Charge migration in caffeine: A real-time time-dependent density functional theory study

We present a real-time time dependent density functional theory approach to simulate charge migration of caffeine molecule after selective photoionization. According to the fact that the time evolution of the created hole can affect the molecular reaction and subsequent dynamics difference from its original structure, we studied the dominant frequency variation of caffeine charge oscillation after ionization of outer shell orbital. In this work, we determined the main orbitals participating in the total charge migration process by comparing the dipole moment peaks to the Fourier signals of the time dependent molecular orbitals (MOs) occupation. Having used the Bader charge analysis, we showed that different valence ionization of molecule resulted in a distinct charge migration between different regions, functional groups, and atoms of cations. We represented two interesting different cases: 1-for ionization out from 46th MO of caffeine, the MOs interfere of the cation resulted in a charge migration between oxygen atoms and upper nitrogen of five-membered ring, 2- for ionization out from 39th MO of caffeine, a different behavior of hole occurred according to which the hole spread throughout the whole molecule. Therefore, a specific MO ionization could result in a dramatically different migration and active site of cation.