Adiabatic Channel Research Articles

Measurements of Specific Rate Constants for Dissociation of Substituted Aromatic Hydrocarbons. A Direct Acces to Thermal Dissociation Rate Constants

AbstractSpecific rate constants for unimolecular bond fission reactions of substituted aromatic hydrocarbons were measured directly using excimer laser excitation at 193 nm followed by internal conversion. For methyl substituted benzenes C —H bond cleavage is the dominant process while C —C bond fission in the side chain is most important for α‐methyl substituted toluenes. Calibration of statistial adiabatic channel modell (SACM) calculation with these experimental results allows for prediction of thermal decomposition rate constants which are difficult to measure directly.

Uni molecular Reaction Dynamics on Ab Initio Potential Energy Surfaces

AbstractOne‐parameter fit versions of unimolecular rate theory for complex elimination and simple bond fission reactions are compared with detailed experiments. The agreement generally is so good that no further extension of this operational version of the theory is required. However, the relation between the fit parameter and properties of the potential surface not always is transparent. The article discusses the implementation of ab initio calculations of the potential by constructing adiabatic channel potential curves. Explicit results for charge‐dipole interactions are presented. SACM‐modified PST applies for selected conditions. Strategies for handling more complex potentials are outlined. The treatment closes the gap between operational fit parameters and potential properties.

Understanding Unimolecular Dissociations with Loose Transition States: Photofragmentation Dynamics of Ketene at the Singlet Threshold

AbstractPhotofragment excitation (PHOFEX) spectra of ketene (CH2CO) were taken by probing rovibronic transitions of the singlet methylene fragment while scanning the photolysis laser frequency. CH2(ã1A1) was detected by laser‐induced fluorescence in a supersonic free jet. The dissociation threshold of the reaction, CH2CO → 1CH2 + CO was determined to be 30116.2 ± 0.4 cm1. The nuclear spin is conserved during the dissociation. Sharp rises in the 1CH2 PHOFEX spectra are observed whenever a new CO rotational state becomes energetically accessible. Phase Space Theory (PST) is in excellent agreement with the experimental data. The Statistical Adiabatic Channel Model (SACM) and the related SACM‐PST theory fail to fit the experimental PHOFEX curves except in the limit where they become indistinguishable from PST (i.e. α/β > 0.7). The data show that there are no barriers to dissociation and that essentially every accessible product quantum state is equally likely to be formed. The implications of these findings for the theory of unimolecular dissociation are discussed.

Pressure drop characteristics of two-phase flow in a vertical channel with offset strip fins

Flow visualization photographs and measured pressure drop data are presented for vertical two-phase flow in an adiabatic channel with offset strip fins. Pressure drop data were obtained for two-phase flow of R-113 at atmospheric pressure over ranges of mass flux and quality typically encountered in small refrigeration and air-conditioning systems. Values of the two-phase multiplier for flow in the offset strip fin matrix were found to be 40–50% higher than those for a round tube under the same flow conditions. Particularly large deviations were observed at high-quality conditions where the pressure drop due to fin form drag becomes more significant. A theoretical analysis of the pressure drop associated with two-phase flow in channels with offset strip fins is also presented. This model accounts for the individual contributions of wall shear and form drag to the overall pressure gradient. While this model represents the trends of the measured data rather well, it slightly underpredicts the two-phase multipliers over the range of conditions explored here. A semiempirical superposition model of the frictional component of pressure drop for two-phase flow in offset fin matrices is also developed. Two-phase multipliers predicted using this model are in good agreement with experimental measurements, with a mean deviation of 15%.

Adiabatic decomposition of mass-selected alkali clusters

The decomposition of metastable photoionized mass-selected alkali clusters is investigated using a tandem time-of-flight spectrometer. Na+n and K+n are found to decompose mainly by the evaporation of either a single neutral atom or a neutral dimer in a time scale of about 10 μs. The predominant fragmentation channels are found to follow the adiabatic dissociation channels associated with the lowest energies accordingly to our CI calculations. In the light of our experimental and theoretical results a comparison between the different calculations of the absolute atomization energies available in the literature is presented.

Statistical adiabatic channel model for ion–molecule capture processes

State-resolved and thermally averaged rate coefficients for ion–molecule capture processes are determined by detailed statistical adiabatic channel calculations. The equivalence of this treatment with alternative recent approaches is demonstrated. The advantage of the derived results is their general analytical form which facilitates an application over a wide range of conditions. Particular emphasis is given to ion + nonlinear dipole collisions at low temperatures.

The effects of far-side dominance on the j-dependence of the medium energy 116Sn(d, p) 117Sn reaction

We report measurements of the 116Sn(d, p) 117Sn reaction to the d 3 2 , g 7 2 , and h 11 2 low-lying states at 79.0 MeV. Transfer reaction calculations which include an adiabatic deuteron channel wavefunction, finite range including the deuteron S- and D-state, and non-locality corrections are performed for these transitions. A separation of the reaction amplitude into the near- and far-side components shows that the g 7 2 and h 11 2 reaction calculations are far-side dominated. In this context, the proton channel spin-orbit distortions modify the j-dependence of the A y vector analyzing power and introduce a strong j-dependence in the A yy tensor analyzing power.

Specific rate constants k(E,J) and product state distributions in simple bond fission reactions. II. Application to HOOH→OH+OH

Detailed and simplified statistical adiabatic channel calculations of specific rate constants k(E,J) and product quantum state distributions for the simple bond fission reaction HOOH→2 OH are compared with recent measurements of state-resolved dissociation rates, product state distributions, and thermally averaged rate coefficients. A simple modification of phase space theory based on the statistical adiabatic channel model successfully predicts product state distributions and rate constants as well. Because of the amount of experimental data and theoretical analysis available, the dissociation of hydrogen peroxide is becoming a model case for simple unimolecular bond fission processes.

Nonempirical parameter estimate for the statistical adiabatic theory of unimolecular fragmentation

For the C-H bond fission in CH3, the reaction path, the potential profile and the frequencies of the normal modes perpendicular to the reaction path are calculated by means of ab initio quantum chemical approaches (UHF, second-order Moller-Plesset perturbation theory). On the whole, these data confirm the assumptions made in the semiempirical statistical adiabatic channel model of Quack and Troe qualitatively; in particular, the decrease of the frequencies of reactant bending modes which transform into free rotations of the product is well reproduced by an exponential switching function with a common parameter. However, this single-parameter switching function cannot be used for all vibrations changing their eigenvalues. Furthermore, within the quantum chemical approaches employed, the Morse function with a constant parameter β is a crude approximation to the potential energy profile. On the basis of these theoretically estimated data, adiabatic rovibronic energy terms are constructed and high-pressure th...

Rotational distributions from resonant and direct scattering in H+CO and tests of statistical theories

Coupled-channel scattering calculations are reported for H+CO using an ab initio potential energy surface. Rotational state-to-state transition probabilities are presented for resonant and direct scattering for J=0 and treating CO as a rigid rotor. The presence of a barrier to complex formation in this system is found to have a crucial role in determining which asymptotic rotational states couple strongly to the resonances. In addition, a propensity for even-Δj transitions is seen for direct scattering and also for rotational states which do not couple to the resonances for resonant scattering. The features of the potential surface which are responsible for these results are uncovered by using distorted wave Born and Franck–Condon approximations. Tests of phase space theory and the statistical adiabatic channel model are made for resonant scattering. One important conclusion of these tests is that the bending motion of the HCO transition state is apparently not adiabatic.

Simplified statistical adiabatic channel model calculations of the reaction H+CH3→CH4 on a b i n i t i o potential energy surfaces

The simplified version of the statistical adiabatic channel model has been applied to calculate limiting high pressure rate coefficients for the title recombination reaction between 200–2000 K. The energy pattern of the channel threshold energies was analyzed in terms of recently reported ab initio potential energy surfaces. The calculated rate coefficients are in very good agreement with quasiclassical trajectory, variational transition state theory, and canonical variational transition state theory calculations performed on the same surfaces. An analysis with a factorized formalism allows one to study separately the role that the motion on the reaction coordinate and the degrees of freedom orthogonal to them, play on the rate coefficients. A recent experimental rate coefficient lies between the values calculated with the ab initio surfaces constructed with data of Duchovic, Hase, and Schlegel–Brown and Truhlar, and with similar information from Hirst. In addition, translational energy dependent thermal averaged cross sections were computed and compared with quasiclassical trajectory results. Based on an experimental limiting low pressure rate coefficient, the average energy transferred per each He–CH4 collision was calculated using the low pressure range unimolecular rate theory with explicit account of the potential energy surface features. Confrontation with direct measurement of collisional energy transfer shows that the abovementioned surfaces also give the best agreement between the experimental and calculated values.

Unimolecular reactions near threshold: The overtone vibration initiated decomposition of HOOH (5νOH)

Laser induced fluorescence detection of the fragments from the unimolecular dissociation of hydrogen peroxide initiated by direct excitation of the fourth overtone of the OH stretching transition (5νOH) is a means of measuring both vibrational overtone predissociation spectra and populations of individual quantum states of the OH products. Because the excitation adds insufficient energy to break the OO bond, the measurements detect reactions of molecules that initially possess substantial thermal vibrational–rotational energy (≥1100 cm−1). The spectroscopic data, in which hot band and high energy transitions are particularly prominent, are fit by a vibrational–torsional model that adiabatically separates the low frequency torsional vibration from the higher frequency OH and OO stretching vibrations. The spectroscopic analysis shows that the effective trans barrier in the torsional potential increases by 75 cm−1 for each quantum of OH stretching excitation but decreases by 50 cm−1 upon excitation of the OO stretching vibration. The product state population distributions demonstrate that the lowest rotational level is preferentially populated in the unimolecular decomposition. This result is inconsistent with phase space theory, which predicts substantially more rotational excitation, but agrees with the statistical adiabatic channel model. The predictions of the latter model agree with the data for HOOH (5νOH) as well as with previous results for HOOH (6νOH). The statistical adiabatic channel model contains one parameter in addition to the ones required in phase space theory, and the value of this parameter that provides the best agreement with the measurements is consistent with an independent analysis of thermal recombination data. The experiment serves to test statistical calculations critically because it explores reactions near the threshold energy, where the predicted product state distributions are most sensitive to the details of the theory.

Statistical adiabatic channel model of ion-neutral dipole capture rate constants

The statistical adiabatic channel model is applied to thermal ion-molecule reactions, which are determined by capture rates, and to ion-molecule association reactions in the high-pressure limit. The radial part of the long-range potential leads to the known ion-locked-dipole rate expression. The anisotropy of the potential introduces rigidity described by an ion-dipole locking constant C. To a first approximation one derives, for dominant permanent dipole contributions. C = 1 − 0.614exp[−( B/kT) 2], where B denotes the rotational constant of the dipolar molecule. This result is often close to recent approximate quantum scattering and classical trajectory calculations.

Theory of thermal unimolecular reactions at high pressures. II. Analysis of experimental results

The simplified version, proposed in Part I, of the statistical adiabatic channel model for thermal unimolecular bond fission reactions and the reverse radical association reactions is applied to 26 experimental systems in which limiting high pressure rate coefficients have been determined. From the analysis, information on the global character of the potential energy surfaces involved is obtained and expressed by the characteristic looseness parameter α of the angular part of the potential and the Morse parameter β of the radial part. It is investigated to what extent empirical ‘‘standard values’’ of α, or of α/β, can be defined, by which limiting high pressure rate coefficients become predictable without knowing further details of the potential energy surfaces.

Electron donor acceptor orbital correlations

A classification of the 16 possible dual (syn + anti) 4 donor + 4 acceptor orbital correlation schemes is developed to describe cofacial complexes of benzene, naphthalene, anthracene and pyrene with benzoquinone, chloranil and tetracyanoquinodimethane, belonging to C 2v and Cs point groups. None of these forms an exciplex and the type of correlation is not indicative of the preferred donor acceptor orientation in the EDA complex ground state. Although the correlations are not uniquely defined in Cs symmetry they provide a basis for identifying adiabatic channels for geminate charge neutralization of radical-ion pairs.

The influence of potential energy parameters on the reaction H + CH 3 CH 4

The simplified statistical adiabatic channel model is used to calculate the limiting high-pressure rate constant of the thermal recombination-dissociation reaction H + CH 3 ▪ CH 4. The dependence of the rate constant on the characteristic parameters of the radial and angular parts of the HCH 3 interaction potential is demonstrated.

High-pressure falloff curves and specific rate constants for the reactions atomic hydrogen + molecular oxygen .dblharw. perhydroxyl .dblharw. hydroxyl + atomic oxygen

The recombination reaction H + O/sub 2/ + M ..-->.. HO/sub 2/ + M is studied by laser flash photolysis in the bath gases M = Ar, N/sub 2/, and CH/sub 4/ at pressures between 1 and 200 bar. By extrapolation of the falloff curves, a limiting high-pressure rate coefficient k/sub infinity/ = 7.5 x 10/sup -11/ cm/sup 3/ molecule/sup -1/ s/sup -1/ is derived at 298 K. An analysis of this result in terms of the statistical adiabatic channel model leads to a looseness parameter of a similarly ordered 0.94 A/sup -1/. The corresponding specific rate constants k(E,J) for dissociation of HO/sub 2/ are constructed and compared with trajectory calculations. The results are also compared with data on the reaction HO + O ..-->.. H + O/sub 2/ for which a nearly temperature-independent rate constant of 2.8 x 10/sup -11/ cm/sup 3/ molecule/sup -1/ s/sup -1/ is predicted. An analysis of the limiting low-pressure rate coefficients with respect to energy-transfer properties is made. The results are compared with trajectory calculations of energy transfer of excited HO/sub 2/. 77 references, 8 figures, 5 tables.

Computation of the direct adiabatic channel for the relaxation of electronically excited C 2 H 5 radical to give H + C 2 H 4

Ab initio 4-31G + 3s RHF computations of the lowest excited state of the ethyl radical show that this species should be either directly dissociative or predissociative. There exists a direct excited-state surface taking the excited state adiabatically to the ground state of ethylene + H.

Rates of reaction of propyl radicals with molecular oxygen

The rates of reaction of 1-propyl and 2-propyl radicals with O/sub 2/ have been measured in the gas phase at room temperature. The rate constants are k/sub 1/(1-C/sub 3/H/sub 7/+O/sub 2/) = (5.5 +/- 0.9) x 10/sup -12/ cm/sup 3/ molecule/sup -1/ s/sup -1/ and k/sub 2/(2-C/sub 3/H/sub 7/+O/sub 2/) = (14.1 +/- 2.4) x 10/sup -12/ cm/sup 3/ molecule/sup -1/ s/sup -1/. Limited pressure studies suggest that both of these reactions are at or near their high-pressure limits at pressures of a few torr. The logarithms of these and previously measured rate constants show an excellent correlation with the ionization potential of the alkyl radical. Adiabatic channel model calculations have been carried out for these reactions with various long-range potential functions. It is concluded that some mixing of the charge-transfer surface (R/sup +/ + O/sub 2//sup -/) into the neutral surface (R + O/sub 2/) is responsible for the correlation of log (k) with ionization potential.

Rates of reaction of alkyl radicals with ozone

The rate constants for a series of alkyl radicals reacting with ozone have been measured at 298 K and 2 torr by using a photoionization mass spectrometer. The radical (R) + ozone rate constants (in units of 10/sup -12/ cm/sup 3/ molecule/sup -1/ s/sup -1/) are 2.53 +/- 0.54 for CH/sub 3/, 25.3 +/- 5.8 for C/sub 2/H/sub 5/, 24.4 +/- 5.9 for 1-C/sub 3/H/sub 7/, 46.5 +/- 10.6 for 2-C/sub 3/H/sub 7/, and 54.5 +/- 11.4 for t-C/sub 4/H/sub 9/. As was observed for the R + O/sub 2/ reactions the R + O/sub 3/ reactions show a trend of increasing rate constant k/sub 1/ with decreasing ionization potential (IP) of the radical. When log (k/sub 1/) vs. IP(R) - EA(O/sub 2/ or O/sub 3/) is plotted, where EA represents electron affinity, the two families of rate constants are brought into close proximity. In addition, rate constants for fluorescence quenching of substituted benzenes by O/sub 2/ are cited which extend the log (k) vs. IP-EA plot to lower ionization potential. Adiabatic channel model calculations have been done by using a long-range potential resulting from the perturbation between the R/sup +/ + O/sub 3//sup -/ and R + O/sub 3/more » surfaces. These calculations give good agreement with the experimental cross sections and they predict rate constants faster than those derived from the classical harpoon model at low IP-EA.« less