88 research outputs found
Constructing grids for molecular quantum dynamics using an autoencoder
A challenge for molecular quantum dynamics (QD) calculations is the curse of
dimensionality with respect to the nuclear degrees of freedom. A common
approach that works especially well for fast reactive processes is to reduce
the dimensionality of the system to a few most relevant coordinates.
Identifying these can become a very difficult task, since they often are highly
unintuitive. We present a machine learning approach that utilizes an
autoencoder that is trained to find a low-dimensional representation of a set
of molecular configurations. These configurations are generated by trajectory
calculations performed on the reactive molecular systems of interest. The
resulting low-dimensional representation can be used to generate a potential
energy surface grid in the desired subspace. Using the G-matrix formalism to
calculate the kinetic energy operator, QD calculations can be carried out on
this grid. In addition to step-by-step instructions for the grid construction,
we present the application to a test system.Comment: 24 pages, 6 figures, articl
Cavity sideband cooling of trapped molecules
The efficiency of cavity sideband cooling of trapped molecules is
theoretically investigated for the case where the IR transition between two
rovibrational states is used as a cycling transition. The molecules are assumed
to be trapped either by a radio-frequency or optical trapping potential,
depending on whether they are charged or neutral, and confined inside a
high-finesse optical resonator which enhances radiative emission into the
cavity mode. Using realistic experimental parameters and COS as a
representative molecular example, we show that in this setup cooling to the
trap ground state is feasible
Visualizing Conical Intersection Passages via Vibronic Coherence Maps Generated by Stimulated Ultrafast X--Ray Raman Signals
The rates and outcomes of virtually all photophysical and photochemical
processes are determined by Conical Intersections. These are regions of
degeneracy between electronic states on the nuclear landscape of molecules
where electrons and nuclei evolve on comparable timescales and become strongly
coupled, enabling radiationless relaxation channels upon optical excitation.
Due to their ultrafast nature and vast complexity, monitoring Conical
Intersections experimentally is an open challenge. We present a simulation
study on the ultrafast photorelaxation of uracil, which demonstrates a new
window into Conical Intersections obtained by recording the transient
wavepacket coherence during this passage with an x-ray free electron laser
pulse. We report two major findings. First, we find that the vibronic coherence
at the conical intersection lives for several hundred femtoseconds and can be
measured during this entire time. Second, the time-dependent energy splitting
landscape of the participating vibrational and electronic states is directly
extracted from Wigner spectrograms of the signal. These offer a novel physical
picture of the quantum Conical Intersection pathways through visualizing their
transient vibronic coherence distributions. The path of a nuclear wavepacket
around the Conical Intersection is directly mapped by the proposed experiment.Comment: 7 pages, 5 Figures, to be published in PNA
Photochemical formation of intricarene
Sunlight is the ultimate driver of biosynthesis but photochemical steps late in biosynthetic pathways are very rare. They appear to play a role in the formation of certain furanocembranoids isolated from Caribbean corals. One of these compounds, intricarene, has been suspected to arise from an intramolecular 1,3-dipolar cycloaddition involving an oxidopyrylium. Here we show, by a combination of experiments and theory, that the oxidopyrylium forms under photochemical conditions and that its cycloaddition occurs via a triplet state. The formation of a complex by-product can be rationalized by another photochemical step that involves a conical intersection. Our work raises the question whether intricarene is biosynthesized in the natural habitat of the corals or is an artefact formed during workup. It also demonstrates that the determination of exact irradiation spectra, in combination with quantum chemical calculations, enables the rationalization of complex reaction pathways that involve multiple excited states
Hole-transfer induced energy transfer in perylene diimide dyads with a donorâspacerâacceptor motif
We investigate the photoinduced dynamics of perylene diimide dyads based on a donorâspacerâacceptor motif with polyyne spacers of varying length by pumpâprobe spectroscopy, time resolved fluorescence, chemical variation and quantum chemistry. While the dyads with pyridine based polyyne spacers undergo energy transfer with near-unity quantum efficiency, in the dyads with phenyl based polyyne spacers the energy transfer efficiency drops below 50%. This suggests the presence of a competing electron transfer process from the spacer to the energy donor as the excitation sink. Transient absorption spectra, however, reveal that the spacer actually mediates the energy transfer dynamics. The ground state bleach features of the polyyne spacers appear due to the electron transfer decay with the same time constant present in the rise of the ground state bleach and stimulated emission of the perylene energy acceptor. Although the electron transfer process initially quenches the fluorescence of the donor it does not inhibit energy transfer to the perylene energy acceptor. The transient signatures reveal that electron and energy transfer processes are sequential and indicate that the donorâspacer electron transfer state itself is responsible for the energy transfer. Through the introduction of a Dexter blocker unit into the spacer we can clearly exclude any through bond Dexter-type energy transfer. Ab initio calculations on the donorâspacer and the donorâspacerâacceptor systems reveal the existence of a bright charge transfer state that is close in energy to the locally excited state of the acceptor. Multipoleâmultipole interactions between the bright charge transfer state and the acceptor state enable the energy transfer. We term this mechanism coupled hole-transfer FRET. These dyads represent a first example that shows how electron transfer can be connected to energy transfer for use in novel photovoltaic and optoelectronic devices
Stereoselective Csp3âCsp2 CrossâCouplings of Chiral Secondary Alkylzinc Reagents with Alkenyl and Aryl Halides
We report palladiumâcatalyzed crossâcoupling reactions of chiral secondary nonâstabilized dialkylzinc reagents, prepared from readily available chiral secondary alkyl iodides, with alkenyl and aryl halides. This method provides αâchiral alkenes and arenes with very high retention of configuration (dr up to 98:2) and satisfactory overall yields (up to 76â% for 3 reaction steps). The configurational stability of these chiral nonâstabilized dialkylzinc reagents was determined and exceeded several hours at 25â°C. DFT calculations were performed to rationalize the stereoretention during the catalytic cycle. Furthermore, the crossâcoupling reaction was applied in an efficient total synthesis of the sesquiterpenes (S)â and (R)âcurcumene with control of the absolute stereochemistry
Photoprotecting uracil by coupling with lossy nanocavities
We analyze how the photorelaxation dynamics of a molecule can be controlled by modifying its electromagnetic environment using a nanocavity mode. In particular, we consider the photorelaxation of the RNA nucleobase uracil, which is the natural mechanism to prevent photodamage. In our theoretical work, we identify the operative conditions in which strong coupling with the cavity mode can open an efficient photoprotective channel, resulting in a relaxation dynamics twice as fast as the natural one. We rely on a state-of-the-art chemically detailed molecular model and a non-Hermitian Hamiltonian propagation approach to perform full-quantum simulations of the system dissipative dynamics. By focusing on the photon decay, our analysis unveils the active role played by cavity-induced dissipative processes in modifying chemical reaction rates, in the context of molecular polaritonics. Remarkably, we find that the photorelaxation efficiency is maximized when an optimal trade-off between light-matter coupling strength and photon decay rate is satisfied. This result is in contrast with the common intuition that increasing the quality factor of nanocavities and plasmonic devices improves their performance. Finally, we use a detailed model of a metal nanoparticle to show that the speedup of the uracil relaxation could be observed via coupling with a nanosphere pseudomode, without requiring the implementation of complex nanophotonic structuresThis work has been funded by the
European Research Council through Grants ERC-2016-StG-
714870 (S. Felicetti, J. Feist, and J. Fregoni) and ERC-2015-
CoG-681285 (J. Fregoni, PI Stefano Corni) and by the
Spanish Ministry for Science, Innovation, and Universities -
Agencia Estatal de InvestigacioÌn through Grants RTI2018-
099737-B-I00, PCI2018-093145 (through the QuantERA
program of the European Commission), and MDM-2014-
0377 (through the MarıaÌ de Maeztu program for Units of
Excellence in R&D). T. Schnappinger and R. de Vivie-Riedle
gratefully acknowledge the DFG Normalverfahren. S. Reiter
gratefully acknowledges financial support by the International Max Planck Research School of Advanced Photon Science
(IMPRS-APS
Substituent effects on the relaxation dynamics of furan, furfural and ÎČ-furfural: a combined theoretical and experimental approach
For the series furan, furfural and ÎČ-furfural we investigated the effect of substituents and their positioning on the photoinduced relaxation dynamics in a combined theoretical and experimental approach. Using time resolved photoelectron spectroscopy with a high intensity probe pulse, we can, for the first time, follow the whole deactivation process of furan through a two photon probe signal. Using the extended 2-electron 2-orbital model [Nenov et al., J. Chem. Phys., 2011, 135, 034304] we explain the formation of one central conical intersection and predict the influence of the aldehyde group of the derivatives on its geometry. This, as well as the relaxation mechanisms from photoexcitation to the final outcome was investigated using a variety of theoretical methods. Complete active space self consistent field was used for on-the-fly calculations while complete active space perturbation theory and coupled cluster theory were used to accurately describe critical configurations. Experiment and theory show the relaxation dynamics of furfural and ÎČ-furfural to be slowed down, and together they disclose an additional deactivation pathway, which is attributed to the nO lonepair state introduced with the aldehyde group
- âŠ