John F. Hershberger

Kinetics of NCO + hydrocarbon reactions

Abstract The kinetics of NCO reactions with hydrocarbon molecules were investigated using time-resolved infrared diode laser absorption spectroscopy. NCO was probed at 1905.6 cm −1 , and HNCO reaction products were probed at 2280.1 cm −1 . Rate constants at 296 K were measured to be −14 , (7.04 ± 1.20) × 10 −14 , (2.29 ± 0.13) × 10 −13 , (6.13 ± 0.32) × 10 −13 , (8.78 ± 0.42) × 10 −13 , (7.47 ± 0.8) × 10 −14 , and (2.85 ± 0.22) × 10 −12 cm 3 molecule −1 s −1 , for CH 4 , C 2 H 6 , C 3 H 8 , n -C 4 H 10 , n -C 5 H 12 , C 2 H 2 , and C 2 H 4 , respectively. Hydrogen abstraction to form HNCO was observed to be a major product channel for reactions with alkanes and for C 2 H 2 . For C 2 H 2 , HNCO is formed in negligible yield, indicating that addition—elimination is the primary mechanism active.

Kinetics and product branching ratios of the CN+NO2 reaction

The CN+NO2 reaction was studied using time‐resolved infrared diode laser absorption spectroscopy. The total rate constant for this reaction over the temperature range 298–650 K was fit to the expression k1=10−10.30±0.03 exp[(171±32)/T], in reasonable agreement with previous measurements. Branching ratios at room temperature were also measured. The dominant product channel is NCO+NO, which accounts for 86.8±6.0% of the total reaction rate. The CO2+N2 and CO+N2O channels account for 5.6±6.0 and 7.6±3.2%, respectively. The dominant mechanism for this reaction is formation of an NCONO intermediate followed by O–N bond cleavage.

Kinetics of the CN+CH2CO and NCO+CH2CO reactions

Abstract The CN+CH 2 CO and NCO+CH 2 CO reactions were studied over the temperature ranges 296–567 K and 296–556 K, respectively, using time-resolved infrared diode laser absorption and visible laser-induced fluorescence spectroscopy. The total rate constant data were fit to the following expressions: k 1 (CN+CH 2 CO) = (2.37±0.5)×10 −11 exp[552.6±97/ T ] and k 2 (NCO+CH 2 CO) = (1.71±0.27)×10 −12 exp[713.2±56/ T ] cm 3 molecule −1 s −1 . Detection and quantification of CO product yields suggests that an addition–elimination mechanism producing CH 2 CN+CO or CH 2 NCO+CO dominates these reactions, and that hydrogen abstraction to produce HCN+HCCO or HNCO+HCCO is a minor or negligible product channel.

Reinvestigation of the branching ratio of the CN + O2 reaction.

The reaction of the CN radical with O(2) was studied using infrared diode laser absorption spectroscopy. Detection of NO and secondary N(2)O products was used to directly measure the product branching ratio. After consideration of possible secondary chemistry and comparison to kinetic modeling simulations, the branching ratio of the CN + O(2) reaction into the NO + CO channel was determined to be phi (NO + CO) = 0.20 +/- 0.02, with little or no temperature dependence over the range 296-475 K.

Product branching ratios of the CD + NO reaction

The reaction of CD radicals with NO was studied by time-resolved diode laser absorption spectroscopy. CD radicals were generated by the 266-nm photolysis of CDBr[sub 3]. Xenon buffer gas was used to relax excited CD(a[sup 4][sigma]) radicals. Detected products include DCN, CO, and CO[sub 2] and N[sub 2]O products of secondary reactions. An upper limit was placed on the formation of CN products. After consideration of important secondary reactions, the following branching ratios were obtained: 47.5 [+-] 12.2% for DCN + O, 18.8 [+-] 5.5% for NCO + D, an upper limit of <7.5% for CN + OD, and 33.7 [+-] 13.8% for the sum of the CO + DN and DCO + N channels (error bars represent one standard deviation). 54 refs., 3 figs., 2 tabs.

Kinetics of the O + HCNO Reaction

The kinetics of the O + ICN reaction was studied using a relative rate method, with O + C(2)H(2) as the competing reaction. Carbon monoxide products formed in the competing reaction and subsequent secondary chemistry were detected as a function of reagent ICN pressure to obtain total rate constants for the O + ICN reaction. Analysis of the experimental data yields rate constants of k(1) = (3.7 ± 1.0 to 26.2 ± 4.0) × 10(-14) cm(3) molecule(-1) s(-1) over the total pressure range 1.5-9.5 Torr. Product channel NCO + I, the only bimolecular exothermic channel of the reaction, was investigated by detection of N(2)O in the presence of NO and found to be insignificant. An ab initio calculation of the potential energy surface (PES) of the reaction at the CCSD(T)/CEP-31G//DFT-B3LYP/CEP-31G level of theory was also performed. The pathways leading to bimolecular product channels are kinetically unfavorable. Formation and subsequent stabilization of an ICNO adduct species appears to dominate the reaction, in agreement with the experimentally observed pressure dependent rate constants.

Product channels of the CN + HCNO reaction.

The kinetics of the reaction of CN radical with fulminic acid (HCNO) was studied by transient infrared absorption spectroscopy with the primary goal of resolving whether the dominant product channel is NO + HCCN (1a) or HCN + NCO (1b). HCN, HCCN, and NO reaction products were directly detected. In some experiments, (15)N(18)O reagent was included in the reaction mixtures in order to suppress possible secondary chemistry due to NCO radicals. Several other possible secondary reactions were also investigated and found to be very slow. The resulting product branching fractions of φ(1a) = 0.98 ± 0.07 for NO + HCCN and φ(1b) ≤ 0.07 for HCN + NCO, respectively, were obtained at 298 K. The potential energy surface (PES) of the reaction was calculated by ab initio methods at several levels of coupled-cluster theory. The calculations show pathways to channels (1a) and (1b) with nearly identical energetics and a substantial dependence on the level of theory used, suggesting that multireference calculations are needed to accurately predict the experimental results.

Kinetics of the SiH3 + O2 and SiH3 + NO2 reactions

Abstract The reactions SiH3+O2 and SiH3+NO2 were studied over the temperature range 235–512 K using time-resolved infrared diode laser absorption spectroscopy. Observed rate constants are kSiH3+O2 = (2.07±0.34) × 10−12 exp[(469±60)/T] and kSiH3 + NO2 = (2.25±0.48) × 10−11 exp[(264±80)/T] cm3 molecule−1 s−1. These results are consistent with initial SiH3O2 and SiH3NO2 adduct formation followed by possible rearrangement and subsequent dissociation.

Kinetics of the CN+OCS reaction

The CN+OCS reaction was studied using flash photolysis/time-resolved infrared laser spectroscopy. The reaction is fast, with a rate constant of (9.75±0.5)×10−11 cm3 molecule−1 s−1 at 296 K and 2.2 Torr pressure. Detection of CO products indicates that the CO+NCS product channel dominates this reaction, and that the CO is produced vibrationally cold, with >98% in the vibrational ground state.

A diode laser study of the product branching ratio of the CH+N2O reaction

Abstract The reactions of CH and CD radicals with N 2 O were studied at room temperature by an excimer photolysis/infrared absorption technique, using multiphoton photolysis of CHBr 3 or CDBr 3 at 248 nm. Two product channels were detected: HCN+NO in 72±4% yield, and CO+H+N 2 in 28±4% yield. No detectable change in the branching ratios upon deuteration was observed.