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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:   499 - 813 K
Pressure:  0.13 bar
Rate expression:   6.8x10-10 [±2.2x10-10 cm3/molecule s] e-39743 [±1663 J/mole]/RT
Bath gas:   Ar
Category:  Experiment
Data type:   Absolute value measured directly
Pressure dependence:   Rate constant is pressure independent
Experimental procedure:   Static or low flow - Data taken vs time
Excitation technique:   Flash photolysis (laser or conventional)
Time resolution:   In real time
Analytical technique:   Laser induced fluorescence
Comments:   Al atoms generated by 248 nm KrF laser photolysis of Trimethylaluminum or Triethylaluminum. Al atoms were detected by LIF near 394.4 nm and AlF products detected by LIF near 227.5 nm. Typically 100 torr Argon with 60 ppm TMA or 20 ppm TEA. SF6 concentrations greater than 100 times Al or AlF concentrations ensuring first-order conditions.

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]
499 4.70E-14
500 4.79E-14
525 7.56E-14
550 1.14E-13
575 1.67E-13
600 2.36E-13
625 3.24E-13
650 4.35E-13
675 5.72E-13
700 7.36E-13
725 9.31E-13
750 1.16E-12
775 1.43E-12
800 1.73E-12
813 1.90E-12