利用間質隔離法研究喹啉鹽陽離子及喹啉鹽基各種異構物在para-H2間質中之紅外光譜

Abstract

外太空未指認的紅外光譜(UIR)主要的譜帶被觀測於3.3、6.2、7.7、8.6、11.3 μm。而質子化的多環芳香烴(H+PAH)及氮雜多環芳香烴(H+PANH)被認為可能是此譜帶的主要來源，其中H+PANH分子上的N取代會使得其C=C振動模較H+PAH分子的C=C振動模更吻合UIR中的6.2 m譜帶，所以在實驗室研究H+PANH之光譜以與UIR光譜比較成為一重要的研究課題。 本實驗將喹啉(C9H7N)與p-H2氣體沉積於3.2 K的樣品靶上，同時以電子束不斷撞擊靶面上之p-H2產生H3+及H。H3+的質子可轉移至喹啉上，形成質子化(protonated)的喹啉，即喹啉鹽陽離子；質子化的喹啉進行電中和反應以及喹啉分子與H之反應，均可產生氫化(hydrogenated)的喹啉，即喹啉鹽基。將此間質放置在黑暗中一段時間，待間質中之電子緩慢擴散與喹啉鹽陽離子作用或氫原子與喹啉作用，可觀測到喹啉鹽陽離子譜線之吸收強度減少，而喹啉鹽基譜線之吸收強度增加。藉由比較實驗光譜與B3LYP/6-311++G(d,p)理論計算預測之非簡諧振動波數及紅外吸收強度可知道所觀測到的譜線分別屬於1-喹啉鹽陽離子及1-喹啉鹽基。理論計算預測1-喹啉鹽陽離子及1-喹啉鹽基皆為異構物中最穩定者，與實驗結果一致。 本實驗並進一步以光解法研究喹啉在p-H2間質中的氫化反應，觀測到不同氫化位置的喹啉鹽基。C9H7N/Cl2/p-H2間質經紫外光(365 nm)照射產生Cl原子再與經紅外光激發的H2反應，產生H原子，其會與間質中的喹啉分子反應形成喹啉鹽基。光譜中新生成之譜線可藉由不同波長之二次光解所造成譜線減少之比例不同而分為五個群組，與B3LYP/6-311++G(d,p)理論計算預測之紅外光譜比較後，可分別指派為氫化位置在1、3、4、7及8的喹啉鹽基。吾人亦以B3LYP/6-311++G(d,p)理論計算探討H原子與喹啉產生各喹啉鹽基異構物之過渡態能量及H原子依序在各C或N原子跳動之過渡態能量，探討其可能的反應機構。 雖然因為喹啉分子的大小不夠大，並不預期1-喹啉鹽陽離子之譜線可以和UIR之譜帶吻合，但與1-萘鹽陽離子實驗比較，1-萘鹽陽離子位於1618.7(~6.18 μm)、1580.8(~6.33 μm)、1510.0(~6.62 μm) cm−1之C=C振動模因為受到1-喹啉陽離子中的N原子之影響而藍移(blue-shifted)至1641.4(~6.09 μm)、1598.4(~6.26 μm)、1562.0(~6.40 μm) cm-1，故推測對於較大的PAH及PANH分子中，H+PANH在6 μm附近之譜線較相對應的H+PAH分子更接近UIR譜帶中之6.2 μm吸收譜帶。

Large protonated polycyclic aromatic hydrocarbons (H+PAH) and polycyclic aromatic nitrogen heterocycles (H+PANH) have been proposed as possible carriers of unidentified infrared emission (UIR) bands from galactic objects. The nitrogen atom in H+PANH is expected to induce a blue shift of the C=C stretching band near 6.2 μm so that their emission bands might agree with the UIR band better than those of H+PAH. In this work, we report the IR spectrum of protonated quinoline and its neutral species measured upon electron bombardment during deposition of a mixture of quinoline and para-hydrogen at 3.2 K. Observed features were assigned to 1-quinolinium (1-C9H7NH+) and quinolinyl (1-C9H7NH), indicating that the protonation and hydrogenation occur at the N-atom site. The intensities of features of 1-C9H7NH+ diminished when the matrix was maintained in darkness for ~10 h, whereas those of 1-C9H7NH increased. Spectral assignments were made according to comparison of experimental results with anharmonic vibrational wavenumbers and IR intensities predicted with the B3LYP/6-311++G(d,p) method. Assignments of 1-C9H7NH were further supported by the observation of similar spectra when a Cl2/C9H7N/p-H2 matrix was irradiated first at 365 nm and followed by irradiation with IR light to generate H atoms to induce the H + C9H7N reaction. Furthermore, we employed this photolytic method to confirm the spectral assignments of quinolinyl radicals 3-, 4-, 7-, and 8-C9H8N. Although agreement between the observed spectrum of the 1-C9H7NH+ and the UIR emission bands is unsatisfactory, presumably because the size of quinoline is too small, we did observe a C=C stretching bands at 1641.4, 1598.4, 1562.0 cm-1, blue-shifted from those at 1618.7, 1580.8, 1510.0 cm−1 of the corresponding protonated PAH (C10H9+), pointing to the direction of the UIR bands.