Spectroscopy, the response of matter to electromagnetic radiation of different wavelengths, is a powerful experimental tool for interrogating a molecule’s structure and dynamics as it interacts with its environment. However, relating a spectroscopic signature to a molecular picture relies on sophisticated computational approaches in order to identify structures, intermolecular interactions, and their correlation with spectroscopic response. This thesis focuses on the question of how to correlate a molecule’s structure and interactions with its environment via the ab initio calculation of spectroscopic parameters.
To build a molecular picture of CO2 dynamics in ionic liquids (ILs), I performed quantum chemical calculations on small gas-phase CO 2-IL clusters, qualitatively reproducing the experimental ordering for CO2’s asymmetric vibrational stretch (∣3) peak position as a function of the anion. To uncover the physical origin of the shift, the language of decomposition analysis based on absolutely localized molecular orbitals (ALMO-EDA) was translated from energies to vibrational frequencies. Geometric distortion of CO2, as a result of charge transfer (CT) from the anion into the CO2, is the driving force for differentiating the CO2 v3 shift in different IL anions.
After validating these simple models, I further decomposed the CT contribution into equilibrium structure and potential energy surface curvature mechanisms, finding that CT is a significant contributor in both the geometry optimization and frequency calculation steps. Comparing ALMO-EDA and symmetry-adapted perturbation theory (SAPT) showed that while dispersion dominates the binding energy, DFT-based ALMO-EDA showed excellent correlation with wavefunction-based SAPT, which enabled the construction of a spectroscopic map based on chemically-intuitive descriptors at lower cost.
This work presents the first application of ALMO-EDA to construct complex spectroscopic maps, however ALMO-EDA is not generally applicable to arbitrary spectroscopies. I reformulated the canonical linear response equations for use with ALMOs to provide a direct connection between EDA terms and their corresponding contribution to spectra. Test calculations indicate that allowing CT is equally important in both the underlying groundstate wavefunction and the response calculations and should not be confused with basis set superposition error.
|School:||University of Pittsburgh|
|School Location:||United States -- Pennsylvania|
|Source:||DAI-B 80/02(E), Dissertation Abstracts International|
|Subjects:||Chemistry, Physical chemistry|
|Keywords:||Absolutely localized molecular orbital, Carbon dioxide, Energy decomposition analysis, Infrared spectroscopy, Ionic liquids, Linear response|
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