Intramolecular Energy Redistribution Research Articles

SF6 Negative Ion Formation in Charge Transfer Experiments.

In the present work, we report an update and extension of the previous ion-pair formation study of Hubers, M.M.; Los, J. Chem. Phys.1975, 10, 235-259, noting new fragment anions from time-of-flight mass spectrometry. The branching ratios obtained from the negative ions formed in K + SF6 collisions, in a wide energy range from 10.7 up to 213.1 eV in the centre-of-mass frame, show that the main anion is assigned to SF5- and contributing to more than 70% of the total ion yield, followed by the non-dissociated parent anion SF6- and F-. Other less intense anions amounting to <20% are assigned to SF3- and F2-, while a trace contribution at 32u is tentatively assigned to S- formation, although the rather complex intramolecular energy redistribution within the temporary negative ion is formed during the collision. An energy loss spectrum of potassium cation post-collision is recorded showing features that have been assigned with the help of theoretical calculations. Quantum chemical calculations for the lowest-lying unoccupied molecular orbitals in the presence of a potassium atom are performed to support the experimental findings. Apart from the role of the different resonances participating in the formation of different anions, the role of higher-lying electronic-excited states of Rydberg character are noted.

Transient-absorption spectroscopy of dendrimers via nonadiabatic excited-state dynamics simulations.

The efficiency of light-harvesting and energy transfer in multi-chromophore ensembles underpins natural photosynthesis. Dendrimers are highly branched synthetic multi-chromophoric conjugated supra-molecules that mimic these natural processes. After photoexcitation, their repeated units participate in a number of intramolecular electronic energy relaxation and redistribution pathways that ultimately funnel to a sink. Here, a model four-branched dendrimer with a pyrene core is theoretically studied using nonadiabatic molecular dynamics simulations. We evaluate excited-state photoinduced dynamics of the dendrimer, and demonstrate on-the-fly simulations of its transient absorption pump-probe (TA-PP) spectra. We show how the evolutions of the simulated TA-PP spectra monitor in real time photoinduced energy relaxation and redistribution, and provide a detailed microscopic picture of the relevant energy-transfer pathways. To the best of our knowledge, this is the first of this kind of on-the-fly atomistic simulation of TA-PP signals reported for a large molecular system.

Electronic-state chaos, intramolecular electronic energy redistribution, and chemical bonding in persisting multidimensional nonadiabatic systems.

We study the chaotic, huge fluctuation of electronic state, resultant intramolecular energy redistribution, and strong chemical bonding surviving the fluctuation with exceedingly long lifetimes of highly excited boron clusters. Those excited states constitute densely quasi-degenerate state manifolds. The huge fluctuation is induced by persisting multidimensional nonadiabatic transitions among the states in the manifold. We clarify the mechanism of their coexistence and its physical significance. In doing so, we concentrate on two theoretical aspects. One is quantum chaos and energy randomization, which are to be directly extracted from the properties of the total electronic wavefunctions. The present dynamical chaos takes place through frequent transitions from adiabatic states to others, thereby making it very rare for the system to find dissociation channels. This phenomenon leads to the concept of what we call intramolecular nonadiabatic electronic-energy redistribution, which is an electronic-state generaliztion of the notion of intramolecular vibrational energy redistribution. The other aspect is about the peculiar chemical bonding. We investigate it with the energy natural orbitals (ENOs) to see what kind of theoretical structures lie behind the huge fluctuation. The ENO energy levels representing the highly excited states under study appear to have four robust layers. We show that the energy layers responsible for chaotic dynamics and those for chemical bonding are widely separated from each other, and only when an event of what we call "inter-layer crossing" happens to burst can the destruction of these robust energy layers occur, resulting in molecular dissociation. This crossing event happens only rarely because of the large energy gaps between the ENO layers. It is shown that the layers of high energy composed of complex-valued ENOs induce the turbulent flow of electrons and electronic-energy in the cluster. In addition, the random and fast time-oscillations of those high energy ENOs serve as a random force on the nuclear dynamics, which can work to prevent a concentration of high nuclear kinetic energy in the dissociation channels.

Методика расчета синтетических спектров излучения триплетных состояний молекулярного азота в полярных сияниях

A method for calculating synthetic emission spectra of systems of triplet levels of molecular nitrogen is presented: Vegard- VK N2(A^3 Σ_u^+→X^1 Σ_g^+), the first positive system 1PG N2(〖 B〗^3 Π_g→A^3 Σ_u^+), the second positive system 2PG N2(C^3 Π_u→B^3 Π_g). The individual structures of the corresponding molecules are taken into account. The populations of the vibrational levels of the triplet states are calculated taking into account the energy transfer between triplet levels in collisional reactions and the intramolecular redistribution of energy. Examples of calculation of synthetic spectra are presented.

Quantum Chaos in the Dynamics of Molecules.

Quantum chaos is reviewed from the viewpoint of "what is molecule?", particularly placing emphasis on their dynamics. Molecules are composed of heavy nuclei and light electrons, and thereby the very basic molecular theory due to Born and Oppenheimer gives a view that quantum electronic states provide potential functions working on nuclei, which in turn are often treated classically or semiclassically. Therefore, the classic study of chaos in molecular science began with those nuclear dynamics particularly about the vibrational energy randomization within a molecule. Statistical laws in probabilities and rates of chemical reactions even for small molecules of several atoms are among the chemical phenomena requiring the notion of chaos. Particularly the dynamics behind unimolecular decomposition are referred to as Intra-molecular Vibrational energy Redistribution (IVR). Semiclassical mechanics is also one of the main research fields of quantum chaos. We herein demonstrate chaos that appears only in semiclassical and full quantum dynamics. A fundamental phenomenon possibly giving birth to quantum chaos is "bifurcation and merging" of quantum wavepackets, rather than "stretching and folding" of the baker's transformation and the horseshoe map as a geometrical foundation of classical chaos. Such wavepacket bifurcation and merging are indeed experimentally measurable as we showed before in the series of studies on real-time probing of nonadiabatic chemical reactions. After tracking these aspects of molecular chaos, we will explore quantum chaos found in nonadiabatic electron wavepacket dynamics, which emerges in the realm far beyond the Born-Oppenheimer paradigm. In this class of chaos, we propose a notion of Intra-molecular Nonadiabatic Electronic Energy Redistribution (INEER), which is a consequence of the chaotic fluxes of electrons and energy within a molecule.

Tracking Energy Transfer across a Platinum Center.

Rigid, conjugated alkyne bridges serve as important components in various transition-metal complexes used for energy conversion, charge separation, sensing, and molecular electronics. Alkyne stretching modes have potential for modulating charge separation in donor–bridge–acceptor compounds. Understanding the rules of energy relaxation and energy transfer across the metal center in such compounds can help optimize their electron transfer switching properties. We used relaxation-assisted two-dimensional infrared spectroscopy to track energy transfer across metal centers in platinum complexes featuring a triazole-terminated alkyne ligand of two or six carbons, a perfluorophenyl ligand, and two tri(p-tolyl)phosphine ligands. Comprehensive analyses of waiting-time dynamics for numerous cross and diagonal peaks were performed, focusing on coherent oscillation, energy transfer, and cooling parameters. These observables augmented with density functional theory computations of vibrational frequencies and anharmonic force constants enabled identification of different functional groups of the compounds. Computations of vibrational relaxation pathways and mode couplings were performed, and two regimes of intramolecular energy redistribution are described. One involves energy transfer between ligands via high-frequency modes; the transfer is efficient only if the modes involved are delocalized over both ligands. The energy transport pathways between the ligands are identified. Another regime involves redistribution via low-frequency delocalized modes, which does not lead to interligand energy transport.

Potential Nonstatistical Effects on the Unimolecular Decomposition of H2O2.

An attempt is made to evaluate the nonstatistical effects in the thermal decomposition of hydrogen peroxide (H2O2). Previous experimental studies on this reaction reported an unusual pressure dependence of the rate constant indicating broader falloff behavior than expected from conventional theory. In this work, the possibility that the rate constant is affected by nonstatistical effects is investigated based on classical trajectory calculations on the global potential energy surfaces of H2O2 and H2O2 + Ar. The emphasis is on the intramolecular energy redistribution from the K-rotor, that is, the external rotor for rotation around the principal axis of least moment of inertia. The calculations for the H2O2 molecules excited above the dissociation threshold suggest that the energy redistribution from the torsion and K-rotor to vibrations can be competitive with dissociation. In particular, the slow redistribution of the energy associated with the K-rotor significantly affects the dissociation rate. The successive trajectory calculations for collisions of H2O2 with Ar show that the energy associated with the K-rotor can be collisionally transferred more efficiently than the vibrational energy. On the basis of these results and several assumptions, a simple model is proposed to account for the nonstatistical effects on the pressure-dependent thermal rate constants. The model predicts significant broadening of the falloff curve of the rate constants but still cannot fully explain the experimental data.

Primary Electronic and Vibrational Dynamics of Cytochrome c Observed by Sub-10 fs NUV Laser Pulses.

The primary reaction mechanism of cytochrome c (Cyt c) was elucidated for two redox forms of ferric (oxidized) and ferrous (reduced) Cyt c by measuring their transient absorption (TA) spectra using a homemade sub-10 fs broadband NUV laser pulses system. The TA traces measured in the broad probe wavelength region were analyzed by the global analysis method to study the electronic dynamics. The difference of relaxation dynamics dependent on the excitation bandwidth enabled us to elucidate that the 2.5 ps component in ferrous Cyt c can be assigned to intramolecular vibration energy redistribution and not to vibrational cooling, which was not clear until this work. The temporal resolution of 10 fs observes TA signal modulation caused by the molecular vibration in the time domain, which can be used to calculate the instantaneous frequency of the molecular vibration mode. The observed vibrational dynamics has visualized that the heme structure changes in 0.8 ps for ferric Cyt c and in >1.0 ps for ferrous Cyt c. These estimated lifetimes of vibrational dynamics reflect vibrational relaxation in the ground state of ferric Cyt c and electronic transition from the S2 state to the S1 state in ferrous Cyt c, respectively.

Vibrational energy redistribution during donor-acceptor electronic energy transfer: criteria to identify subsets of active normal modes.

Photoinduced electronic energy transfer in conjugated donor-acceptor systems is naturally accompanied by intramolecular vibrational energy redistributions accepting an excess of electronic energy. Herein, we simulate these processes in a covalently linked donor-acceptor molecular dyad system by using nonadiabatic excited state molecular dynamics simulations. We analyze different complementary criteria to systematically identify the subset of vibrational normal modes that actively participate on the donor → acceptor (S2→ S1) electronic relaxation. We analyze energy transfer coordinates in terms of state-specific normal modes defined according to the different potential energy surfaces (PESs) involved. On one hand, we identify those vibrations that contribute the most to the direction of the main driving force on the nuclei during electronic transitions, represented by the non-adiabatic derivative coupling vector between donor and acceptor electronic states. On the other hand, we monitor normal mode transient accumulations of excess energy and their intramolecular energy redistribution fluxes. We observe that the subset of active modes varies according to the PES on which they belong and these modes experience the most significant rearrangements and mixing. Whereas the nuclear motions that promote donor → acceptor energy funneling can be localized mainly on one or two normal modes of the S2 state, they become spread out across multiple normal modes of the S1 state following the energy transfer event.

Selective excitation of vibrational modes and probe for asymmetric intramolecular energy redistribution

Femtosecond time- and frequency-resolved coherent anti-Stokes Raman scattering spectroscopy was performed on liquid nitrobenzene (NB) to explore intramolecular vibrational energy redistribution in the electronic ground state. While high-frequency C–H stretching vibrational modes (vCH ∼ 3080 cm−1) were selectively excited, vibrational energy redistribution to the nitro group and phenyl ring breathing modes ( ∼ 1335 cm−1, ∼ 1514 cm−1 and ∼1584 cm−1) was described. While lower frequency modes of the nitro group and phenyl ring breathing were selectively excited, vibrational energy redistribution to C–H stretching modes with higher efficiency was detected. There is counter-intuitive vibrational energy flow in NB. Vibrational energy redistribution was discussed in the view of coherent population transfer between parent modes. Coherent coupling of these relevant modes and the selectivity of vibrational energy redistribution were further analyzed from the viewpoint of vibrational symmetry.

Intensified C≡C Stretching Vibrator and Its Potential Role in Monitoring Ultrafast Energy Transfer in 2D Carbon Material by Nonlinear Vibrational Spectroscopy.

In this work, an intensity-enhanced C≡C stretching infrared (IR) absorption is observed in hexakis[(trimethylsilyl)ethynyl]benzene (HTEB), whose IR transition dipole magnitude becomes comparable to that of a typical C═O stretch, and the enhancement is believed to be due to a joint effect of π-π conjugation and hyperconjugation associated with a terminal trimethylsilyl group. Using dynamical time-dependent two-dimensional infrared (2D IR) spectroscopy, a picosecond intramolecular energy redistribution process is observed between two nondegenerate C≡C stretching modes, whose symmetry breaking is attributed to a noncovalent halogen-bonding interaction between HTEB and solvent CH2Cl2. The rigid structure of HTEB and limited structural dynamics are also inferred from the insignificant initial spectral diffusion value extracted from the 2D IR spectra. This work provides the first nonlinear infrared investigation of the conventionally weak C≡C stretch. The methods outlined are particularly important for detailed understanding of the structure-related processes such as vibrational energy transfer in novel C≡C species containing materials such as graphdiyne.

Ab initio surface hopping molecular dynamics on the dissociative recombination of CH3+

Ab initio molecular dynamics (AIMD) simulations were carried out to investigate dynamics in the dissociative recombination of CH3+ at the state-averaged complete active space self-consistent field (SA-CASSCF) level. Nonadiabatic transitions between adiabatic electronic states were taken into account by the Tully's fewest switches algorithm. The direct process was examined by trajectories starting from the crossing points of the potential energy surfaces of the CH3+ ground state and CH3 dissociative state, while the indirect process was investigated by trajectories starting from the approximate crossing points of the CH3 Rydberg state and dissociative states. It is shown that, in the direct process, the dominant products, CH2 + H, are generated in 96% of trajectories, while in the indirect process, the ratio of the other products, CH + 2H, CH + H2, C + H2 + H, increases extensively due to the intramolecular energy redistribution while descending through multiple potential energy surfaces, which is more consistent with the experimental observation.

Let Digons be Bygones: The Fate of Excitons in Curved π-Systems.

We explore the diverse origins of unpolarized absorption and emission of molecular polygons consisting of π-conjugated oligomer chains held in a bent geometry by strain controlled at the vertex units. For this purpose, we make use of atomistic nonadiabatic excited-state molecular dynamics simulations of a bichromophore molecular polygon (digon) with bent chromophore chains. Both structural and photoexcited dynamics were found to affect polarization features. Bending strain induces exciton localization on individual chromophore units of the conjugated chains. The latter display different transition dipole moment orientations, a feature not present in the linear oligomer counterparts. In addition, bending makes exciton localization very sensitive to molecular distortions induced by thermal fluctuations. The excited-state dynamics reveals an ultrafast intramolecular energy redistribution that spreads the exciton equally among spatially separated chromophore fragments within the molecular system. As a result, digons become virtually unpolarized absorbers and emitters, in agreement with recent experimental studies on the single-molecule level.

Energy transfer and spatial scrambling of an exciton in a conjugated dendrimer.

Photoexcitation of multichromophoric light harvesting molecules induces a number of intramolecular electronic energy relaxation and redistribution pathways that can ultimately lead to ultrafast exciton self-trapping on a single chromophore unit. We investigate the photoinduced processes that take place on a phenylene-ethynylene dendrimer, consisting of nine equivalent linear chromophore units or branches. meta-Substituted links between branches break the conjugation giving rise to weak couplings between them and to localized excitations. Our nonadiabatic excited-state molecular dynamics simulations reveal that the ultrafast internal conversion process to the lowest excited state is accompanied by an inner → outer inter-branch migration of the exciton due to the entropic bias associated with energetically equivalent conjugated segments. The electronic energy redistribution among chromophore units occurs through several possible pathways in which through-bond transport and through-space exciton hopping mechanisms can be distinguished. Besides, triple bond excitations coincide with the localization of the electronic transition densities, suggesting that the intramolecular energy redistribution is a concerted electronic and vibrational energy transfer process.

Tracking intramolecular energy redistribution dynamics in aryl halides: the effect of halide mass.

Selective excitation of C–H, C–C, CX1 and CX2 stretching vibrational modes in an orderly manner, detection of intramolecular energy redistribution and vibrational coupling in the electronic ground state of aryl halides are performed by time- and frequency-resolved Coherent Anti-Stokes Raman Scattering (CARS) spectroscopy. Intramolecular energy flow from parent modes to daughter modes is observed in the experiment. According to the experimental results, it is found that the up-hill vibrational energy flow from lower frequency modes to higher frequency ones is counterintuitive and energy redistribution efficiencies are controlled by the mass of the halide. The selectivity and directionality of energy flow are also discussed in view of vibrational symmetry.

Correction: Tracking intramolecular energy redistribution dynamics in aryl halides: the effect of halide mass

Correction for ‘Tracking intramolecular energy redistribution dynamics in aryl halides: the effect of halide mass’ by Xiaosong Liu et al., RSC Adv., 2018, 8, 29775–29780.

Control of vibrational distribution functions in nonequilibrium molecular plasmas and high-speed flows

The control of the vibrational distribution of nitrogen by energy transfer to CO2 is studied in two closely related experiments. In the first experiment, the time-resolved N2(v = 0–3) vibrational level populations and temperature in the afterglow of a diffuse filament nanosecond pulse discharge are measured using broadband coherent anti-Stokes Raman spectroscopy. The rotational–translational temperature in the afterglow is inferred from the partially rotationally resolved structure of the N2(v = 0) band. The measurements are performed in nitrogen, dry air, and their mixtures with CO2. N2 vibrational excitation in the discharge occurs by electron impact, with subsequent vibration–vibration (V–V) energy transfer within the N2 vibrational manifold, vibration–translation (V–T) relaxation, and near-resonance V–V′ energy transfer from the N2 to CO2 asymmetric stretch vibrational mode. The results show that rapid V–V′ energy transfer to CO2, followed by collisional intramolecular energy redistribution to the symmetric stretch and bending modes of CO2 and their V–T relaxation, accelerate the net rate of energy thermalization and temperature increase in the afterglow. In the second experiment, injection of CO2 into a supersonic flow of vibrationally excited nitrogen demonstrates the effect of accelerated vibrational relaxation on a supersonic shear layer. The nitrogen flow is vibrationally excited in a repetitive nanosecond pulse/DC sustainer electric discharge in the plenum of a nonequilibrium flow supersonic wind tunnel. A transient pressure increase as well as an upward displacement of the shear layer between the supersonic N2 flow and the subsonic CO2 injection flow are detected when the source of N2 vibrational excitation is turned on. CO2 injection leads to the reduction of the N2 vibrational temperature in the shear layer, demonstrating that its displacement is caused by accelerated N2 vibrational relaxation by CO2, which produces a static temperature and a pressure increase in the test section. This demonstrates the significant potential of accelerated vibrational relaxation for nonequilibrium flow control, by injection of rapid ‘relaxer’ species at a desired location, resulting in the rapid thermalization of vibrational energy in nitrogen and air flows, and producing a significant effect on the flow field.

Mechanism of short-pulse-induced solute migration in comparison to continuous-light-driven thermal diffusion

Short-pulse-induced and continuous-light-driven solute migrations are distinguished based on the composite time rate of photo absorption and intra-molecular excess energy redistribution relative to that of inter-molecular excess energy transfer.

Vibrationally Coherent Preparation of the Transition State for Photoisomerization of the Cyanine Dye Cy5 in Water

Femtosecond pump-continuum probe spectroscopy with impulsive excitation was employed to observe coherent wavepacket motions of the cyanine dye Cy5 in water that promote photoisomerization after optical preparation of the first excited singlet state, S1. The chief component in the excited-state vibrational coherence is a resonance Raman-inactive, 273 cm(-1) mode of mixed carbon-carbon bond length alternation and out-of-plane or twisting character. The ultrafast (30 fs) damping of these motions arises from trajectories that irreversibly cross the transition state barrier; after several recurrences to the transition state region, vibrational cooling traps a significant fraction of the excited-state molecules in the planar, Franck-Condon region of the potential energy surface. Motion in the 273 cm(-1) promoting mode is apparently launched by a change in conformation of the conjugated polyene backbone during the first few vibrations of the high-frequency C-C and C═C bond length alternation coordinates that principally contribute to the initial displacement from the Franck-Condon structure. To our knowledge, this work provides the first direct observations of the intramolecular redistribution of excited-state potential energy into reactive motions that are rapidly damped by transition state barrier-crossing events leading to photoisomerization in a conjugated polyene. These results provide insight into the vibrational dynamics that contribute to the photoisomerization of retinal protonated Schiff bases in the rhodopsins and to the formation of intramolecular charge transfer character in carotenoids in photosynthetic light-harvesting proteins.

Applicability of the protein environment equilibrium approximation for describing ultrafast biophysical processes

The theoretical description of ultrafast processes in biological systems, in particular, electron transfer in photosynthetic reaction centers, is an important problem in modern biological physics. Because these processes occur in a protein medium with which an energy exchange is possible, methods of the quantum theory of open systems must be used to describe them. But because of a high process rate and the specifics of the protein environment, basic approximations of this theory might be inapplicable. We study the applicability of the approximation of the protein environment (bath) state invariance for the dissipative dynamics of charge transfer between molecule-pigments contained in reaction centers. For this, we use model systems whose parameters are close to real ones. We conclude that this approximation can be used to describe both the monotonic and the oscillating dynamics of the reaction subsystem in large biological molecules. We consider various mechanisms for bath thermalization and show that the bath thermalization occurs not because of the intramolecular redistribution of the vibrational energy in it but because of its coupling to the reaction subsystem.