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.     

Vibrational coherence transfer characterized with Fourier-transform 2D IR spectroscopy

Two-dimensional infrared (2D IR) spectroscopy of the symmetric and asymmetric C[Triple Bond]O stretching vibrations of Rh(CO)(2)acac in hexane has been used to investigate vibrational coherence transfer, dephasing, and population relaxation in a multilevel vibrational system. The transfer of coherence between close-lying vibrational frequencies results in extra relaxation-induced peaks in the 2D IR spectrum, whose amplitude depends on the coherence transfer rate. Coherence transfer arises from the mutual interaction of the bright CO stretches with dark states, which in this case reflects the mutual d-pi(*) back bonding of the Rh center to both the terminal carbonyls and the acetylacenonate ligand. For 2D IR relaxation experiments with variable waiting times, coherent dynamics lead to the modulation of peak amplitudes, while incoherent population relaxation and exchange results in the growth of the relaxation-induced peaks. We have modeled the data by propagating the density matrix with the Redfield equation, incorporating all vibrational relaxation processes during all three experimental time periods and including excitation reorientation effects arising from relaxation. Coherence and population transfer time scales from the symmetric to the asymmetric stretch were found to be 350 fs and 3 ps, respectively. We also discuss a diagrammatic approach to incorporating all vibrational relaxation processes into the nonlinear response function, and show how coherence transfer influences the analysis of structural variables from 2D IR spectroscopy.     

Prediction of vibrational frequencies of UO2(2+) at the CCSD(T) level

Electronic structure calculations at the coupled cluster (CCSD(T)) and density functional theory levels with relativistic effective core potentials and large basis sets were used to predict the isolated uranyl ion frequencies. The effects of anharmonicity and spin-orbit corrections on the harmonic frequencies were calculated. The anharmonic effects are larger than the spin-orbit corrections, but both are small. The anharmonic effects decreased all the frequencies, whereas the spin-orbit corrections increased the stretches and decreased the bend. Overall, these two corrections decreased the harmonic asymmetric stretch frequency by 6 cm-1, the symmetric stretch by 3 cm-1, and the bend by 3 cm-1. The best calculated values for UO22+ for the asymmetric stretch, symmetric stretch, and bend were 1113, 1032, and 174 cm-1, respectively. The separation between the asymmetric and the symmetric stretch band origins was predicted to be 81 cm-1, which is consistent with experimental trends for substituted uranyls in solution and in the solid state. The anharmonic vibrational frequencies of the isoelectronic ThO2 molecule also were calculated and compared to experiment to calibrate the UO22+ results.     

High-resolution infrared studies in slit supersonic discharges: CH2 stretch excitation of jet-cooled CH2Cl radical

First high-resolution infrared spectra are presented for jet-cooled CH2 35Cl and CH2 37Cl radicals in the symmetric (nu1) CH2 stretching mode. A detailed spectral assignment yields refined lower and upper state rotational constants, as well as fine structure spin-rotation parameters from least-squares fits to the sub-Doppler line shapes for individual transitions. The rotational constants are consistent with a nearly planar structure, but do not exclude substantial large amplitude bending motion over a small barrier to planarity accessible with zero-point excitation. High level coupled cluster (singles/doubles/triples) calculations, extrapolated to the complete basis set limit, predict a slightly nonplanar equilibrium structure (theta approximately 11 degrees), with a vibrationally adiabatic treatment of the bend coordinate yielding a v = 1<--0 anharmonic frequency (393 cm(-1)) in excellent agreement with matrix studies (nu(bend) approximately 400 cm(-1)). The antisymmetric CH2 stretch vibration is not observed despite high sensitivity detection (signal to noise ratio >20:1) in the symmetric stretch band. This is consistent with density functional theory intensity calculations indicating a >35-fold smaller antisymmetric stretch transition moment for CH2Cl, and yet contrasts dramatically with high-resolution infrared studies of CH2F radical, for which both symmetric and antisymmetric CH2 stretches are observed in a nearly 2:1 intensity ratio. A simple physical model is presented based on a competition between bond-dipole and "charge-sloshing" contributions to the transition moment, which nicely explains the trends in CH2X symmetric versus asymmetric stretch intensities as a function of electron withdrawing group (X = D,Br,Cl,F).     

On the thermal expansion of molecules

Experimental results for a variety of molecules have shown that their bond lengths expand appreciably when the molecules are heated, as expected from the asymmetric Morse-like potentials characterizing the bonds. However, in a series of papers on structures determined by gas-phase electron diffraction, Giricheva et al. claimed that, for very hot MX₃ molecules, effects of out-of-plane vibrations cancel the thermal expansion of the M–X bonds. This is incorrect. Although the computations to support their claim were correct as far as they went, the authors neglected the effects of asymmetric vibrational modes and centrifugal stretches of the bonds. In the present report, we show that quantum chemical computations for LaI₃ reveal the crucial roles played by the terms neglected by Giricheva et al., which terms are responsible for thermal bond stretches of approximately 0.023 Å at 1142 K. In addition, because the iodine atoms in LaI₃ are further apart in the mean structure than the sum of their Pauling van der Waals radii, the geminal nonbonded interactions are attractive. This accounts for the fact that the Morse asymmetry constant for the symmetric stretch mode is smaller than that for the asymmetric stretch. It also helps to explain the very large amplitude of the out-of-plane puckering mode, which tends to decrease the La–I bond length during the puckering trajectory.     

Solvation dynamics in Ni+ (H2O)n clusters probed with infrared spectroscopy

Infrared photodissociation spectroscopy is reported for mass-selected Ni+ (H2O)n complexes in the O-H stretching region up to cluster sizes of n = 25. These clusters fragment by the loss of one or more intact water molecules, and their excitation spectra show distinct bands in the region of the symmetric and asymmetric stretches of water. The first evidence for hydrogen bonding, indicated by a broad band strongly red-shifted from the free OH region, appears at the cluster size of n = 4. At larger cluster sizes, additional red-shifted structure evolves over a broader wavelength range in the hydrogen-bonding region. In the free OH region, the symmetric stretch gradually diminishes in intensity, while the asymmetric stretch develops into a closely spaced doublet near 3700 cm(-1). The data indicate that essentially all of the water molecules are in a hydrogen-bonded network by the size of n = 10. However, there is no evidence for the formation of clathrate structures seen recently via IR spectroscopy of protonated water clusters.     

IR and FTMW-IR spectroscopy and vibrational relaxation pathways in the CH stretch region of CH3OH and CH3OD

Infrared spectra of jet-cooled CH(3)OD and CH(3)OH in the CH stretch region are observed by coherence-converted population transfer Fourier transform microwave-infrared (CCPT-FTMW-IR) spectroscopy (E torsional species only) and by slit-jet single resonance spectroscopy (both A and E torsional species, CH(3)OH only). Twagirayezu et al. reported the analysis of ν(3) symmetric CH stretch region (2750-2900 cm(-1); Twagirayezu et al. J. Phys. Chem. A 2010, 114, 6818), and the present work addresses the more complicated higher frequency region (2900-3020 cm(-1)) containing the two asymmetric CH stretches (ν(2) and ν(9)). The additional complications include a higher density of coupled states, more extensive mixing, and evidence for Coriolis as well as anharmonic coupling. The overall observed spectra contain 17 interacting vibrational bands for CH(3)OD and 28 for CH(3)OH. The sign and magnitude of the torsional tunneling splittings are deduced for three CH stretch fundamentals (ν(3), ν(2), ν(9)) of both molecules and are compared to a model calculation and to ab initio theory. The number and distribution of observed vibrational bands indicate that the CH stretch bright states couple first to doorway states that are binary combinations of bending modes. In the parts of the spectrum where doorway states are present, the observed density of coupled states is comparable to the total density of vibrational states in the molecule, but where there are no doorway states, only the CH stretch fundamentals are observed. Above 2900 cm(-1), the available doorway states are CH bending states, but below, the doorway states also involve OH bending. A time-dependent interpretation of the present FTMW-IR spectra indicates a fast (～200 fs) initial decay of the bright state followed by a second, slower redistribution (about 1-3 ps). The qualitative agreement of the present data with the time-dependent experiments of Iwaki and Dlott provides further support for the similarity of the fastest vibrational relaxation processes in the liquid and gas phases.     

High resolution spectroscopic study of the first overtone of the N---H stretch and of the fundamentals of the C---H stretches in pyrrole

We have examined the first overtone N---H stretching region and the fundamental C---H stretching region of gas phase pyrrole (C₄H₅N), using high resolution Fourier transform spectra. The first overtone N---H stretch has been rotationally analysed using an asymmetric top model and was found to exhibit two separate perturbations. These perturbations produce line splittings and anomalous intensity patterns in the spectrum which are briefly discussed. Hot band transitions, one of them red-shifted and others likely to overlap the main cold transition are also discussed. Three bands observed around 3100 cm⁻¹ were also rotationally analysed, using a symmetric top Hamiltonian, and assigbed to three of the four closely overlapping fundamentals of the C---H stretch vibrations. Evidence was obtained for the fourth expected C---H fundamental.     

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.     

Direct dynamics study of ultrafast vibrational energy relaxation in ice Ih

Car-Parrinello molecular dynamics (CPMD) and a previously developed wave packet model are used to study ultrafast relaxation in water clusters. Water clusters of 15 water molecules are used to represent ice Ih. The relaxation is studied by exciting a symmetric or an asymmetric stretch mode of the central water molecule. The CPMD results suggest that relaxation occurs within 100 fs. This is in agreement with experimental work by Woutersen and Bakker and the earlier wave packet calculations. The CPMD results further indicate that the excitation energy is transferred both intramolecularly and intermolecularly on roughly the same time scale. The intramolecular energy transfer occurs predominantly between the symmetric and asymmetric modes while the bend mode is largely left unexcited on the short time scale studied here.     

Classical vs quantum vibrational energy relaxation pathways in solvated polyatomic molecules

Vibrational energy relaxation (VER) of solvated polyatomic molecules can occur via different pathways. In this paper, we address the question of whether treating VER classically or quantum-mechanically can lead to different predictions with regard to the preferred pathway. To this end, we consider the relaxation of the singly excited asymmetric stretch of a rigid, symmetrical, and linear triatomic molecule (A-B-A) in a monatomic liquid. In this case, VER can occur either directly to the ground state or indirectly via intramolecular vibrational relaxation (IVR) to the symmetric stretch. We have calculated the rates of these two different VER pathways via classical mechanics and the linearized semiclassical (LSC) method. When the mass of the terminal A atoms is significantly larger than that of the central B atom, we find that LSC points to intermolecular VER as the preferred pathway, whereas the classical treatment points to IVR. The origin of this trend reversal appears to be purely quantum-mechanical and can be traced back to the significantly weaker quantum enhancement of solvent-assisted IVR in comparison to that of intermolecular VER.     

Infrared spectroscopy of Sc(+)(H(2)O) and Sc(2+)(H(2)O) via argon complex predissociation: The charge dependence of cation hydration

Singly and doubly charged scandium-water ion-molecule complexes are produced in a supersonic molecular beam by laser vaporization. These ions are mass analyzed and size selected in a specially designed reflectron time-of-flight spectrometer. To probe their structure, vibrational spectroscopy is measured for these complexes in the O-H stretching region using infrared laser photodissociation and the method of rare gas atom predissociation, also known as "tagging." The O-H stretches in these systems are shifted to lower frequency than those for the free water molecule, and the intensity of the symmetric stretch band is strongly enhanced relative to the asymmetric stretch. These effects are more prominent for the doubly charged ions. Partially resolved rotational structure for the Sc(+)(H(2)O)Ar complex shows that the H-O-H bond angle is larger than it is in the free water molecule. Fragmentation and spectral patterns indicate that the coordination of the Sc(2+) ion is filled with six ligands (one water and five argons).     

On the structure of the matrix isolated water trimer

Infrared spectra of partially deuterated water trimers have been investigated. It is found that HDO(H(2)O)(2) has a single, bound OD stretching fundamental, (HDO)(2)H(2)O two bound OD stretches. (HDO)(3) has a single, bound OD stretch and (H(2)O)(3) has a pair of bound OH stretches. Ab initio and discrete Fourier transform (DFT) calculations predict that the water trimer has C(1) symmetry with six different, isoenergetic minima. These calculations consequently give three numerically different OD stretches for HDO(H(2)O)(2), six for (HDO)(2)H(2)O, three for (HDO)(3), and three bound OH stretches for (H(2)O)(3). The connection between the observations and the pseudorotation of the trimer is discussed with the help of Wales' pseudorotation model. It is found that pseudorotation is sufficiently fast to average the effective symmetry of the A(3) trimer to C(3h) and to eliminate the difference between the different ab initio minima for A(2)B. The only exception is (H(2)O)(3) where the splitting between the different bound OH stretches is largest. Here a doublet is observed due to incomplete averaging. DFT calculations indicate that the D-bonded form of HDO(H(2)O)(2) is between 50 and 60 cm(-1) more stable than the H-bonded form. The energy difference is determined by differences in zero point vibration energy of intermolecular librations of the two forms. Attempts to measure the energy difference indicate that the energy difference is larger, of the order of 100 cm(-1).     

Infrared spectroscopy of Cr+(H2O) and Cr2+(H2O): the role of charge in cation hydration

Singly and doubly charged chromium-water ion-molecule complexes are produced by laser vaporization in a pulsed-nozzle cluster source. These species are detected and mass-selected in a specially designed time-of-flight mass spectrometer. Vibrational spectroscopy is measured for these complexes in the O-H stretching region using infrared photodissociation spectroscopy and the method of rare gas atom predissociation. Infrared excitation is not able to break the ion-water bonds in these systems, but it leads to elimination of argon, providing an efficient mechanism for detecting the spectrum. The O-H stretches for both singly and doubly charged complexes are shifted to frequencies lower than those for the free water molecule, and the intensity of the symmetric stretch band is strongly enhanced relative to the asymmetric stretch. Partially resolved rotational structure for both complexes shows that the H-O-H bond angle is greater than it is in the free water molecule. These polarization-induced effects are enhanced in the doubly charged ion relative to its singly charged analog.     

Infrared and Raman spectra and normal coordinate calculations for methylisothiocyanate

The infrared spectra (3200-50 cm^?1) of gaseous and solid CH₃NCS and CD₃NCS and the Raman spectra (3200-10 cm^?1) of the liquids and solids have been recorded. The spectra have been interpreted on the basis of a “pseudo-symmetric top” with C_3v symmetry. An assignment of the fundamental vibrations in both molecules, based on their infrared band contours, depolarization values and group frequencies, is given and discussed. Particularly interesting is the low-frequency region where band maxima were observed at 152 and 80 cm^?1 for CH₃NCS and 139 and 71 cm^?1 for CD₃NCS in the infrared spectra of the gases. A normal coordinate analysis has also been carried out based on C_3v symmetry. Considerable mixing was found between the C_αN stretch and NCS symmetric stretch in both isotopic species. The other normal modes in CH₃NCS are reasonably pure but, for the CD₃NCS molecule, considerable mixing was found between the CD₃ stretches and NCS antisymmetric stretch. The proposed vibrational assignment and the results of the normal coordinate calculations are discussed and compared with the results obtained for similar molecules.     

Short‐time stretch relaxation of entangled polymer solutions investigated using full Rouse model predictions

We conduct a systematical investigation into the short‐time stretch relaxation behavior (i.e., shorter than the Rouse time but sufficiently longer than the glassy time) of entangled polymer liquid in single‐step strain flows, on the basis of theory/data comparisons for a broad series of type‐A entangled polymer solutions. First, within existing normal‐mode formulations, the Rouse model predictions on a full‐chain stretch relaxation in single‐step strain flows are derived for a popular 1‐D model proposed within the Doi–Edwards tube model, as well as for the original 3‐D model for nonentangled systems. In addition, an existing formula for the aforementioned 1‐D model that, however, rested upon a consistent‐averaging or the so‐called uniform‐chain‐stretch approximation is simultaneously examined. Subsequently, the previously derived formulas on chain stretch relaxation are directly incorporated into a reliable mean‐field tube model that utilizes the linear relaxation spectrum and the Rouse time constant consistently determined from linear viscoelastic data. It is found that the predictions of the 1‐D model differ substantially from that of the original 3‐D model at short times. Theory/data comparisons further indicate that the 1‐D model without approximations seems able to describe fairly well the nonlinear relaxation data under investigation. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1199–1211, 2006     

Vibrational relaxation of C-D stretching vibrations in CDCl3, CDBr3, and CDI3

We present time-resolved transient grating measurements of the vibrational relaxation rates of the C-D stretching vibrations of deuterated haloforms in benzene and acetone. We compare our results with previous measurements of excited C-H stretches in the same solvents to obtain insight into the solvent effect on the vibrational relaxation. In deuterated molecules, there are more low-order-coupled states and the states are closer in energy to the C-D stretch than in the unlabeled isotopologs. Therefore, the relaxation is faster for the deuterated molecules. The relaxation also shows a significant solvent dependence. Bromoform and iodoform form charge-transfer complexes with both benzene and acetone which enhance the relaxation rate. For chloroform, hydrogen bonding to acetone is expected to be a more favorable interaction. Surprisingly, however, the vibrational relaxation of CDCl(3) is slower in acetone than in benzene.     

Vibrational relaxation of CN stretch of pseudo-halide anions (OCN-, SCN-, and SeCN-) in polar solvents

The vibrational relaxation dynamics of pseudo-halide anions XCN- (X = O, S, Se) in polar solvents were studied to understand the effect of charge on solute-to-solvent intermolecular energy transfer (IET) and solvent assisted intramolecular vibrational relaxation (IVR) pathways. The T1 relaxation times of the CN stretch in these anions were measured by IR pump/IR probe spectroscopy, in which the 0-1 transition was excited, and the 0-1 and 1-2 transitions were monitored to follow the recovery of the ground state and decay of the excited state. For these anions in five solvents, H2O, D2O, CH3OH, CH3CN, and (CH3)2SO, relaxation rates followed the trend of OCN- > SCN- > SeCN-. For these anions and isotopes of SCN-, the relaxation rate was a factor of a few (2.5-10) higher in H2O than in D2O. To further probe the solvent isotope effect, the relaxation rates of S12C14N-, S13C14N-, and S12C15N- in deuterated methanols (CH3OH, CH3OD, CH3OH, CD3OD) were compared. Relaxation rate was found to be affected by the change of solvent vibrational band at the CN- stretching mode (CD3 symmetric stretch) and lower frequency regions, suggesting the presence of both direct IET and solvent assisted IVR relaxation pathways. The possible relaxation pathways and mechanisms for the observed trends in solute and solvent dependence were discussed.     

Vibrational Spectra and Adsorption of Trisiloxane Superspreading Surfactant at Air/Water Interface Studied with Sum Frequency Generation Vibrational Spectroscopy

The C-H stretch vibrational spectra of the trisiloxane superspreading surfactant Silwet L-77((CH3)3Si-O-Si(CH3)(C3H6)(OCH2CH2)7-8OCH3)-O-Si(CH3)3) at the air/water interface are measured with the surface Sum Frequency Generation Vibrational Spectroscopy (SFG-VS). The spectra are dominated with the features from the –Si-CH3 groups around 2905 cm-1 (symmetric stretch or SS mode) and 2957 cm-1 (mostly the asymmetric stretch or AS mode), and with the weak but apparent contribution from the -O-CH2- groups around 2880 cm-1 (symmetric stretch or SS mode). Comparison of the polarization dependent SFG spectra below and above the critical aggregate or micelle concentration (CAC) indicates that the molecular orientation of the C¡H related molecular groups remained unchanged at different surface densities of the Silwet L-77 surfactant. The SFG-VS adsorption isotherm suggested that there was no sign of Silwet L-77 bilayer structure formation at the air/water interface. The Gibbs adsorption free energy of the Silwet surfactant to the air/water interface is -42.2±0.8kJ/mol, indicating the unusually strong adsorption ability of the Silwet L-77 superspreading surfactant