Communication: mode-specific photodissociation of vibrationally excited pyrrole

Laser-based spectroscopies coupled with molecular beam techniques facilitated the monitoring of H fragments released in ultraviolet photodissociation of pre-excited isoenergetic vibrational levels of pyrrole. Most noticeably, there was an order of magnitude larger reactivity for an eigenstate primarily consisting of two quanta of ring deformation than for another with one quantum of symmetric C-H stretch. The dynamics, the intramolecular interactions controlling the energy flow, and the mode-selectivity within a medium-sized, ten atom molecule, is discussed.     

Intra- and intermolecular vibrational energy transfer in tungsten carbonyl complexes W(CO)5(X) (X=CO, CS, CH3CN, and CD3CN)

Vibrational energy relaxation of degenerate CO stretches of four tungsten carbonyl complexes, W(CO)6, W(CO)5(CS), W(CO)5(CH3CN), and W(CO)5(CD3CN), is observed in nine alkane solutions by subpicosecond time-resolved infrared (IR) pump-probe spectroscopy. Between 0 and 10 ps after the vibrational excitation, the bleaching signal of the ground-state IR absorption band shows anisotropy. Decay of the anisotropic component corresponds either to the rotational diffusion of the molecule or to the intramolecular vibrational energy transfer among the degenerate CO stretch modes. The time constant of the anisotropy decay, tauaniso, shows distinct solvent dependence. By comparing the results for the T1u CO stretch of W(CO)6 and the A1 CO stretch of W(CO)5(CS), the time constant of the rotational diffusion, taur, and the time constant of the intramolecular energy transfer among the three degenerate vibrational modes, taue, are determined as 12 and 8 ps, respectively. The tauaniso value increases as the number of carbon atoms in the alkane solvent increases. After 10 ps, the recovery of the bleaching becomes isotropic. The isotropic decay represents the vibrational population relaxation, from v=1 to v=0. In heptane, the time constant for the isotropic decay, tau1, for W(CO)5(CS) and W(CO)6 was 140 ps. The tau1 for the two acetonitrile-substituted complexes, however, shows a smaller value of 80 ps. The vibrational energy relaxation of W(CO)5(CH3CN) and W(CO)5(CD3CN) is accelerated by the intramolecular energy redistribution from the CO ligand to the acetonitrile ligand. In the nine alkane solutions, the tau1 value of W(CO)6 ranges between 124 and 158 ps, showing the apparent V-shaped solvent dependence with its minimum in decane, while the tau1 value shows little solvent dependence for W(CO)5(CH3CN) and W(CO)5(CD3CN).     

Ultrafast Decay Dynamics of N-Ethylpyrrole Excited to the S1 Electronic State: A Femtosecond Time-Resolved Photoelectron Imaging Study

N-ethylpyrrole is one of ethyl-substituted derivatives of pyrrole and its excited-state decay dynamics has never been explored. In this work, we investigate ultrafast decay dynamics of N-ethylpyrrole excited to the S₁ electronic state using a femtosecond time-resolved photoelectron imaging method. Two pump wavelengths of 241.9 and 237.7 nm are employed. At 241.9 nm, three time constants, 5.0±0.7 ps, 66.4±15.6 ps and 1.3±0.1 ns, are derived. For 237.7 nm, two time constants of 2.1±0.1 ps and 13.1±1.2 ps are derived. We assign all these time constants to be associated with different vibrational states in the S₁ state. The possible decay mechanisms of different S₁ vibrational states are briefly discussed.     

Dissociation energy and vibrational predissociation dynamics of the ammonia dimer

Experiments using infrared excitation of either the intramolecular symmetric N-H stretch (ν(NH,S)) or the intramolecular antisymmetric N-H stretch (ν(NH,A)) of the ammonia dimer ((NH(3))(2)) in combination with velocity-map ion imaging provide new information on the dissociation energy of the dimer and on the energy disposal in its dissociation. Ion imaging using resonance enhanced multiphoton ionization to probe individual rovibrational states of one of the ammonia monomer fragments provides recoil speed distributions. Analyzing these distributions for different product states gives a dissociation energy of D(0) = 660 ± 20 cm(-1) for the dimer. Fitting the distributions shows that rotations are excited up to their energetic limit and determines the correlation of the fragment vibrations. The fragments NH(3)(ν(2) = 3(+)) and NH(3)(ν(2) = 2(+)) have a vibrational ground-state partner NH(3)(ν = 0), but NH(3)(ν(2) = 1(+)) appears in partnership with another fragment in ν(2) = 1. This propensity is consistent with the idea of minimizing the momentum gap between the initial and final states by depositing a substantial fraction of the available energy into internal excitation.     

2D IR spectroscopy of the C-D stretching vibration of the deuterated formic acid dimer

We report transient grating and 2D IR spectra of the C-D stretching vibration of deuterated formic acid dimer. The C-D stretching transition is perturbed by an accidental Fermi resonance interaction that gives rise to a second transition. The transient grating results show that the population lifetime of these states, which are in rapid equilibrium, is 11 ps. 2D IR spectroscopy reveals the energies of the eigenstates in the regions of one quantum and two quanta of C-D stretching excitation. Using these eigenstate energies, we construct a simplified model for the zeroth-order states that we then use to simulate the 2D IR spectrum. The results of this simulation suggest that the model captures the essential features of the vibrational spectroscopy in the region of the C-D stretching transition and compares well with previous gas-phase spectroscopy of the C-D stretch of deuterated formic acid dimer.     

Computational experiments on the intramolecular energy flow in macromolecules

The microscopic details of the flow of energy in a single chain of polyethylene containing 300 atoms is discussed. The intramolecular dynamics of the polyethylene molecule is studied as a function of CH stretch excitation, temperature, and pressure. The rate of energy flow from CH stretching modes is found to be very rapid and irreversible, occurring on a timescale of less than 0.5 ps at low temperatures, and increases with temperature. A general characteristic two-phase energy flow behavior is observed, where there is initially a very rapid flow (due to the decay of the initial excitation) followed by a slower flow (due to energy redistribution throughout the system). The mechanism for the initial facile energy flow is shown to involve strong resonant pathways. In particular, a CH stretch/HCH bend Fermi (1:2) resonance is shown to dominate the short-time dynamics and facilitates the overall process of energy redistribution. The increase in the rate of energy flow as a function of the backbone temperature is found to be due to the increase in the density of the bath states for energy redistribution which subsequently results in the formation of new low-order resonant interactions (1:1, etc). The long-time dynamics, associated to complete redistribution of the initial CH stretch energy with all of the 894 available vibrational modes, occurs within a time of 2 ps. This timescale corresponds to the time for intramolecular redistribution. A comparison of the intramolecular redistribution time to that of intermolecular redistribution (redistribution in the condensed or solid phase as opposed to a single chain) is also made. A preliminary study of energy flow in a crystal of polyethylene (system containing 19 polyethylene chains) shows that the energy flow exhibits two very different time behaviors. The first is for the intramolecular redistribution as in the single chain study and the second is for intermolecular (chain-to-chain) redistribution. The timescale for intermolecular redistribution is found to be on the order of 0.2 ns at room temperature and pressure, about two orders of magitude larger than the intramolecular timescale.     

Experimental and quantum chemical study of pyrrole self-association through N-H?π hydrogen bonding

An FT-IR study of pyrrole self-association in CCl₄ solutions was carried out. According to the IR measurements, pyrrole forms self-associated dimeric species via N-H?π hydrogen bonding. This was also confirmed by quantum chemical calculations for pyrrole monomer and dimer at B3LYP/6-31++G(d,p) level of theory. A T-shaped minimum was located on B3LYP/6-31++G(d,p) PES of pyrrole dimer characterized with a hydrogen bond of an N-H?π type, with centers-of-mass separation of monomeric units of 4.520 Å, H?π distance of 2.475 Å, the interplanar angle between the two monomeric units being 72.9°. The anharmonic vibrational frequency shift upon dimer formation calculated on the basis of 1D DFT vibrational potentials is in excellent agreement with the experimental data (84 vs. 87 cm⁻¹). Harmonic vibrational analysis predicts somewhat smaller shift (68 cm⁻¹). On the basis of NIR spectroscopic data, anharmonicity constants for the 2ν(N-H) and 2ν(N-H?π) vibrational transitions were calculated. The orientational dynamics of monomeric and self-associated pyrrole species was studied within the framework of the transition dipole moment time correlation function formalism. The period of essentially free rotation in the condensed phase reduces from 0.05 ps for the monomeric pyrrole to 0.02 ps for the proton-donor molecule within the dimer.     

Intramolecular vibrational dynamics of diacetylene and diacetylene-d1 via eigenstate-resolved overtone spectroscopy

The high resolution spectra of several CH overtone bands in diacetylene and diacetylene-d₁ were measured using optothermally detected excitation of a collimated molecular beam. The first overtone of the acetylenic CH stretches in these two molecules were recorded in a single resonance scheme using a 1.5 μm color center laser. The second overtone spectra were taken using sequential infrared/infrared double resonance with a 3.0 and a 1.5 μm color center lasers. The perturbations in the spectra have been analyzed to obtain information about the nature and timescales of the underlying intramolecular vibrational redistribution processes. The uncovered dynamical features appear to be dominated by anharmonic couplings and exhibit regular, not chaotic, behavior. The first and second overtone spectra of diacetylene-d₁ are consistent with a coupling model which involves coupling through a doorway state and then subsequent coupling to the bath. In diacetylene, a combination band was also recorded which, in the local mode picture, is equivalent to putting two quanta in one acetylenic CH stretch and one quanta at the other end of the molecule. Comparison of this spectrum with the spectrum obtained by putting three quanta in the same CH stretch, is consistent with earlier observations that delocalized combination bands are less perturbed than nearly isoenergetic pure overtone states.     

Ultrafast vibrational dynamics of adenine-thymine base pairs in DNA oligomers

N-H stretching excitations of DNA oligomers containing 23 alternating adenine-thymine base pairs are studied in femtosecond two-color pump-probe experiments. For a DNA film in a zero relative humidity atmosphere, transient vibrational spectra and their time evolution up to 10 ps demonstrate negligible spectral diffusion and allow for discerning different N-H stretching bands and the O-H stretching absorption of residual water molecules. Lifetimes on the order of 0.5 ps are found for both N-H and O-H stretching modes. The time-dependent pump-probe anisotropies of the different N-H excitations point to a pronounced coupling among them, whereas the O-H stretching anisotropy remains essentially constant.     

A femtosecond study of the infrared-driven cis-trans isomerization of nitrous acid (HONO)

We investigate the dynamics and mechanism of the IR-driven cis-trans isomerization of nitrous acid (HONO) in a low-temperature krypton matrix applying ultrafast time resolved IR spectroscopy. After excitation of the OH-stretching mode the trans HONO state decays biexponentially on a 8 and 260 ps time scale. The initially excited cis HONO state decays on a 20 ps time scale. Cis HONO isomerizes with 10% quantum yield on a 20 ps time scale to trans HONO. The quantum yield we observe is significantly smaller than the previously reported 100%, which could imply that additional, much slower reaction channels exist. We furthermore developed a four-dimensional model of the system, which includes the three proton intramolecular degrees of freedom of HONO fully quantum mechanically and one intermolecular translational degree of freedom of the molecule in the crystal cage. We find that cis-trans isomerization necessarily is accompanied by a translation of the molecule as a whole in the crystal cage. The translational degree of freedom tunes the intramolecular proton states of HONO with respect to each other. When resonances occur, the proton states might couple and transfer population. We suggest a possible reaction pathway, where the cis OH-stretch excited state first couples to a high cis torsional mode, which then may transfer almost instantaneously to the trans side. The model qualitatively explains all experimental observations.     

Multidimensional infrared spectroscopy of the N-H bond motions in formamide

The heterodyned two-dimensional (2D) IR spectra and equilibrium dynamics of the N-H stretching motion of DCONHD in deuterated formamide, DCOND(2), were studied with 80 fs pulses at 3 microm. The time evolution of the heterodyned 2D IR spectra, pump-probe spectra, and photon echo peak shift demonstrate that interstate dynamics is occurring by relaxation of the original N-H excitation. The N-H vibrational frequency correlation function can be expressed as a sum of three exponentials with correlation times 0.24 ps, 0.8 ps, and 11 ps. The intermediate component is attributed to motions of the N-Hcdots, three dots, centeredO unit involving only slight angular variations of the N-H bond. The slow component is attributed to the structure breaking and making. The anisotropy decay confirmed that the significant angular N-H bond motion occurs on the 11 ps time scale. The fast component, which is the least well determined, might correspond to the modulation of the H-bond distance without angular motion. The correlation coefficient between the pumped and relaxed state distributions was +0.51, implying that the excited state phase memory is only slightly diminished by the relaxation of the N-H excitation. The relaxed modes are concluded to be local to the driven N-H mode.     

Mimicking trimeric interactions in the aromatic side chains of the proteins: a gas phase study of indole...(pyrrole)2 heterotrimer

Aromatic trimeric interactions are extremely significant in the stabilization of the specific structures of the proteins as well as protein-protein, and protein-ligand interactions. Here we have reported a direct evidence of the observation of a cyclic asymmetric structure of indole...(pyrrole)(2) trimer bound by three N-H...π hydrogen bonding interactions in a supersonic jet. The experiment has been performed by using resonant two-photon ionization (R2PI), IR-UV, and UV-UV double resonance spectroscopic techniques. Density functional theory (DFT) calculations nicely corroborate the experimental results showing one weakly allowed IR-active band due to symmetric stretch of the N-H bonds and two strongly allowed IR-active bands due to two types of asymmetric stretches of the N-H bonds in the trimer. The present spectroscopic investigation demonstrates that the strength of the three N-H...π bound intermolecular interactions in the cyclic asymmetric trimer is quite different unlike the corresponding interactions of similar strength in a cyclic symmetric trimer.     

Time scales and pathways of vibrational energy relaxation in liquid CHBr3 and CDBr3

Molecular dynamics simulations are used in conjunction with Landau-Teller, fluctuating Landau-Teller, and time-dependent perturbation theories to investigate energy flow out of various vibrational states of liquid CHBr3 and CDBr3. The CH stretch overtone is found to relax with a time scale of about 1 ps compared to the 50 ps rate for the fundamental. The relaxation pathways and rates for the CD stretch decay in CDBr3 are computed in order to understand the changes arising from deuteration. While the computed relaxation rate agrees well with experiments, the pathway is found to be more complex than anticipated. In addition to the above channels for CH(D) stretch relaxation that involve only the hindered translations and rotations of the solvent, routes involving off-resonant and resonant excitations of solvent vibrational modes are also examined. Finally, the decay of energy from low frequency states to near-lying solute states and solvent vibrations are studied.     

Vibrational energy relaxation of the OH(D) stretch fundamental of methanol in carbon tetrachloride

The lifetimes of the hydroxyl stretch fundamentals of two methanol isotopomers, MeOH and MeOD, in carbon tetrachloride solvent are calculated through the use of the perturbative Landau-Teller and fluctuating Landau-Teller methods. Examination of these systems allows for insight into the nature of the vibrational couplings that lead to intramolecular vibrational energy transfer. While both systems display energy transfer to nearly degenerate modes, MeOD also displays strong coupling to an off-resonant vibration. The relaxation of MeOH and MeOD occurs through transitions involving a total change in the vibrational quanta of 4 and 3, respectively. We calculate vibrational energy relaxation lifetimes of 4-5 ps for MeOH and 2-3 ps for MeOD that agree well with the experimentally determined values.     

Determining the vibrational pattern via overtone cold spectra: C-H methyl stretches of propyne

Vibrationally mediated photodissociation and photoacoustic (PA) spectroscopy were employed for studying the intramolecular dynamics of propyne initially excited to the first through fourth overtone of methyl C-H stretching modes. Room-temperature PA and jet-cooled action spectra, monitoring the absorption of the parent and the yield of the ensuing H photofragments, respectively, were obtained. The PA spectra exhibit mainly broad features, while the action spectra, due to inhomogeneous structure reduction, expose multiple peaks of recognizable shapes in the differing overtone manifolds. Symmetric rotor simulations of the band contours of the action spectra allowed retrieving of band origins and linewidths. The linewidths of the bands in each manifold enabled estimates for energy redistribution times out of the corresponding states to the bath states, the times ranging from 18+/-6 ps for two quanta of C-H excitation to subpicosecond for five quanta. The data were also analyzed in terms of a normal-mode model and a joint local-/normal-mode model. These models enabled determination of harmonic frequencies, anharmonicities, and interaction parameters reproducing the observed data in all monitored regions and provided spectral assignments. The measured Doppler profiles were well fitted by Gaussians with widths suggesting low average translational energies for the released H photofragments. These low energies and their similarities to those for dissociation of propyne isotopomers preexcited to acetylenic C-H stretches were ascribed to an indirect dissociation process occurring after internal conversion to the ground electronic state and isomerization to allene.     

Ab initio quantum dynamical study of the multi-state nonadiabatic photodissociation of pyrrole

There has been a substantial amount of theoretical investigations on the photodynamics of pyrrole, often relying on surface hopping techniques or, if fully quantal, confining the study to the lowest two or three singlet states. In this study we extend ab initio based quantum dynamical investigations to cover simultaneously the lowest five singlet states, two π-σ? and two π-π? excited states. The underlying potential energy surfaces are obtained from large-scale MRCI ab initio computations. These are used to extract linear and quadratic vibronic coupling constants employing the corresponding coupling models. For the N-H stretching mode Q(24) an anharmonic treatment is necessary and also adopted. The results reveal a sub-picosecond internal conversion from the S(4) (π-π?) state, corresponding to the strongly dipole-allowed transition, to the S(1) and S(2) (π-σ?) states and, hence, to the ground state of pyrrole. The significance of the various vibrational modes and coupling terms is assessed. Results are also presented for the dissociation probabilities on the three lowest electronic states.     

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.     

Vibrational energy flow rates for cis- and trans-stilbene isomers in solution

Transient electronic absorption following excitation of the first C-H stretching overtone (2nu(CH)) or a C-H stretch-bend combination (nu(CH) + nu(bend)) monitors the flow of vibrational energy in cis-stilbene and in trans-stilbene. Following a rapid initial rise as energy flows into states interrogated by the probe pulse, the absorption decays with two time constants, which are about a factor of 2 longer for the cis-isomer than for the trans-isomer. The decay times for cis-stilbene are tau2(cis) = (2.6 +/- 1.5) ps and tau3(cis) = (24.1 +/- 2.1) ps, and those for trans-stilbene are tau2(trans) = (1.4 +/- 0.6) ps and tau3(trans) = (10.2 +/- 1.1) ps. The decay times are essentially the same in different solvents, suggesting that the relaxation is primarily intramolecular. The two decay times are consistent with the sequential flow of energy through sets of coupled states within the molecule, and the difference in the rates for the two isomers likely reflects differences in coupling among the states arising from the different structures of the isomers. The similarity of the time evolution following excitation of the first C-H overtone at 5990 cm(-1) and the stretch-bend combination at 4650 cm(-1) is consistent with a subset of states, whose structure is similar for the two vibrational excitation energies, controlling the observed flow of energy.     

Vibrational dynamics of deoxyguanosine 5'-monophosphate following UV excitation

The relaxation dynamics of the DNA nucleotide deoxyguanosine 5'-monophosphate (dGMP) following 266 nm photoexcitation has been studied by transient IR spectroscopy with femtosecond time resolution. The induced dynamics of the amide I (carbonyl) stretch, the asymmetric guanine ring stretch and the phosphate asymmetric stretch are monitored in the region 1000-1800 cm(-1). Excitation and subsequent rapid internal conversion to a "hot" ground state is reflected by depletion of the vibrational ground states of the amide I stretch and guanine ring stretch. However, the vibrational ground state of the phosphate is left unperturbed, indicating the absence of vibrational coupling between the guanine ring system and the phosphate group. The vibrational ground state of the amide I is repopulated in 2.5 ps (±0.2 ps) while it takes 3.7 ps (±0.5 ps) to repopulate the guanine ring vibration. This article discusses two possible relaxation pathways of dGMP, as well as the implications of the weak phosphate dynamics.