On non-exponential unimolecular dissociation of molecules prepared by vibrational overtone excitation

The sensitivity of vibrational overtone excitation experiments to non-RRKM behavior is considered. It is proposed that the experimental results are not inconsistent with intrinsically non-RRKM unimolecular lifetime distribution.     

Infrared overtone spectroscopy and unimolecular decay dynamics of peroxynitrous acid

Peroxynitrous acid (HOONO) is generated in a pulsed supersonic expansion through recombination of photolytically generated OH and NO(2) radicals. A rotationally resolved infrared action spectrum of HOONO is obtained in the OH overtone region at 6971.351(4) cm(-1) (origin), providing definitive spectroscopic identification of the trans-perp (tp) conformer of HOONO. Analysis of the rotational band structure yields rotational constants for the near prolate asymmetric top, the ratio of the a-type to c-type components of the transition dipole moment for the hybrid band, and a homogeneous linewidth arising from intramolecular vibrational energy redistribution and/or dissociation. The quantum state distribution of the OH (nu=0,J(OH)) products from dissociation is well characterized by a microcanonical statistical distribution constrained only by the energy available to products, 1304+/-38 cm(-1). This yields a 5667+/-38 cm(-1) [16.2(1) kcal mol(-1)] binding energy for tp-HOONO. An equivalent available energy and corresponding binding energy are obtained from the highest observed OH product state. Complementary high level ab initio calculations are carried out in conjunction with second-order vibrational perturbation theory to predict the spectroscopic observables associated with the OH overtone transition of tp-HOONO including its vibrational frequency, rotational constants, and transition dipole moment. The same approach is used to compute frequencies and intensities of multiple quantum transitions that aid in the assignment of weaker features observed in the OH overtone region, in particular, a combination band of tp-HOONO involving the HOON torsional mode.     

Unimolecular dissociation and thermochemistry of CH3OOH

The unimolecular dissociation of CH3OOH is investigated by exciting the molecule in the region of its 5nu(OH) band and probing the resulting OH fragments using laser-induced fluorescence. The measured OH fragment rotational and translational energies are used to determine the CH3O-OH bond dissociation energy, which we estimate to be approximately 42.6+/-1 kcal/mol. Combining this value with the known heats of formation of the fragments also gives an estimate for the heat of formation of CH3OOH which at 0 K we determine to be deltaH(f)0=-27+/-1 kcal/mol. This experimental value is in good agreement with the results of ab initio calculations carried out at the CCSD(T)/complete basis set limit which finds the heat of formation of CH3OOH at 0 K to be deltaH(f)0=-27.3 kcal/mol.     

Unimolecular processes in CH2OH below the dissociation barrier: O-H stretch overtone excitation and dissociation

The OH-stretch overtone spectroscopy and dynamics of the hydroxymethyl radical, CH(2)OH, are reported in the region of the second and third overtones, which is above the thermochemical threshold to dissociation to H+CH(2)O (D(0)=9600 cm(-1)). The second overtone spectrum at 10 484 cm(-1) is obtained by double resonance IR-UV resonance enhanced multiphoton ionization (REMPI) spectroscopy via the 3p(z) electronic state. It is rotationally resolved with a linewidth of 0.4 cm(-1) and displays properties of local-mode vibration. No dissociation products are observed. The third overtone spectra of CH(2)OH and CD(2)OH are observed at approximately 13 600 cm(-1) by monitoring H-atom photofragments while scanning the excitation laser frequency. No double resonance REMPI spectrum is detected, and no D fragments are produced. The spectra of both isotope analogs can be simulated with a linewidth of 1.3 cm(-1), indicating dissociation via tunneling. By treating the tunneling as one dimensional and using the calculated imaginary frequency, the barrier to dissociation is estimated at about 15 200 cm(-1), in good agreement with theoretical estimations. The Birge-Sponer plot is linear for OH-stretch vibrations 1nu(1)-4nu(1), demonstrating behavior of a one-dimensional Morse oscillator. The anharmonicity parameter derived from the plot is similar to the values obtained for other small OH containing molecules. Isomerization to methoxy does not contribute to the predissociation signal and the mechanism appears to be direct O-H fission via tunneling. CH(2)OH presents a unique example in which the reaction coordinate is excited directly and leads to predissociation via tunneling while preserving the local-mode character of the stretch vibration.     

Vibrational overtone spectroscopy and intramolecular dynamics of ethene

The first through fourth C-H stretching overtone regions of ethene were measured by photoacoustic spectroscopy of room-temperature molecules and action spectroscopy of jet-cooled molecules. The rotational cooling led to improved resolution in the action spectra, turning these spectra into key players in determining the multiple band appearance in each region, their types, and origins. These manifolds arise from strong couplings of the C-H stretches to doorway states and were analyzed in terms of a simplified joint local-mode/normal-mode (LM)/(NM) model and an equivalent NM model, accounting for principal resonances. The diagonalization of the LM/NM and NM vibrational Hamiltonians and the least-square fittings revealed model parameters, enabling assignment of A- and B-type bands. These bands behave differently through the V = 2-4 manifolds, showing coupling to doorway states for the former but not for the latter. The energy flow out of the fourth C-H overtone is governed by the interaction with bath states due to the increase in the density of states.     

Model calculation of local versus hyperspherical mode selective dissociation of H2O

Hyperspherical H₂O^* resonances excited to energies ≲1 eV above the electronic ground state H₂O^* → H + OH dissociation threshold have lifetimes ≈ 45 ps, at least ten times longer than near-degenerate local H₂O^* resonances. The results are evaluated and analysed using fast-Fourier-transform (FFT) propagation of quantum wavefunctions representing H₂O stretching vibrations modeled by coupled Morse oscillators.     

Vibrational overtone spectroscopy of phenol and its deuterated isotopomers

We have measured the OH- and OD-stretching fundamental and overtone spectra of phenol and its deuterated isotopomers under jet-cooled conditions using nonresonant ionization detection spectroscopy and vapor-phase infrared (IR) and near-infrared (NIR) spectra at room temperature using conventional and photoacoustic spectroscopy. The OH- and OD-stretching bands in the jet-cooled spectra are about 1-10 cm(-1) wide and generally show a few Lorentzian shaped peaks. The bands in the room-temperature spectra have widths of 20-30 cm(-1) and display clear rotational profiles. The band profiles in the jet-cooled spectra arise mostly from nonstatistical intramolecular vibrational redistribution (IVR) with specific coupling to "doorway" states, which are likely to involve CH- and CD-stretching vibrations. The transition dipole moment that determines the rotational structure is found to rotate significantly from the fundamental to the third overtone and is not directed along the OH(D) bond. We use these calculated transition dipole moments to simulate the rotational structure. We determine the rotational temperature in the jet-cooled spectra to be about 0.5 K. Anharmonic oscillator local mode calculations of frequencies and intensities of the OH- and OD-stretching transitions are compared with our measured results. The calculated intensities are in good agreement with the absolute intensities obtained from conventional spectroscopy and with the relative intensities obtained from the room-temperature laser spectroscopy.     

Molecular description of indigo oxidation mechanisms initiated by OH and OOH radicals

In this work, we report a quantum chemistry mechanistic study of the hydroxyl (?OH) and hydroperoxyl (?OOH) radicals initiated oxidation of indigo, within the density functional theory framework. All possible hydrogen abstraction and radical addition reaction pathways have been considered. We find that the reaction between a free indigo molecule and an ?OH radical occurs mainly through two competing mechanisms: H-abstraction from an NH site and ?OH addition to the central C═C double bond. Although the latter is favored, both channels occur, the indigo chromophore group structure is modified, and thus the color is changed. This mechanism adequately accounts for the loss of chromophore in urban air, including indoor air such as in museums and in urban areas. Regarding the reactivity of indigo toward ?OOH radicals, only ?OOH-addition to the central double bond is thermodynamically feasible. The corresponding transition state free energy value is about 10 kcal/mol larger than the one for the ?OH initiated oxidation. Therefore, even considering that the ?OOH concentration is considerably larger than the one of ?OH, this reaction is not expected to contribute significantly to indigo oxidation under atmospheric conditions.     

Semiclassical formulation of the unimolecular multiphoton dissociation. Dressing of energy levels of the active mode.

Using the minimum coupling Hamiltonian for deascribing the system active mode-laser fields, we have calculated the shifts of the energy levels of the active mode, produced by thr intense laser fields, by means of a methods upon a space-translation transformation. The laser field has been considered classically.     

Second OH overtone excitation and statistical dissociation dynamics of peroxynitrous acid

The second OH overtone transition of the trans-perp conformer of peroxynitrous acid (tp-HOONO) is identified using infrared action spectroscopy. HOONO is produced by the recombination of photolytically generated OH and NO(2) radicals, and then cooled in a pulsed supersonic expansion. The second overtone transition is assigned to tp-HOONO based on its vibrational frequency (10 195.3 cm(-1)) and rotational band contour, which are in accord with theoretical predictions and previous observations of the first overtone transition. The transition dipole moment associated with the overtone transition is rotated considerably from the OH bond axis, as evident from its hybrid band composition, indicating substantial charge redistribution upon OH stretch excitation. The overtone band exhibits homogeneous line broadening that is attributed to intramolecular vibrational redistribution, arising from the coupling of the initially excited OH stretch to other modes that ultimately lead to dissociation. The quantum state distributions of the OH X (2)Pi (nu=0) products following first and second OH overtone excitation of tp-HOONO are found to be statistical by comparison with three commonly used statistical models. The product state distributions are principally determined by the tp-HOONO binding energy of 16.2(1) kcal mol(-1). Only a small fraction of the OH products are produced in nu=1 following the second overtone excitation, consistent with statistical predictions.     

Isotope selective infrared laser-induced unimolecular dissociation of dimeric bis (1,1,1.5,5,5-hexafluoropentane-2,4-dionato) dioxouranium (vi)

The uranium isotope selective gas phase decomposition of [UO₂ (hfacac)₂]₂ has been measured following absorption of infrared laser radiation by either of two strong absorption features near 10.6 μm These results show that isotopic selectivity can be attained with high yield at Ion laser fluence.     

State-resolved unimolecular dissociation of cis-cis HOONO: Product state distributions and action spectrum in the 2nuOH band region

Nascent OH fragment product state distributions arising from unimolecular dissociation of room temperature HOONO, initiated by excitation in the region of the 2nu(OH) band, are probed using laser-induced fluorescence at sub-Doppler resolution. Phase-space simulations of the measured OH rotational distributions are consistent with the dissociation dynamics being statistical and confirm that all major features in the room temperature action spectrum belong to the cis-cis conformer. The phase-space simulations also allow us to estimate the HO-ONO bond dissociation energy of cis-cis HOONO to be D(0)=19.9+/-0.5 kcal/mol, which when combined with the known heat-of-formation data for the OH and NO(2) fragments gives DeltaH(f) (0)(cis-cis HOONO)=-2.5 kcal/mol. In addition to fragment energy release, spectral features in the cis-cis HOONO action spectrum are examined with respect to their shifts upon (15)N isotope substitution and through ab initio spectral simulation using a two-dimensional dipole surface that takes into account the influence of HOON torsional motion on the OH stretching overtone. The two-dimensional spectral simulations, using CCSD(T)/cc-pVTZ dipole surface, qualitatively reproduces features appearing in the action spectrum and suggest that the strong broad feature occurring approximately 570 cm(-1) to the blue of the cis-cis HOONO 2nu(OH) peak, likely involve excitation of HOON-torsion/OH-stretch combination bands originating from thermally populated excited torsional states. A closer examination of the predictions of the two-dimensional model with experiments also reveals its limitations and suggests that a more elaborate treatment, one which includes several additional modes, will likely be required in order to fully explain the room temperature action spectrum. Ab initio calculations of the HOON torsional potential at the CCSD(T)/cc-pVTZ level of theory are also presented and confirm that cis-perp configuration does not correspond to a bound localized minimum on the HOONO potential energy surface.     

Vibrational overtone spectroscopy of saturated hydrocarbons dissolved in liquefied Ar, Kr, Xe, and N2

This article presents a collection of vibrational overtone spectra of hydrocarbons in cryogenic solutions. Vibrational overtone spectra of ethane and propane dissolved in liquid argon and n-butane and isobutane dissolved in liquid krypton were recorded between 5000 and 14,000 cm(-1). Spectral regions for the first four overtones were measured using a Fourier transform spectrophotometer. The fifth overtone (Deltaupsilon = 6) spectra were recorded with a double beam (pump-probe) thermal lens technique using concentrations as low as 10-3 mole fraction. We obtained the C-H (Deltaupsilon = 6) spectra of (a) liquid ethane at 100 K and ethane in solutions in liquid Ar at 92 K and liquid N2 at 85 K, (b) liquid propane at 148 K and propane in liquid Ar at 93 K, (c) n-butane in liquid Kr at 129 K, (d) n-pentane in liquid Xe at 160 K, and (e) isobutane liquid at 135 K and isobutane in liquid Kr at 130 K. Local-mode parameters were calculated for primary, secondary, and tertiary C-H oscillators in solution and compared with gas-phase local-mode parameters. The peak frequency shift (Deltaomega) from gas phase to solution is explained by the change in harmonic frequency and anharmonicity in solution with respect to the gas-phase values. The bandwidth (Deltaomega1/2) of the (Deltaupsilon = 6) C-H absorption bands of ethane in solution can be explained in terms of collisions with the solvent molecules.     

Gas phase unimolecular decomposition and chemiluminescence of tetramethyldioxetane initiated by a tea CO2 laser

Time resolved chemiluminescence spectra of pure tetramethyldioxetane are obtained for the first time using pulsed infrared laser excitation. The only major product, acetone, is shown to be formed by at least two pathways: a unimolecular, collision free, decomposition and a collision induced mechanism. Spectral and temporal behaviour indicate that neither singlet nor triplet acetone are the primary products. Other possibilities are discussed.     

Theory of multichannel thermal unimolecular reactions. 2. Application to the thermal dissociation of formaldehyde

The thermal dissociation of formaldehyde proceeds on three channels, the molecular-elimination channel H2CO --> H2 + CO (1), the radical-forming bond-fission channel H2CO --> H + HCO (2), and the bond-fission-initiated, intramolecular-hydrogen-abstraction channel H2CO --> H...HCO --> H2 + CO (3) which also forms molecular products. The kinetics of this system in the low-pressure range of the unimolecular reaction is shown to be governed by a subtle superposition of collisional channel coupling to be treated by solving a master equation, of rotational channel switching accessible through ab initio calculations of the potential as well as spectroscopic and photophysical determinations of the threshold energies and channel branching above the threshold energy for radical formation which can be characterized through formaldehyde photolysis quantum yields as well as classical trajectory calculations. On the basis of the available information, the rate coefficients for the formation of molecular and radical fragments are analyzed and extrapolated over wide ranges of conditions. The modeled rate coefficients in the low-pressure range of the reaction (neglecting tunneling) over the range 1400-3200 K in the bath-gas Ar in this way are represented by k0,Mol/[Ar] approximately 9.4 x 10(-9) exp(-33,140 K/T) cm3 molecule(-1) s(-1) and k0,Rad/[Ar] approximately 6.2 x 10(-9) exp(-36,980 K/T) cm3 molecule(-1) s(-1). The corresponding values for the bath-gas Kr, on which the analysis relies in particular, are k0,Mol/[Kr] approximately 7.7 x 10(-9) exp(-33,110 K/T) and k0,Rad/[Kr] approximately 4.1 x 10(-9) exp(-36 910 K/T) cm3 molecule(-1) s(-1). While the threshold energy E0,2 for channels 2 and 3 is taken from spectroscopic measurements, the threshold energy E0,1 for channel 1 is fitted on the basis of experimental ratios k0,Rad/k0,Mol in combination with photolysis quantum yields. The derived value of E0,1(1) = 81.2 (+/-0.9) kcal mol(-1) is in good agreement with results from recent ab initio calculations, 81.9 (+/-0.3) kcal mol(-1), but is higher than earlier results derived from photophysical experiments, 79.2 (+/-0.8) kcal mol(-1). Rate coefficients for the high-pressure limit of the reaction are also modeled. The results of the present work markedly depend on the branching ratio between channels 2 and 3. Expressions of this branching ratio from classical trajectory calculations and from photolysis quantum yield measurements were tested. At the same time, a modeling of the photolysis quantum yields was performed. The formaldehyde system so far presents the best characterized multichannel dissociation reaction. It may serve as a prototype for other multichannel dissociation reactions.     

193-nm photodissociation of acryloyl chloride to probe the unimolecular dissociation of CH2CHCO radicals and CH2CCO

The work presented here uses photofragment translational spectroscopy to investigate the primary and secondary dissociation channels of acryloyl chloride (CH2==CHCOCl) excited at 193 nm. Three primary channels were observed. Two C-Cl fission channels occur, one producing fragments with high kinetic recoil energies and the other producing fragments with low translational energies. These channels produced nascent CH2CHCO radicals with internal energies ranging from 23 to 66 kcal/mol for the high-translational-energy channel and from 50 to 68 kcal/mol for the low-translational-energy channel. We found that all nascent CH2CHCO radicals were unstable to CH2CH + CO formation, in agreement with the G3//B3LYP barrier height of 22.4 kcal/mol to within experimental and computational uncertainties. The third primary channel is HCl elimination. All of the nascent CH2CCO coproducts were found to have enough internal energy to dissociate, producing CH2C: + CO, in qualitative agreement with the G3//B3LYP barrier of 39.5 kcal/mol. We derive from the experimental results an upper limit of 23 +/- 3 kcal/mol for the zero-point-corrected barrier to the unimolecular dissociation of the CH2CHCO radical to form CH2CH + CO.     

Crystal Structure of two Lanthanide(III) Glycolate Complexes, [Lu(HOCH2COO)2(H2O)4][Lu(HOCH2COO)4] and Dy2(OCH2COO)2(HOCH2COO)2 · 4H2O. An X‐Ray Diffraction and Vibrational Spectroscopic Study

The crystal structure of [Lu(HOCH₂COO)₂(H₂O)₄][Lu(HOCH₂COO)₄] ( 1 ) and Dy₂(OCH₂COO)₂(HOCH₂COO)₂ · 4H₂O ( 2) were determined by X‐ray crystallography. The space group of 1 and 2 are P2/c and P2₁/c, respectively. In 1 , discrete anions and cations held together by hydrogen bonds form the lattice, while the structure of 2 is a 3‐D network of cross‐linked metal‐ligand chains. The lanthanides are eight‐coordinated by chelating glycolate ligands and water molecules with distorted dodecahedral coordination. The core of 2 is a centrosymmetric dimer complex formed by two dysprosium atoms bridged by two oxygen atoms from deprotonated hydroxyl groups of glycolate ligands. The vibration spectra of the crystals were also measured and compared to each other.     

Quasiclassical trajectory study of formaldehyde unimolecular dissociation: H(2)CO-->H2 + CO, H + HCO

We report quasiclassical trajectory calculations of the dynamics of the two reaction channels of formaldehyde dissociation on a global ab initio potential energy surface: the molecular channel H(2)CO-->H(2) + CO and the radical H(2)CO-->H + HCO. For the molecular channel, it is confirmed that above the threshold of the radical channel a second, intramolecular hydrogen abstraction pathway is opened to produce CO with low rotation and vibrationally hot H(2). The low-j(CO) and high-nu(H(2) ) products from the second pathway increase with the total energy. The competition between the molecular and radical pathways is also studied. It shows that the branching ratio of the molecular products decreases with increasing energy, while the branching ratio of the radical products increases. The results agree well with very recent velocity-map imaging experiments of Suits and co-workers and solves a mystery first posed by Moore and co-workers. For the radical channel, we present the translational energy distributions and HCO rotation distributions at various energies. There is mixed agreement with the experiments of Wittig and co-workers, and this provides an indirect confirmation of their speculation that the triplet surface plays a role in the formation of the radical products.     

Vibrational overtone induced elimination reactions within hydrogen-bonded molecular clusters: the dynamics of water catalyzed reactions in CH2FOH.H2On

The dynamics of the light initiated OH-overtone induced elimination reactions CH(2)FOH.(H(2)O)(n) + hnu--> HF + CH(2)O + n(H(2)O), n = 1-3, are studied using classical trajectory simulations where the ab initio potential energy surface is computed "on-the-fly". Hydrogen bonding to the water is found to lower the barrier to reaction by over 20 kcal mol(-1) and modifies the mechanism to a concerted multiple H-atom transfer process. The reaction process is found to occur on a rapid timescale, <100 fs, and involves the hydronium ion as an intermediate. An essential aspect of dynamics is the successful competition of reaction with energy dissipation through water evaporation from the cluster.