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Author(s):   Parker, J.K.; Garland, N.L.; Nelson, H.H.
Title:   Kinetics of the Reaction Al + SF6 in the Temperature Range 499-813 K
Journal:   J. Phys. Chem. A
Volume:   106
Page(s):   307 - 311
Year:   2002
Reference type:   Journal article
Squib:   2002PAR/GAR307-311

Reaction:   SF6 + AlSF5 + AlF
Reaction order:   2
Temperature:   500 - 800 K
Rate expression:   2.2x10-10 [cm3/molecule s] e-45187 [J/mole]/RT
Category:  Theory
Data type:   Ab initio
Comments:   Rate expression given is based on MP2/6-311++G(2d) energies (and geometries).

In this work, ab initio and DFT calculations were also used to compute transition states and heats of reactions. MP2, BH&HLYP, and B3LYP theories were used with 6-31G(D), 6-311++G(2d), aug-cc-pVTZ basis sets. No transition state could be found using B3LYP theory. Computed heats of reaction were found to be ?58-65) kcal/mol (exothermic) depending on level of theory compared to ?8.0(3.8) kcal/mol for experimental value or generally at least 3-5 kcal/mol high (less exothermic). Computed barriers were on the order of 10-13 kcal/mol depending on level of theory compared to experimental derived activation energy of 9.5(0.4) kcal/mol or on the order of 1-2 kcal/mol high. Computed pre-exponential factors were on the order of 1-2 E-10 cm3/molecule/s compared to 6.8(2.2) cm3/molecule/s or on the order of 3-4 times lower. Converting these into rate constants, the ab initio derived rate constants were on the order of 6-10 times lower than the experimental value. The low A factor was interpreted to mean that the computed transition states were too tight.

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Rate constant values calculated from the Arrhenius expression:

T (K)k(T) [cm3/molecule s]
500 4.19E-15
525 7.02E-15
550 1.12E-14
575 1.73E-14
600 2.56E-14
625 3.68E-14
650 5.14E-14
675 7.01E-14
700 9.35E-14
725 1.22E-13
750 1.57E-13
775 1.98E-13
800 2.47E-13