Fotoestabilidade da vitamina C : dinâmica de Surface hopping e análise de transferência de carga intramolecular

Abstract

Vitamin C is an essential nutrient for humans, being important in several biochemical, photochemical and antioxidant activities. However, many mechanisms of vitamin C activity are not yet fully understood, such as, for example, an intramolecular charge transfer, photochemical stability and the time decay of their excited states. In this sense, the present work had as main objective the analysis of the first 12 excited states of vitamin C, classifying them in local excitations, charge transfer or Rydberg states. In addition, the decay time for each excited state was estimated using the Surface Hopping molecular dynamics (DMSH). In DMSH, the Khon-Sham orbitals were expanded using flat waves with 75 Ry cut-off, PBE exchange and correlation functional and Troullier-Martins of conserved norm pseudopotential. The temperature was controlled at 300 K using a chain of three Nosé-Hoover thermostats. The UV-vis spectra were chosen using TD-DFT with the CAMB3LYP, B3LYP and PBE exchange and correlation functionals and the 6-31G base set. The effect of the aqueous solvation was included in the calculations explicitly and implicitly. The results of the calculations show that the geometric parameters obtained theoretically have good agreement with the experimental values. It is also observed that the 𝜆𝑚𝑎𝑥 obtained with the implicit and explicit inclusion of the aqueous solvent was 246.40 nm and 245.43 nm, respectively. These results when compared with the λmax in gas phase represent a displacement of 21.5 and 20.53 nm, respectively. In 33,33 % of the analyzed excited states, charge transfer occurs, and the excitations are predominantly of the type π → π*. Of all the DMSH trajectories analyzed, photodegradation was not observed, that is, vitamin C shows photochemical stability, which makes it possible, for example, to use this molecule in photoelectric devices and sunscreens.