Date of Award
Master of Science in Chemical Sciences (MSCB)
In this computational work, vibrational infrared and Raman spectra of small size gas-phase molecules and complexes are studied using molecular dynamics (MD) method. Additionally, polarizability driven molecular dynamics method is employed to assign prominent vibrational modes beyond the constraint of the harmonic approximation. These methods are applied to the benchmark systems CH4, H5O2+, and N4H+. Classical MD uses existing fitting procedures of the polarizability tensor surface (PTS) done via permutationally invariant polynomials developed by our collaborator at Emory University. This new PTS method has advantages in describing polarizability with increased computational effectiveness and sufficient accuracy. The PTS functional form is used to describe the polarizabilities ofthe complexes. Various polarizability models are tested for the methane molecule as cheaper and more accurate alternatives to the Applequist’s point dipole model. The Thole linear model showed a good correlation to the non-linear model in a non-linear parameter space used to calculate Raman spectra. CH4 Raman active modes include the bending mode at 1550 cm-1, symmetric stretch at 3063 cm-1, and asymmetric stretch at 3206 cm-1. IR and Raman spectra of the protonated water dimer, H5O2+ and its deuterium isotopologues are calculated using PTS in the analytical form. MD calculations show Raman intensities for the OO stretch, OH+O perpendicular proton transfer, and OH symmetric stretch modes with corresponding frequencies at 561 cm-1, 1550 cm-1, and 3733 cm-1, respectively. The PTS analytical form of another benchmark complex, N2...H+...N2 together with its existing potential energy surface are used to resolve anharmonic spectral features, in particular, Fermi resonance between gerade symmetric N-N stretch and the ungerade N...H+...N bend overtone. Raman activity in the proton transfer region shows broad anharmonic vibrational features that correspond to an asymmetric proton transfer overtone and other combinations bands.
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