The unimolecular dissociation of the C3H4 isomers allene and propyne has been examined using two complementary shock-tube techniques:  laser schlieren (LS) and time-of-flight (TOF) mass spectrometry. The LS experiments cover 1800−2500 K and 70−650 Torr, in 1, 2, and 4% propyne/Kr and 1 and 2% allene/Kr, whereas the TOF results extend from 1770 and 2081 K in 3% allene or propyne in Ne. The possible channels for unimolecular dissociation in the C3H4 system of isomers are considered in detail, using new density functional theory calculations of the barriers for insertion of several C3H2 into H2 to evaluate the possibility of H2 elimination as a dissociation route. The dominant path clearly remains CH fission, from either isomer, as suggested in earlier work, although some small amount of H2 elimination may be possible from allene. Rate constants for the CH fission of both allene and propyne were obtained by the usual model-assisted extrapolation of LS profiles to zero time using an extensive mechanism constructed to be consistent with both the time variation of LS gradients and the TOF product profiles. This procedure then provides rate constants effectively independent of both the near-thermoneutral isomerization of the allene/propyne and of secondary chain reactions. Derived rate constants show a strong, persistent pressure dependence, i.e., a quite unexpected deviation (falloff) from second-order behavior. These rate constants are nearer first than second order even for T > 2000 K. They are also anomalously large; RRKM rates using literature barriers and routine energy-transfer parameters are almost an order of magnitude too slow. The two isomers show slightly differing rates, and falloff is slightly less in allene. It is suggested that isomerization is probably slow enough for this difference to be real. The anomalously large rates and falloff are both consistent with an unusually large low-pressure-limit rate in this system. Extensive isomerization of these C3H4 is possible for energies well below their CH fission barriers, and this can become hindered internal rotation in the activated molecule. On the C3H4 surface we identify three such accessible rotors. State densities for the molecule including these rotors are calculated using a previous general classical formulation. Insertion of these state densities into the RRKM model results in rates quite close to the measured magnitudes, and showing much of the observed falloff. The increase in the low-pressure rate is as much as a factor of 8; a necessary but nonetheless remarkable effect of anharmonicity on the unimolecular rate. This again demonstrates the importance of accessible isomerization and consequent hindered internal rotation on the rate of dissociation of unsaturated species.

Rate constants for the thermal decomposition of CHCl3 in Kr diuent have been measured by the laser schlieren density gradient method. The only decomposition process indicated is molecular elimination giving the singlet carbene, CCl2, and HCl. Rate constants are determined under different conditions of density over the temperature range 1282−1878 K, giving k(±15%) = 4.26 × 1016 exp(−22 516 K/T) cm3 mol-1 s-1. Electronic structure calculations have provided models for both the transition state and molecule. With these models, both semiempirical Troe and Rice−Ramsperger−Kassel−Marcus unimolecular theoretical calculations are carried out. The experimental results agree with theory provided E 0 = 56.0 kcal mol-1 and 〈ΔE〉down = (820 ± 30) cm-1, suggesting that the barrier for back reaction is 3.8 kcal mol-1. Cl-atom atomic resonance absorption spectrometric (ARAS) experiments, also in Kr diluent, are then carried out, confirming that atom formation is entirely due to the thermal reactivity of CCl2. On the basis of Cl-atom yield measurements, a mechanism for Cl-atom formation is devised. Chemical simulations of the absolute Cl-atom profile data then provide estimates of the temperature dependences for the rate constants used in the mechanism. These results are discussed in terms of unimolecular reaction rate theory suggesting that the heat of formation for CCl radicals is 100 ± 4 kcal mol-1 at 0 K. Our calculated results (R-CCSD(T)) extrapolated to the complete basis set limit give values of Δf = 53.0 and Δf = 102.5 kcal mol-1 and are consistent with the experimental results reported herein. Additionally, the results suggest that CCl2 undergoes dissociative recombination with a substantial activation energy.

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A standard low-pressure limit Rice–Ramsperber–Kassel–Marcus rate constant is shown to significantly underestimate, by factors of three or more, the measured thermal dissociation rates for HCCH and HCN if the correct value of the bond-dissociation energy is used. An explanation for this discrepancy is sought by examining anharmonic effects due to isomerization. Classical expressions for the density of states and partition function are developed which include isomerization anharmonicity and can be substituted in the standard rate constant expression for corresponding harmonic terms. These expressions are then applied to HCN and HCCH. For HCN, the resulting expression can be compared both to experiment and to a previous quantum mechanical study using the same Hamiltonian form and potential for isomerization. The classical and quantum mechanical agreement is excellent. Good agreement with experiment is obtained with the consensus dissociation energy. For HCCH, electronic structure calculations are performed to produce the required potential for isomerization. With this potential, comparison between measured rate constants and those calculated with the consensus dissociation energy is also good. In both of these applications, adiabatic influences from the two stretching frequencies are argued to reduce the effective isomerization barrier and increase the effective mass of the rotation. Based on these detailed applications, an approximate, closed-form multiplicative factor for the rate constant expression is derived. This expression can be regarded as a generalization of one-dimensional hindered rotor formulas for the inherently multidimensional hindered rotors of isomerization. The expression is parametrized by the height of the hindered-rotor barrier. With the correct barrier height, this expression reproduces the more detailed calculations on HCN and HCCH. Its application to other systems indicates that the kinetic importance of isomerization in olefins is a rather general effect, not relegated only to small molecules.