Temperature dependence of vibrational energy transfer in the “low” pressure competitive two-channel 1,1-cyclopropane-d2 system

Intermolecular energy transfer has been studied in the two-channel competitive isomerization of 1,1-cyclopropane-d₂, both in the neat system and in the presence of helium bath gas at values of k/k^? centered around 0.02. The competing path ways differ in threshold energy by ≈ 0.6 kcal. The temperature range 773 K to 973 K was covered. Several methods of treating the data, whether by isotopic ratios of rate constants or by temperature dependence of fall off, are each independent of a knowledge of collision cross sections. Used in conjunction, they provide measurements of these quantities. Cyclopropane is an operationally strong collider (β_ω = 1) for itself at 773 K with an average down step size, <ΔE/s> >/ 10 kcal mole ^?1 (>/ 3500 cm^?1). At 973 K the substrate is no longer a strong collider; β_ω declines to ≈ 0.55 with <ΔE/s> ≈ 5.2 kcal mole^?1. For helium the corresponding quantities are β_ω ≈ 0.078 <ΔE/s> ≈ 1.1 kcal mole ^?1 declining to β_ω ≈ 0.010 and <ΔE/s> ≈ 0.53 kcal mole^?1. The several methods of measuring these quantities give excellent independent agreement. Comparison with the earlier theoretical formulation of Tardy and Rabinovitch gives good agreement, the temperature dependence of β_ω for the weak collider, helium, follows the relation β_ω ∝ T^?m, where m /s> 2.     

Vibrational energy transfer and relaxation in O2 and H2O

Near-resonant vibrational energy exchange between oxygen and water molecules is an important process in the Earth's atmosphere, combustion chemistry, and the chemical oxygen iodine laser (COIL). The reactions in question are (1) O2(1) + O2(0) --> O2(0) + O2(0); (2) O2(1) + H2O(000) --> O2(0) + H2O(000); (3) O2(1) + H2O(000) <--> O2(0) + H2O(010); (4) H2O(010) + H2O(000) --> H2O(000) + H2O(000); and (5) H2O(010) + O2(0) --> H2O(000) + O2(0). Reanalysis of the data available in the chemical kinetics literature provides reliable values for rate coefficients for reactions 1 and 4 and strong evidence that reactions 2 and 5 are slow in comparison with reaction 3. Analytical solution of the chemical rate equations shows that previous attempts to measure the rate of reaction 3 are unreliable unless the water mole fraction is higher than 1%. Reanalysis of data from the only experiment satisfying this constraint provides a rate coefficient of (5.5 +/- 0.4) x 10(-13) cm3/s at room temperature, between the values favored by the atmospheric and laser modeling communities.     

Vibrational energy transfer in H2 + He

A semiclassical approach is developed to study vibrational energy transfer in H₂ + He by use of the a priori interaction potential including all nonzero impact parameter collisions. The calculated values of the rate coefficient are found to be in excellent agreement with experimental data which are available in the temperature ranges 60–450 K and 1350-3000 K. The temperature dependence is shown to seriously deviate from the Landau-Teller prediction below 1000 K. The calculation was carried out over the temperature range of 30 to 10000 K.     

Vibrational energy transfer in CH2F2

Infrared fluorescence has been observed from the ν₁, ν₆, 2ν₉, ν₈ and ν₄ levels of CH₂F₂ following excitation by a 9.6 μ Q-switch CO₂ laser. All the observed states exhibit a single exponential decay rate of approximately 44 msec^?1 torr^?1. The rare gas dependence of this rate has also been measured and found to be up to 20 times slower than the rate for the pure gas. Measurements of the risetimes of the observed fluorescence signals yielded an upper limit of 5 μsec at 1 torr for the ν₁, ν₆ and ν₈ levels. The 2ν₉ and ν₄ risetimes were effectively instantaneous under the experimental conditions that prevailed. The relative magnitudes of the measured rate are discussed in terms of existing V-T/R theories and collisional energy transfer processes.     

Collisional energy transfer in the two-channel decomposition of 1,1,2,2-tetrafluorocyclobutane and 1-methyl-2,2,3,3-tetrafluorocyclobutane, I. Gas/gas collisons

The two-channel thermal decomposition of 1,1,2,2-tetrafluorocyclobutane (TFCB) and 1-methyl-2,2,3,3-tetrafluorocyclobutane (MTFCB) have been studied in the temperature range of 730–805 K at pressures varied from 1.1 Pa up to 4.6 kPa. In the pressure independent range, Arrhenius expressions were obtained for TFCB decomposition into 2 CH₂CF₂ (k₁) and C₂H₄+C₂F₄ (k₂), respectively. The same kinetic equations were determined for the decomposition of MTFCB into C₃H₄F₂+C₂H₂F₂ (k₃) and C₃H₆+C₂F₄(k₄). From the study of the pressure dependence of the homogeneous decomposition rates, the average downward energy transfer values of 1800±200 cm^–1 and 1600±200 cm^–1 were obtained for the TFCB and MTFCB molecules, respectively.     

Vibrational energy transfer in methanol and deuterated methanols

The vibrational energy transfer probabilities in methanol and deuterated methanols are calculated by Schwartz, Slawsky and Herzfeld breathing-sphere theory in the temperature range 300–1000 K. The required breathing-sphere parameters are obtained through normal coordinate analysis. All the modes of a manifold, either low-lying or upper vibrational levels, are coupled through rapid V-V exchange processes, whereas the cross-coupling transition probabilities between levels in the different manifolds are smaller by several orders of magnitude.     

Vibrational energy transfer in N2-N2 collisions: a new semiclassical study

The vibrational energy relaxation in collisions between N2 molecules in the low- and medium-lying vibrationally excited levels was revisited using the semiclassical coupled-state method and the use of two different potential-energy surfaces having the same short-range potential recently determined from ab initio calculations but with different long-range interactions. Compared to the data reported in the classical work by Billing and Fisher [Chem. Phys. 43, 395 (1979)], the newly calculated vibration-to-translation rate constant K(1,0 / 0,0) is in much better agreement with the available experimental data over a large temperature interval, from T = 200 K up to T = 6000 K. Nevertheless, as far as the vibration-to-translation exchanges are concerned, the lower-temperature regime remains quite critical in that the new rate constants do not completely account for the rate constant curvature suggested by the experiments for temperatures lower than T = 500 K. The dependence of the state-selected vibration-to-vibration rate constants, K(v,v-delta v / 0,1), both upon the vibrational quantum number v and the gas temperature are calculated. The substantial deviations from previously found behaviors could have major consequences for the vibrational kinetic modeling of N2-containing gas mixtures.     

Intermolecular energy transfer in two-channel unimolecular reactions: the pyrolysis of 1-iodopropane

Pressure-dependent unimolecular reaction rate coefficients have been obtained for the two channels of decomposition of 1-iodopropane (dilute in Ne), using very low-pressure pyrolysis (VLPP). The interpretation, taking finite diffusion rates into account, gives convincing evidence for “weak” gas/gas collisions and “strong” gas/wall collisions.     

Vibrational and rotational energy transfer in Ar + HF

State-to-state energy transfer cross sections for Ar + HF (v = 2, 4, and 6; J = 4, 6, 8, and 10) were computed using quasiclassical trajectories. Rotational energy transfer is invariant with increasing v, but vibrational energy transfer is significantly enhanced by increasing J.     

Vibrational dephasing and resonance energy transfer in molecular liquids

The influence of vibrational band-broadening mechanisms (pure dephasing, resonance transfer, depopulation processes) on the vibrational correlation functions of dense fluids is discussed. The coupling between the vibrational and the rotational-translational subsystems is assumed to be weak. It is found that even for weak coupling the homogeneously broadened linewidth (the inverse dephasing time τ^?1) cannot be represented as a sum of widths related to the individual mechanisms. Using an exponential repulsive interaction potential, we obtain numerical estimates for the dephasing time of the 1 ← 0 transition of liquid nitrogen, which agree very well with experimental observations. It turns out that the most significant contribution for τ arises from the weak anharmonicities of the molecular oscillator.     

Vibrational energy transfer in hydrogen liquid and its isotopes

The transfer of vibrational energy (V-V) from H₂ to isotopic impurities (HD or D₂) has been studied in the liquid state, between 15 and 30 K. The subsequent relaxation (V-T) of the excited impurity by the H₂ liquid host has also been measured and contrasted with the vibrational relaxation behaviour of pure H₂ and D₂ liquids. The isothermal density dependence of both V-V and V-T transfer has been investigated in the fluid state at 30 K. High density relaxation rates are also compared to our data in the pure gases and to other available gas phase results. Measurements in the solid, near the triple point temperature, are equally reported for each process studied.     

Vibrational energy transfer in CH3I

Previous works have reported vibration—vibration and vibration—translation transfer rates in the methyl halides. Using the technique of laser induced infrared fluorescence we have studied energy transfer in the concluding member of this series, CH₃I. Following excitation by resonant lines of a Q-switch CO₂ laser, infrared fluorescence has been observed from the v₂, v₅ as well as the 2v₅, v₁, v₄ vibrational energy levels of CH₃I. All the observed states exhibit a single exponential decay rate of 23 ± 2 ms^?1 torr^?1. Measurements have also been made on deactivation of the various modes by rare gases. The risetime of the v₂, v₅ levels was found to be approximately 101 ± 20 ms^?1 torr^?1, while that of the 2v₅, v₁, v₄ levels was approximately 225 ± 45 ms^?1 torr^?1. Fluorescence was not detected from the v₃ level. These results are discussed in terms of SSH type theoretical calculations, and comparison is made with the results obtained for other members of the methyl halide series, namely CH₃F, CH₃Cl and CH₃Br.     

Vibrational energy transfer in ozone by infrared-ultraviolet double resonance

Absorption transients at 254 nm have been observed in O₃-O₂ mixtures following laser irradiation at 9.64 μm. From analysis of these transients, we are able to determine vibrational relaxation rate constants (O₃-O₂ λ₁^?1/[O₂] = (2560±370) Torr^?1 S^?1, λ₂^?1/[O₂] = (640±50) Torr^?1 S^?1, and also a v₁-v₃ equilibration rate constant (O₃-O₃) of (1.5±1.0) × 10⁶ Torr^?1 S^?1.     

Vibrational energy pathways in N2O

In previous papers a method has been proposed to find out the relative importance of the different paths in the decay of the ν₃ mode of N₂O, but the VT transfer constants involved in the kinetic model were barely known. As new values of VT constants have just been measured, the calculations of the kinetic model have been performed again; they qualitatively confirm the results already obtained with estimated VT constants.     

Vibrational energy transfer in non-reactive molecular collisions: An information-theoretic analysis

Experimental and (trajectory) computed rates of vibrational relaxation of molecules in selected internal states are analyzed. A dynamic constraint is identified. The entire array of (vibrational) manifold-to-manifold rate constants (at a given temperature) is shown to be characterized by this constraint and hence yield a linear surprisal plot. It is argued that bulk measurements of the vibrational relaxation time in a buffer gas will suffice to determine the single parameter in the dynamic constraint.     

Valence isomerization in dendrimers by photo-induced electron transfer and energy transfer from the dendrimer backbone to the core

A series of poly(aryl ether) dendrimers with a norbornadiene (NBD) group attaching to the core (Gn-NBD), generations 1–4, were synthesized and characterized, and their photophysical and photochemical properties were examined. The fluorescence of the dendrimer backbone is quenched by the norbornadiene group as a result of the electron transfer and energy transfer from the dendrimer backbone to the norbornadiene group in Gn-NBD. Selective excitation of the dendrimer backbone results in an isomerization of the norbornadiene group to the quadricyclane (QC) group. The intramolecular electron transfer and energy transfer efficiencies are ca. 0.93, 0.73, 0.54, 0.30 in dichloromethane, and ca. 0.90, 0.70, 0.55, 0.34 in tetrahydrofuran for generations 1–4, respectively, with the rate constant ca. 10¹⁰ s⁻¹. The light-harvesting ability of these dendritic molecules is demonstrated by the enhanced valence isomerization rate of NBD to QC with increasing generation.     

On the energy transfer at the radiolysis of CO-H2S gas mixture

The kinetics of hydrogen formation at various amounts of H₂S /1–60%/ in the radiolysis of CO–H₂S mixture has been studied. The ratio of the reaction rate constants for reactions CO^x+COproduct and CO^x+H₂SH+SH=CO, which amounts to 5×10^–2, has been estimated. The effective activation energy of hydrogen formation /E_eff/ has been determined at various amounts of H₂S in the temperature interval 323–573 K.     

5-氟胞嘧啶气相及水助质子转移异构化的理论研究

采用密度泛函B3LYP/6-311G**方法,对6种5-氟胞嘧啶异构体孤立分子的稳定性及质子转移引起的酮式-烯醇式、氨基式-亚胺式互变异构反应机理进行了计算研究,获得了零点能、吉布斯自由能及质子转移过程的反应焓、活化能、活化吉布斯自由能和速率常数等参数.计算结果表明,气相中烯醇-氨基式FC4是最稳定的异构体.分子内质子转移设计了FC1→FC2和FC1→FC6两条通道,分别标记为P(1)和P(2),各通道速控步骤的活化能和速率常数分别为155.9 kJ·mol-1,4.70×10-15 s-1和173.1 kJ·mol-1,1.41×10-18 s-1.水助催化时,相应通道P(3) 和P(4) 速控步骤的活化能和速率常数分别为51.0 kJ·mol-1,1.41×103 s-1和88.2 kJ·mol-1,4.53×10-3 s-1.可见,水分子的加入极大地降低了质子转移的活化能垒.另外发现,水分子参与形成协同的双质子转移机理比水助单质子转移机理更利于降低活化能垒.