High-Resolution FTIR Spectroscopy Of Cyclopropylamine,Benzaldehyde And Benzonitrile In The Far-Infrared Region

The formation mechanisms of polycyclic aromatic hydrocarbons(PAHs)play an important role in the understanding how chemistry evolves beyond planet Earth.High-resolution laboratory spectroscopy of small cyclic molecules,as potential precursors to polycyclic aromatic hydrocarbons,offers the scientific basis for their detection in interstellar medium.Additionally,the high-resolution spectroscopy provides accurate information on the energy landscape of molecular potential energy surfaces(PES)which govern molecular dynamics in the form of effective Hamiltonians.In this thesis,rotationally-resolved FTIR spectroscopy of cyclopropylamine,benzaldehyde and benzonitrile in the far-infrared region were measured using Fourier Transform Infrared Spectrometer coupled to a synchrotron source at the Canadian Light Source(unapodized resolution of 0.00064 cm^-1).For benzaldehyde,the ro-vibrational analysis of the–CHO torsional mode led to the unambiguous assignment of the torsional fundamental at 109.415429(20)cm^-1,followed by the tentative assignment of the first and second hot bands at 107.58 cm^-1and 105.61 cm^-1,respectively.This assignment is different from any previous low resolution infrared studies and resulted in a potential barrier height of 1533.6 cm^-1.An anharmonic resonance between the second torsional overtone and the in-plane formyl bending mode could be responsible for wavenumber shifts,leading to the discrepancy between theoretical and experimental barrier heights.Also,the band origins of the benzene skeleton motion modes?₃₂ and?₃₁ were determined at 688.727591(24)cm^-1and 741.890230(17)cm^-1,respectively.Based on the ro-vibrational analysis of these low-lying large amplitude motions,the energy level of ground state and some excited states from potential profiles of benzaldehyde was accurately determined.For trans-cyclopropylamine,the successful ro-vibrational assignment of–NH₂ torsion fundamental and first hot band was at 253.8708741(26)cm^-1 and 246.9595772(37)cm^-1,respectively.Combined with assigned pure rotational transitions in the Terahertz region(35-62 cm^-1),the vibrational ground state and the first and second excited state of the–NH₂ torsion have been fully characterized.For benzonitrile,the c-type ring skeleton vibrationl mode?₁₉ was assigined at688.518330(55)cm^-1 unambiguous based on previously available millimeter study.The resultant Hamiltonian models provides important information about vibrational ground state which is used for the assignment for other skeleton modes in benzonitrile.Collectively,in this work,the vibrational modes of these three small cyclic molecules were analyzed using high-resolution spectroscopy in the traditionally difficult far-IR regions.Through spectral assignment,the accurate characterization of the molecular structure and the resulting potential energy functions were obtained.In comparison with high level theoretical calculation,assigned transitions and resultant accurate molecular structure information based on the high-resolution spectroscopy can effectively validate and modify the current theoretical model,making it closer to the real situation of vibrational modes.In addition,the effective Hamiltonians of these three molecules will be used for their prospected interstellar detection.