Dissection of the Multichannel Reaction O(3P) + C2H2: Differential Cross-Sections and Product Energy Distributions

The O(³P) + C₂H₂ reaction plays an important role in hydrocarbon combustion. It has two primary competing channels: H + HCCO (ketenyl) and CO + CH₂ (triplet methylene). To further understand the microscopic dynamic mechanism of this reaction, we report here a detailed quasi-classical trajectory study of the O(³P) + C₂H₂ reaction on the recently developed full-dimensional potential energy surface (PES). The entrance barrier TS1 is the rate-limiting barrier in the reaction. The translation of reactants can greatly promote reactivity, due to strong coupling with the reaction coordinate at TS1. The O(³P) + C₂H₂ reaction progress through a complex-forming mechanism, in which the intermediate HCCHO lives at least through the duration of a rotational period. The energy redistribution takes place during the creation of the long-lived high vibrationally (and rotationally) excited HCCHO in the reaction. The product energy partitioning of the two channels and CO vibrational distributions agree with experimental data, and the vibrational state distributions of all modes of products present a Boltzmann-like distribution.     

Quantum-classical wave packet calculations for the O(1D) + N2O → NO + NO/N2 + O2 reaction

Mixed quantum-classical calculations have been carried out for the O(¹D) + N₂O reaction with an emphasis on the effect of the relative translational energy as well as initial vibrational state of N₂O on the NO + NO/N₂ + O₂ product branching. The calculations were done within a planar constraint using a five-dimensional analytical potential energy surface previously developed by our group. Three vibrational coordinates in the N₂O molecule were treated with a quantum wave packet technique while other two degrees of freedom, translational and angular motions of O(¹D) with respect to N₂O, were described with classical mechanics. We have found that the initial orientation angle significantly affects the NO + NO/N₂ + O₂ product branching similar to our previous classical trajectory result using the same potential surface. It has been also found that the branching ratio decreases as the translational energy increases except for a low energy region. Excitation of the initial vibrational state of the N₂O reactant does not largely affect the reaction dynamics.     

On the catalytic role of Re+ in the oxygen transport activation of N2O by CO

The mechanism of the cyclic reaction N₂O(¹∑) + CO(¹∑⁺) → N₂(¹∑ _g ⁺ ) + CO₂(¹∑ _g ⁺ ) catalyzed by Re⁺ has been investigated on quintet and septet potential energy surfaces (PES). The reactions were studied by the B3LYP density functional method and the CCSD(T) theory. The calculated results of different PES show that the reaction proceeds in a two-step manner and spin crossing between different PES occurs. The involving crossing points (CPs) between the quintet and septet PES have been discussed by means of the intrinsic reaction coordinate approach. And the O-atom affinities testified that Re⁺ can capture O from N₂O and transfer O atom to CO in the two spin state, which are thermodynamically allowed. Furthermore, the spin–orbit coupling (SOC) is calculated between electronic states of different multiplicities at the CPs. For CP1 and CP2, the computed SOC constants are 8.34 and 10.09 cm^?1, respectively, obtained by using one-electron spin–orbit Hamiltonian in GAMESS. Therefore, the intersystem crossing at CP1 and CP2 occurs with a little probability because of the small SOC involved.     

Rapid sensitization of vibrational energy levels of N2O in collisions with SF6 excited by a CO2 laser

Experimental investigations of mixtures containing predominantly N₂O and small amounts of SF₆ demonstrate that rapid interspecies pooling of vibrational energy can occur to produce a pulse of excess vibrational energy in the ν₃ mode of N₂O following excitation of SF₆ by a Q-switch CO₂ laser. This increased population in the ν₃ mode of N₂O can occur on a time scale shorter than that on which collision-induced VV processes redistribute vibrational energy among the modes of SF₆. The equilibration takes place in three discernible stages: (1) a rapid pooling of energy between a limited number of levels of the SF₆ and N₂O, then (2) a slower collision-dependent VV process that equilibrates all the vibrational modes in the system, with (3) a subsequent VT,R process that returns the system to its initial state. Argon is shown to accelerate selectively process (2) with an efficiency consistent with the previously measured ability of argon to accelerate the VV process in pure SF₆. The experimental evidence indicates that other modes in N₂O do not become involved on the time scale on which direct crossing to ν³ occurs. Additionally, on the time scale preceding the SF₆ VV equilibration, a fast collision-dependent process competes with the transfer of excitation to N₂O. The production of a pulse of excitation in N₂O is eliminated when isotopically substituted N₂O (¹⁴N¹⁵NO) is used instead under the same conditions because the crossing rate to the ν₃ mode of N₂O is decreased sufficiently when ¹⁵N is substituted for ¹⁴N that it no longer can compete with the VV equilibration among the modes in SF₆.     

Vibrational relaxation of ethylene oxide and ethylene oxide-rare-gas mixtures

The collision-induced vibrational energy relaxation of ethylene oxide (C₂H₄O) was studied by means of laser-induced fluorescence. The time-dependent population of the vibrational modes v₃ and v₅/v₁₂ was measured after excitation of CH-stretching vibrations near 3000 cm^?1. Rate constants for the vibrational energy transfer by collisions with C₂H₄O and the rare gases are deduced, and a simplified model for the vibrational relaxation of C₂H₄O is discussed.     

Features of the fluorine‐substituted acetaldehydes dynamics in the low‐lying electronic states: Quantum mechanical study of the CHF2CHO molecule lowest excited singlet state

The potential energy surface (PES) for the CHF₂CHO molecule in the excited S₁ state is calculated by the CASSCF method. The features of the 1‐ and 2‐D cross‐sections of PES are considered in comparison with those of the relative molecules. The vibrational frequencies are calculated in harmonic approximation and the vibrational energy levels for the inversion motion of the carbonyl fragment CCH_aO and for the torsion motion of the CHF₂‐top are calculated in anharmonic approximation by the 1‐ and 2‐D variational methods. The calculated data are compared with the experimental ones. The problems of the experimental data interpretation are considered. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Velocity dependence of vibrational branching ratios in electronic energy transfer collisions of metastable argon atoms with nitrogen

Measurements have been made of the vibrational branching ratio (υ′=0)/(υ′=1) in N^*₂ (C³Π_u) formed in electronic energy transfer collisions between argon metastable atoms and ground state nitrogen molecules, using crossed molecular beams. In the relative collision energy range, 0.08–0.20 eV, this ratio is 3.5±0.2.     

Non-adiabatic unimolecular dissociation of polyatomic molecules. II. The thermal dissociation of nitrous oxide

The theory presented in part I of this series is applied to the non-adiabatic spin-forbidden thermal dissociation N₂O(¹Σ⁺)→N₂(¹Σ⁺_g)+O(³P) as a test case. The molecular model is multidimensional and includes all vibrational modes of the molecule. Specifically considered is the fact that the initial singlet state of N₂O is linear and the final triplet state is bent. The best available data are used for describing the intersection of singlet and triplet potential energy surfaces. Calculated microcanonical rate constants are averaged over Boltzmann distribution of energies and compared with k_co, the high-pressure rate constant deduced from experiment. The agreement between theory and experiment is satisfactory. Analysis of the calculations shows that the driving force for the N₂O dissociation is the flow of energy into the bending vibrations. This is because the bendings have very different equilibrium angles in the initial and final states.     

Dissociative excitation of water by metastable argon

The reaction Ar(²P_2,0) + H₂O → Ar + H + OH(A²Σ⁺)was studied in crossed molecular beams by observing the luminescence from OH(A²Σ⁺). No significant dependence of the spectrum on collision energy was found over the 22–52 meV region. Spectral simulation was used to obtain the OH(A) vibrational distribution and rotational temperature, assuming a Boltzmann rotational distribution. Since predissociation is known to strongly affect the rovibrational distribution, the individual rotational state lifetimes were included in the simulation program and were used to obtain the average vibrational state lifetimes. Excellent agreement with experiment was obtained for vibrational population ratios N₀/N₁/N₂ of 1.00/ 0.40/0.013 and a rotational temperature of 4000 K. Correction for the different average vibrational lifetimes gave formation rate ratios P₀/P₁/P₂ of 1.00/0.49/0.25. The differences between these results and those from flowing afterglow studies on the same system are discussed. Three reaction mechanisms are considered, and the vibrational prior distributions are calculated from a simple density-of-states model. Only fair agreement with experiment is obtained. The best agreement for the mechanisms giving OH(A) in two 2-body dissociation steps is obtained by assuming 1.0 eV of internal energy remains in the second step. The OH(A) vibrational population distribution of the present work is similar to that found in the photolysis of H₂O at 122 nm, where there is 1.10 eV of excess internal energy.     

C2O(Ã 3Πi↔-~X3Σ−: laser induced excitation and fluorescence spectra

Laser induced fluorescence of C₂O is observed following the 266 nm laser photodissociation Of C₃O₂. Excitation spectra of C₂O(óΠ_i?-~X³Σ^? are consistent with previous absorption studies of C₂O. A number of new transitions are identified and assigned. Fluorescence spectra have been recorded following single vibrational level laser excitation. Bands are assigned to ground state vibrational progressions. Values of 1967 and 1063 cm^?1 are found for υ₁″ and υ₃″ stretching vibrations in the X?³Σ ^? state. A subband structure in the fluorescence spectrum is observed and discussed.     

How Frustrated Lewis Acid/Base Systems Pass through Transition‐State Regions: H2 Cleavage by [tBu3P/B(C6F5)3]

We investigate the transition‐state (TS) region of the potential energy surface (PES) of the reaction tBu₃P+H₂+B(C₆F₅)₃→tBu₃P‐H⁽⁺⁾+^(?)H?B(C₆F₅)₃ and the dynamics of the TS passage at room temperature. Owing to the conformational inertia of the phosphane???borane pocket involving heavy tBu₃P and B(C₆F₅)₃ species and features of the PES E(P???H, B???H | B???P) as a function of P???H, B???H, and B???P distances, a typical reactive scenario for this reaction is a trajectory that is trapped in the TS region for a period of time (about 350 fs on average across all calculated trajectories) in a quasi‐bound state (scattering resonance). The relationship between the timescale of the TS passage and the effective conformational inertia of the phosphane???borane pocket leads to a prediction that isotopically heavier Lewis base/Lewis acid pairs and normal counterparts could give measurably different reaction rates. Herein, the predicted quasi‐bound state could be verified in molecular collision experiments involving femtosecond spectroscopy.     

Effects of zero point vibration on the reaction dynamics of water dimer cations following ionization

Reactions of water dimer cation following ionization have been investigated by means of a direct ab initio molecular dynamics method. In particular, the effects of zero point vibration and zero point energy (ZPE) on the reaction mechanism were considered in this work. Trajectories were run on two electronic potential energy surfaces (PESs) of : ground state (²A″‐like state) and the first excited state (²A′ ‐ like state). All trajectories on the ground‐state PES lead to the proton‐transferred product: H₂O⁺(Wd)‐H₂O(Wa) → OH(Wd)‐H₃O⁺(Wa), where Wd and Wa refer to the proton donor and acceptor water molecules, respectively. Time of proton transfer (PT) varied widely from 15 to 40 fs (average time of PT = 30.9 fs). The trajectories on the excited‐state PES gave two products: an intermediate complex with a face‐to‐face structure (H₂O‐OH₂)⁺ and a PT product. However, the proton was transferred to the opposite direction, and the reverse PT was found on the excited‐state PES: H₂O(Wd)‐H₂O⁺ (Wa) → H₃O⁺(Wd)‐OH(Wa). This difference occurred because the ionizing water molecule in the dimer switched between the ground and excited states. The reaction mechanism of and the effects of ZPE are discussed on the basis of the results. © 2017 Wiley Periodicals, Inc.     

Theoretical investigation on the reaction of N2O and CO catalyzed by Fe(C6H6)

The reaction of N₂O with CO, catalyzed by Fe⁺(C₆H₆) and producing N₂ and CO₂, has been investigated at the UB3LYP/6-311+G(d) level. The computation results revealed that the reaction of Fe⁺(C₆H₆), N₂O and CO, is an O-atom abstraction mechanism. For the reaction channels, the geometries and the vibrational frequencies of all species have been calculated and the frequency modes analysis also have been given to elucidate the reaction mechanism. On the basis for geometry optimizations, the thermodynamic data of these reactions channels have been calculated using the statistical theory at 295.15 K and pressure of 0.35 Torr. Using Eyring transition state theory with Wigner correction, the activation thermodynamic data, rate constant and frequency factors for the these reaction channels also have been given. The results showed that CO and N₂O do not react without catalyst and Fe⁺(C₆H₆) can excellently mediate the reaction of N₂O and CO.     

β‐Carotene Revisited by Transient Absorption and Stimulated Raman Spectroscopy

β‐Carotene in n‐hexane was examined by femtosecond transient absorption and stimulated Raman spectroscopy. Electronic change is separated from vibrational relaxation with the help of band integrals. Overlaid on the decay of S₁ excited‐state absorption, a picosecond process is found that is absent when the C₉‐methyl group is replaced by ethyl or isopropyl. It is attributed to reorganization on the S₁ potential energy surface, involving dihedral angles between C₆ and C₉. In Raman studies, electronic states S₂ or S₁ were selected through resonance conditions. We observe a broad vibrational band at 1770 cm^?1 in S₂ already. With 200 fs it decays and transforms into the well‐known S₁ Raman line for an asymmetric C=C stretching mode. Low‐frequency activity (<800 cm^?1) in S₂ and S₁ is also seen. A dependence of solvent lines on solute dynamics implies intermolecular coupling between β‐carotene and nearby n‐hexane molecules.     

The A 1Π(u)←X 1Σ Electronic Transition of CCN+ and CNC+

The electronic absorption spectra of the A ¹Π_(u)←X ¹Σ transition of CCN⁺ and CNC⁺ have been observed in a 5 K Ne matrix after mass selection of C₂N⁺. CCN⁺ has the origin band at 462.0(2) nm. The vibrational structure with frequencies 1223(20) and 1725(20) cm⁻¹ corresponds to the symmetric and antisymmetric stretching modes in the excited state. The origin band of CNC⁺ is observed at 325.7(2) nm, and the system shows extensive vibrational excitation. Calculations of the potential energy functions of CCN⁺ and CNC⁺ in their X ¹Σ ground state and the A ¹Π state of CCN⁺ followed by variational evaluation of the rovibronic energy levels allows the assignment of the observed spectra. These spectroscopic data open the way to gas‐phase studies of the astrophysically important C₂N⁺ ions.     

Chemically pumped N2O laser

Optical gain and laser oscillation has been achieved in N₂O through selective excitation of the (001) state by vibrational energy transfer from CO². The CO² is produced by the flash photolysis initiated chemical reaction: O + CS → CO² + S.     

Potential energy surfaces and vibrational spectra for isotopomers of N2O

Potential energy surfaces and vibrational spectra for the four isotopomers (^l5N¹⁴N¹⁶O,^l4N^I5N¹⁶O,¹⁵N₂ ¹⁶O and¹⁵N₂ ¹⁸O) of N₂O have been investigated with the vibrational self-consistent field-configuration interaction method. It is shown that the isotopomers with the same end atom have similar values of the potential parameters, and that substitution with different end atoms can affect the potential obviously. The calculated vibrational levels are in good agreement with the observed values by the optimization of several potential parameters (f ₁ ⁽¹⁾,f ₁₃ ⁽⁰⁾,f ₃ ⁽¹⁾ which are sensitive to isotopic substitutions. Project supported by the National Natural Science Foundation of China (Grant No. 29673029).     

Direct observation of rotational energy transfer in ammonia by time-resolved infrared—microwave double resonance

An N₂O laser is used to pump the ground vibrational state (8,7) inversion doublet of ¹⁴NH₃ while simultaneously monitoring other ground state doublets. Time-resolved rotational energy transfer signals are observed in accordance with known selection values. Absolute rates of rotational energy transfer processes are estimated.     

High-Resolution Experimental Study on Photodissocaition of N2O

We study the photodissociation dynamics of nitrous oxide using the time-sliced ion velocity imaging technique at three photolysis wavelengths of 134.20, 135.30, and 136.43 nm. The O(¹S_J=0)+N₂(X¹∑_g⁺) product channels were investigated by measuring images of the O(¹S_J=0) products. Vibrational states of N₂(X¹∑_g⁺) products were fully resolved in the images. Product total kinetic energy releases (TKER) and the branching ratios of vibrational states of N₂ products were determined. It is found that the most populated vibrational states of N₂ products are v=2 and v=3. The angular anisotropy parameters (β values) were also derived. The β values are very close to 2 at low vibrational states of the correlated N₂(X¹∑_g⁺) products at all three photolysis wavelengths, and gradually decrease to about 1.4 at v=7. This indicates the dissociation is mainly through a parallel transition state to form products at lower vibrational states, and the highly vibrational exited products are from a more bent configuration. This is consistent with the observed shift of the most intense rotational structure in the TKER as the vibrational quantum number increases.     

Photoelectron spectra and electronic structure of derivatives of imidazoline

We have obtained for the first time the photoelectron spectra (PES) of 20 diamagnetic and paramagnetic derivatives of imidazole. On the basis of calculations in the MNDO approximation, analysis of the vibrational structure, and comparisons in a series we have interpreted the bands in the range 7–11 eV. In the nitroxyl radicals of 3-imidazoline the highest occupied molecular orbital is the _NO ^* orbital, having also a contribution on the C⁴ atom. On introducing N-oxide groups there is a considerable rearrangement of the electronic structure of the radicals. In the PES of the nitroxyl radicals of 3-imidazoline-3-oxide there is multiplet splitting of the ionization band of the n_o·MO, amounting to 0.3–0.4 eV. Replacement of the methyl groups on the C² atom by methoxyl groups leads to an increase in the interactions of the unshared pairs of the O atoms of the nitronium and nitroxyl groups, while the same replacement on the C⁵ atoms leads to a considerable decrease in this interaction. When the substituent on the N¹ atom in derivatives of 3-imidazoline-3-oxide is varied the energy of ionization of the _C-NO MO decreases in the series: CH₃ < H OH < O < NO.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1769–1777, August, 1990.