MATI Spectroscopy of Fused Ring Aromatic Molecules

• Main Authors: Yi-Chang Li, 李奕璋
• Other Authors: Sheng-Hsen Lin
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/93694945136248561918

碩士 === 國立臺灣大學 === 化學研究所 === 95 === Part I We applied high-resolution resonant two photon ionization and mass analyzed threshold ionization spectroscopy to investigate the cationic properties of benzimidazole and benzotriazole. The obtained experimental data include the first electronic transition energy (E1) and ionization energy (IE) as well as the active vibrations in the electronically excited S1 and cationic ground D0 states. The E1 and IE of benzimidazole are determined to be 36022 cm-1 and 67552 cm-1, whereas those of benzotriazole are 34917 cm-1 and 70474 cm-1, respectively. We also performed ab initio and density functional calculations to support our experimental findings. Analysis of our experimental results shows that most of the active modes of benzimidazole in the S1 and D0 states are related to in-plane vibrations of the aromatic bicyclic ring, whereas those of benzotriazole correspond to the out-of-plane N2–H and skeleton N1N2N3 bending vibrations. It is found that the IEs of benzimidazole benzotriazole are greater than that of indole by 4961 cm-1 and 7883 cm-1, respectively. This indicates that the IE of these aza-aromatic bicyclic molecules increases with the number of the N atoms. This finding may be attributed to the nature and the N atoms in the five-membered ring. Part II We applied high-resolution resonant two photon ionization and mass analyzed threshold ionization spectroscopy to study the molecular properties of 2, 4 and 7-methylindole (2, 4 and 7MI) in the electronically excited S1 and cationic ground D0 states. Analysis of the obtained spectra give information about the precise first electronic transition energy (E1) and ionization energy (IE) as well as the active vibrations in the S1 and D0 states. The E1’s of 2, 4 and 7MI are determined to be 35161cm-1、35105cm-1 and 35176cm-1, whereas the IEs are determined to be 60482, 61222 and 61490cm-1, respectively. The observed active vibrations include CH3 torsion and the in-plane and out-of-plane vibrations of the indole ring. It is found that the low-frequency CH3 torsion are active for the 2MI and 7MI cations but inactive for the 4MI cation. This indicates that the nature and the location of the substituent can influence the molecular vibrations in addition to the transition energies. Comparing these data with those of indole and 1-methylindole suggests that the methyl substitution on the pyrrole part leads to a grater redshift in the IE than on the benzene part. These experimental findings are well supported by our theoretical calculations.