Kossoski Fábris, Mendes Mónica, Lozano Ana I, Rodrigues Rodrigo, Jones Nykola C, Hoffmann Søren V, da Silva Filipe Ferreira
Laboratoire de Chimie et Physique Quantiques (UMR 5626), Université de Toulouse, CNRS, UPS, France.
CEFITEC, Department of Physics, NOVA School of Science and Technology, NOVA University Lisbon, Caparica, 2829-516, Portugal.
Phys Chem Chem Phys. 2025 May 8;27(18):9687-9701. doi: 10.1039/d5cp00198f.
As a prototypical molecule in the important class of halopyrimidines, 2-chloropyrimidine has been the subject of numerous spectroscopic studies. However, its absorption spectrum under vacuum ultraviolet (VUV) radiation has not yet been reported. Here, we close this gap by presenting high-resolution VUV photoabsorption cross-sections in the 3.7-10.8 eV range. Based on time-dependent density functional theory (TDDFT) calculations performed within the nuclear ensemble approach, we are able to characterize the main features of the measured spectrum. By comparing the present results for 2-chloropyrimidine with those of 2-bromopyrimidine and pyrimidine, we find that the effect of the halogen atom increases remarkably with the photon energy. The two lowest-lying absorption bands are overall similar for the three molecules, apart from some differences in the vibrational progressions in band I (3.7-4.6 eV) and minor energy shifts in band II (4.6-5.7 eV). Larger shifts appear in band III (5.7-6.7 eV), especially when comparing pyrimidine with the two halogenated species. The three molecules absorb more strongly in the region of band IV (6.7-8.2 eV), where the bands look qualitatively different because the mixing of excited configurations is strongly dependent on the species. At higher energies (8.2-10.8 eV) the three spectra no longer resemble each other. An important finding of this study is the very satisfactory comparison between experiment and theory, as the combination of TDDFT calculations with the nuclear ensemble approach yields cross-sections much closer to experiments than the simpler vertical approximation, in shape and magnitude, and across the whole spectral range surveyed here.
作为卤代嘧啶这一重要类别中的典型分子，2-氯嘧啶一直是众多光谱研究的对象。然而，其在真空紫外（VUV）辐射下的吸收光谱尚未见报道。在此，我们通过给出3.7 - 10.8 eV范围内的高分辨率VUV光吸收截面来填补这一空白。基于在核系综方法内进行的含时密度泛函理论（TDDFT）计算，我们能够表征测量光谱的主要特征。通过将2-氯嘧啶的当前结果与2-溴嘧啶和嘧啶的结果进行比较，我们发现卤原子的效应随光子能量显著增加。对于这三个分子，两个最低的吸收带总体相似，只是在带I（3.7 - 4.6 eV）的振动进程上存在一些差异，以及在带II（4.6 - 5.7 eV）有较小的能量位移。在带III（5.7 - 6.7 eV）出现了更大的位移，特别是将嘧啶与两种卤代物种进行比较时。这三个分子在带IV（6.7 - 8.2 eV）区域吸收更强，在该区域各吸收带在性质上有所不同，因为激发态构型的混合强烈依赖于物种。在更高能量（8.2 - 10.8 eV）下，这三个光谱不再彼此相似。本研究的一个重要发现是实验与理论之间非常令人满意的比较，因为TDDFT计算与核系综方法相结合所得到的截面在形状和大小上，以及在此处测量的整个光谱范围内，都比更简单的垂直近似更接近实验结果。
