CH Stretch Excitation Research Articles

High-resolution infrared spectroscopy of jet cooled CH2Br radicals: The symmetric CH stretch manifold and absence of nuclear spin cooling.

Direct laser absorption of a slit supersonic discharge expansion provides the first high-resolution spectroscopic results on the symmetric CH stretch excitation (ν1) of the bromomethyl (CH2Br) radical in the ground electronic state. Narrowband (<1 MHz) mid-infrared radiation is produced by difference-frequency generation of two visible laser beams, with the open shell halohydrocarbon radical generated by electron dissociative attachment of CH2Br2 in a discharge and rapidly cooled to Trot = 18 ± 1 K in the subsequent slit-jet supersonic expansion. A rovibrational structure in the radical spectrum is fully resolved, as well as additional splittings due to spin-rotation effects and 79Br/81Br isotopologues in natural abundance. Spectroscopic constants and band origins are determined by fitting the transition frequencies to a non-rigid Watson Hamiltonian, yielding results consistent with a vibrationally averaged planar radical and an unpaired electron in the out-of-plane pπ orbital. Additionally, extensive satellite band structure from a vibrational hot band is observed and analyzed. The hot band data is compared to CFOUR/VPT2 (CCSD(T)cc-pVQZ) ab initio anharmonic predictions of the vibration rotation alpha matrix, which permits unambiguous assignment to CH2 symmetric-stretch excitation built on the singly excited CH2 out-of-plane bending mode (ν1 + ν4 ← ν4). Longitudinal cooling of the Doppler width in the slit-jet expansion geometry also reveals partially resolved hyperfine structure on transitions out of the lowest angular momentum states in excellent agreement with predictions based on microwave studies. High level ab initio MOLPRO calculations [CCSD(T)-f12b/VnZ-f12 (n = 3, 4, CBS)] are also performed with explicitly correlated f12 electron methods for the out-of-plane CH2 bending mode over the halogen series CH2X (X = F, Cl, Br, I), which clearly reveals a non-planar geometry for X = F (with a ΔE ≈ 0.3 kcal/mol barrier) and yet planar equilibrium geometries for X = Cl, Br, and I. Finally, a detailed Boltzmann analysis of the transition intensities provides support for negligible collisional equilibration of the entangled H atom nuclear spin states on the few hundred microsecond time scale and high collision densities of a slit supersonic expansion.

High resolution spectroscopy of jet cooled phenyl radical: The ν1 and ν2 a1 symmetry C-H stretching modes.

A series of CH stretch modes in phenyl radical (C6H5) has been investigated via high resolution infrared spectroscopy at sub-Doppler resolution (∼60 MHz) in a supersonic discharge slit jet expansion. Two fundamental vibrations of a1 symmetry, ν1 and ν2, are observed and rotationally analyzed for the first time, corresponding to in-phase and out-of-phase symmetric CH stretch excitation at the ortho/meta/para and ortho/para C atoms with respect to the radical center. The ν1 and ν2 band origins are determined to be 3073.968 50(8) cm(-1) and 3062.264 80(7) cm(-1), respectively, which both agree within 5 cm(-1) with theoretical anharmonic scaling predictions based on density functional B3LYP/6-311g++(3df,3dp) calculations. Integrated band strengths for each of the CH stretch bands are analyzed, with the relative intensities agreeing remarkably well with theoretical predictions. Frequency comparison with previous low resolution Ar-matrix spectroscopy [A. V. Friderichsen et al., J. Am. Chem. Soc. 123, 1977 (2001)] reveals a nearly uniform Δν ≈ + 10-12 cm(-1) blue shift between gas phase and Ar matrix values for ν1 and ν2. This differs substantially from the much smaller red shift (Δν ≈ - 1 cm(-1)) reported for the ν19 mode, and suggests a simple physical model in terms of vibrational mode symmetry and crowding due to the matrix environment. Finally, the infrared phenyl spectra are well described by a simple asymmetric rigid rotor Hamiltonian and show no evidence for spectral congestion due to intramolecular vibrational coupling, which bodes well for high resolution studies of other ring radicals and polycyclic aromatic hydrocarbons. In summary, the combination of slit jet discharge methods with high resolution infrared lasers enables spectroscopic investigation of even highly reactive combustion and interstellar radical intermediates under gas phase, jet-cooled (Trot ≈ 11 K) conditions.

Fermi Resonance Effects in the Vibrational Spectroscopy of Methyl and Methoxy Groups

A theoretical model Hamiltonian [J. Chem. Phys. 2013, 138, 064308] for describing vibrational spectra associated with the CH stretch of CH2 groups is extended to molecules containing methyl and methoxy groups. Results are compared to the infrared (IR) spectroscopy of four molecules studied under supersonic expansion cooling in gas phase conditions. The molecules include 1,1-diphenylethane (DPE), 1,1-diphenylpropane (DPP), 2-methoxyphenol (guaiacol), and 1,3-dimethoxy-2-hydroxybenzene (syringol). Transforming the bending normal mode vibrations of CH3 groups to local scissor vibrations leads to model Hamiltonians which share many features present in our model Hamiltonian for the stretching vibrations of CH2 Fermi coupled to scissor modes. The central difference arises from the greater scissor-scissor coupling present in the CH3 case. Comparing anharmonic couplings between these modes and the stretch-bend Fermi coupling for a variety of systems, it is observed that the anharmonic couplings are robust; their values are similar for the four molecules studied as well as for ethane and methanol. Similar results are obtained with both density functional theory and coupled-cluster calculations. This robustness suggests a new parametrization of the model Hamiltonian that reduces the number of fitting parameters. In contrast, the harmonic contributions to the Hamiltonian vary substantially between the molecules leading to important changes in the spectra. The resulting Hamiltonian predicts most of the major spectral features considered in this study and provides insights into mode mixing and the consequences of the mixing on dynamical processes that follow ultrafast CH stretch excitation.

Theoretical Study of the Validity of the Polanyi Rules for the Late-Barrier Cl + CHD3 Reaction.

The Polanyi rules, which state that vibrational energy is more efficient in promoting a late-barrier reaction than translational energy, were questioned recently by an experimental unexpected finding that the CH stretch excitation is no more effective in promoting the late-barrier Cl + CHD3 reaction than the translational energy. However, the present quantum dynamics study on the best-available potential energy surface for the title reaction reveals that the CH stretch excitation does promote the reaction significantly, except at low collision energies. Further studies should be carried out to solve the disagreements between theory and experiment on the reaction.

Vibrational Energy Dynamics of Normal and Deuterated Liquid Benzene

Ultrafast Raman spectroscopy with infrared (IR) excitation is used to study vibrational energy dynamics of ambient temperature liquids benzene and benzene-d(6). After IR pumping of a CH-stretch or CD-stretch parent excitation, the redistribution of vibrational energy is probed with anti-Stokes Raman. Ten benzene or 12 benzene-d(6) vibrations out of 30 total have large enough cross sections to be observed. The pathways, quantum yields, and lifetimes for energy transfer among these vibrations are quantified. Using a CCl(4) molecular thermometer, we demonstrate an ultrafast Raman calorimetry method which allows measurement of the rate that benzene vibrational energy is dissipated into the bath. On the basis of energy conservation, we then determine the time-dependent dissipation of aggregate vibrational energy from the unobserved, "invisible" vibrations. During the approximately 1 ps IR excitation process, vibrational energy is coherently redistributed to several vibrational modes ("coherently" means the rate is faster than (T(2))(-1) of the pumped transition). This energy is then further redistributed in an incoherent intramolecular vibrational relaxation process with a 6 ps T(1) time constant. The subsequent dynamics involve energy transfer processes accompanied by vibrational energy dissipation to the bath. This vibrational cooling process has a half-life of 30 ps in benzene and 20 ps in benzene-d(6), and thermalization is complete in approximately 100 ps. The observed strongly Raman-active vibrations have about the same amount of energy per mode as the invisible vibrations. The invisible vibrational energy in benzene decays somewhat faster than the observed energy. These two decay rates are about the same in benzene-d(6).

Vibrational Energy Dynamics of Glycine,N-Methylacetamide, and Benzoate Anion in Aqueous (D2O) Solution

Ultrafast infrared-Raman spectroscopy is used to study vibrational energy dynamics of three molecules in aqueous solution (D(2)O) that serve as models for the building blocks of peptides. These are glycine-d(3) zwitterion (GLY), N-methylacetamide-d (NMA), and benzoate anion (BZ). GLY is the simplest amino acid, NMA a model compound with a peptide bond, and BZ a model for aromatic side chains. An ultrashort IR pulse pumps a parent CH-stretch on each solute. Anti-Stokes Raman monitors energy flow through the solutes' strongly Raman-active transitions. Stokes Raman of D(2)O stretching functions as a molecular thermometer to monitor energy dissipation from solute to solvent. A three-stage model is used to summarize the vibrational energy redistribution process and to provide a framework for discussing energy dynamics of different molecules. The initial CH-stretch excitation is found to be delocalized over some or all of the solute molecule in NMA and BZ but not in GLY. The overall time constants for energy dissipation are 7.2 ps for GLY, 4.9 ps for NMA, and 8.0 ps for BZ. CH-stretch energy in GLY is redistributed in a nearly statistical manner among observed GLY vibrations. In NMA the energy is distributed among about one-half of the observed vibrations, and in BZ much of the observed energy is channeled along a CH-stretch to the ring stretch pathway. The strongly Raman-active vibrations accurately represent the flow of vibrational energy through NMA but not through GLY or BZ.

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.

Vibrational relaxation of an amino acid in aqueous solution

Time-resolved Raman spectroscopy is used to study vibrational energy redistribution after CH-stretch excitation, of glycine zwitterion in aqueous solution. Anti-Stokes Raman monitors the glycine vibrations and Stokes Raman monitors heating of the surrounding water, which acts as a molecular thermometer. A three-step mechanism is proposed for vibrational cooling (VC) that includes 0.8 ps decay of the parent CH-stretch, 1.0 ps relaxation of the midrange vibrations and 1.2 ps relaxation of the lower-energy vibrations. The overall VC process observed by the molecular thermometer is represented by a 1.8 ps time constant.

Seven-dimensional quantum dynamics study of the O(P3)+CH4 reaction

The initial state selected time-dependent wave packet calculations have been carried out to study the title reaction with seven degrees of freedom included by restricting the nonreacting CH3 group under C3V symmetry and the CH bond length in the group. Total reaction probabilities as well as integral cross sections were calculated for the ground and four vibrationally excited reagent states. Our calculation shows that the reactivity is very small for the reaction for collision energy up to 1.0eV for all the initial states. Initial vibration excitation of CH4, in particular, the CH stretch excitation, enhances the reactivity, but only part of the excitation energy deposited can be used to reduce the reaction threshold. The rate constant for the ground initial state agrees rather well with that from a recent quasiclassical trajectory study and is larger than that from the semirigid vibrating rotor target calculations, in particular, in the low temperature region. On the other hand, the thermal rate constant calculated from the integral cross sections for these five vibrational states is about a factor of 20 smaller than that from the multiconfiguration time-dependent Hartree study.

Vibrational relaxation of the CH stretch fundamental in liquid CHBr3

In continuation of our work on haloforms, the decay of CH stretch excitation in bromoform is modeled using molecular dynamics simulations. An intermolecular force field is obtained by fitting ab initio energies at select CHBr3 dimer geometries to a potential function. The solvent forces on vibrational modes obtained in the simulation are used to compute relaxation rates. The Landau-Teller approach points to a single acceptor state in the initial step of CH stretch relaxation. The time scale for this process is found to be 50-90 ps, which agrees well with the experimental value of 50 ps. The reason for the selectivity of the acceptor is elaborated. Results from a time-dependent approach to the decay rates are also discussed.

CH stretch/internal rotor dynamics in ethyl radical: High-resolution spectroscopy in the CH3-stretch manifold

High-resolution IR absorption spectra of supersonically cooled ethyl radicals (Trot approximately 20 K) have been obtained in a slit supersonic jet discharge expansion, revealing first rotationally resolved data for CH-stretch excitation of the methyl group. Three different vibrational bands are observed, one parallel (k=0<--0) and two perpendicular (/k/1<--0), which for a nearly decoupled methyl rotor framework would correspond to symmetric and (nearly degenerate) asymmetric CH-stretch excitations. However, the splitting between the two asymmetric CH-stretch excitations is anomalously large (approximately 125 cm(-1)), signaling the presence of interactions between the CH2 radical moiety and the opposing CH bond on the methyl group. This suggests an improved zeroth-order vibrational description as an isolated CH stretch, strongly redshifted by hyperconjugation, with localized vibrations in the remaining CH bonds split into symmetric and asymmetric stretches. Such a dynamical picture highlights a remarkably strong coupling between methyl CH-stretch vibrations and C-C torsional geometry and begins to elucidate discrepancies with previous matrix observations.

Intramolecular vibrational energy redistribution in toluene: a nine-dimensional quantum mechanical study using the MCTDH algorithm

Abstract The intramolecular vibrational energy redistribution within the methyl group of toluene is investigated. A nine-dimensional model potential energy surface in curvilinear coordinates, derived by Cavagnat and Lespade, is adopted along with an exact nine-dimensional kinetic operator in polyspherical coordinates. The quantum calculations are performed with the multiconfiguration time-dependent Hartree (MCTDH) method. MCTDH and some recent extensions of the algorithm are briefly reviewed. We investigate the vibrational energy transfer which is initiated by a local CH-stretch excitation of 1–5 quanta. The redistribution of vibrational energy strongly depends on the initial excitation, being most efficient for the vCH=3 excitation. Selected energy levels are also computed, using a recently developed relaxation scheme as well as the filter-diagonalisation approach.

Overtone spectroscopy in the blue region by cavity-enhanced absorption spectroscopy with a mode-locked femtosecond laser: application to acetylene

The wide spectral coverage and high sensitivity of mode-locked cavity enhanced absorption spectroscopy (ML-CEAS) are illustrated by the observation of a very high overtone transition of C 2H 2 in the blue spectral region (420 nm, 8 quanta of CH stretch excitation), which was easily accessed by frequency-doubling a mode-locked femtosecond Ti:Sa laser. The detection limit is about 10 −8/cm. The rotational analysis of this Σ u +–Σ g + parallel band, centred at 23 813.244 cm −1, is presented and the vibrational assignment is discussed.

Vibrationally mediated photodissociation of CH 3Cl: the v=3 and 4 CH stretch overtone levels

The photodissociation of jet-cooled vibrationally excited CH 3Cl near 235 nm has been investigated by detection of the Cl( 2 P 3/2) and Cl( 2 P 1/2) photofragments through resonance enhanced multiphoton ionization (REMPI) in a time-of-flight mass spectrometer. Vibrationally excited molecules with three or four quanta of CH stretch excitation were prepared by irradiation with tunable near infrared (NIR) laser radiation, and a single UV laser beam was used for both photolysis and fragment detection. The action spectra of the photofragments were much more highly resolved than the room-temperature photoacoustic (PA) spectrum. The differing intensities of vibrational bands in the PA and action spectra are discussed.

Observation of the Highly Excited Vibrational Levels of HCP: Application of IR-UV-SEP Triple Resonance Spectroscopy

Abstract Highly excited vibrational levels of HCP involving both CH stretch and bend modes excitation, vibrational energy of which is 15600 - 17000 cm−1, in the ground electronic state were observed by IR-UV double resonance pumped dispersed fluorescence and stimulated emission pumping spectroscopies.

Computational experiments on the migration of internal energy in macromolecular systems

The mechanistic details of internal energy flow in models of polyethylene containing up to 300 000 atoms (with explicit treatment of hydrogens) is discussed. The intra- and intermolecular dynamics of the macromolecular systems is studied as a function of CH-stretch excitation, temperature, pressure, crystal structure, and phase (solid, melt, or gas) by employing the quasiclassifical trajectory method coupled with computational neural networks. The rate of energy flow local and normal CH stretching modes is found to be very rapid and irreversible, occuring on a time scale of less than 1.0 picosecond at low temperatures and increases with rising temperature. The flow of energy follows a pathway that traces out multiple stages, with an initial rapid flow due to the decay of the excitation followed by a slower flow related to redistribution throughout the system. The mechanism for the facile energy flow is shown to involve strong nonlinear couplings dominated by a CH-stretch/HCH bend Fermi (1:2) resonance. This strong dynamical interaction facilitates the overall process of energy flow away from CH stretching sites in all of the various macromolecular systems that were examined. A second type of energy relaxation process is observed in the long-time dynamics which demonstrates two primary components: the time required to redistribute the initial energy intramolecularly (within a chain) and the time associated for complete redistribution among all of the available vibrational modes (intermolecularly, chain to chain). Intramolecular redistribution occurs on a 2 picosecond time scale while the intermolecular process requires up to 270 ps (two orders of magnitude longer). However, both processes are coupled, even on a picosecond time scale, thereby leading to intermolecularly assisted intramolecular energy transfer. Overall, the results demonstrate that there are invariant pathways for energy redistribution in polyethylene (marked by strong nonlinear coupling such as Fermi resonances). However, due to the differences in the intermolecular interactions for the various environments (different phases), the processes occur over a range of time scales. A thermal conductivity of 0.253 J/ (K m s) and a rate for heat diffusion of 1.6 Km/s were determined for a highly crystalline model of polyethylene based on the simulations. The thermal conductivity tends to decrease substantially as the model is allowed to have more amorphous content and approaches a value near 0.15 J/ (K m s).

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.

Neural network predictions of energy transfer in macromolecules

A neural network is trained to predict the dynamical behavior of internal energy in macromolecules. Energy flow out of localized sites within a polyethylene chain is studied as a function of time, excitation level, and temperature. Computations were examined for CH stretch excitation levels between υ=2 and 14, temperatures between T=20 and 350 K, and time from 0 to 200 ps. The results show that the neural network predictions are accurate to within 8% error of those calculated from molecular dynamics. This difference is related to statistical fluctuations in the molecular dynamics calculations which result from small ensemble averages

An adjusted global potential surface for HCN based on rigorous vibrational calculations

We report extensive trial and error modifications of the Murrell–Carter–Halonen potential surface for HCN to improve agreement with experiments on highly excited stretching and bending states. The vibrational calculations make use of an exact Hamiltonian for nonrotating HCN and use an exact formalism to obtain energies. Two experimental data bases are used to compare against the calculations. One is for highly excited stretch states, but with no bend excitation, and the other is for highly excited bend and CN stretch states, but with no CH stretch excitation. The combined data base consists of 58 vibrational energies for nonrotating HCN. The modifications applied are angular and stretch coordinate scaling and an angular-dependent potential scaling. In addition, the saddle point position is adjusted to agree with the results of a recent ab initio calculation.

The sensitivity of IVR in benzene to bend–stretch potential energy coupling

Quasiclassical trajectory calculations have been performed to study intramolecular vibrational energy redistribution (IVR) from CH overtone states in benzene. The rate and extent of this redistribution is sensitive to details of the potential energy surface. A particularly important potential energy surface property is attenuation of the HCC bending frequency upon CH stretch excitation. This property gives rise to non-irreversible IVR when the CH stretch is highly excited. An analog to quantum beats is observed in phase averaged quasiclassical trajectories.