Intramolecular Vibrational Energy Redistribution Rate Research Articles

Photodissociation dynamics of m- and p-cresol in the S1 state: Interplay between the mode-randomization and H atom tunneling reaction.

The H atom tunneling dissociation dynamics of the S1 state of meta- or para-cresol has been investigated by using the picosecond time-resolved pump-probe spectroscopy in a state-specific manner. The S1 state lifetime (mainly due to the H atom tunneling reaction) is found to be mode-dependent whereas it quickly converges and remains constant as the rapid intramolecular vibrational energy redistribution (IVR) starts to participate in the S1 state relaxation with the increase of the S1 internal energy (Eint). The IVR rate and its change with increasing Eint have been reflected in the parent ion transients taken by tuning the total energy (hνpump + hνprobe) just above the adiabatic ionization threshold (so that the dissipation of the initial mode-character could be monitored as a function of the reaction time), indicating that the mode randomization rate into the S1 isoenergetic manifolds exceeds the tunneling rate quite early in terms of Eint for m-cresol (≤∼1200cm-1) or p-cresol (≤∼800cm-1) compared to the case of phenol (≤∼1800cm-1). Though the H atom tunneling dynamics of phenol (S1) seems to be little influenced by the methyl substitution on the either m- or p-position, the IVR rate has been found to be strongly accelerated due to the sharply-increasing (S1) density of states with increasing Eint due to the pivotal role of the low-frequency CH3 torsional mode.

Antenna-coupled infrared nanospectroscopy of intramolecular vibrational interaction

Many photonic and electronic molecular properties, as well as chemical and biochemical reactivities are controlled by fast intramolecular vibrational energy redistribution (IVR). This fundamental ultrafast process limits coherence time in applications from photochemistry to single quantum level control. While time-resolved multidimensional IR-spectroscopy can resolve the underlying vibrational interaction dynamics, as a nonlinear optical technique it has been challenging to extend its sensitivity to probe small molecular ensembles, achieve nanoscale spatial resolution, and control intramolecular dynamics. Here, we demonstrate a concept how mode-selective coupling of vibrational resonances to IR nanoantennas can reveal intramolecular vibrational energy transfer. In time-resolved infrared vibrational nanospectroscopy, we measure the Purcell-enhanced decrease of vibrational lifetimes of molecular vibrations while tuning the IR nanoantenna across coupled vibrations. At the example of a Re-carbonyl complex monolayer, we derive an IVR rate of (25±8) cm-1 corresponding to (450±150) fs, as is typical for the fast initial equilibration between symmetric and antisymmetric carbonyl vibrations. We model the enhancement of the cross-vibrational relaxation based on intrinsic intramolecular coupling and extrinsic antenna-enhanced vibrational energy relaxation. The model further suggests an anti-Purcell effect based on antenna and laser-field-driven vibrational mode interference which can counteract IVR-induced relaxation. Nanooptical spectroscopy of antenna-coupled vibrational dynamics thus provides for an approach to probe intramolecular vibrational dynamics with a perspective for vibrational coherent control of small molecular ensembles.

Ultrafast vibrational dynamics of the tyrosine ring mode and its application to enkephalin insertion into phospholipid membranes as probed by two-dimensional infrared spectroscopy.

Enkephalins are small opioid peptides whose binding conformations are catalyzed by phospholipid membranes. Binding to opioid receptors is determined by the orientation of tyrosine and phenylalanine side chains. In this work, we investigate the effects of different charged phospholipid headgroups on the insertion of the tyrosine side chain into a lipid bilayer using a combination of 2D IR spectroscopy, anharmonic DFT calculations, and third order response function modeling. The insertion is probed by using the ∼1515cm-1 tyrosine ring breathing mode, which we found exhibits rich vibrational dynamics on the picosecond timescale. These dynamics include rapid intramolecular vibrational energy redistribution (IVR), where some of the energy ends up in a dark state that shows up as an anharmonically shifted combination band. The waiting-time dependent 2D IR spectra also show an unusual line shape distortion that affects the extraction of the frequency-frequency correlation function (FFCF), which is the dynamic observable of interest that reflects the tyrosine side chain's insertion into the lipid bilayer. We proposed three models to account for this distortion: a hot-state exchange model, a local environment dependent IVR model, and a coherence transfer model. A qualitative analysis of these models suggests that the local environment dependent IVR rate best explains the line shape distortion, while the coherence transfer model best reproduced the effects on the FFCF. Even with these complex dynamics, we found that the tyrosine ring mode's FFCF is qualitatively correlated with the degree of insertion expected from the different phospholipid headgroups.

Vibrational predissociation of aniline-Inert gas cluster cations

The molecular structures and predissociation dynamics of aniline (Phenylamine, PhNH2)-inert gas cluster cations PhNH2X+ (X=Ne, Ar, and Kr) were investigated by infrared (IR) spectroscopy coupled with density functional theory (DFT) calculations and IR photofragmentation studies. The ring isomer was the minimum energy structure for PhNH2Ne+ and PhNH2Ar+ whereas the NH isomer was more stable in PhNH2Kr+. The decay constants of PhNH2X+ for ejection of X excited with IR depended strongly on the structure of PhNH2X+, presumably because the rate of intramolecular vibrational energy redistribution (IVR) was governed by the structure. As a result, the dissociation was faster for the larger PhNH2X+ clusters, because the rate of IVR was faster for the NH type structure than the ring type. The decay constants deviate from those expected from the Rice–Ramsperger–Kassel–Marcus (RRKM) theory calculation by the factors of 104 or larger, manifesting that the dissociation occurred mainly via nonstatistical pathways.

The VENUS/NWChem software package. Tight coupling between chemical dynamics simulations and electronic structure theory

The interface for VENUS and NWChem, and the resulting software package for direct dynamics simulations are described. The coupling of the two codes is considered to be a tight coupling since the two codes are compiled and linked together and act as one executable with data being passed between the two codes through routine calls. The advantages of this type of coupling are discussed. The interface has been designed to have as little interference as possible with the core codes of both VENUS and NWChem. VENUS is the code that propagates the direct dynamics trajectories and, therefore, is the program that drives the overall execution of VENUS/NWChem. VENUS has remained an essentially sequential code, which uses the highly parallel structure of NWChem. Subroutines of the interface that accomplish the data transmission and communication between the two computer programs are described. Recent examples of the use of VENUS/NWChem for direct dynamics simulations are summarized. Program summaryProgram title: VENUS/NWChemCatalogue identifier: AERS_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AERS_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Open Source Educational Community LicenseNo. of lines in distributed program, including test data, etc.: 10,831,970No. of bytes in distributed program, including test data, etc.: 77,141,871Distribution format: tar.gzProgramming language: Fortran 77 with some C in NWChem, MPI.Computer: All Linux based workstations and parallel supercomputers.Operating system: Linux.Has the code been vectorized or parallelized?: Venus is a sequential code; NWChem can run in parallel.Classification: 16.8. Subprograms used:Cat IdTitleReferenceAEGI_v1_0NWChemCPC 181(2010)1477Nature of problem:Direct dynamics simulations play an important role in investigating and understanding atomic-level chemical dynamics information such as atomistic reaction mechanisms, unimolecular and bimolecular rate constants, intramolecular vibrational energy redistribution rates, etc. The ability to couple direct dynamics with electronic structure methods brings a level of fidelity to the simulations that is important for complex systems. However, a tight coupling between two codes that have their own development teams and schedules can be challenging.Solution method:The VENUS/NWChem interface is designed to link the general electronic structure program (NWChem) and classical chemical dynamics simulation program (VENUS) to perform direct dynamics simulation in which the trajectories “on the fly” with the potential and its derivatives obtained directly from electronic structure theory. One of the design goals is to build interfaces that require as little interference in NWChem and VENUS as possible so that each of the code developments can continue independently. This is especially important since VENUS is currently a sequential code and NWChem is a parallel code and being able to compute the energies, gradients, and Hessian in parallel is an important aspect of making the software useful to users. In this manuscript, the tight coupling interface between the two codes is described and examples of its use are given. In the classical chemical dynamics simulation an ensemble of trajectories is calculated, and the initial sampling represents the conditions of the reactants for the chemical reaction under investigation. Each trajectory is evaluated by numerically integrating either Hamilton’s or Newton’s equations of motion. The Schrödinger equation is solved and the energy and energy gradient are calculated in the electronic structure program (NWChem), and this information is passed to the classical trajectory program (VENUS) to solve the equations of motion.Additional comments:Full documentation is provided in the distribution file. This includes a README file giving the names and brief description of all the files that make up the package and instructions on the installation and execution of the program. Sample input and output data for test run will also be provided.The software is free to download and use once a signed license agreement has been received. The agreement will be displayed when the program is requested.Running time:The running time depends on the size of the chemical system, simulation time, complexity of the ab initio method and number of CPUs. The ab initio method is the most time consuming part of each step in the calculations and scaling for different types of systems and levels of theory is available in Valiev et al. (2010). Again, there are many factors that affect the running time for the full simulation and it can range from several hours for simulations of a few atoms with DFT and a small basis set running on a single compute node to several days for the simulation of tens of heavy atoms with larger basis set running parallel.

A classical trajectory study of the intramolecular dynamics, isomerization, and unimolecular dissociation of HO2

The classical dynamics and rates of isomerization and dissociation of HO2 have been studied using two potential energy surfaces (PESs) based on interpolative fittings of ab initio data: An interpolative moving least-squares (IMLS) surface [A. Li, D. Xie, R. Dawes, A. W. Jasper, J. Ma, and H. Guo, J. Chem. Phys. 133, 144306 (2010)] and the cubic-spline-fitted PES reported by Xu, Xie, Zhang, Lin, and Guo (XXZLG) [J. Chem. Phys. 127, 024304 (2007)]. Both PESs are based on similar, though not identical, internally contracted multi-reference configuration interaction with Davidson correction (icMRCI+Q) electronic structure calculations; the IMLS PES includes complete basis set (CBS) extrapolation. The coordinate range of the IMLS PES is limited to non-reactive processes. Surfaces-of-section show similar generally regular phase space structures for the IMLS and XXZLG PESs with increasing energy. The intramolecular vibrational energy redistribution (IVR) at energies above and below the threshold of isomerization is slow, especially for O-O stretch excitations, consistent with the regularity in the surfaces-of-section. The slow IVR rates lead to mode-specific effects that are prominent for isomerization (on both the IMLS and XXZLG) and modest for unimolecular dissociation to H + O2 (accessible only on the XXZLG PES). Even with statistical distributions of initial energy, slow IVR rates result in double exponential decay for isomerization, with the slower rate correlated with slow IVR rates for O-O vibrational excitation. The IVR and isomerization rates computed for the IMLS and XXZLG PESs are quantitatively, but not qualitatively, different from one another with the largest differences ascribed to the ~2 kcal/mol difference in the isomerization barrier heights. The IMLS and XXZLG results are compared with those obtained using the global, semi-empirical double-many-body expansion DMBE-IV PES [M. R. Pastrana, L. A. M. Quintales, J. Brandão, and A. J. C. Varandas, J. Chem. Phys. 94, 8073 (1990)], for which the surfaces-of-section display more irregular phase space structure, much faster IVR rates, and significantly less mode-specific effects in isomerization and unimolecular dissociation. The calculated IVR results for all three PESs are reasonably well represented by an analytic, coupled three-mode energy transfer model.

Effect of ring torsion on intramolecular vibrational redistribution dynamics of 1,1′-binaphthyl and 2,2′-binaphthyl

The role of ring torsion in the enhancement of intramolecular vibrational energy redistribution (IVR) in aromatic molecules was investigated by conducting excitation and dispersed fluorescence spectroscopy of 1,1'-binaphthyl (1,1'-BN) and 2,2'-BN. The dispersed fluorescence spectra of 1,1'-BN in the origin region of S(1)-S(0) were well resolved, which presented 25-27 cm(-1) gaps of torsional mode in the ground state. The overall profile of the dispersed spectra of 1,1'-BN is similar to that of naphthalene. In contrast, the spectra of 2,2'-BN were not resolved due to the multitude of the active torsional modes. In both cases, dissipative IVR was observed to take place with a relatively small excess vibrational energy: 237.5 cm(-1) for 1,1'-BN and 658 cm(-1) for 2,2'-BN, which clearly shows that ring torsion efficiently enhances the IVR rate. Ab initio and density functional theory calculations with medium-sized basis sets showed that the torsional potential of 1,1'-BN has a very flat minimum over the range of torsional angles from ca. 60° to 120°, whereas that of 2,2'-BN showed two well-defined potential minima at ca. 40° and 140°, in resemblance to the case of biphenyl. In this work, we propose that aromatic molecules be classified into "strong" and "weak" torsional hindrance cases: molecules with strong hindrance case show shorter torsional progressions and more effective IVR dynamics than do those with weak hindrance.

Quantum dynamics simulations of energy redistribution in HO–SO 2

Quantum dynamics simulations of HO–SO 2 using the coupled coherent state methodology are described in detail. Motivated by the assumption of fast intramolecular vibrational energy redistribution (IVR) within the nascent collision complex in measurements of the association rate coefficients using the ‘proxy’ method, we examine IVR within HO–SO 2. Like our earlier classical dynamics calculations [D.R. Glowacki, S.K. Reed, M.J. Pilling, D.V. Shalashilin, E. Martínez-Núñez, Phys. Chem. Chem. Phys. 11 (2009) 963], the quantum dynamics results suggest that OH vibrational excitation is deactivated within HO–SO 2 prior to its dissociation, although the quantum IVR rates are greater than those in the classical simulations. The ubiquitous question of zero point energy in classical dynamics calculations is also considered. Reducing the quantity of zero point energy included in classical dynamics calculations decreases the HO–SO 2 dissociation rate and increases the deactivation of the OH stretch thereby producing vibrational energy distributions for the dissociated OH that more closely resemble those from the quantum dynamics calculations.

Rotational Contour Analysis of Jet-Cooled Methyl Hydroperoxide Action Spectra in the Region of the 2νOH and 3νOH Bands

State-selected photodissociation is used to record the partially rotationally resolved action spectra of CH(3)OOH in the region of its first and second OH-stretching overtones (2nu(OH) and 3nu(OH)) under free-jet expansion conditions. From an analysis of the rotational band contours for the OH-stretching states and their corresponding COOH torsion combination bands, effective rotational constants and transition dipole moment orientations are determined for the vibrational eigenstates. The level splitting between the lowest symmetric and antisymmetric pair of COOH torsion levels, 0(+) and 0(-), associated with the 2nu(OH) overtone state is found to be approximately 3.9 cm(-1). Comparison of spectra in the region of the 2nu(OH) and 2nu(OH) + nu(COOH) bands in CH(3)OOH and CD(3)OOH reveals that the spectral features in CH(3)OOH are substantially more perturbed compared to those of its deuterated counterpart, suggesting that modes involving the methyl rotor contribute significantly to promoting intramolecular vibrational energy redistribution (IVR) in CH(3)OOH. Furthermore, a comparison of the average rotational line widths in both CH(3)OOH and CD(3)OOH for the 2nu(OH) and 2nu(OH) + nu(COOH) bands appears to suggest that at these energies, adding one quanta of low-frequency COOH torsional motion does not enhance the IVR rate relative to that of the pure OH-stretching overtone.

Picosecond IR-UV pump–probe spectroscopic study on the vibrational energy flow in isolated molecules and clusters

Intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP) from the XH stretching vibrations, where X refers to O or C atom, of aromatic molecules and their hydrogen(H)-bonded clusters are investigated by picosecond time-resolved IR-UV pump probe spectroscopy in a supersonic beam. For bare molecules, we mainly focus on IVR of the OH stretch of phenol. We describe the IVR of the OH stretch by a two-step tier model and examine the effect of the anharmonic coupling strength and the density of states on IVR rate and mechanism by using isotope substitution. In the H-bonded clusters of phenol, we show that the relaxation of the OH stretching vibration can be described by a stepwise process and then discuss which process is sensitive to the H-bonding strength. We discuss the difference/similarity of IVR/VP between the "donor" and the "acceptor" sites in phenol-ethylene cluster by exciting the CH stretch vibrations. Finally, we study the vibrational energy transfer in the isolated molecules having the alkyl chain, namely phenylalcanol (PA). In this system, we measure the rate constant of the vibrational energy transfer between the OH stretch and the vibrations of benzene ring which are connected at the both ends of the alkyl chain. This energy transfer can be called "through-bond IVR". We investigate the three factors which are thought to control the energy transfer rate; (1) "OH <--> next CH(2)" coupling, (2) chain length and (3) conformation. We discuss the energy transfer mechanism in PAs by examining these factors.

Picosecond IR-UV pump-probe spectroscopic study of the dynamics of the vibrational relaxation of jet-cooled phenol. I. Intramolecular vibrational energy redistribution of the OH and CH stretching vibrations of bare phenol.

The intramolecular vibrational energy redistribution (IVR) of the OH stretching vibration of jet-cooled phenol-h6 (C6H8OH) and phenol-d8 (C6D8OH) in the electronic ground state has been investigated by picosecond time-resolved IR-UV pump-probe spectroscopy. The OH stretching vibration of phenol was excited with a picosecond IR laser pulse, and the subsequent temporal evolutions of the initially excited level and the redistributed ones due to the IVR were observed by multiphoton ionization detection with a picosecond UV pulse. The IVR lifetime for the OH stretch vibration of phenol-h6 was determined to be 14 ps, while that of the OH stretch for phenol-d8 was found to be 80 ps. This remarkable change of the IVR rate constant upon the dueteration of the CH groups strongly suggests that the "doorway states" for the IVR from the OH level would be the vibrational states involving the CH stretching modes. We also investigated the IVR rate of the CH stretching vibration for phenol-h6. It was found that the IVR lifetime of the CH stretch is less than 5 ps. The fast IVR is described by the strong anharmonic resonance of the CH stretch with many other combinations or overtone bands.

Intramolecular vibrational energy redistribution and intermolecular energy transfer of benzene in supercritical CO 2: measurements from the gas phase up to liquid densities

Femtosecond pump probe spectroscopy was employed to measure intramolecular vibrational energy redistribution (IVR) and intermolecular vibrational energy transfer (VET) of benzene in the gas phase and in supercritical (sc) CO 2. We observe two IVR time scales the faster of which proceeds within τ IVR (1)<0.5 ps . The slower IVR component has a time constant of τ IVR (2)=(48±5) ps in the gas phase and in scCO 2 is accelerated by interactions with the solvent. At the highest CO 2 density it is reduced to τ IVR (2)=(6±1) ps . The corresponding IVR rate constants show a similar density dependence as the VET rate constants. Model calculations suggest that both quantities correlate with the local CO 2 density in the immediate surrounding of the benzene molecule.

Vibrational Dynamics of Terminal Acetylenes: I. Comparison of the Intramolecular Vibrational Energy Redistribution Rate of Gases and the Total Relaxation Rate of Dilute Solutions at Room Temperature

The population relaxation rate of the first excited state of the acetylenic C−H stretch is compared for room-temperature gas-phase and solution-phase samples of 10 terminal acetylenes. The gas-phase sample pressure is less than 1 atm for all measurements, ensuring that the dynamics occur under collision-free conditions. The relaxation rates are measured using two-color transient absorption picosecond spectroscopy. The population of the excited state is monitored directly through the anharmonically shifted v = 1−v = 2 excited-state transition. The relaxation rates of isolated and solvated molecules are strongly correlated and follow a relationship expected for a parallel relaxation process in solution: the total rate in solution is the sum of a molecule-dependent rate related to the isolated molecule dynamics and a molecule-independent solvent-induced relaxation rate. For the terminal acetylenes, the vibrational normal-mode frequencies of the acetylene chromophore and their anharmonic interactions are highly conserved for all terminal acetylenes. Therefore, the observation that a single solvent relaxation contribution to the total relaxation rate describes the solution dynamics for all terminal acetylenes is consistent with the idea that solvent-induced energy relaxation pathways are dominated by the vibrational motions that are in close proximity to the excited state.

Intra- and Intermolecular Vibrational Energy Relaxation of C−H Overtone Excited Benzonitrile,para-Difluorobenzene, and Pyrazine in Solution

Femtosecond IR-pump−UV-probe spectroscopy allows us to directly observe the intramolecular vibrational energy redistribution (IVR) and the intermolecular vibrational energy transfer (VET) of selectively excited aromatic molecules in solution (CF2ClCFCl2). In this article, we report global IVR and VET rate coefficients for benzonitrile, p-difluorobenzene, and pyrazine in a weakly interacting (nonpolar) solvent which are excited in overtones or combination bands of C−H stretch vibrations. The experimental findings are compared with recent results for benzene, toluene, and α,α,α-trifluorotoluene. While we have found a characteristic variation of the relaxation dynamics of the aromatic systems upon chemical substitution, the remarkable similarity of the time scales of IVR for a large group of molecules investigated is striking. Furthermore, the intermolecular vibrational relaxation is characteristically different for different chemical substitutions; however, the correlation of VET rates with the presence of ...

Structure and vibrational dynamics of the halogen-substituted butynes HCCCH2CH2X (X = F,Cl,Br) from molecular-beam high-resolution microwave and infrared spectroscopyElectronic supplementary information (ESI) available: Microwave transition frequencies. See http://www.rsc.org/suppdata/cp/b2/b211597b/

The pure rotational and high-resolution acetylenic C–H stretch rovibrational spectra of a series of substituted butynes, HCCCH2CH2X (X = F,Cl,Br), are reported. For each of the molecules the pure rotational spectrum of two conformational isomers (trans and gauche) has been assigned. For the Cl and Br substituted compounds the pure rotational spectrum of two isotopic species has been assigned (35Cl, 37Cl and 79Br, 81Br) for each conformer. An analysis of the nuclear quadrupole hyperfine structure in the pure rotational spectrum shows good agreement with structural parameters obtained through electronic structure calculations. The rotational transitions are used to obtain full rotational assignment of the acetylenic C–H stretch vibrational band of the more stable trans conformers through infrared-microwave double-resonance spectroscopy. The rotational band contours are predominantly a-type for all three halobutynes. The rotational structure of the band displays separations characteristic of the trans conformer. This result reflects the fact that the bright state for the vibrational spectrum retains the ground state conformation. From the eigenstate-resolved spectra we determine the timescale for intramolecular vibrational energy redistribution (IVR). The IVR rate obtained from the high-resolution spectrum provides an upper limit to the isomerization rate following coherent vibrational excitation. The IVR lifetimes of the acetylenic C–H stretch have been determined to be 1.5 ns for trans-4-fluorobut-1-yne, 3.5 ns for trans-4-chlorobut-1-yne, and 2.0 ns for trans-4-bromobut-1-yne. In all cases, the upper limit isomerization rate inferred from the vibrational spectrum is three orders-of-magnitude slower than the RRKM rate calculated using the ab initio barrier heights. Although direct dynamical information about the isomerization rate cannot be obtained from the spectrum, evidence is found for conformational isomerization occurring through the J-dependent growth of the measured rovibrational state density.

Vibrational Energy Relaxation of Selectively Excited Aromatic Molecules in Solution: The Effect of a Methyl Rotor and Its Chemical Substitution

Transient femtosecond IR-pump-UV-probe spectroscopy is employed to investigate the intramolecular vibrational energy redistribution (IVR) and the intermolecular vibrational energy transfer (VET) of benzene, toluene (CH 3 -C 6 H 5 ), and α,α,α,-trifluorotoluene (CF 3 -C 6 H 5 ) selectively excited in overtones or combination bands of C-H stretch vibrations in solution. Global IVR and VET rate coefficients are derived from the measured transient absorption profiles using a simple kinetic model. The study reveals the effect of a methyl rotor and the effect of methyl rotor fluorination on the mechanisms and time scales of IVR and VET in aromatic model systems. For the present case, it turned out that the methyl rotor in toluene is not simply an enhancer for IVR; however, its fluorination accelerates IVR significantly. These results suggest that the methyl rotor effect on an aromatic ring in solution is more subtle than expected from previous gas-phase studies. In particular, the corresponding relaxation rates in this case are not simply governed by the number of lowest order resonances, such as found for aliphatic molecules. Instead, in aromatic molecules also, the very large number of higher order anharmonic resonances may play a pronounced role. Because the IVR rates are not at all correlated with the total density of states, we conclude that intramolecular vibrational energy relaxation of a zeroth order C-H stretch overtone or combination vibration in these molecules is not in its statistical limit and that hierarchical IVR, such as known for isolated molecules, still survives to some extent in solution. Our results further suggest that VET rates are not always simply correlated with the lowest frequency modes of the molecules.

Intramolecular vibrational redistribution of CH 2I 2 dissolved in supercritical Xe

Intramolecular vibrational energy redistribution (IVR) of CH 2I 2 in supercritical Xe has been studied. The first overtone of the C–H stretching mode was excited with a near infrared laser pulse and the transient UV absorption near 390 nm was monitored. Signals showed a rise and decay profile, which gave the IVR and VET (intermolecular vibrational energy transfer) rates, respectively. Solvent density dependence of each rate was obtained by tuning the pressure at a constant temperature. The IVR rate in supercritical Xe increased with increasing solvent density and asymptotically reached a limiting value. This result suggests that the IVR process of CH 2I 2 in condensed phase is a solvent-assisted process.

Real-Time Observation of Intra- and Intermolecular Vibrational Energy Flow of Selectively Excited Alkyl Iodides in Solution: The Effect of Chemical Substitution

Intramolecular vibrational energy redistribution (IVR) and intermolecular vibrational energy transfer (VET) of two alkyl iodides (CF3CH2I and CH3CH2I) selectively excited in the two quanta overtone region of the CH stretch vibration (υCH = 2) were measured in real time in solution. In this study, we have focused on the effect of chemical substitution on the mechanisms and time scales of IVR and VET of this family of molecules. With a simple model, we have obtained global IVR and VET rate coefficients for both molecules. The magnitude and the variation of the relaxation rates upon chemical substitution provide evidence for a survival of hierarchical IVR in these solvated molecules, which is governed by specific low-order intramolecular interactions and which can be rationalized with a simple low-order coupling model. At the same time, the general assumption that VET is simply dominated by the lowest-frequency modes in a molecule is not supported.

Intramolecular and intermolecular vibrational energy relaxation of CH 2I 2 dissolved in supercritical fluid

A pump–probe experiment was performed to examine vibrational population relaxation of diiodomethane (CH 2I 2) molecule dissolved in supercritical CO 2. Using an apparatus with femtosecond time resolution, we observed the contributions of intramolecular vibrational energy redistribution (IVR) and intermolecular vibrational energy transfer (VET) separately. IVR and VET rates were measured with varying solvent densities at a constant temperature. It is shown that the IVR rate is not density dependent while the VET rate increases with increasing density from 0.4 to 0.8 g cm −3 . This observation suggests that the rate of the VET process is determined by solute–solvent collisions whereas the IVR rate is not much affected by solute–solvent interaction.

Theoretical Studies of the Decomposition of RDX in Liquid Xenon

The unimolecular dissociation of RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) in liquid xenon is investigated to determine condensed-phase effects on the N−N bond fission and ring-opening reactions. The dependence of the rate constants on pressure at a fixed temperature is studied using molecular dynamics simulations, and the result is consistent with the experimental finding that the ring-opening channel is suppressed in a condensed-phase environment. The effects of intramolecular vibrational energy redistribution (IVR) and intermolecular energy transfer on reaction rates are also studied by putting a “hot” RDX molecule in liquid xenon. The reaction rates are calculated using a statistical approach and direct simulations. The statistical rate for the bond fission is 45% larger than the corresponding dynamical one, indicating that the rate of IVR is not faster than that of reaction.