Vacuum ultraviolet spectroscopy of pyrimidine derivatives: the effect of halogenation.

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.