Trajectory studies of energy transfer: CH3NC with He,Xe, H2 and N2

A trajectory program was used to simulate the collisions of CH₃NC with He, Xe, H₂ and N₂. Calculated energy transfer is in accord with experiment. The pattern of CH₃NC vibrational mode energization is found to be noticeably non-random. The approximate sampling methods used in thermal unimolecular trajectory studies produce a more uniform state distribution.     

Threshold energies for production of CH2(3B1) and CH2(1A1) from ketene photolysis. The CH2(3B1) ↔ CH2(1A1) energy splitting

By measuring the relative CO quantum yields from ketene photolysis as a function of photolysis wavelength we have determined the threshold energy at 25° for CH₂CO(¹A₁) → CH₂(³B₁) + CO(¹Σ⁺) to be 75.7 ± 1.0 kcal/mole. This corresponds to a value of 90.7 ± 1.0 kcal/mole for ΔH_f298⁰[CH₂(³B₁)]. By measuring the relative ratio of CH₂(¹A₁)/CH₂(³B₁) from ketene photolysis as a function of photolysis wavelength we have determined the threshold energy at 25°C for CH₂CO(¹A₁) → CH₂(¹A₁) + CO(¹Σ⁺) to be 84.0 ± 0.6 kcal/mole. This corresponds to a value of 99.0 ± 0.6 kcal/mole for ΔH_f298⁰[CH₂(¹A₁)]. Thus a value for the CH₂(³B₁) ? CH₂(¹A₁) energy splitting of 8.3 ± 1 kcal/mole is determined, which agrees with three other recent independent experimental estimates and the most recent quantum theoretical calculations.     

Product energetics of the reaction C(1D) + H2 → CH + H

The product CH radical of the subject reaction was monitored by laser-induced fluorescence following vacuum ultraviolet photolysis of C₃O₂. As expected from consideration of reaction energetics, only the ground vibrational level was observed. The rotational population distribution in CH(ν″ = 0) matches well a calculated RRHO prior distribution which is averaged over a 300 K Boltzmann distribution of available reactant energy. The experimental results are compared to rotational population distributions derived from previously reported classical trajectory calculations.     

Vibrational state analysis of unrelaxed BaI from the reactions Ba + CH3I and Ba + CH2I2

The reactions Ba + CH₃I → BaI + CH₃ and Ba + CH₂I₂ → BaI + CH₂I have been investigated by the method of laser-induced fluorescence. Excitation spectra are reported for BaI products formed under single-collision conditions in a “beam-gas” arrangement. The production of BaI for Ba + CH₂I₂ is found to be a major reaction pathway with a cross section about twice that for Ba + CH₃I. The relative vibrational populations show for both reactions bell-shaped distributions peaking close to υ = 21 for Ba + CH₃I and υ = 39 for Ba + CH₂I₂. The corresponding average fraction of the total reaction exoergicity that appears as BaI vibration is $\text{f}$_υ = 0.18 for Ba + CH₃I and $\text{f}$_υ = 0.29 for Ba + CH₂I₂. In the case of Ba + CH₃I, an estimate for the average relative translational energy of the products, obtained from the primitive angular distribution measurements of Lin, Mims and Herm, can be combined with the average vibrational excitation of BaI to provide evidence that the internal excitation of the methyl radical exceeds that of BaI. A model is discussed which postulates an electron jump in the exit valley of the Ba + CH₃I reaction to account for this feature of the reaction dynamics.     

On the mechanism of the ion-molecule reaction CH3+ + CH4 = C2H5+ + H2

Experimental evidence supporting the “direct” reaction model and the “intermediate complex” model for the reaction CH₃⁺(CH₄, H₂)C₂H₅⁺ are analysed. It is shown that the evidence for the former can equally well be interpreted in terms of a proposed model of persistent complex formation and decay. The plausibility of a “direct” mechanism is discussed and is found to be poor.     

Excitation and decay of the C2Σ+u state of N+2 following collisions of He+ ions with N2 isotopes

A quantitative analysis is made of the N⁺₂ “2nd negative” emission (“2N”: C²Σ⁺_u → X²Σ⁺_g) produced by the impact of 500 eV to 25 keV He⁺ beams on ¹⁴N₂, ¹⁴N¹⁵N and ¹⁵N₂. Above about 5 keV, the relative 2N emission rates from the various vibrational levels of the C state are the same as those observed for ? 2 keV Ne⁺, or > / 90 eV electron-impact. These limiting distributions are compared to those predicted for a Franck-Condon excitation of the C state, modified by configuration interaction. The weakening in 2N emission at the vibrational levels ν′ > / 3 is ascribed to spontaneous C-state predissociation. The data fully confirm recent reports that this predissociation extends over a wide range of ν′ and that it is subject to a strong isotope effect. The ratios of the rates of C-state predissociation to 2N emission are obtained for the levels ν′ = 3 to 8 of each nitrogen isotope. By means of these data it is shown that near-resonant charge transfer dominates the distribution of vibrational excitation probabilities only at energies below about 10 eV. A comparison is made of absolute cross-sections for C-state emission with those for N⁺ and N⁺₂ production in He⁺/¹⁴N₂ collisions at energies between 5.5 eV and 25 keV.     

CH2(1A1) from CH2N2 photolyses at 4358 and 3660 Å vibrational energy distributions upon reaction with cyclobutane

Approximate vibrational energy distribution for CH^*₂(¹A₁) from diazomethane photolyses at 4358 and 3660 Å have been determined to be reasonably broad. These distributions apply to CH^*₂(¹A₁) at the time of reaction with cyclobutane and were deduced from the internal energy distribution of the formed chemically activated methylcyclobutane. An apparent anomaly in the pressure dependence of the decompositions of CH₂(¹A₁) generated chemically molecules is explained. The anomaly pertains to the relative behavior of systems utilizing ketene and diazomethane photolyses as CH₂(¹A₁) sources. The explanation offered is that the vibrational energy distributions for CH^*₂(¹A₁) are narrow for ketene photolyses at 3340 or 3130 Å and broad for diazomethane photolyses at 4358 or 3660 Å.     

Asymmetric dissociation of a symmetric molecule: Photofragmentation dynamics of Cd (CH3)2

A theoretical analysis of the dissociation of energy-rich triatomic molecules shows that asymmetric dissociation is expected to be the major decay mode. Both experimental and computational (classical trajectories) results for the photofragmentation of Cd(CH₃)₂ are in accord with the theoretical analysis.     

Vibrationally excited NO2 from CH3NO2 and 2-C3H7NO2 photodissociation

We observe vibrationally excited NO₂ from photodissociation of CH₃NO₂ and 2-C₃H₇NO₂ by means of laser induced fluorescence. This approximate method shows very large vibrational excitation in all frequencies of NO₂. The result is interpreted as an indirect predissociation.     

The exchange reaction H2 (ν = 1) + H = H + H2

The classical trajectory method is applied to calculate the total cross section for the exchange reaction H₂(ν = 1) + H = H + H₂. The vibrational excitation is shown to influence efficiently the threshold value. Partial reaction rate-constants calculated on the basis of the cross sections obtained are in good agreement with those measured in H-maser experiments.     

Absolute yields I(2P1/2 as a function of wavelength in the photolysis of n-C3F7I

The absolute branching ratio I(²P_1/2)/I(²P_3/2) in the photolysis of n-C₃F₇I is examined optoacoustically as a functionof excitation wavelength. The yield of I(²P_1/2) is much less than the normally assumed value of unity. Comparison in terms of the sub-bands present is made with CH₃I, CD₃I and CF₃I.     

UO2F4: A new uranyl complex ion

The synthesis, vibrational and electronic spectra of [(CH₃)₄N]₂ UO₂F₄ are described. The data indicate that this compound contains the previously unknown UO₂F₄²⁻ ion which has D_4h symmetry.     

Theoretical study of the reaction H + ClCH3 → HCl + CH3

The reaction H + ClCH₃ has theoretically studied in a LEPS potential energy surface with a single-particle approximation for the methyl group. The LEPS adjustable parameters were selected to reach a good agreement with experimental values of activation energy and exothermicity. A wide set of quasi-classical trajectories for that system has been calculated within a energy range covering the significative values of relative velocities at temperatures between 300 and 1000 K. Calculated reactive cross sections increase with translational energy and with the initial vibrational level, but they are not influenced by rotational excitation of the reactants. Microscopic and total reaction rate constants have been obtained within the temperature range and agree quite well with available experimental results. Final energy distribution shows that most of the exoergicity is consumed in increasing the relative velocity of the products, while HCl molecules remain in their vibrational ground state.     

Selectivity in resonant vibrational excitation by e− impact (0.3–4.5 eV) on formaldehyde,acetaldehyde and acetone

Vibrational excitation by e⁻ impact via low-energy resonances has been investigated in acetaldehyde and acetone and compared with similar results in formaldehyde. Despite the large number of vibrational modes involved, the three systems exhibit a selective excitation of only several modes. Besides an important excitation of the CO stretch modes (dominant in H₂CO), excitation of CH stretch, CH₃ stretch and CH₃ deformation are also observed in CH₃CHO and (CH₃)₂CO. Interpretation of the energy loss spectra is given in terms of recently developed symmetry considerations together with the character of the LUMO occupied by the extra electron to form the transitory negative ion (resonance). Differential cross sections versus electron energy are presented for elastic and several inelastic processes. Weak oscillations (of the “boomerang” type) are observed on the inelastic cross sections for acetaldehyde, whereas no structure appears for acetone. This is in contrast with the pronounced oscillations observed for H₂CO, and reveals a shorter lifetime for the CH₃CHO⁻ and (CH₃)₂CO⁻ resonant states, compared to H₂CO⁻.     

Search for quasiperiodic motion in vibrationally excited formaldehyde formed by S1 → So internal conversion

A quasi-classical trajectory calculation is performed to simulate formaldehyde decomposition following S₁ → S_o internal conversion. No mode specificity is observed in the unimolecular decay rates. Each of the trajectories exhibited chaotic motion. These findings suggest that the S₁ → S_o internal conversion step does not prepare long-lived vibrational states in ground-state formaldehyde.     

Quasi-classical trajectory and direct-dynamics CVT study on the initiation steps of methanol combustion

Direct-dynamics canonical variational transition-state theory (CVT) and quasi-classical trajectory (QCT) calculations have been performed to study the dynamics of the initiation steps in the methanol combustion at high oxygen concentration. The initiation steps in combustion of methanol is hydrogen abstraction from carbon or oxygen in methanol to produce hydroxymethyl radical (CH₂OH) or methoxy radical (CH₃O), respectively, and hydroperoxyl radical (HO₂). A new analytical potential energy function driven from our DFT calculations is constructed to study the dynamics of the title reactions. Reactive cross sections and reaction probabilities at various relative translational energies and initial vibrational and rotational reactant excitation were obtained to calculate the rate constants. The calculated rate constants from CVT and QCT calculations are compared.     

Collision-assisted multiple step excitation of CH3F by irradiation with a TEA CO2 laser

By irradiating a mixture of CH₃F highly diluted in Ar with a TEA CO₂ laser, the v₃ overtone emission rises with a rate much larger than the “up-the-ladder” V-V pumping rate indicating that the CH₃F is excited up to the 3v₃ level almost instantly. This occurs via multiple step excitation assisted by rotational transitions in collisions.     

(CF3)2Cd·D und (CF3)2Zn·D: neue reagenzien für difluorcarben-reaktionen [1]

During the thermal decomposition of (CF₃)₂Cd and (CF₃)₂Zn complexes primarily difluorocarbene is eliminated. The formation of CF₂ is unambiguously proved by matrix i.r. spectroscopy. Both (CF₃)₂Cd·D and (CF₃)₂Zn·D are excellent CF₂ sources, which can easily be prepared and handled, and which undergo CF₂ reactions even at low temperature. CF₂ insertion was found during the reaction of (CF₃)₂Te with (CH₃)₂Cd via the intermediate formation of a CF₃Cd compound to form CH₃TeCF₂CH₃. (CF₃)₂Cd·glyme reacts with (CH₃)₃Si(OCOCF₃) to CF₃Cd(OCOCF₃)·glyme; during this reaction CF₂ is also eliminated.     

反式2-甲基-2-丁烯酸甲酯与臭氧反应机理的计算研究

采用G3B3方法构建反式2-甲基-2-丁烯酸甲酯与O3反应体系以及后续Criegee自由基有、无水分子参与下异构化反应的势能面剖面.结果表明,反式2-甲基-2-丁烯酸甲酯与O3反应首先生成一个稳定的五元环中间体,此中间体按断键位置不同后续裂解反应存在两条路径,分别生成产物P1(CH3CHOO+CH3OC(O)C(CH3)O)和P2(CH3CHO+CH3OC(O)C(CH3)OO).利用经典过渡态理论(TST)并结合Wigner矫正模型计算了200-1200 K温度区间内标题反应的速率常数kTST/W.计算结果显示,294 K时,该反应速率常数为7.55×10-18cm3molecule-1s-1,与Bernard等对类似反应所测实验值非常接近.生成的Criegee自由基(CH3CHOO和CH3OC(O)C(CH3)OO)可分别与水分子发生α-加成及β-氢迁移反应,其中Criegee自由基与水的α-加成反应较其与水的β-氢迁移反应具有优势.另外与无水分子参与CH3CHOO和CH3OC(O)C(CH3)OO异构化反应相比,水分子的参与使得异构化反应较为容易进行.