THE ABUNDANCES OF HYDROCARBON FUNCTIONAL GROUPS IN THE INTERSTELLAR MEDIUM INFERRED FROM LABORATORY SPECTRA OF HYDROGENATED AND METHYLATED POLYCYCLIC AROMATIC HYDROCARBONS
THE ABUNDANCES OF HYDROCARBON FUNCTIONAL GROUPS IN THE INTERSTELLAR MEDIUM INFERRED FROM LABORATORY SPECTRA OF HYDROGENATED AND METHYLATED POLYCYCLIC AROMATIC HYDROCARBONS
复制标题
DOI：
10.1088/0067-0049/208/2/26
复制
发表时间：
2013-08
期刊：
影响因子：
--
通讯作者：
M. Steglich;Cornelia Jager;F. Huisken;M. Friedrich;W. Plass;H. Rader;K. Mullen;T. Henning
中科院分区：
文献类型：
--
作者：
M. Steglich;Cornelia Jager;F. Huisken;M. Friedrich;W. Plass;H. Rader;K. Mullen;T. Henning
在室温（颗粒状）和低温（Ne基质）下分别测定了含甲基（CH 3）、亚甲基（CH 2）或类金刚石CH基团的多环芳烃（PAHs）的红外吸收光谱和气相缩合制备的甲基化和氢化多环芳烃混合物的红外吸收光谱。此外，还通过高效液相色谱法、核磁共振光谱法和基质辅助激光解吸/电离飞行时间质谱法对多环芳烃混合物进行了深入的分子结构分析。在密度泛函理论水平计算的支持下，实验室结果被用于详细分析3.4 μm弥漫星际介质的脂肪族吸收复合物，并确定烃官能团的丰度。假设多环芳烃主要锁定在颗粒中，脂肪族CHx基团（x = 1，2，3）对3.4 μm特征的贡献大致相等（NCHx/NH = 10−5-2 × 10−5）。然而，如果3.4 μm特征的主要贡献来自气相中的分子，则丰度可能会降低2至4倍。芳香CH基团似乎在某些视线中几乎不存在，但在其他方向上几乎与每个脂肪族组分一样丰富（/NH = 2 × 10−5;谷物的上限值）。由于相对较低的结合能，天文红外发射源不会显示出如此重的过量氢化。在最好的情况下，特别是在原行星状星云中，CH 2基团与芳香分子结合，即，只有分子外围的多余氢才能在附近的星星存在下存活下来。
Infrared (IR) absorption spectra of individual polycyclic aromatic hydrocarbons (PAHs) containing methyl (CH3), methylene (CH2), or diamond-like CH groups and IR spectra of mixtures of methylated and hydrogenated PAHs prepared by gas-phase condensation were measured at room temperature (as grains in pellets) and at low temperature (isolated in Ne matrices). In addition, the PAH blends were subjected to an in-depth molecular structure analysis by means of high-performance liquid chromatography, nuclear magnetic resonance spectroscopy, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Supported by calculations at the density functional theory level, the laboratory results were applied to analyze in detail the aliphatic absorption complex of the diffuse interstellar medium at 3.4 μm and to determine the abundances of hydrocarbon functional groups. Assuming that the PAHs are mainly locked in grains, aliphatic CHx groups (x = 1, 2, 3) would contribute approximately in equal quantities to the 3.4 μm feature (NCHx/NH ≈ 10−5–2 × 10−5). The abundances, however, may be two to four times lower if a major contribution to the 3.4 μm feature comes from molecules in the gas phase. Aromatic CH groups seem to be almost absent from some lines of sight, but can be nearly as abundant as each of the aliphatic components in other directions (/NH ≲ 2 × 10−5; upper value for grains). Due to comparatively low binding energies, astronomical IR emission sources do not display such heavy excess hydrogenation. At best, especially in protoplanetary nebulae, CH2 groups bound to aromatic molecules, i.e., excess hydrogens on the molecular periphery only, can survive the presence of a nearby star.