Deuterium (D) is one of the light elements created in the Big Bang. As the Galaxy evolves, the D/H abundance in the interstellar medium (ISM) decreases from its primordial value due to astration. However, the observed gas-phase D/H abundances of some sightlines in the local Galactic ISM are substantially lower than the expected reduction by astration. The missing D could have been depleted onto polycyclic aromatic hydrocarbon (PAH) molecules which are ubiquitous and abundant in interstellar regions. To quantitatively explore the hypothesis of PAHs as a possible reservoir of interstellar D, we quantum-chemically compute the infrared vibrational spectra of monodeuterated PAHs and their cations. We find that, as expected, when H in PAHs is replaced by D, the C–H stretching and bending modes at 3.3, 8.6, and 11.3 μm shift to longer wavelengths at ∼4.4, 11.4, and 15.4 μm, respectively, by a factor of ∼, the difference in reduced mass between the C–H and C–D oscillators. From the computed spectra we derive the mean intrinsic band strengths of the 3.3 μm C–H stretch and 4.4 μm C–D stretch to be and for neutral deuterated PAHs which would dominate the interstellar emission at 3.3 and 4.4 μm. By comparing the computationally derived mean band-strength ratio of for neutral PAHs with the mean ratio of the observed intensities of , we find that the degree of deuteration (i.e., the fraction of peripheral atoms attached to C atoms in the form of D) is ∼2.4%, corresponding to a D enrichment of a factor of ∼1200 with respect to the interstellar D/H abundance.

氘（D）是大爆炸中产生的轻元素之一。随着星系的演化，星际介质（ISM）中的D / H丰度由于星变而从其原始值减小。但是，在局部银河系ISM中观察到的某些视线的气相D / H丰度大大低于预期的减少。丢失的D可能已经耗尽到星际区域中普遍存在且丰富的多环芳烃（PAH）分子上。为了定量研究PAHs可能是星际D的储层的假设，我们用量子化学方法计算了单氘代PAHs及其阳离子的红外振动光谱。我们发现，如预期的那样，当在多环芳烃H由d取代，该C-H伸缩和在3.3弯曲模式，8.6，和11.3 μm个移位至较长的波长在~4.4，11.4，和15.4 μ米，分别由〜倍，在C-H和C-d振荡器之间减少的质量的差。从所计算的光谱，我们推导出3.3的平均固有带强度μ M C-H伸缩和4.4 μ米C-d的拉伸是和中性氘化多环芳烃这将在3.3和4.4主宰星际发射μ米。通过比较计算得出的中性PAHs的平均带强度比与观察到的强度的平均比，我们发现氘的程度（即，以D的形式连接到C原子的外围原子的分数）约为2.4％，相对于星际D / H的D富集度约为1200。丰富。