Molecular Raman Scattering And Absorption Spectra Of A Number Of New Theoretical And Experimental Research

Raman scattering and electronic absorption are powerful tools for studying the molecular structures and properties. In this thesis, some new problems in Raman scattering and absorption spectroscopies, related to the antisymmetric polarizability and intermolecular interaction, have been studied using theoretical, experimental and quantum chemical computational methods. The thesis is divided to seven chapters.Chapter I introduced the basic knowledge and new developments in Raman scattering, molecular polarizability, and the DFT calculations of molecular spectroscopy. The research backgrounds and main contents of the thesis were also presented in this chapter.Chapter II studied the UV near-resonant Raman (UVRR) spectroscopy of 1,1'-bi-2-naphthol (BN) solutions. The normal and UV near-resonant Raman spectra of 1,1'-bi-2-naphthol in basic solution were measured and analyzed. Density functional theory (DFT) calculations were carried out to study the ground state geometry structure, vibrational frequencies, off-resonance Raman intensities, and depolarization ratios of 1,1'-bi-2-naphtholate dianion (BN^2-). On the basis of the calculated and experimental results, the observed Raman bands were assigned in detail. The 1612 cm^-1 Raman band of BN in basic solution was found dramatically enhanced in the UV resonance Raman spectrum in comparison with the normal Raman spectrum. Analyzing the depolarization ratios of the 1366 and 1612 cm^-1 bands in the RR spectra manifests that both the symmetric and antisymmetric parts of transition polarizabilities contribute to the 1366 cm^-1 band, but only symmetric part contributes to the 1612 cm^-1 band.Chapter III investigated the electronic and vibrational spectra of octaethylporphyrin diacid (H₄OEP²⁺). The ground-state structure and electronic and vibrational spectra of octaethylporphyrin diacid have been studied with the density functional theory. The geometrical parameters computed with B3LYP, PBE1PBE and mPW1PW91 functionals and 6-31G(d) basis sets are well consistent with the experimental values. Electronic absorption spectrum of H₄OEP²⁺ has been studied with the time-dependent DFT method, and the calculated excitation energies and oscillator strengths are compared with the experimental results. The Raman and IR spectra of H₄UOEP²⁺ and the Raman spectrum of its N-deuterated analogue (D₄OEP²⁺) were measured. The observed Raman and IR bands have been assigned based on the frequency calculations at the B3LYP/6-31G(d) level of theory.Chapter IV investigated the resonance Raman spectra (RRS) of aggregated tetra(4-pyridyl) porphyrin diacid (H₈TPyP⁶⁺). The photophysical properties of the segregated monomers and aggregates have been studied with UV-Vis absorption, fluorescence emission, and resonance light scattering (RLS). The absorption bands of the aggregate are found exhibiting obvious shift and broadening, which is attributed to the excitonic coupling between porphyrin rings. Resonance Raman spectra of aggregated tetra(4-pyridyl)porphyrin diacid excited near exciton absorption bands of 470 nm were measured and analyzed. The observed Raman bands of monomeric and aggregated H₈TPyP⁶⁺ were assigned on the basis of the deuterial shifted and by comparing with the well-studied meso-tetraphenylporphyrin diacid (H₄TPP²⁺). The aggregation causes the enhancement of resonance Raman intensities in the aggregates, not just in low-frequency region, but also in the high-frequency region.Chapter V investigated the structure and UV-visible spectrum of pyridine-chloranil charge-transfer complex. The ground-state structure of the charge-transfer complex formed by pyridine (Py) as electron donor and chloranil (CA) as acceptor has been studied by full geometry optimization at the MP2 and DFT levels of theory. Binding energies was calculated and counterpoise corrections were used to correct the BSSE. Both MP2 and DFT indicate that the pyridine binds with chloranil to form an inclined T-shape structure, with the pyridine plane perpendicular to the chloranil. The CP and ZPE corrected binding energies were calculated to be 14.21 kJ/mol by PBEPBE/6-31G(d) and 23.21 kJ/mol by MP2/6-31G(d). The charge distribution of the ground state Py-CA complex was evaluated with the natural population analysis, showing a neat charge transfer from Py to CA. Analysis of the frontier molecular orbitals reveals aσ-Ï€interaction between CA and Py, and the binding is reinforced by the attraction of the O₇ atom of CA with the H₂₃ atom of Py. TD-DFT calculations have been performed to analyze the UV-visible spectrum of Py-CA complex, revealing both the charge transfer transitions and the weak symmetry-relieved chloranilÏ€-Ï€^* transition in the UV-visible region.Chapter VI investigated the difference between stokes and anti-stokes Raman shifts and time-reversal symmetry. The difference between the Stokes and anti-Stokes Raman shifts is proposed and theoretically explored on the basis of the time-reversal symmetry of light-scattering experiments. In the literature of Raman scattering, the Stokes and anti-Stokes Raman shifts for some specific systems show experimentally major differences in the vibrational, electronic, Raman spectra and Brillouin scattering, which has not been systematically investigated yet. Some examples of such experiments are briefly reviewed. The anti-Stokes scattering is defined as the corresponding time-reversed process of the Stokes scattering. Based on the time-reversal symmetry the differences between one Stokes and its corresponding anti-Stokes Raman shifts have been theoretically demonstrated for the systems with Kramers-degenerate ground or final states. In the former cases the Stokes and its corresponding anti-Stokes Raman lines should display on same side by the excited line. In the latter cases, they show major differences on two sides by the excited line. However, whole normal Stokes Raman spectrum of a system may be still symmetric to whole anti-Stokes spectrum, except in the cases of resonant, or Laser, or circularly polarized excitation Raman scattering. This is a preliminary theoretical predication which needs more experimental and theoretical investigations.Chapter VII presented and discussed a generalization of the equations of laser-induced circular dichroism (LICD) derived for linear molecules. LICD for freely rotating molecules by a resonant circularly-polarized pump field is an optical analogue of the Faraday effect in isotropic media. Its fundamental formulae and existent conditions have been deduced using the density matrix theory with a two-level model and time-reversal arguments. The antisymmetric rotational polarizability and LICD of symmetric and asymmetric top molecule are studied concretely using perturbation theory and angular momentum algebra. The order of magnitude of LICD and the physical phenomena closely related with it have been discussed. The results show that the LICD effect is exhibited by all freely rotating molecules and may provide useful information on optical activity and antisymmetric polarizability.