Intramolecular Vibrational Energy Redistribution Process Research Articles

Investigation on the vibrational relaxation and ultrafast electronic dynamics of S1 state in 2,4-difluoroanisole.

Intramolecular vibrational energy redistribution (IVR) has a profound impact on dynamic processes. We have studied two types of IVR processes, restricted and dissipative, and ultrafast dynamics of the S1 state of 2,4-difluoroanisole using time-resolved photoelectron spectroscopy and time-of-flight mass spectroscopy. The restricted IVR occurs in the intermediate regime of 219cm-1 vibrational level, and the dissipative IVR occurs in the statistical regime of 1200cm-1. The lifetimes of IVR processes are measured to be 90 and 11ps, respectively, depending on the internal energies of the S1 state and differ by a factor of eight. Similar subsequent dynamics were observed at two vibrational levels in the S1 state. The population undergoes IVR following the initial excitation and subsequently leaks into a triplet state, accompanied by intersystem crossing within ∼400ps followed by a slower nonradiative relaxation of the triplet state on the nanosecond time scale. Furthermore, the values of 3s and 3px Rydberg states of 2,4-difluoroanisole were experimentally determined to be 5.02 and 6.28eV.

Intramolecular vibrational energy redistribution in nucleobases: Excitation of NH stretching vibrations in adenine-uracil + H2O.

Redistribution of vibrational energy in the adenine-uracil base pair is studied when the base pair undergoes an intermolecular interaction with an overtone-bending vibration excited H2O(2νbend) molecule. Energy transfer is calculated using the structural information obtained from density functional theory in the solution of the equations of motion. Intermolecular vibrational energy transfer (VET) from H2O(2νbend) to the uracil-NH stretching mode is efficient and rapidly followed by intramolecular vibrational energy redistribution (IVR) resulting from coupling between vibrational modes. An important pathway is IVR carrying energy to the NH-stretching mode of the adenine moiety in a subpicosecond scale, the energy build-up being sigmoidal, when H2O interacts with the uracil-NH bond. The majority of intermolecular hydrogen bonds between the base pair and H2O are weakened but unbroken during the ultrafast energy redistribution period. Lifetimes of intermolecular HB are on the order of 0.5ps. The efficiency of IVR in the base pair is due to near-resonance between coupled CC and CN vibrations. The resonance also exists between the frequencies of H2O bend and NH stretch, thus facilitating VET. When H2O interacts with the NH bond at the adenine end of the base pair, energy flow in the reverse direction to the uracil-NH stretch is negligible, the unidirectionality discussed in terms of the effects of uracil CH stretches. The energy distributed in the CH bonds is found to be significant. The IVR process is found to be nearly temperature independent between 200 and 400K.

Elucidating the Coupling Mechanisms of Rapid Intramolecular Vibrational Energy Redistribution in Nitromethane: Ab Initio Molecular Dynamics Simulation.

Ab initio molecular dynamics simulations are presented to investigate the intramolecular vibrational energy redistribution (IVR) of an isolated nitromethane molecule. A number of IVR processes are simulated by monitoring the kinetic energy of vibrational modes under selective low-lying vibrational excitations from their ground states (Δν = 1 or 2). Evolution of the normal-mode kinetic energy gives the ultrafast energy transfer processes from parent modes to daughter modes intuitively. From the ultrafast vibrational transfer made by Fourier transformation of the time-dependent normal-mode kinetic energy, we can capture that the symmetry of the normal modes plays an important role in the anharmonic coupling between the vibrational modes. The results show three symmetry-dependent coupling mechanisms: direct symmetric coupling, overtone-assisted coupling, and rotation-assisted coupling. Furthermore, the calculated efficiencies of IVR also coincide with these mechanisms.

Ultrafast intramolecular vibrational energy transfer in carbon nitride hydrocolloid examined by femtosecond two-dimensional infrared spectroscopy.

In this work, ultrafast vibrational and structural processes in a graphitic carbon nitride hydrocolloid system were studied using a combination of linear infrared and nonlinear two-dimensional infrared (2D IR) spectroscopies. The experimentally observed three IR line shapes in the C=N stretching vibration frequency region were analyzed and attributed to the rigid and conjugated molecular frame of the prepared g-CN molecular species, which is believed to be a dimeric tris-s-triazine, as well as attributed to insignificant solvent influence on the delocalized C=N vibrations. Vibrational transition density cubes were also computed for the proposed g-CN dimer, confirming the heterocyclic C=N stretching nature of the three IR absorption peaks. Intramolecular vibrational energy transfer dynamics and spectral diffusion of the g-CN system were characterized by examining a series of time-dependent 2D IR spectra. A picosecond intramolecular vibrational energy redistribution process was found to occur among these delocalized C=N stretching modes, acting as an efficient vibrational energy transfer channel. This work reasonably connects the experimentally observed IR signature to a specific g-CN structure and also provides the first report on the ultrafast intramolecular processes of such carbon nitride systems. The obtained results are fundamentally important for understanding the molecular mechanisms of such carbon-nitride based functional materials.

Efficient Intramolecular Vibrational Excitonic Energy Transfer in Ru3(CO)12 Cluster Revealed by Two-Dimensional Infrared Spectroscopy.

Trinuclear transition-metal carbonyl complex dodecacarbonyl triruthenium (Ru3(CO)12) is considered as one of the paradigms in cluster chemistry, which plays an important role in photocatalysis, photoenergy conversion, and synthetic chemistry. Due to structural symmetry (D3h point group), 12 carbonyl (C≡O) groups in the Ru3(CO)12 complex contribute to mainly three excitonic carbonyl stretching modes: E' (radial), A2″ (axial), and E' (axial). In this work, efficient intramolecular vibrational energy redistribution (IVR) processes among the three modes in this Ru-CO complex were observed to occur on the time scale of tens of picoseconds. The IVR processes were characterized in detail using a kinetic model and fitting to the waiting-time-dependent diagonal and off-diagonal signals of ultrafast two-dimensional infrared spectroscopy. In addition, the diagonal anharmonicities of the three C≡O stretching modes were determined to be quite close to one another, and the coupling-induced cross peaks were invariant because this Ru3(CO)12 cluster does not show picosecond fluxionality and hence their contributions were neglected in modeling the IVR processes. Our results provide a benchmark for understanding the excitonic nature of the vibrational excited states of the carbonyl vibrators and the associated efficient vibrational energy-flow pathways, in such multicentered transition-metal complexes, which are of key importance to their functions.

Central-metal effect on intramolecular vibrational energy transfer of M(CO)5Br (M = Mn, Re) probed by two-dimensional infrared spectroscopy.

Vibrational energy transfer in transition metal complexes with flexible structures in condensed phases is of central importance to catalytical chemistry processes. In this work, two molecules with different metal atoms, M(CO)5Br (where M = Mn, Re), were used as model systems, and their axial and radial carbonyl stretching modes as infrared probes. The central-metal effect on intramolecular vibrational energy redistribution (IVR) in M(CO)5Br was investigated in polar and nonpolar solvents. The linear infrared (IR) peak splitting between carbonyl vibrations increases as the metal atom changes from Mn to Re. The waiting-time dependent two-dimensional infrared diagonal- and off-diagonal peak amplitudes reveal a faster IVR process in Re(CO)5Br than in Mn(CO)5Br. With the aid of density functional theory (DFT) calculations, the central-metal effect on IVR time linearly correlates with the vibrational coupling strength between the two involved modes. In addition, the polar solvent is found to accelerate the IVR process by affecting the anharmonic vibrational potentials of a solute vibration mode.

Experimental Evidence of Long-Range Intramolecular Vibrational Energy Redistribution through Eight Covalent Bonds: NIR Irradiation Induced Conformational Transformation of E-Glutaconic Acid.

Long-range intramolecular vibrational energy redistribution (IVR) driven conformational changes were investigated in a matrix-isolated open-chain, asymmetrical dicarboxylic acid, E-glutaconic acid. Although the analysis was challenging due to the presence of multiple backbone conformers and short lifetimes of the prepared higher energy cis conformers, it was shown that the selective excitation of the O-H stretching overtone of one of the carboxylic groups can induce the conformational change (trans to cis) of the other carboxylic group, located at the other end of the E-glutaconic acid molecule. This is a direct proof that the IVR process can act through eight covalent bonds in a flexible molecule before the excess energy completely dissipates into the matrix. The lifetime of the prepared higher energy conformers (averaged over the different backbones) was measured to be 12 s.

Understanding the connection between conformational changes of peptides and equilibrium thermal fluctuations.

Despite the increasing evidence that conformational transitions in peptides and proteins are driven by specific vibrational energy pathways along the molecule, the current experimental techniques of analysis do as yet not allow to study these biophysical processes in terms of anisotropic energy flows. Computational methods offer a complementary approach to obtain a more detailed understanding of the vibrational and conformational dynamics of these systems. Accordingly, in this work we investigate jointly the vibrational energy distribution and the conformational dynamics of trialanine peptide in water solution at room temperature by applying the Instantaneous Normal Mode analysis to the results derived from equilibrium molecular dynamics simulations. It is shown that conformational changes in trialanine are triggered by the vibrational energy accumulated in the low-frequency modes of the molecule, and that excitation is caused exclusively by thermal fluctuations of the solute-solvent system, thus excluding the possibility of an intramolecular vibrational energy redistribution process.

Vibrational energy relaxation of the amide I mode of N-methylacetamide in D₂O studied through Born-Oppenheimer molecular dynamics.

The vibrational relaxation of the amide I mode of deuterated N-methylacetamide in D2O solution is studied through nonequilibrium simulations using the semiempirical Born-Oppenheimer molecular dynamics (SEBOMD) approach to describe the whole solute-solvent system. Relaxation pathways and lifetimes are determined using the instantaneous normal mode (INM) analysis. The relaxation of the amide I mode is characterized by three different time scales; most of the excess energy (80%) is redistributed through intramolecular vibrational energy redistribution processes, with a smaller contribution (20%) of intermolecular energy flowing into the solvent. The amide II mode is found to contribute modestly (7%) to the relaxation mechanism. The amide I mode and the total vibrational energy decay curves obtained using SEBOMD and INM are in satisfactory agreement with the experimental measurements.

Study on the ultrafast dynamics of o-xylene cation by combined fs-photoelectron imaging-photofragmentation spectroscopy

Ultrafast dynamics of o-xylene cation has been studied by time resolved fs-photofragmentation (PF) spectroscopy in combination with photoelectron imaging (PEI). In the experiment, multiphoton ionization is used to prepare the o-xylene cation characterized by PEI. The ultrafast dynamics of o-xylene ions are measured by monitoring the time dependent parent-ion depletion and the fragment-ion formation, simultaneously. An ultrafast relaxation time of the parent ion of 734 (±61)fs has been observed. The PEI-PF measurements support the interpretation of this relaxation channel to a combination of internal conversion between the two ionic states (D0 and D1) and intramolecular vibrational-energy redistribution process within the D0 state.

Picosecond Rotational Interconversion Adjacent to a C═O Bond Studied by Two-Dimensional Infrared Spectroscopy

Molecular conformations around the C═O group of carbonyl compounds like ketones and aldehydes play an important role in determining their reaction properties in solutions, including reaction rate, mechanism, steric structure, and chirality of products. Investigating different rotational conformers and their rapid exchange at room temperature will provide information on the rotational barrier and insights into how different rotamers may contribute to fundamental reactions in chemistry. We applied two-dimensional infrared (2D IR) spectroscopy and polarization-dependent IR transient grating technique to the study of 4,4-dimethyl-2-pentanone in CCl(4). Spectroscopic evidence showed that the internal rotation around the single carbon-carbon bond adjacent to the C═O group takes place on a picosecond time scale. DFT calculations suggested the presence of three different rotational conformations, one eclipsed and two staggered forms. Spectral simulation utilized the stochastic Liouville equation with a three-state jump model and incorporated the polarization factors that take into account the different direction of transition dipole moment in the three rotamers. The effects of the intramolecular vibrational energy redistribution process on the waiting time dependence of the 2D absorptive spectra were also included. Through comprehensive simulation of the observed spectral features, the exchange time constants between the three rotamers were determined: 5.4 ps from the eclipsed to staggered forms and 1.7 ps for the reverse direction.

High-resolution infrared spectrum of jet-cooled methyl acetate in the C=O stretching region: Internal rotations of two inequivalent methyl tops

The jet-cooled high resolution infrared (IR) spectrum of methyl acetate (MA), CH(3)-C(=O)-O-CH(3), in the C=O fundamental band region was recorded by using a rapid scan IR laser spectrometer equipped with an astigmatic multipass cell. No high resolution IR analyses of the ro-vibrational transitions between the ground and non-torsionally excited vibrational states have hitherto been reported for molecules with two inequivalent methyl rotors. Because of the two chemically different methyl tops in MA, i.e., the acetyl -CH(3) and methoxy -CH(3), each rotational energy level is split into more than two torsional sublevels by internal rotations of these methyl groups. We were able to assign ro-vibrational transitions of four torsional species by using the ground state combination differences calculated from the molecular constants of the vibrational ground state recently determined by a global fit of the microwave and millimeter wave lines [M. Tudorie, I. Kleiner, J. T. Hougen, S. Melandri, L. W. Sutikdja, and W. Stahl, J. Mol. Spectrosc. 269, 211 (2011)]. The assigned lines were successfully fitted using the BELGI-Cs-IR program to an overall standard deviation which is comparable to the measurement accuracy. This study is also of interest in understanding the role of methyl rotors in the intramolecular vibrational-energy redistribution processes in mid-size organic molecules.

Conformational isomerization kinetics of pent-1-en-4-yne with 3,330 cm −1 of internal energy measured by dynamic rotational spectroscopy

We demonstrate the application of molecular rotational spectroscopy to measure the conformation isomerization rate of vibrationally excited pent-1-en-4-yne (pentenyne). The rotational spectra of single quantum states of pentenyne are acquired by using a combination of IR-Fourier transform microwave double-resonance spectroscopy and high-resolution, single-photon IR spectroscopy. The quantum states probed in these experiments have energy eigenvalues of approximately 3,330 cm(-1) and lie above the barrier to conformational isomerization. At this energy, the presence of intramolecular vibrational energy redistribution (IVR) is indicated through the extensive local perturbations found in the high-resolution rotation-vibration spectrum of the acetylenic C-H stretch normal-mode fundamental. The fact that the IVR process produces isomerization is deduced through a qualitatively different appearance of the excited-state rotational spectra compared with the pure rotational spectra of pentenyne. The rotational spectra of the vibrationally excited molecular eigenstates display coalescence between the characteristic rotational frequencies of the stable cis and skew conformations of the molecule. This coalescence is observed for quantum states prepared from laser excitation originating in the ground vibrational state of either of the two stable conformers. Experimental isomerization rates are extracted by using a three-state Bloch model of the dynamic rotational spectra that includes the effects of chemical exchange between the stable conformations. The time scale for the conformational isomerization rate of pentenyne at total energy of 3,330 cm(-1) is approximately 25 ps and is 50 times slower than the microcanonical isomerization rate predicted by the statistical Rice-Ramsperger-Kassel-Marcus theory.

Quantum Tunneling in the Midrange Vibrational Fundamentals of Tropolone

The Fourier transform infrared spectrum of tropolone(OH) vapor in the 1175-1700 cm(-1) region is reported at 0.0025 and 0.10 cm(-1) spectral resolutions. The 12 vibrational fundamentals in this region of rapidly rising vibrational state density are dominated by mixtures of the CC, CO, CCH, and COH internal coordinates. Estimates based on the measurement of sharp Q branch peaks are reported for 11 of the spectral doublet component separations DS(v) = |Delta(v) +/- Delta(0)|. Delta(0) = 0.974 cm(-1) is the known zero-point splitting, and three a(1) modes show tunneling splittings Delta(v) approximately Delta(0), four b(2) modes show splittings Delta(v) approximately 0.90Delta(0), and the remaining four modes show splittings Delta(v) falling 5-14% from Delta(0.) Significantly, the splitting for the nominal COH bending mode nu(8) (a(1)) is small, that is, 10% from Delta(0). Many of the vibrational excited states demonstrate strong anharmonic behavior, but there are only mild perturbations on the tautomerization mechanism driving Delta(0). The data suggest, especially for the higher frequency a(1) fundamentals, the onset of selective intramolecular vibrational energy redistribution processes that are fast on the time scale of the tautomerization process. These appear to delocalize and smooth out the topographical modifications of the zero-point potential energy surface that are anticipated to follow absorption of the nu(v) photon. Further, the spectra show the propensity for the Delta(v) splittings of b(2) and other complex vibrations to be damped relative to Delta(0).

Observation of a simple vibrational wavepacket in a polyatomic molecule via time-resolved photoelectron velocity-map imaging: A prototype for time-resolved IVR studies

We have prepared a coherent superposition of the two components of a Fermi resonance in the S1 state of toluene at approximately 460 cm(-1) with a approximately 1 ps laser pulse and monitored time-resolved photoelectron velocity-map images. The photoelectron intensities oscillate with time in a manner that depends on their kinetic energy, even though full vibrational resolution in the cation is not achieved. Analysis of the time-dependent photoelectron spectra enables information on the composition of the S1 wavepacket to be deduced. Such an experiment, in which a whole set of partially dispersed cation vibrational states are detected simultaneously, suggests an efficient method of studying intramolecular vibrational energy redistribution processes in excited states.

Dynamics of intramolecular vibrational redistribution in propargylchloride molecule studied by time-resolved Raman spectroscopy

We have studied the dynamics of intramolecular vibrational energy redistribution (IVR) from the initially excited mode ν 1 (acetylene-type H–C bond) in H–C C–CH 2Cl molecules in the gaseous phase by means of anti-Stokes spontaneous Raman scattering. The energy relaxation from ν 1 due to IVR was estimated to occur on the time-scale τ ≈ 750 ps, which is one of the slowest IVR time-scales reported so far. The deactivation due to molecular collisions occurred with a rate constant of ≈8.5 μs −1 Torr −1, and was slower than the IVR process. A theoretical model which is based on the idea of statistical nature of couplings and incompleteness of IVR (expressed in the form of the so-called dilution factor) is proposed. The model provides rationalization for the observed kinetics and suggests that the IVR process is mediated by not only anharmonic but also vibrational–rotational interactions.

Picosecond IR-UV pump-probe spectroscopic study on the intramolecular vibrational energy redistribution of NH2 and CH stretching vibrations of jet-cooled aniline

Intramolecular vibrational energy redistribution (IVR) of the NH2 symmetric and asymmetric stretching vibrations of jet-cooled aniline has been investigated by picosecond time-resolved IR-UV pump-probe spectroscopy. A picosecond IR laser pulse excited the NH2 symmetric or asymmetric stretching vibration of aniline in the electronic ground state and the subsequent time evolutions of the excited level as well as redistributed levels were observed by a picosecond UV pulse. The IVR lifetimes for symmetric and asymmetric stretches were obtained to be 18 and 34 ps, respectively. In addition, we obtained the direct evidence that IVR proceeds via two-step bath states; that is, the NH2 stretch energy first flows into the doorway state and the energy is further dissipated into dense bath states. The rate constants of the second step were estimated to be comparable to or slower than those of the first step IVR. The relaxation behavior was compared with that of IVR of the OH stretching vibration of phenol [Y. Yamada, T. Ebata, M. Kayano, and M. Mikami J. Chem. Phys. 120, 7400 (2004)]. We found that the second step IVR process of aniline is much slower than that of phenol, suggesting a large difference of the "doorway state increasing the dense bath states" anharmonic coupling strength between the two molecules. We also observed IVR of the CH stretching vibrations, which showed much faster IVR behavior than that of the NH2 stretches. The fast relaxation is described by the interference effect, which is caused by the coherent excitation of the quasistationary states.

Picosecond time-resolved photoelectron spectroscopy as a means of elucidating mechanisms of intramolecular vibrational energy redistribution in electronically excited states of small aromatic molecules

Preliminary findings are reported following the detailed analysis of intramolecular vibrational energy redistribution (IVR) in S1 para-fluorotoluene following the excitation of one quantum in the CF stretching mode ν7. It is shown that the method proposed has the potential to enable not only the determination of a rate constant for IVR, but also the identification of the dark states into which energy has redistributed. In addition energy flow has been observed from a bright state (111) previously thought to be below the onset for IVR in this molecule, and evidence of mode-specificity in the IVR process.

Spectroscopy and relaxation dynamics of photoexcited anisole and anisole-d3 molecules in a supersonic jet

The vibronic structures of the S1–S0 electronic transitions of jet-cooled anisole and anisole-d3 molecules have been investigated in detail using the laser induced fluorescence and single vibrational level dispersed fluorescence (DF) spectroscopy. Normal mode frequencies of the ground and excited states including the methyl and methoxy internal rotations were determined by analyses of dispersed fluorescence spectra and molecular orbital calculations. Strong vibrational mixing in the S1 state was observed in several DF spectra. Duschinsky rotation between 6a and b modes prominently appeared for both molecules and it was found that methyl deuteration depressed the second-order vibronic coupling of these modes. Another explicit Duschinsky rotation was found in 10b and 16a modes of both molecules. However, in the case of the deuterated molecule the mixing cannot be explained by only Duschinsky rotation. A Fermi resonance due to level proximity should be involved in the mixing scheme. Vibronic bands in the higher frequency regions exhibit broadened and structureless fluorescence due to intramolecular vibrational energy redistribution (IVR). The onset of the IVR process inferred from the DF spectra was found to be increased by methyl deuteration on the contrary to general propensity rule. The deuteration effect is characteristic of anisole molecules and indicates considerable decrease in the interaction with the dark bath modes. Fluorescence lifetime measurements suggest the enhancement of intersystem crossing in the levels with out-of-plane vibrational components. The observed broadening of DF spectra corresponded to non-radiative decay rates from levels with the in-plane vibrational modes. This suggests that the energy flow into out-of-plane bath modes through the IVR process should dominate non-radiative rates on initially excited in-plane vibrational levels. Our analysis clarified that the out-of-plane vibrations accompanying the methoxy motion make large contribution to the relaxation dynamics.

Classical trajectory calculations of intramolecular vibrational energy redistribution. II. Phenol-water complex.

Ab initio classical trajectory calculations have been applied to the intramolecular vibrational energy redistribution process of an O-H stretching vibration for phenol cation, [phenol]+, and its hydrogen-bonded water complex, [phenol-water]+. In phenol cation, a single narrow peak in the power spectrum, obtained by Fourier transformation of the autocorrelation function of its total momentum, indicates that the initial energy given to the O-H stretching oscillator of the phenol moiety is conserved and no energy flow occurs. On the other hand, for phenol-water cation, the calculated broadened power spectrum implies that the initial energy is not conserved and the energy flow causes an energy redistribution among various vibrational modes.