Abstract
The distinction of enantiomers is a key aspect of chemical analysis. In mass spectrometry the distinction of enantiomers has been achieved by ionizing the sample with circularly polarized laser pulses and comparing the ion yields for light of opposite handedness. While resonant excitation conditions are expected to be most efficient, they are not required for the detection of a circular dichroism (CD) in the ion yield. However, the prediction of the size and sign of the circular dichroism becomes challenging if non-resonant multiphoton excitations are used to ionize the sample. Employing femtosecond laser pulses to drive electron wavepacket dynamics based on ab initio calculations, we attempt to reveal underlying mechanisms that determine the CD under non-resonant excitation conditions. Simulations were done for (R)-1,2-propylene oxide, using time-dependent configuration interaction singles with perturbative doubles (TD-CIS(D)) and the aug-cc-pVTZ basis set. Interactions between the electric field and the electric dipole and quadrupole as well as between the magnetic field and the magnetic dipole were explicitly accounted for. The ion yield was determined by treating states above the ionization potential as either stationary or non-stationary with energy-dependent lifetimes based on an approved heuristic approach. The observed population dynamics do not allow for a simple interpretation, because of highly non-linear interactions. Still, the various transition pathways are governed by resonant enantiospecific n-photon excitation, with preferably high transition dipole moments, which eventually dominate the CD in the ionized population.
Highlights
Mass spectrometry (MS) is a widely used and exceptionally sensitive analytic technique
For (R)-3-methylcyclopentanone we have proven that the circular dichroism (CD) in the ion yield after resonance enhanced multiphoton ionization (REMPI) can be predicted by laser-driven quantum electron dynamics based on ab initio calculations, namely time-dependent configuration interaction singles with perturbative doubles (TD-CIS(D)).[10,11]
In this paper we extended our methodology to calculate the CD in ion yields via laser driven electron wavepacket dynamics by several aspects
Summary
Circular dichroism in ion yields can be detected employing shaped circular polarized femtosecond (fs) laser pulses.[8]. Electric quadrupole interactions will be introduced into the interaction Hamiltonian, and their contribution to resonant and non-resonant multiphoton excitations will be studied Based on these extensions we are able to clarify which (enantiospecific) transitions are responsible for the CD in the ion yield after nonresonant laser pulse ionization. Our approach differs in several ways from the one of Ma and Salam, who studied the related problem of optimal selective excitation of enantiomers in a racemic mixture by circular polarized laser pulses.[13] Their investigations were based on the analytical description of a two-level model system, for which resonant and off-resonant one-photon absorptions, mediated by electric and magnetic dipole interactions, were compared showing that off-resonant conditions significantly decreased the enantioselectivity. Appendices A to E give more insight into the quality of the ab initio results, and more detailed explanations for the more complex findings, partly based on a two-level model system
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
