Simulation of the Resonance Raman Spectra for 5‑Halogenated (F, Cl, and Br) Uracils

The resonance Raman spectra of the 5-halogenated (F, Cl, and Br) uracils are simulated via the Herzberg–Teller (HT) short-time dynamics formalism. The gradient of the S₁ excited state is computed at the CAMB3LYP/aug-cc-pVTZ level of theory in the conductor-like polarizable continuum model for water (C-PCM, H₂O), based on the equilibrium geometry determined using PBE0/aug-cc-pVTZ in H₂O (C-PCM). The simulated resonance Raman spectra show good agreement with the experimental spectra in terms of both peak positions and intensities. The differences between the resonance Raman spectra of the three 5-halogenated uracils, caused by the effect of halogen substitution, are examined in terms of ground-state normal-mode eigenvectors and excited-state Cartesian gradients, according to the HT formalism. The differences in the normal-mode eigenvectors and excited-state Cartesian gradients between 5-fluorouracil and 5-chlorouracil are used to interpret the dissimilarity between their resonance Raman spectra. Meanwhile, the similarity between the spectra of 5-chlorouracil and 5-bromouracil is explained by the correspondence between their normal modes and excited-state gradients.