Vibrational relaxation of normal and deuterated liquid nitromethane

Anti-Stokes Raman scattering is used to monitor vibrational energy redistribution in the ambient temperature liquids nitromethane (NM-h3) and perdeuterated nitromethane (NM-d3) after ultrafast IR excitation of either the symmetric or asymmetric CH- or CD-stretch transitions. The instantaneous populations of most of the fifteen NM vibrations are determined with good accuracy, and a global fitting procedure with a master equation is used to fit all the data. The pump pulses excite not only CH- or CD-stretches but also certain combinations of bending and nitro stretching fundamentals. The coupled vibrations that comprise the initial state are revealed via the instantaneous rise of the anti-Stokes transients associated with each vibrational fundamental. In contrast to many other polyatomic liquids studied previously, there is little energy exchange among the CH-stretch (or CD-stretch) excitations, which is attributed to the nearly free rotation of the methyl group in NM. The vibrational cooling process, which is the multistep return to a thermalized state, occurs in three stages in both NM-h3 and NM-d3. In the first stage, the parent CH- or CD-stretch decays in a few picoseconds, exciting all lower-energy vibrations. In the second stage, the midrange vibrations decay in 10-15 ps, exciting the lower-energy vibrations. In the third stage, these lower-energy vibrations decay into the bath in tens of picoseconds. The initial excitations are thermalized in approximately 150 ps in NM-h3 and there is little dependence on which CH-stretch is excited. VC is somewhat faster in NM-d3 with more dependence on the initial CD-stretch, taking approximately 100 ps with symmetric CD-stretch excitation and approximately 120 ps with asymmetric CD-stretch excitation. Comparison is made with earlier nonequilibrium molecular dynamics simulations of VC [Kabadi, V. N.; Rice, B. M. Molecular dynamics simulations of normal mode vibrational energy transfer in liquid nitromethane. J. Phys. Chem. A 2004, 108, 532-540]. The simulations do a good job of reproducing the observed VC process and in addition they predicted the slow interconversion among CH-stretch excitations and the slower relaxation of the asymmetric CH-stretch now observed here.     

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).     

CH2Cl2分子中CH的摇摆振动与伸缩和弯曲振动的耦合

Infrared absorption spectra of gaseous CH2Cl2 in the regions of 1200-12000 cm-1 were measured using a Bruker IFS 120HR Fourier transform spectrometer in conjunction with a multipass cell. 47 vibrational levels of overtone and combinational spectral lines of the CH stretching (v1, v6), bending (v2), and rocking(v8) modes were analyzed and assigned. Utilizing the normal mode model and considering the coupling among CH stretching, bending and rocking vibrations, values of the harmonic frequency ωi, the anharmonic constant xij, and the coefficients of Fermi and the Darling-Dennison resonances of v1, v6, v2 and v8 modes were also determined from experimental spectral data with nonlinear least-square fitting. These spectral constants reproduced the experimental levels very well. These results showed that Fermi resonance between CH stretching and rocking vibrations (ki88=-254.63 cm-1) is stronger than that between CH stretching and bending vibrations (k122 = 54.87 cm-1); and that Darling-Dennison resonances between CH stretching and bending vibrations (k1166=-215.28 cm-1) is also much stronger than that between CH bending and rocking vibrations (k2288=-5.72 cm-1).     

High-resolution infrared spectroscopy of jet-cooled vinyl radical: symmetric CH2 stretch excitation and tunneling dynamics

First high-resolution IR spectra of jet-cooled vinyl radical in the C-H stretch region are reported. Detailed spectral assignments and least squares fits to an A-reduction Watson asymmetric top Hamiltonian yield rotational constants and vibrational origins for three A-type bands, assigned to single quantum excitation of the symmetric CH(2) stretch. Two of the observed bands arise definitively from ground state vinyl radical, as rigorously confirmed by combination differences predicted from previous midinfrared CH(2) wagging studies of Kanamori et al. [J. Chem. Phys. 92, 197 (1990)] as well as millimeter wave rotation-tunneling studies of Tanaka et al. [J. Chem. Phys. 120, 3604 (2004)]. The two bands reflect transitions out of symmetric (0(+)) and antisymmetric (0(-)) tunneling levels of vinyl radical populated at 14 K slit-jet expansion temperatures. The band origins for the lower-lower (0(+)<--0(+)) and upper-upper (0(-)<--0(-)) transitions occur at 2901.8603(7) and 2901.9319(4) cm(-1), respectively, which indicates an increase in the tunneling splitting and therefore a decrease in the effective tunneling barrier upon CH(2) symmetric stretch excitation. The third A-type band with origin at 2897.2264(3) cm(-1) exhibits rotational constants quite close to (but at high-resolution distinguishable from) the vinyl radical ground state, consistent with a CH(2) symmetric stretch hot band built on one or more quanta of excitation in a low frequency vibration. The observed CH(2) symmetric stretch bands are in excellent agreement with anharmonically scaled high level density functional theory (DFT) calculations and redshifted considerably from previous low resolution assignments. Of particular dynamical interest, Boltzmann analysis indicates that the pair of 0(+) and 0(-) tunneling bands exhibits 1:1 nuclear spin statistics for K(a)=even:odd states. This differs from the expected 3:1 ratio for feasible exchange of the two methylenic H atoms but is consistent with a 4:4 ratio predicted for interchange between all three H atoms. This suggests the novel dynamical possibility of large amplitude "roaming" of all three H atoms in vinyl radical, promoted by high internal vibrational excitation arising from dissociative electron attachment in the discharge.     

Reactions of trimethylindium on TiO2 nanoparticles: experimental and computational study

This article reports the results of an experimental and computational study on the reaction of trimethylindium, In(CH3)(3), adsorbed on TiO2 nanoparticle films. Experimentally, Fourier transform infrared (FTIR) spectra have been measured by varying In(CH3)(3) dosing pressure, UV irradiation time in the absence and presence of oxygen, and surface annealing temperature on both "clean" and HO-covered TiO2 nanoparticle films. Computationally, adsorption energies, molecular structures, and vibrational frequencies of possible adsorbates have been predicted by first-principles calculations based on the density functional theory (DFT) and the pseudopotential method. Three important reactions involving CH3 elimination, CH4 elimination, and CH3 migration from the adsorbed trimethylindium have been elucidated in detail. CH(3 migration is the only exothermic process with the lowest reaction barrier. On the basis of experimental and computational results, the two sharpest peaks at 2979 and 2925 cm(-1), detected in the dosage and UV irradiation experiments in the absence of oxygen, are attributable to the asymmetric and symmetric C-H vibrations of methyl groups in In(CH3)3(a) and its derivatives, (H3C)2In(a), H3CIn(a), and H3CO(a). In the UV irradiation experiment in the presence of oxygen, the methyl groups attached to the In atom were quickly oxidized to the methoxy with the C-H vibrations at 2925 and 2822 cm(-1) and to the carboxyl group with vibrations at 2888 cm(-1) (vs(CH)), 1577 cm(-1) (va(OCO)), 1380 cm(-1) (delta(CH)), and 1355 cm(-1) (vs(OCO)). Finally, from the computed energies with vibrational analysis, the adsorbed structure of the carboxyl group was confirmed to involve two oxygen atoms doubly adsorbed on two surface Ti atoms.     

CH stretch vibrations of adsorbed molecules studied by infrared spectroscopy; CH3O on Cu(100)

The first high-resolution CH stretch vibrations in submonolayers of molecules adsorbed on a single-crystalline surface have been obtained by infrared spectroscopy. Investigation of symmetric and asymmetric vibrational modes of CH₃O adsorbed on Cu(l00) shows that the molecules are tilted on the surface, that nearest-neighbour interactions are important and that the intrinsic peak widths are large.     

Coherent vibrational quantum beats as a probe of Langmuir-Blodgett monolayers

We combine frequency- and femtosecond time-domain measurements of vibrational coherences for spectroscopic characterization of surface monolayer films, utilizing 3-wave mixing as the surface-selective technique. Frequency-domain spectra in the CH-stretch region are obtained by infrared + visible sum frequency generation (SFG). Time-domain coherences are measured using SFG free induction decay (SFG-FID), where a 75 fs IR pulse excites several vibrational modes and a delayed 40 fs visible pulse probes the oscillating surface polarization. A unified framework based on optical Bloch equations is used to simultaneously analyze the time- and frequency-domain data. We compare molecular organization of monolayers in different two-dimensional phases. Highly ordered films transferred at high surface pressure are dominated by two transitions in the frequency domain, CH3 symmetric stretch (2875 cm(-1)) and CH3-Fermi resonance with bend overtone (2935 cm(-1)), and a coherent quantum beat in the time-domain at the difference frequency (approximately 540 fs period). At lower surface pressure, relative amplitudes change and additional transitions emerge (CH3 asymmetric stretch and CH2 modes), indicating changes in molecular orientation and onset of disorder. Information redundancy in the combined frequency- and time-domain data allows more accurate determination of the spectral parameters than purely frequency- or time-domain techniques.     

Jet-cooled infrared spectroscopy in slit supersonic discharges: symmetric and antisymmetric CH2 stretching modes of fluoromethyl (CH2F) radical

The combination of shot noise-limited direct absorption spectroscopy with long-path-length slit supersonic discharges has been used to obtain first high-resolution infrared spectra for jet-cooled CH2F radicals in the symmetric (nu1) and antisymmetric (nu5) CH2 stretching modes. Spectral assignment has yielded refined lower- and upper-state rotational constants and fine-structure parameters from least-squares fits to the sub-Doppler line shapes for individual transitions. The rotational constants provide indications of large amplitude vibrational averaging over a low-barrier double minimum inversion-bending potential. This behavior is confirmed by high-level coupled cluster singles/doubles/triples calculations extrapolated to the complete basis set limit and adiabatically corrected for zero point energy. The calculations predict a nonplanar equilibrium structure (theta approximately 29 degrees, where theta is defined to be 180 degrees minus the angle between the C-F bond and the CH2 plane) with a 132 cm(-1) barrier to planarity and a vibrational bend frequency (nu(bend) approximately 276 cm(-1)), in good agreement with previous microwave estimates (nu(bend) = 300 (30) cm(-1)) by Hirota and co-workers [Y. Endo et al., J. Chem. Phys. 79, 1605 (1983)]. The nearly 2:1 ratio of absorption intensities for the symmetric versus antisymmetric bands is in good agreement with density functional theory calculations, but in sixfold contrast with simple local mode CH2 bond dipole predictions of 1:3. This discrepancy arises from a surprisingly strong dependence of the symmetric stretch intensity on the inversion bend angle and provides further experimental support for a nonplanar equilibrium structure.     

Sub-Doppler infrared spectroscopy of CH2D radical in a slit supersonic jet: isotopic symmetry breaking in the CH stretching manifold

First high-resolution infrared absorption spectra in the fundamental symmetric/asymmetric CH stretching region of isotopically substituted methyl radical, CH(2)D, are reported and analyzed. These studies become feasible in the difference frequency spectrometer due to (i) high density radical generation via dissociative electron attachment to CH(2)DI in a discharge, (ii) low rotational temperatures (23 K) from supersonic cooling in a slit expansion, (iii) long absorption path length (64 cm) along the slit axes, and (iv) near shot noise limited absorption sensitivity (5 × 10(-7)/√(Hz)). The spectra are fully rovibrationally resolved and fit to an asymmetric top rotational Hamiltonian to yield rotational/centrifugal constants and vibrational band origins. In addition, the slit expansion collisionally quenches the transverse velocity distribution along the laser probe direction, yielding sub-Doppler resolution of spin-rotation structure and even partial resolution of nuclear hyperfine structure for each rovibrational line. Global least-squares fits to the line shapes provide additional information on spin-rotation and nuclear hyperfine constants, which complement and clarify previous FTIR studies [K. Kawaguchi, Can. J. Phys. 79, 449 (2001)] of CH(2)D in the out-of-plane bending region. Finally, analysis of the spectral data from the full isotopomeric CH(m)D(3-m) series based on harmonically coupled Morse oscillators establishes a predictive framework for describing the manifold of planar stretching vibrations in this fundamental combustion radical.     

State-selected dynamics of the complex-forming bimolecular reaction Cl- +CH3 Cl'-->ClCH3+Cl'-: a four-dimensional quantum scattering study

Time-independent quantum scattering calculations have been carried out on the Walden inversion S(N)2 reaction Cl(-)+CH(3)Cl(')(v(1),v(2),v(3))-->ClCH(3)(v(1) ('),v(2) ('),v(3) ('))+Cl('-). The two C-Cl stretching modes (quantum numbers v(3) and v(3) (')) and the totally symmetric internal modes of the methyl group (C-H stretching vibration, v(1) and v(1) ('), and inversion bending vibration, v(2) and v(2) (')) are treated explicitly. A four-dimensional coupled cluster potential energy surface is employed. The scattering problem is formulated in hyperspherical coordinates using the exact Hamiltonian and exploiting the full symmetry of the problem. Converged state-selected reaction probabilities and product distributions have been calculated up to 6100 cm(-1) above the vibrational ground state of CH(3)Cl, i.e., up to initial vibrational excitation (2,0,0). In order to extract all scattering resonances, the energetic grid was chosen to be very fine, partly down to a resolution of 10(-12) cm(-1). Up to 2500 cm(-1) translational energy, initial excitation of the umbrella bending vibration, (0,1,0), is more efficient for reaction than exciting the C-Cl stretching mode, (0,0,1). The combined excitation of both vibrations results in a synergic effect, i.e., a considerably higher reaction probability than expected from the sum of both independent excitations, even higher than (0,0,2) up to 1500 cm(-1) translational energy. Product distributions show that the umbrella mode is strongly coupled to the C-Cl stretching mode and cannot be treated as a spectator mode. The reaction probability rises almost linearly with increasing initial excitation of the umbrella bending mode. The effect with respect to the C-Cl stretch is five times larger for more than two quanta in this mode, and in agreement with previous work saturation is found. Exciting the high-frequency C-H stretching mode, (1,0,0), yields a large increase for small energies [more than two orders of magnitude larger than (0,0,0)], while for translational energies higher than 2000 cm(-1), it becomes a pure spectator mode. For combined initial excitations including the symmetric C-H stretch, the spectator character of the latter is even more pronounced. However, up to more than 1500 cm(-1) translational energy, the C-H vibration does not behave adiabatically during the course of reaction, because only 20% of the initial energy is found in the same mode of the product molecule. The distribution of resonance widths and peak heights is discussed, and it is found that individual resonances pertinent to intermediate complexes Cl(-)...CH(3)Cl show product distributions independent of the initial vibrational state of the reactant molecule. The relatively high reactivity, of resonance states with respect to excitation of any mode, found in previous work is confirmed in the present calculations. However, reactivity of intermediate states and reactivity with respect to initial vibrational excitation have to be distinguished. There is a strong mixing between the vibrational states reflected in numerous avoided crossings of the hyperspherical adiabatic curves.     

CH-stretching overtone spectra of a fast rotating methyl group: 2-CH3 and 2-CHD2 pyridines

The CH-stretching overtone spectra of the methyl group in gaseous 2-CH(3) and 2-CHD(2) methylpyridines are recorded with conventional Fourier transform near-infrared spectroscopy in the Deltav(CH) = 1-4 regions and by intracavity laser photoacoustic spectroscopy in the Deltav(CH) = 5 and 6 regions. All spectra exhibit a complex structure. They are analyzed with a theoretical model that incorporates, within the adiabatic approximation, the coupling of the anharmonic CH-stretch vibrations described by Morse potentials with the quasifree internal rotation of the methyl group and with isoenergetic combination states involving the six angle deformation modes of the methyl group. The molecular vibrations are calculated in terms of redundant internal coordinates in an unambiguous canonical form. A simultaneous analysis of different isotopic derivatives is thus achieved. The Fermi resonance coupling parameters are those previously determined for toluene. The technique of diabatic rotations is used to disentangle the multiple avoided crossings occurring along the internal rotation coordinate theta in the calculated spectra, which become rapidly very dense owing to the low symmetry of the system. This simulation is successful in reproducing the experimental spectra. In addition, the transferrability of the Fermi resonance coupling parameters between two parent molecules is demonstrated.     

Infrared vacuum-ultraviolet laser pulsed field ionization-photoelectron study of CH3Br+ (X(2)E(3/2))

By preparing methyl bromide (CH3Br) in selected rotational levels of the CH3Br(X(1)A1; v1 = 1) state with infrared (IR) laser excitation prior to vacuum-ultraviolet (VUV) laser pulsed field ionization-photoelectron (PFI-PE) measurements, we have observed rotationally resolved photoionization transitions to the CH3Br(+)(X(2)E(3/2); v1(+) = 1) state, where v1 and v1(+) are the symmetric C-H stretching vibrational mode for the neutral and cation, respectively. The VUV-PFI-PE origin band for CH3Br(+)(X(2)E(3/2)) has also been measured. The simulation of these IR-VUV-PFI-PE and VUV-PFI-PE spectra have allowed the determination of the v1(+) vibrational frequency (2901.8 +/- 0.5 cm(-1)) and the ionization energies of the origin band (85 028.3 +/- 0.5 cm(-1)) and the v1(+) = 1 <-- v1 = 1 band (84 957.9 +/- 0.5 cm(-1)).     

Vibrationally controlled chemistry: mode- and bond-selected reaction of CH3D with Cl

Selective vibrational excitation controls the competition between C-H and C-D bond cleavage in the reaction of CH(3)D with Cl, which forms either HCl + CH(2)D or DCl + CH(3). The reaction of CH(3)D molecules with the first overtone of the C-D stretch (2nu(2)) excited selectively breaks the C-D bond, producing CH(3) exclusively. In contrast, excitation of either the symmetric C-H stretch (nu(1)), the antisymmetric C-H stretch (nu(4)), or a combination of antisymmetric stretch and CH(3) umbrella bend (nu(4) + nu(3)) causes the reaction to cleave only a C-H bond to produce solely CH(2)D. Initial preparation of C-H stretching vibrations with different couplings to the reaction coordinate changes the rate of the H-atom abstraction reaction. Excitation of the symmetric C-H stretch (nu(1)) of CH(3)D accelerates the H-atom abstraction reaction 7 times more than excitation of the antisymmetric C-H stretch (nu(4)) even though the two lie within 80 cm(-1) of the same energy. Ab initio calculations and a simple theoretical model help identify the dynamics behind the observed mode selectivity.     

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.     

State-to-state reaction dynamics of CH3I photodissociation at 304 nm

The detailed reaction dynamics of CH(3)I photodissociation at 304 nm were studied by using high-resolution long time-delayed core-sampling photofragment translation spectroscopy. The vibrational state distributions of the photofragment, i.e., CH(3), are directly resolved due to the high kinetic resolution of this experiment for the first time. CH(3) radicals produced from I((3)Q(0+)), I((1)Q(1) <--( 3)Q(0+)), and I((3)Q(1)) channels are populated in different vibrational state distributions. The I((3)Q(0+)) and I((3)Q(1)) channels show only progressions in the nu2'(a2") umbrella bending mode, and the I((1)Q(1) <-- (3)Q(0+)) channel shows both progression in the nu2' umbrella bending mode and a small amount of excitation in the nu1'(a1') C-H stretching mode. The photodissociation processes from the vibrational hot band of CH(3)I (upsilon3 = 1, upsilon3 = 2) were also detected, primarily because of the absorption probability from the vibrational excited states, i.e., hot bands are relatively enhanced. Photofragments from the hot bands of CH(3)I show a cold vibrational distribution compared to that from the vibrational ground state of CH(3)I. The I* quantum yield and the curve crossing possibility were also studied for the ground vibrational state of CH(3)I. The potential energy at the curve crossing point was calculated to be 32 790 cm(-1) by using the one-dimensional Landau-Zener model.     

Infrared laser spectroscopy of CH3...HF in helium nanodroplets: The exit-channel complex of the F + CH4 reaction

High-resolution infrared laser spectroscopy is used to study the CH3...HF and CD3...HF radical complexes, corresponding to the exit-channel complex in the F + CH4 --> HF + CH3 reaction. The complexes are formed in helium nanodroplets by sequential pickup of a methyl radical and a HF molecule. The rotationally resolved spectra presented here correspond to the fundamental v = 1 <-- 0 H-F vibrational band, the analysis of which reveals a complex with C(3v) symmetry. The vibrational band origin for the CH3...HF complex (3797.00 cm(-1)) is significantly redshifted from that of the HF monomer (3959.19 cm(-1)), consistent with the hydrogen-bonded structure predicted by theory [E. Ya. Misochko et al., J. Am. Chem. Soc. 117, 11997 (1995)] and suggested by previous matrix isolation experiments [M. E. Jacox, Chem. Phys. 42, 133 (1979)]. The permanent electric dipole moment of this complex is experimentally determined by Stark spectroscopy to be 2.4+/-0.3 D. The wide amplitude zero-point bending motion of this complex is revealed by the vibrational dependence of the A rotational constant. A sixfold reduction in the line broadening associated with the H-F vibrational mode is observed in going from CH3...HF to CD3...HF. The results suggest that fast relaxation in the former case results from near-resonant intermolecular vibration-vibration (V-V) energy transfer. Ab initio calculations are also reported (at the MP2 level) for the various stationary points on the F + CH4 surface, including geometry optimizations and vibrational frequency calculations for CH3...HF.     

Comparing the dynamical effects of symmetric and antisymmetric stretch excitation of methane in the Cl+CH4 reaction

The effects of two nearly isoenergetic C-H stretching motions on the gas-phase reaction of atomic chlorine with methane are examined. First, a 1:4:9 mixture of Cl(2), CH(4), and He is coexpanded into a vacuum chamber. Then, either the antisymmetric stretch (nu(3)=3019 cm(-1)) of CH(4) is prepared by direct infrared absorption or the infrared-inactive symmetric stretch (nu(1)=2917 cm(-1)) of CH(4) is prepared by stimulated Raman pumping. Photolysis of Cl(2) at 355 nm generates fast Cl atoms that initiate the reaction with a collision energy of 1290+/-175 cm(-1) (0.16+/-0.02 eV). Finally, the nascent HCl or CH(3) products are detected state-specifically via resonance enhanced multiphoton ionization and separated by mass in a time-of-flight spectrometer. We find that the rovibrational distributions and state-selected differential cross sections of the HCl and CH(3) products from the two vibrationally excited reactions are nearly indistinguishable. Although Yoon et al. [J. Chem. Phys. 119, 9568 (2003)] report that the reactivities of these two different types of vibrational excitation are quite different, the present results indicate that the reactions of symmetric-stretch excited or antisymmetric-stretch excited methane with atomic chlorine follow closely related product pathways. Approximately 37% of the reaction products are formed in HCl(v=1,J) states with little rotational excitation. At low J states these products are sharply forward scattered, but become almost equally forward and backward scattered at higher J states. The remaining reaction products are formed in HCl(v=0,J) and have more rotational excitation. The HCl(v=0,J) products are predominantly back and side scattered. Measurements of the CH(3) products indicate production of a non-negligible amount of umbrella bend excited methyl radicals primarily in coincidence with the HCl(v=0,J) products. The data are consistent with a model in which the impact parameter governs the scattering dynamics.     

Infrared laser spectroscopy of the CH3-HCN radical complex stabilized in helium nanodroplets

The CH3-HCN and CD3-HCN radical complexes have been formed in helium nanodroplets by sequential pickup of a CH3 (CD3) radical and a HCN molecule and have been studied by high-resolution infrared laser spectroscopy. The complexes have a hydrogen-bonded structure with C3v symmetry, as inferred from the analysis of their rotationally resolved nu = 1 <-- 0 H-CN vibrational bands. The A rotational constants of the complexes are found to change significantly upon vibrational excitation of the C-H stretch of HCN within the complex, DeltaA = A'-A" = -0.04 cm(-1) (for CH3-HCN), whereas the B rotational constants are found to be 2.9 times smaller than that predicted by theory. The reduction in B can be attributed to the effects of helium solvation, whereas the large DeltaA is found to be a sensitive probe of the vibrational averaging dynamics of such weakly bound systems. The complex has a permanent electric dipole moment of 3.1 +/- 0.2 D, as measured by Stark spectroscopy. A vibration-vibration resonance is observed to couple the excited C-H stretching vibration of HCN within the complex to the lower-frequency C-H stretches of the methyl radical. Deuteration of the methyl radical was used to detune these levels from resonance, increasing the lifetime of the complex by a factor of 2. Ab initio calculations for the energies and molecular parameters of the stationary points on the CN+CH4 --> HCN+CH3 potential-energy surface are also presented.     

Four-dimensional quantum study on exothermic complex-forming reactions: Cl- + CH3Br-->ClCH3+Br-

The exothermic gas-phase bimolecular nucleophilic substitution (S(N)2) reaction Cl(-)+CH(3)Br (upsilon1',upsilon2',upsilon3')-->ClCH(3) (upsilon1,upsilon2,upsilon3)+Br- and the corresponding endothermic reverse reaction have been studied by time-independent quantum scattering calculations in hyperspherical coordinates on a coupled-cluster potential-energy surface. The dimensionality-reduced model takes four degrees of freedom into account [Cl-C and C-Br stretching modes (quantum numbers upsilon3' and upsilon3); totally symmetric modes of the methyl group, i.e., C-H stretching (upsilon1' and upsilon1) and umbrella bending vibrations (upsilon2' and upsilon2)]. Diagonalization of the Hamiltonian was performed employing the Lanczos algorithm with a variation of partial reorthogonalization. A narrow grid in the total energy was employed so that long-living resonance states could be resolved and extracted. While excitation of the reactant umbrella bending mode already leads to a considerable enhancement of the reaction probability, its combination with vibrational excitation of the broken C-Br bond, (0, 1, 1), results in a strong synergic effect that can be rationalized by the similarity with the classical transitional normal mode. Exciting the C-H stretch has a non-negligible effect on the reaction probability, while for larger translational energies this mode follows the expected spectatorlike behavior. Combination of C-Br stretch and symmetric C-H, (1,0,1), stretch does not show a cooperative effect. Contrary to the spectator mode concept, energy originally stored in the C-H stretching mode is by no means conserved, but almost completely released in other modes of the reaction products. Products are most likely formed in states with a high degree of excitation in the new C-Cl bond, while the internal modes of the methyl group are less important. Reactants with combined umbrella/C-Br stretch excitation, (0, 1, 1), may yield products with two quanta in the umbrella mode.     

Fourier transform infrared study of the effect of diabetes on rat liver and heart tissues in the CH region

Diabetes mellitus is characterized by hyperglycemia, a relative lack of insulin. The metabolic disturbances in diabetic patients are often associated with cardiac and liver dysfunctions. Generally, experimental diabetic models in animals have been used to study diabetes-related changes in organ function, but the complexity of intact tissues can cause contradictory results. For this reason, different techniques have been used to understand the mechanisms of these dysfunctions in diabetic organs. The purpose of the present study is to investigate the effects of streptozotocin (STZ)-induced diabetes on rat liver and heart tissues at the molecular level by Fourier Transform Infrared (FTIR) spectroscopy. Wistar rats of both sexes, weighing 200-250 g, were made diabetic by a single injection of 50 mg kg(-1) intraperitoneal (i.p.) streptozotocin dissolved in 0.05 M citrate buffer (pH 4.5) and they were kept for 4-5 weeks. The diabetes status was checked by measuring the blood glucose level. In the complex FTIR spectra, the bands in the CH region for example the CH(2) antisymmetric and symmetric stretching, the CH(3) symmetric and asymmetric stretching vibrations due to lipids and proteins in the 3000-2800 cm(-1) region and CH(2) scissoring around 1464 cm(-1) and the CH(3) scissoring at 1454 cm(-1) were analyzed. Characteristic spectral bands of these diabetic samples were compared with those of control group to confirm the effect of diabetes on liver and heart tissues. The FTIR spectra revealed dramatic differences in the band positions and bandwidths, signal intensity values and signal intensity ratios between diabetic and control tissues. Similar differences were observed for diabetic liver and heart tissues. A significant increase in the bandwidth of the CH(2) symmetric and antisymmetric stretching and the CH(3) symmetric and asymmetric stretching bands has been observed for both tissue types. The wavenumber of the CH(3) asymmetric stretching band shifts to lower values, indicating an increase in the order in the deep interior part of the lipid chains. The ratio of the CH(2) symmetric to CH(3) symmetric stretching band (lipid/protein ratio) decreases by 13% for diabetic heart and liver tissues. A decrease is also detected in the ratio of the CH(2) scissoring to the CH(3) scissoring mode. The overall intensity of these bands is seen to increase for diabetic tissues.