Conformational dependence of intramolecular vibrational redistribution in methanol

Previous state-selected spectra of methanol in the 5nu(1) OH stretch overtone region [O. V. Boyarkin, T. R. Rizzo, and D. S. Perry, J. Chem. Phys. 110, 11346 (1999)] revealed a structure indicating an intramolecular vibrational redistribution on three time scales. Whereas in that work, methanol in the 5nu(1) bright state was prepared close to the staggered conformation, methanol in the "partially eclipsed" conformation is prepared here by double resonance excitation through a torsionally excited intermediate state. The excited molecules are detected by infrared laser assisted photofragment spectroscopy. In partially eclipsed methanol, the strong coupling of the nu(1) OH stretch to the nu(2) CH stretch becomes weaker, but the coupling responsible for the widths of the narrowest features becomes stronger.     

Solvent-hindered intramolecular vibrational redistribution

Ultrafast two-dimensional infrared spectroscopy and molecular dynamics simulations of Mn(2)(CO)(10) in a series of linear alcohols reveal that the rate of intramolecular vibrational redistribution among the terminal carbonyl stretches is dictated by the average number of hydrogen bonds formed between the solute and solvent. The presence of hydrogen bonds was found to hinder vibrational redistribution between eigenstates, while leaving the overall T(1) relaxation rate unchanged.     

Torsion-vibration coupling in methanol: diabatic behavior in the CH overtone region

Through a fit to methanol CH overtone data, a previously developed 4-dimensional torsion-vibration Hamiltonian is extended to high CH stretch excitation as well as to high torsional excitation. The strength of the torsion-vibration coupling is found to increase with CH stretch excitation. Systematic patterns of near degeneracy (3-, 4-, and 6-fold) are found in different regions of quantum number space. In the region of the CH fundamentals, an approximate a diabatic separation of the torsion (slow degree of freedom) from the CH stretches (fast degrees of freedom) accounts for the pattern of the energy levels and for the signs of the torsional tunneling splittings. For the higher CH overtones (v(CH) > or = 4), a diabatic representation accounts for the torsional structure obtained from the fully coupled calculation and for certain trends found in the pattern of the energy levels.     

Overlapping resonances in the control of intramolecular vibrational redistribution

Coherent control of bound state processes via the interfering overlapping resonance scenario [Christopher et al., J. Chem. Phys. 123, 064313 (2006)] is developed to control intramolecular vibrational redistribution (IVR). The approach is applied to the flow of population between bonds in a model of chaotic OCS vibrational dynamics, showing the ability to significantly alter the extent and rate of IVR by varying quantum interference contributions.     

Coriolis interactions and intramolecular vibrational redistribution in jet-cooled pyrimidine

Evidence is presented which indicate that the second-order Coriolis interaction play an important role in intramolecular vibrational redistribution.     

Fluorescence polarization and intramolecular vibrational redistribution in pyrimidine vapor

The fluorescence emitted from the initially prepared vibronic level (IPL) in S₁ of pyrimidine vapor is polarized and the degree of polarization varies remarkably for excitation along the rotational contour of various vibronic absorption bands, whereas the broad fluorescence emitted from levels to which the molecule is transferred from the IPL as a result of intramolecular vibrational redistribution (IVR) is nearly depolarized. On the basis of these results, the role of molecular rotation in IVR is discussed.     

Fluorescence spectra and intramolecular vibrational redistribution in jet-cooled perylene

The jet-cooled fluorescence spectra of perylene excited to the S₁ state with E_vib = 0–1600 cm^?1 are recorded and analyzed. For E_vib <800 cm^?1 only the resonant fluorescence was detected. Ground- and excited-state frequencies of 14 low-frequency normal modes are determined. A drastic change in frequency of the “butterfly” modes upon electronic excitation shows that perylene slightly deviates from planarity in its ground state and is more rigid in the excited singlet state. For a number of levels in the E_vib = 800–1600 cm^?1 range, the fluorescence is composed of the resonant emission and of non-resonant (“‘relaxed’”) bands. It is shown that apparently single bands in the fluorescence-excitation spectrum correspond to ovelapping bands pumping different molecular eigenstates resulting from the intrastate coupling. The relative role of the anharmonicity and of the Coriolis interaction are discussed. The data are treated in terms of a selective coupling between doorway and hallway states with the coupling constant rapidly decreasing with the difference in the overall vibrational quantum number between initial and final state.     

Vibrational stretch—bend coupling and the adiabatic approximation

Two different adiabatic methods are applied to the calculation of energy levels for a two-dimensional system modeling an anharmonic XH stretch and an associated harmonic bend coupled through centrifugal stretching. The two methods are compared in their ability to yield accurate eigenvalues for the first several overtones and combination bands, and are both shown to be easily extendible to handle cases of Fermi resonance.     

Coriolis interaction and intramolecular vibrational redistribution: A high-resolution spectroscopic probe of the rotational contribution to vibrational

We present here high-resolution fluorescence excitation spectra of the 12₀² band of pyrimidine in a molecular beam, which provide compelling ev     

Ultrafast dynamics through conical intersections and intramolecular vibrational energy redistribution in styrene

We report a femtosecond time-resolved photoelectron spectroscopy (TRPES) investigation of internal conversion in the first two excited singlet electronic states of styrene. We find that radiationless decay through an S(1)/S(0) conical intersection occurs on a timescale of ～4 ps following direct excitation to S(1) with 0.6 eV excess energy, but that the same process is significantly slower (～20 ps) if it follows internal conversion from S(2) to S(1) after excitation to S(2) with 0.3 eV excess energy (0.9 eV excess energy in S(1)).     

Theoretical investigation of intramolecular vibrational energy redistribution in highly excited HFCO

The present paper is devoted to the simulations of the intramolecular vibrational energy redistribution (IVR) in HFCO initiated by an excitation of the out-of-plane bending vibration [nnu(6)=2,4,6,...,18,20]. Using a full six-dimensional ab initio potential energy, the multiconfiguration time-dependent Hartree (MCTDH) method was exploited to propagate the corresponding six-dimensional wave packets. This study emphasizes the stability of highly excited states of the out-of-plane bending mode which exist even above the dissociation threshold. More strikingly, the structure of the IVR during the first step of the dynamics is very stable for initial excitations ranging from 2nu(6) to 20nu(6). This latter result is consistent with the analysis of the eigenstates obtained, up to 10nu(6), with the aid of the Davidson algorithm in a foregoing paper [Iung and Ribeiro, J. Chem. Phys. 121, 174105 (2005)]. The present study can be considered as complementary to this previous investigation. This paper also shows how MCTDH can be used to predict the dynamical behavior of a strongly excited system and to determine the energies of the corresponding highly excited states.     

Manifestation of intramolecular vibrational energy redistribution on electronic relaxation in large molecules

Some universal characteristics are discussed of the decay lifetimes and fluorescence quantum yields from the S₁ manifold of large molecules, which originate from the coupling between intrastate vibrational energy redistribution and interstate electronic relaxation. The time-resolved total fluorescence decay from the S₁ state of jet-cooled 9-cyanoanthracene exhibits non-exponential decay in the energy range E_v= 1200–1740 cm⁻¹ above the S₁ origin, which does not originate from dephasing but rather manifests the effects of intrastate intermediate level structure for vibrational energy redistribution on intersystem crossing.     

Iridium metal complexes as an unambiguous probe of intramolecular vibrational redistribution

Ultrafast luminescence spectroscopy has been undertaken on three iridium cored phosphorescent complexes, with the Ir(ppy)3 molecule being compared with two Ir(ppy)3 cored dendrimers. Energy dissipation by intramolecular vibrational redistribution (IVR) and cooling shows as a luminescence decay because it decreases the admixture of singlet character to the emitting triplet state. A larger amount of vibrational energy dissipates by IVR in dendrimer complexes. We have therefore found a methodology of obtaining unambiguous information on the IVR process and show its potential to study IVR rates as a function of vibrational energy.     

On the selective elimination of intramolecular vibrational redistribution using strong resonant laser fields

A method is proposed for the selective elimination of intramolecular vibrational redistribution (IVR) in polyatomic molecules by using a strong resonant laser excitation. When the Rabi frequency is larger than the frequency spread of an isolated group of molecular eigenstates, IVR processes are totally suppressed, and the molecule simply oscillates between the ground state and the doorway state. The method may have direct implications on laser-selective chemistry.     

Quasi-classical trajectory simulations of intramolecular vibrational energy redistribution in HONO2 and DONO2

By use of an analytic potential energy surface developed in this work for nitric acid, the quasi-classical trajectory method was used to simulate intramolecular vibrational energy redistribution (IVR). A method was developed for monitoring the average vibrational energy in the OH (or OD) mode that uses the mean-square displacement of the bond length calculated during the trajectories. This method is effective for both rotating and nonrotating molecules. The calculated IVR time constant for HONO(2) decreases exponentially with increasing excitation energy, is almost independent of rotational temperature, and is in excellent agreement with the experimental determination (Bingemann, D.; Gorman, M. P.; King, A. M.; Crim, F. F. J. Chem.Phys. 1997, 107, 661). In DONO(2), the IVR time constants show more complicated behavior with increasing excitation energy, apparently due to 2:1 Fermi-resonance coupling with lower frequency modes. This effect should be measurable in experiments.     

The intramolecular vibrational energy redistribution threshold in S(1) deuterated p-difluorobenzene

The intramolecular vibrational energy redistribution (IVR) in S(1) deuterated p-difluorobenzene (pDFB-d(4) or -d(4)) has been studied to determine the IVR threshold. For this, the S(1) <-- S(0) fluorescence excitation (FE) spectrum of jet-cooled d(4) was investigated in the 2000-3250 cm(-1) vibronic energy range of the S(1) electronic state, and single vibronic level fluorescence (SVLF) spectra have been acquired by exciting selected levels lying between 750 and 2850 cm(-1) in vibrational energy in the S(1) excited state. Congestion of the dispersed fluorescence in this molecule first appears as the vibrational level energy climbs above 2000 cm(-1). By comparing the SVLF spectra of pDFB-d(4) with those of p-difluorobenzene (pDFB or -h(4)), it is obvious that IVR threshold in -d(4) is localized with a few hundreds cm(-1) lower than that in pDFB. This decrease is entirely due to the increase in vibrational state density due to deuteration.     

Effects of rapid intramolecular vibrational redistribution on molecular spectra at microwave frequencies

Some molecules with more than 10 atoms and more than two torsional degrees of freedom have state densities sufficient for rapid (10¹⁰ s^?1) intramolecular vibrational redistribution at energies as low as 0.25 kcal/mol. Predicted features of low-resolution microwave (LRMW) band spectra of rapidly relaxing polar prolate molecules are discussed and compared with LRMW spectra of ethyl esters.     

Theoretical investigation of highly excited vibrational states in DFCO: calculation of the out-of-plane bending states and simulation of the intramolecular vibrational energy redistribution

A previously developed modified Davidson scheme [C. Iung and F. Ribeiro, J. Chem. Phys. 121, 174105 (2005)] is applied to compute and analyze highly excited (nu2,nu6) eigenstates in DFCO. The present paper is also devoted to the simulations of the intramolecular vibrational energy redistribution (IVR) initiated by an excitation of the out-of-plane bending vibration (nnu6, n=2,4,6, . . . ,18, and 20). The multiconfiguration time-dependent Hartree method is exploited to propagate the corresponding six-dimensional wave packets. A comprehensive comparison with experimental data as well as with previous simulations of IVR in HFCO [G. Pasin et al. J. Chem. Phys. 124, 194304 (2006)] is presented.     

Tuning intramolecular anharmonic vibrational coupling in 4-nitroaniline by solvent-solute interaction

Applying a combined experimental and theoretical approach we demonstrate that doublets of the nu(s)(NO(2)) band of 4-nitroaniline which have been observed in several environments originate from Fermi resonances. Changes of the line shapes typical for Fermi resonances are reported also for other isotopomers of 4-nitroaniline, however, for each of them in different solvents and solvent mixtures. Simulations of the infrared spectra based on the solvatochromic frequency shifts of the nu(s)(NO(2)) vibration determined experimentally together with calculated cubic couplings with overtones and combination bands account for the experimental findings.     

On modeling the pressure-dependent photoisomerization of trans-stilbene by including slow intramolecular vibrational energy redistribution

Experimental data for the photoisomerization of trans-stilbene (S(1)) in thermal bath gases at pressures up to 20 bar obtained previously by Meyer, Schroeder, and Troe (J. Phys. Chem. A 1999, 103, 10528-10539) are modeled by using a full collisional-reaction master equation that includes non-RRKM (Rice-Ramsperger-Kassel-Marcus) effects due to slow intramolecular vibrational energy redistribution (IVR). The slow IVR effects are modeled by incorporating the theoretical results obtained recently by Leitner et al. (J. Phys. Chem. A 2003, 107, 10706-10716), who used the local random matrix theory. The present results show that the experimental rate constants of Meyer et al. are described to within about a factor of 2 over much of the experimental pressure range. However, a number of assumptions and areas of disagreement will require further investigation. These include a discrepancy between the calculated and experimental thermal rate constants near zero pressure, a leveling off of the experimental rate constants that is not predicted by theory and which depends on the identity of the collider gas, the need to use rate constants for collision-induced IVR that are larger than the estimated total collision rate constants, and the choice of barrier-crossing frequency. Despite these unsettled issues, the theory of Leitner et al. shows great promise for accounting for possible non-RRKM effects in an important class of reactions.