Rate Constants for the Thermal Decomposition of Ethanol and Its Bimolecular Reactions with OH and D: Reflected Shock Tube and Theoretical Studies
Raghu Sivaramakrishnan, Meng-Chih Su, Joe V. Michael, Stephen J. Klippenstein, Lawrence B. Harding, and Branko Ruscic
Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
J. Phys. Chem. A
114(35), 9425�9439 (2010)
The thermal decomposition of ethanol and its reactions with OH and D have been studied with both shock
tube experiments and ab initio transition state theory based master equation calculations. Dissociation rate
constants for ethanol have been measured at high T in reflected shock waves using OH optical absorption and
high sensitivity H-atom ARAS detection. The three dissociation processes that are dominant at high-T are
C2H5OH → C2H4 + H2O (1)
C2H5OH → CH3 + CH2OH (2)
C2H5OH → C2H5 + OH (3)
The rate coefficient for reaction (3) was measured directly with high sensitivity at 308 nm using a multipass
optical White cell. Meanwhile, H-atom ARAS measurements yield the overall rate coefficient and that for
the sum of reactions (2) and (3), since H-atoms are instantaneously formed from the decompositions of CH2OH
and C2H5 into CH2O + H and C2H4 + H, respectively.
By difference, rate constants for reaction (1) could be obtained. One potential complication is the scavenging of OH
by unreacted ethanol in the OH experiments, and therefore rate constants for
OH + C2H5OH → products (4)
were measured using tert-butyl hydroperoxide (tBH) as the thermal source for OH. The present experiments
can be represented by the Arrhenius expression
k = (2.5 � 0.43) ∙ 10-11 exp(-911 � 191 K/T) cm3 molecule-1 s-1
over the T range 857-1297 K. For completeness, we have also measured the rate coefficient for the reaction
of D atoms with ethanol
D + C2H5OH → products (5)
whose H analogue is another key reaction in the combustion of ethanol. Over the T range 1054-1359 K,
the rate constants from the present experiments can be represented by the Arrhenius expression,
k = (3.98 � 0.76) ∙ 10-10 exp(-4494 � 235 K/T) cm3 molecule-1 s-1
The high-pressure rate coefficients for reactions (2) and (3) were studied with variable reaction coordinate
transition state theory employing directly determined CASPT2/cc-pvdz interaction energies. Reactions (1), (4), and (5)
were studied with conventional transition state theory employing QCISD(T)/CBS energies. For the
saddle point in reaction (1) additional high level corrections are evaluated. The predicted reaction exo and
endothermicities are in good agreement with the current Active Thermochemical Tables values. The transition
state theory predictions for the microcanonical rate coefficients in ethanol decomposition are incorporated in
master equation calculations to yield predictions for the temperature and pressure dependences of reactions (1)�(3).
With modest adjustments (< 1 kcal/mol) to a few key barrier heights the present experimental and
adjusted theoretical results yield a consistent description of both the decomposition, (1)-(3),
and abstraction kinetics, (4) and (5). The present results are compared to earlier experimental and theoretical work.
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