| Summary: | 碩士 === 國立交通大學 === 應用化學系分子科學碩博士班 === 107 === Aniline is a prototypical aromatic amine. It contains two types of basic centers that can accept an approaching proton, namely the amino group and the aromatic ring. The alkaline properties of aniline in solution result from the stabilization of the protonated-nitrogen form when solvated. However, the site of protonation in gaseous aniline is unclear. Previously, studies on protonated aniline in the gaseous phase are mainly based on mass spectrometry; only indirect information about the preferred protonation site of aniline was thus derived. Therefore, understanding the protonation site of aniline without solvent and their spectra is desirable.
A new method for investigating the IR spectra of protonated species and their neutral counterparts was developed in our laboratory using electron bombardment during deposition of a gaseous mixture of para-hydrogen (p-H2) matrix host containing a small proportion of guest molecules. This method produces mainly protonated parent molecules and corresponding neutrals without fragmentation; the IR spectra show true absorption intensity with small linewidths, excellent signal-to-noise ratio, and wide spectral coverage.
In this work, we applied this technique to investigate the IR spectrum of protonated aniline in a solid p-H2 matrix. We distinguished spectral lines into various groups according to their behaviors after maintaining in darkness and upon secondary photolysis; the assignments were derived on comparison with quantum-chemical calculations using the B3LYP/cc-pVTZ method. The results indicate that three protonated isomers with protonation on the para position (absorption lines at 3482.7, 3395.5, 1669.2, 1523.5, 1470.0, 1337.2, 1197.5, 884.2, 816.7, 663.0, and 561.3 cm-1), the ortho position (absorption lines at 3479.3, 3392.1, 1667.0, 1541.9, 1515.7, 1452.0, 1419.3, 1341.6, 1188.1, 786.1, and 736.2 cm-1), and the amino group (absorption lines at 3303.3, 3278.8, 3234.2, 1469.9, and 737.7 cm-1), are observed. Observation of these three isomers with mixing ratios (0.89 ± 0.12), (0.61 ± 0.09) and (0.81 ± 0.79) ppm for para-, ortho-, and amino-, respectively, are consistent with relative energies 0, 8, and 18 kJ mol-1 predicted with the B3LYP/cc-pVTZ method.
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