Abstract The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 µ m are ubiquitously seen in a wide variety of astrophysical regions in the Milky Way and nearby galaxies as well as distant galaxies at redshifts z ≳ 4. The UIE features are characteristic of the stretching and bending vibrations of aromatic hydrocarbon materials. The 3.3 µ m feature which results from the C–H stretching vibration in aromatic species is often accompanied by a weaker feature at 3.4 µ m. The 3.4 µ m feature is often thought to result from the C–H stretch of aliphatic groups attached to the aromatic systems. The ratio of the observed intensity of the 3.3 µ m aromatic C–H feature (I3.3) to that of the 3.4 µ m aliphatic C–H feature (I3.4) allows one to estimate the aliphatic fraction (e.g., NC, aliph/NC, arom, the number of C atoms in aliphatic units to that in aromatic rings) of the carriers of the UIE features, provided that the intrinsic oscillator strengths (per chemical bond) of the 3.3 µ m aromatic C–H stretch (A3.3) and the 3.4 µ m aliphatic C–H stretch (A3.4) are known. In this review we summarize the computational results on A3.3 and A3.4 and their implications for the aromaticity and aliphaticity of the UIE carriers. We use density functional theory and second-order perturbation theory to derive A3.3 and A3.4 from the infrared vibrational spectra of seven polycyclic aromatic hydrocarbon (PAH) molecules with various aliphatic substituents (e.g., methyl-, dimethyl-, ethyl-, propyl-, butyl-PAHs, and PAHs with unsaturated alkyl chains). The mean band strengths of the aromatic (A3.3) and aliphatic (A3.4) C–H stretches are derived and then employed to estimate the aliphatic fraction of the carriers of the UIE features by comparing the ratio of the intrinsic band strength of the two stretches (A3.4/A3.3) with the ratio of the observed intensities (I3.4/I3.3). We conclude that the UIE emitters are predominantly aromatic, as revealed by the observationally-derived mean ratio of ⟨I3.4/I3.3⟩ ≈ 0.12 and the computationally-derived mean ratio of ⟨A3.4/A3.3⟩ ≈ 1.76 which suggest an upper limit of NC, aliph/NC, arom ≈ 0.02 for the aliphatic fraction of the UIE carriers.

中文翻译：

---

未知红外发射特征的载体：来自具有脂肪族侧基的多环芳烃的线索

摘要 在 3.3、6.2、7.7、8.6、11.3 和 12.7 µm 处的未识别红外发射 (UIE) 特征在银河系和附近星系的各种天体物理区域以及红移 z ≳ 4 的遥远星系中无处不在。 . UIE 特征是芳烃材料伸缩振动的特征。由芳香族物种中的 C-H 伸缩振动产生的 3.3 µm 特征通常伴随着 3.4 µm 的较弱特征。3.4 µm 特征通常被认为是由连接到芳族系统的脂肪族基团的 C-H 延伸引起的。3.3 µm 芳香族 C-H 特征 (I3.3) 与 3.4 µm 脂肪族 C-H 特征 (I3.4) 的观察强度之比允许人们估计脂肪族分数（例如，NC， alph/NC, 芳香族, UIE 特征的载体的脂肪族单元中的 C 原子数与芳环中的 C 原子数相比，前提是 3.3 µm 芳族 C-H 拉伸 (A3.3) 的固有振子强度（每个化学键）和3.4 µm 脂肪族 C-H 拉伸 (A3.4) 是已知的。在这篇综述中，我们总结了 A3.3 和 A3.4 的计算结果及其对 UIE 载体的芳香性和脂肪性的影响。我们使用密度泛函理论和二阶微扰理论从具有各种脂肪族取代基（例如甲基-、二甲基-、乙基-、丙基-、丁基-多环芳烃和具有不饱和烷基链的多环芳烃）。芳香族 (A3.3) 和脂肪族 (A3.3) 的平均谱带强度。4) 导出 C-H 拉伸，然后通过比较两个拉伸的固有带强度 (A3.4/A3.3) 与观察到的强度（I3.4/I3.3）。我们得出结论，UIE 发射体主要是芳香的，正如观察得出的平均比率 ⟨I3.4/I3.3⟩ ≈ 0.12 和计算得出的平均比率 ⟨A3.4/A3.3⟩ ≈ 1.76这表明 UIE 载体的脂肪族分数的上限为 NC, aliph/NC, arom ≈ 0.02。