Kinetics and Thermochemistry of SiH3 + NO and SiD3 + NO Reactions: Pressure Falloff and SiH3−NO Bond Energy

The reactions of silyl and silyl- d 3 radicals with nitric oxide, SiH 3 + NO → SiH 3 NO (1) and SiD 3 + NO → SiD 3 NO (2), have been studied using pulsed excimer laser photolysis coupled with time-resolved photoionization mass spectrometry over the temperature range 301−817 K. The rate constants were measured in the ranges 0.75 × 10 16 ≤ [He] ≤ 32.0 × 10 16 molecules cm -3 and 301 ≤ T ≤ 611 K. Both reactions are in the pressure falloff regions considerably far from the true low-pressure limit. RRKM fittings using the Troe factorization approach yielded the low-pressure limit and high-pressure limit rate constants: k 1,tri = (3.12 ± 0.12) × 10 -29 ( T /298) -2.96±0.09 cm 6 molecule -2 s -1, k 2,tri = (9.02 ± 0.40) × 10 -29 ( T /298) -3.36±0.11 cm 6 molecule -2 s -1, k 1,inf = (3.9 ± 1.1) × 10 -11 ( T /298) -1 cm 3 molecule -1 s -1, and k 2,inf = (5.0 ± 1.3) × 10 -11 ( T /298) -1 cm 3 molecule -1 s -1 . Equilibrium constants were measured in the temperature range 711−817 K. The standard enthalpies of reactions 1 and 2 were determined using the third law method. The bond energies determined are DH° 0 (H 3 Si−NO) = 153.7 ± 5.7 kJ mol -1, DH° 298 (H 3 Si−NO) = 158.9 ± 5.7 kJ mol -1, DH° 0 (D 3 Si−NO) = 156.1 ± 5.7 kJ mol -1, and DH° 298 (D 3 Si−NO) = 160.8 ± 5.7 kJ mol -1 . This value of DH° 0 (H 3 Si−NO) is ca . 30 kJ mol -1 higher than previous estimates. Ab initio structural parameters were used to calculate the thermodynamic functions of SiH 3, SiD 3, SiH 3 NO, and SiD 3 NO. Both the singlet and the low-lying triplet states were taken into account for the adduct molecules. The calculated thermodynamic functions were used to obtain the standard entropies of the reactions 1 and 2: Δ S ° 298 (rxn 1) = 143.1 J mol -1 K -1 and Δ S ° 298 (rxn 2) = 146.0 J mol -1 K -1 . The rate constants of the reaction SiH 3 NO + NO → products (3) were measured in the temperature range 301−611 K: k 3 = (2.95 ± 0.38) × 10 -13 ( T /298) -1.59±0.29 cm 3 molecule -1 s -1 .