonding in molecules and molecular clusters is determined by the nature of the electron density distribution between the atoms. Over the past century, different theories such as Lewis’s bond, valence-bond theory, molecular orbital theory, and frontier molecular orbital theory, have explained static chemical bond in terms of electron density and their symmetry. Over the past century, “dynamic” nature of the chemical bond has been explored by monitoring the nuclear dynamics in femtosecond and picosecond time regimes. In my thesis work, mostly a computational attempt has been made to explore electronic-scale dynamic nature of chemical bonding in isolated molecules and molecules in solvent environment. As electron density can move over a few angström distance within attosecond time scale, dynamic nature of chemical bonding can be named as attochemistry. To explore attochemistry, vertical ionization scheme has been adopted along with the time-dependent NBO analysis.1 Our discussion of the “dynamic” nature of the chemical bonding must begin with an intriguing comparative study of the potential-energy curves for neutral H2 and cationic H2+, and analogous neutral Li2 and cationic Li2+ species. These curves are well-documented in the literature at different levels of theory. Note that neutral H2 exhibits stronger bonding than neutral Li2 (the potential well of neutral H2 is quite deeper than that of neutral Li2). On the other hand, respective cationic potential energy curves evidence that while the H-H bond becomes weaker following ionization, the same leads to strengthening of the Li-Li bond, revealing a different fate for the adiabatic ions of the respective species. Here, one may ask an important question very relevant to the subject of the present thesis, “Does the vertical ion incorporate the electronic-scale change in the chemical bonding which ultimately dictates the fate of the adiabatic ion?” Characters of the NBOs are determined from the natural hybrid orbitals (NHOs) shows that neutral σH-H is formed by the overlap of two pure 1s orbitals (NBO analysis predicts more than 99% 1s contribution). Similarly, σLi-Li bond also originates from overlap between (almost) pure 2s orbitals (NBO analysis predicts more than 95% 2s contribution). The vertical ionization of the H2 molecule removes an electron from bonding orbital . NBO analysis again predicts more than 99% contribution of the 1s orbital to singly occupied orbital (with α spin) in the vertical H2+ ion. This makes the H-H bond strength weaker upon vertical ionization and as a result, the H-H bond distance of the adiabatic H2+ ion becomes considerably longer than that of the neutral H2 species. Compared with the neutral Li2, the bonding hybrids of the vertical Li2+ ion develops significant sp-hybrid character, resulting in much more directional nature. The wB97xD/6-311+G(d,p) level of theory predicts that bonding NHOs of the vertical Li2+ ion to be of sp0.25 form (with more than 19% p character). In contrast, the corresponding hybridization in neutral Li2 is sp0.05 (with less than 5% p character). This enhanced p-character in NHOs of the vertical Li2+ ion also leads to greater inter-nuclear separation than that of neutral Li2 at the equilibrium geometry. Thus, above comparison of the chemical bonding in Li2 and Li2+ shows that strength of the chemical bond does not merely depend on the number of electrons in the bonding NBO (bond order); rather, it depends on the difference in the bonding hybrids, particularly with regard to the directional p-character. Similarly using NBO fate of a noncovalent bond can be predicted from NBO analysis. For the present discussion, CO:H2O hydrogen bonded complex is taken as an example. The highest E(2) value for the CO:H2O noncovalently bonded complex can be seen (increased after ionization), which also shows the NBOs responsible for the relevant charge transfer. The nC to σOH* charge transfer dominantly contributes to the O-C…..H noncovalent bonding interaction. NBO analysis suggests that the vertical ionization-induced change of the chemical bonding in the Li2 species, in comparison with the H2 species, occurs at the electronic level without involving any nuclear movement. This electronic change of chemical bonding in the vertical ion dictates the fate (structure and energetics) of the adiabatic ion of the respective species. Using the scheme of the quantum dynamics simulation, one can simulate the temporal evolution of the change of chemical bonding in Li2 species following the vertical ionization scheme. Clearly, time dependent NBO analysis predicts extremely fast periodic expansion and contraction of the electron density in the inter-atomic space following the vertical ionization. This periodic movement represents the initial step of the ionization-induced re-hybridization dynamics of the Li-Li σ-chemical bond. No experiment with attosecond metrology has been performed thus far to unravel this re-hybridization dynamics. Similarly, a systematic theoretical investigation of the attochemistry of solvated molecules would help one design attosecond experiments under ambient conditions to explore the attochemistry in a liquid environment. With this goal in mind, for the first time, we have explored the attochemistry of molecules surrounded by different non-polar solvent environments. To model solvation effects on the attochemistry of molecules containing gold–chalcogen linkages, we have used an implicit solvent model (Polarizable Continuum Model) under the density functional theory (DFT) formalism for non-polar solvents. We have found that the charge migration time scale in molecules becomes faster in the presence of the solvent environment as compared to that under vacuum. Charge oscillation does not damp quickly in molecules surrounded by the solvent environment as compared to that under vacuum. Furthermore, the direction of the charge migration may change in molecules when they are surrounded by the solvent environment as compared to that under vacuum. Thus, the present work has laid the foundation, for the first time, for thinking of the attochemistry into the realm of liquids.2 In addition to the above-mentioned computational efforts, certain preliminary experimental efforts have also been undertaken. Two important steps towards exploring non-equilibrium solvation effect on attosecond chemical bonding are to prepare a suitable model sample and activate liquid beam inside a vacuum. In this regard, an experimental attempt has been made to carry out laser ablation in liquid to synthesize metal-carbyne3 system which can be a good model system for exploring the attosecond non-equilibrium solvation. Finally liquid beam nozzle has been constructed to activate liquid jet inside the vacuum. High harmonic generation spectroscopy is a self-probing spectroscopy which can probe the evolving attosecond electronic structure following ionization of a species. Hence, dependency of the HHG spectrum on the direction of recombination of the ejected electron has been explored.4 In the present contribution, taking the ion, atoms, and molecule as examples, we show that V(x) can be constructed from one-dimensional molecular electrostatic potential (MEP) of the respective cation to access theoretical HHG spectra not only of simple atoms but also of multielectron complex molecules. We have shown that the MEP-based formalism not only successfully reproduces the ionization energy of the molecular system but also gives explanation for the orientation-dependent HHG spectral intensity change in terms of the nodal plane of the molecular orbital from where the electron is removed during the HHG process. Further extension of the present one-dimensional model to the two- or three-dimensional potentials is expected to address many interesting and practical issues such as quantum interference and effects of different shapes of orbitals. We intend to explore those effects in the future. In brief this thesis contributes to our fundamental understanding of electronic scale dynamics of chemical bonding. Our hitherto-taken attempts and the progresses in exploring electronic-scale dynamics of chemical bonding are discussed in the following chapters.