Communication: Oscillating charge migration between lone pairs persists without significant interaction with nuclear motion in the glycine and Gly-Gly-NH-CH3 radical cations

Abstract

Coupled electron-nuclear dynamics has been studied, using the Ehrenfest method, for four conformations of the glycine molecule and a single conformation of Gly-Gly-NH-CH 3 . The initial electronic wavepacket was a superposition of eigenstates corresponding to ionization from the σ lone pairs associated with the carbonyl oxygens and the amine nitrogen. For glycine, oscillating charge migration (when the nuclei were frozen) was observed for the 4 conformers studied with periods ranging from 2 to 5 fs, depending on the energy gap between the lone pair cationic states. When coupled nuclear motion was allowed (which was mainly NH 2 partial inversion), the oscillations hardly changed. For Gly-Gly-NH-CH 3 , charge migration between the carbonyl oxygens and the NH 2 lone pair can be observed with a period similar to glycine itself, also without interaction with nuclear motion. These simulations suggest that charge migration between lone pairs can occur independently of the nuclear motion. © 2014 AIP Publishing LLC. [http://dx

Electron dynamics upon ionization: Control of the timescale through chemical substitution and effect of nuclear motion

“…In para-xylene cation, the initial electron dynamics seems to be too fast for the nuclear motion to have a very significant effect, as was the case for other systems we have studied. 15,17 …”

“…25 Using our complete active space self-consistent field (CASSCF) implementation of the Ehrenfest method, 16 we can study the evolution of a non-stationary electronic wavefunction for fixed nuclei, and where the nuclei are allowed to move, to investigate the differences. 15,17 We can therefore study the influence of the nuclear motion on the electron dynamics and see to what extent the fixed-nuclei approximation is valid. Note that in this work, we focus on the electron dynamics affected by the nuclear motion, not on the nuclear motion (studied in detail in our previous article 26 ).…”

“…A theoretical study by Nest and Ulusoy 14 suggests that the effect of nuclear motion is very small during the first ten or so femtoseconds, as it takes time for the nuclei to move sufficiently to influence the electronic superposition. We have previously studied molecules 15,16 where electron dynamics is affected by the nuclear motion after only a few fs and others 15,17 when it seems to survive for several tens of fs. One of the purposes of this article is to understand further the extent to which neglecting nuclear motion is a reasonable approximation.…”

Using quantum dynamics simulations to follow the competition between charge migration and charge transfer in polyatomic molecules

“…The eMCG basis functions were then run along these trajectories using spline fits of the data and the evolution of the expansion coefficients calculated. This is the procedure that could be used to run an eMCG simulation along Ehrenfest trajectories calculated by a quantum chemistry program, such as Gaussian, which have been generated and used in a number of studies on charge migration [25,26,27].…”

“…Ehrenfest methods provide an alternative approach that calculates all the non-adiabatic couplings along a nuclear trajectory and provides only a single mean-field potential surface for the nuclear motion. Recently Ehrenfest trajectories have been used to study the charge migration in benzene [25], glycine [26] and toluene [27].…”