Molecular Size Dependent Fall-off Rate Constants as long as the Recombination Reactions

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Molecular Size Dependent Fall-off Rate Constants as long as the Recombination Reactions

Engle, Kathleen, Freelance Writer has reference to this Academic Journal, PHwiki organized this Journal Molecular Size Dependent Fall-off Rate Constants as long as the Recombination Reactions of Alkyl Radicals with O2 Akira Miyoshi Department of Chemical Systems Engineering, University of Tokyo Introduction — R (alkyl) + O2 key reactions that lead to chain branching in low-temperature oxidation of hydrocarbons — Challenges resolution of complicated pressure- in addition to temperature- dependent product specific rate constants including second O2 addition reactions to QOOH — Objectives evaluation of universal fall-off rate expression as long as recombination master equation analysis as long as the dissociation/recombination steady-state Computational

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Computational — Quantum Chemical Calculations B3LYP & CBS-QB3 calculations by Gaussian 03 CASPT2 calculations by MOLPRO 2008.1 — TST in addition to VTST Calculations by GPOP including: Pitzer-Gwinn approximation as long as hindered rotors, qPG (after analysis by BEx1D) 1D tunneling correction (asymmetric Eckart), tun rotational con as long as mer distribution partition function, qRCD — RRKM/ME Calculations (E) in addition to k(E) accounting as long as all TST feature (qPG, tun, in addition to qRCD) by modified UNIMOL RRKM program steady-state & transient master equation calculations by SSUMES http://www.frad.t.u-tokyo.ac.jp/~miyoshi/tools4kin.html Hindered Rotor (carbon-centered radical) partition function calculated from eigenstate energies, qexact, is well approximated by qPG(V0 = 100 cm–1) or qFR (free rotor) — Pitzer-Gwinn Approximation Hindered Rotor (RO2) partition function calculated from eigenstate energies, qexact, is well approximated by 2qHO+qHO’ or qHOqRCD — Taken into Account as Rotational Con as long as mers

Rotational Con as long as mers rotational con as long as mer distribution partition function, qRCD — Taken into Account via Partition Function by assuming qi q0 Molecular Size Dependent Fall-off Rate Constants Potential Energy Curves CASPT2(7,5)/aug-cc-pVDZ // B3LYP/6-311G(d,p) potential energy well reproduced experimental k(300 K) within ± 25% B3LYP/6-311G(d,p) potential energy systematically underestimated k(300 K) R (alkyl) + O2 RO2

High-Pressure Limiting Rate Constants, k same as long as primary R’s same as long as secondary R’s class (primary, secondary, or tertiary) determines the rate constant — Size-Independent — Class-Specific Fall-off Calculations Plumb & Ryan, Int. J. Chem. Kinet., 1981, 13, 1011; Slagle et al., J. Phys. Chem., 1984, 88, 3648; Wagner et al., J. Phys. Chem., 1990, 94, 1853. — Energy Transfer Model experimental data as long as C2H5 + O2 in fall-off region were well reproduced by the exponential-down model with: Low-Pressure Limiting Rate Constants, k0 same as long as three C4 R’s irrespective of class (primary, secondary, or tertiary) — Class-Independent — Size-Dependent

Size-Dependent Expression as long as k0 Parameters as long as modified Arrhenius Expression: k0 = A T b exp(–Ea / RT ) nHA = number of heavy (non-hydrogen) atoms — Universal Fall-off Rate Constants as long as R + O2 class-specific k + size-dependent k0 Collapse of Steady-State Assumption Steady-State Distribution of Large RO2 steady-state distribution as long as dissociation rump distribution after major part has gone steady-state distribution as long as chemical-activation Boltzmann distribution Collapse of steady-state assumption or Lindemann-Hinshelwood type mechanism (Miller in addition to Klippenstein, Int. J. Chem. Kinet., 2001, 33, 654–668) k k at high temperatures

Dissociation/Recombination Steady-State R + O2 RO2 Partial Equilibrium — Dissociation/Recombination Steady-State Chemical activation steady state When other channels are not present, there is trivial solution where = Boltzmann distribution more general condition where near F(E) is established Dissociation/Recombination Steady-State — Near Boltzmann Distribution rate constants as long as subsequent isomerization/dissociation reactions of RO2 can be estimated to be in near high-pressure limit

Three “Steady-States” Miller in addition to Klippenstein, Int. J. Chem. Kinet., 2001, 33, 654–668. Clif as long as d, Farrell, DeSain in addition to Taatjes, J. Phys. Chem. A, 2000, 104, 11549–11560. “prompt” “delayed” HO2 as long as mation in C2H5 + O2 C2H5O2 Experimental data by Clif as long as d, Farrell, DeSain in addition to Taatjes, J. Phys. Chem. A, 2000, 104, 11549–11560. k(HO2) k(HO2) at moderate T but in partial equilibrium of R + O2 RO2 Time Dependent Solution Time-dependent solution as long as with n0 = 0 in addition to kin = const. Build-up time kdis,FO–1 Nearly the same with in addition to without concerted HO2 elimination channel

In Autoignition Modeling near partial equilibrium transient Building-Up Transient as long as C8H17O2 collision-free build-up of F(E) with bu–1 kdis, >> kdis,FO build-up of F(E) with bu–1 kdis,FO kdis, build-up of F(E) with bu–1 kdis,FO kdis, (0.01atm) bimodal build-up (10–6 atm) Summary — Size-Dependent Fall-Off Rate Constants as long as R + O2 VTST in addition to RRKM/ME calculations as long as R = C2H5, i-C3H7, n-C4H9, s-C4H9, t-C4H9, n-C6H13, in addition to i-C8H17 k is class-specific but size-independent k0 is size-dependent but class-independent Universal fall-off rate expression as long as arbitrary R + O2 — Collapse of Steady-State Assumption For large RO2 at high temperatures — Dissociation/Recombination Steady-State nss(E) F(E) as long as RO2 in partial equilibrium with R + O2 HPL(k) can be assumed as long as subsequent reactions of RO2 build-up time kdis,FO–1 at low T kdis,–1 at high T irrespective of P bimodal build-up at midium T especially at low P

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