Theoretical Calculation And Rotational Spectroscopy Of Dibenzofuran

Interstellar organic molecules are potentially related to the origin of life in space and the evolution of celestial bodies.Polycyclic aromatic hydrocarbons are the key to the evolution of complex organic molecules in the interstellar medium.The establishment of chemical models is important for the study of the molecular structure and formation mechanism of polycyclic aromatic hydrocarbons.This thesis includes a pure rotational spectroscopy study of the potential interstellar molecule dibenzofuran and quantum chemistry theoretical calculations,resulted in valuable knowledge of the molecular structure.The significance of polycyclic aromatic hydrocarbons in the interstellar medium was summarized in the first chapter,and then broadband Fourier transform spectroscopy was comprehensively introduced focusing on the broad-band chirp-pulse Fourier transform microwave spectroscopy instrument（CP-FTMW）.The working principle of spectroscopy was compared with the pulsed nozzle Fourier transform microwave（PN-FTMW）spectrometer,and the advantage and disadvantage of the CP-FTMW were summarized.Additionally,theoretical background pertaining to the analysis of the pure rotational spectra was explained,such as effective Hamiltonian,moment of inertia,inertia defect,etc.also,the calculation of quantum theory and its application programs were discussed.The experimental part of the research mainly focused on the high-resolution spectral information of the asymmetric top molecule dibenzofuran.The newly developed CP-FTMW spectroscopy technology was applied to collect the pure rotation spectrum of dibenzofuran.Combined with quantum calculations and with the aid of the PGOPHER program,the three rotational constants A,B and C of dibenzofuran were obtained.A total of 40 b-type ground state transition spectrum lines with a frequency range of 2-6 GHz were fitted,withJ_max=19 and K_{a max}=5.The inertial defects of dibenzofuran molecules were calculated using newly obtained rotational constants.The last part of the thesis is the summary and outlook.In terms of electronic arrangement and molecular structure,the symmetry of dibenzofuran and the influence of the substitution of different groups on the structure was discussed.The analysis of experimental data shows that the inertial defect of dibenzofuran is slightly negative,with the possible reason being the influence of low-frequency,out-of-plane vibrational motions of the molecule.The analysis of the highest occupied orbitals of three three-membered ring molecules with similar structures show that when the NH group or O atom replaces the CH₂ group in the central five-membered ring,the atoms on the six-membered ring are polarized and the distribution of ground-state charges has been greatly affected.