H. H. Nelson

Kinetics of the reaction of CH3S with NO, NO2 and O2

Abstract We have used the laser-induced fluorescence technique to study the reactions of the methyl thiyl radical (CH 3 S) with NO, NO 2 , and O 2 as a function of pressure (1–400 Torr N 2 or SF 6 ) and temperature (25–238°C). Rate constants for the reaction with NO are in the transition region between the low- and high-pressure limits and are fit using an expression developed by Troe. The resulting Arrhenius expression is k ∞ (±2σ) = (1.81 ± 0.84) × 10 −12 exp[(1800 ± 320 cal/mol)/ RT ] cm 3 /s. The rate constant for the reaction of CH 3 S with NO 2 is essentially pressure independent and yields k = (8.3 ± 1.4) × 10 −11 exp[(160 ± 120 cal/mol)/ RT ] cm 3 /s. The reaction with O 2 is too slow to measure in our system; we estimate that k ⩽ 2 × 10 −17 cm 3 /s at room temperature.

Kinetics of the reactions of isopropoxy radicals with NO, NO2, and O2

Abstract A fluorescence excitation spectrum of (CH 3 ) 2 CHO (isopropoxy radical) is reported following photolysis of isopropyl nitrite at 355 nm. Rate constants for the reaction of isopropoxy with NO, NO 2 , and O 2 have been measured as a function of pressure (1–50 Torr) and temperature (25–110°C) by monitoring isopropoxy radical concentrations using laser-induced fluorescence. We have obtained the following Arrhenius expressions for the reaction of isopropoxy with NO and O 2 respectively: (1.22±0.28)×10 −11 exp[(+0.62±0.14 kcal)/ RT ]cm 2 /s and (1.51±0.70)×10 −14 exp[(−0.39±0.28)kcal/ RT ]cm 3 /s where the uncertainties represent 2σ. The results with NO 2 are more complex, but indicate that reaction with NO 2 proceeds more rapidly than with NO contrary to previous reports. The pressure dependence of the thermal decomposition of the isopropoxy radical was studied at 104 and 133°C over a 300 Torr range using nitrogen as a buffer gas. The reaction is in the fall-off region over the entire range. Upper limits for the reaction of isopropoxy with acetaldehyde, isobutane, ethylene, and trimethyl ethylene are reported. We have performed the first LIF study of the isopropoxy radical. Arrhenius parameters were measured for the reaction of i -PrO with O 2 , NO, NO 2 , using direct radical measurement techniques. All reactions are in their high-pressure limits at a few Torr of pressure. The rate constant for the reactions of i -PrO with NO and NO 2 reactions exhibit a small negative activation energy. Studies of the i -PrO + NO 2 reaction produce data which indicate that O( 3 P) reacts rapidly with i -PrO. Unimolecular decomposition studies of i -PrO indicate that the reaction is in the fall-off region between 1 and 300 Torr of N 2 and the high-pressure limit is above 1 atmosphere of N 2 .

Temperature-dependent reaction kinetics of NH(a1Δ)

Abstract The gas-phase reactions of NH(a 1 Δ) with H 2 and selected saturated and unsaturated hydrocarbons have been studied over the 250-600 K temperature range. Olefin reactions proceed at near the gas kinetic collision rate and show no temperature dependence. H 2 and saturated hydrocarbons show temperature-dependent reactions rates, with activation energies of = 0.8-2 kcai/mole. No evidence of electronic quenching of NH(a 1 Δ) to the ground state was observed with any of the hydrocarbons studied. First-order reactions rates, Arrhenius A factors and activation energies for the reactions are reported. We discuss a mechanistic interpretation of the kinetics in view of earlier kinetic and reaction-product studies and ab initio SCF Cl calculations.

Optical properties and dynamics of a photochromic bisthienylethene in solution and in a polymer film

Abstract Optical switching dynamics and fatigue were investigated for a photochromic molecular switch, bis(5-pyridyl-2-methylthien-3-yl)perfluorocyclopentene (PMTFC). Femtosecond transient absorption was measured using 270 nm pump and broadband visible probe pulses for samples in ethanol solution and as a dispersion film of poly(butyl acrylate-co-acrylic acid). The optical switching occurs in ≤3 ps in both cases. The spectral evolution observed in solution within 20 ps is absent in the film, suggesting that the switching dynamics are sensitive to the molecular environment. PMTFC in the copolymer film demonstrated evidence of fatigue after ∼1500 complete cycles of optical switching.

Potential chlorofluorocarbon replacements: OH reaction rate constants between 250 and 315 K and infrared absorption spectra

We measured the rate constant for reactions of the OH radical with several potential chlorofluorocarbon replacements over the temperature range 251–314 K using laser photolysis laser-induced fluorescence techniques. The compounds studied and Arrhenius parameters determined from fits to the measured rate constants are as follows: CHF2OCHF2 (E 134), k(T) = (5.4 ± 3.5) × 10−13 cm3 s−1 exp [(−3.1 ± 0.4 kcal mol−1)/RT]; CF3CH2CF3 (FC 236fa), k(T) = (2.0 ± 1.0) × 10−14 cm3 s−1 exp [(−1.8 ± 0.3 kcal mol−1)/RT]; CF3CHFCHF2 (FC 236ea), k(T) = (2.0 ± 0.9) × 10−13 cm3 s−1 exp [(−2.0 ± 0.3 kcal mol−1)/RT]; and CF3CF2CH2F (FC 236cb), k(T)= (2.6 ± 1.6) × 10−13 cm3 s−1 exp [(−2.2 ± 0.4 kcal mol−1)/RT]. The measured activation energies (2–3 kcal mol−1) are consistent with a mechanism of H atom abstraction. The tropospheric lifetimes, estimated from the measured OH reaction rates, and measured integrated infrared absorption cross sections over the range 770 to 1430 cm−1 suggest that E 134 and FC 236fa may have significant global warming potential, while FC 236ea and FC 236cb do not.

Temperature dependence of the kinetics of the reaction Al+O2→AlO+O

Abstract The temperature and pressure dependence of the rate constant for the reaction Al+O2 has been measured by observing both Al disappearance and AlO appearance. No pressure dependence is observed from 10 to 100 Torr total pressure of Ar at room temperature. Over the range 298–1083 K, a fit to the measured data yields the expression k(T) = (1.62±0.13) × 10−10 cm3 s−1 exp[(−0.16±0.08) kcal mol−1/RT].

Excitation and emission spectra of the 2A″2 ↽ → 2E″1 system of the gas-phase cyclopentadienyl radical

Abstract Laser-induced fluorescence excitation and resolved emission spectra of the gas-phase cyclopentadienyl radical are reported. Emission spectra following single vibronic level excitation of prominent absorption features from 338.0 to 325.6 nm are recorded and catalogued. The collision-free fluorescence lifetime is in the range of 12–20 ns. Efficient vibrational relaxation of the 2 A″ 2 state by SF 6 is reported. Based upon analysis of the excitation and emission spectra it is concluded that there are two or more Jahn-Teller active modes in C 5 H 5 . The measured spectra and our analyses agree qualitatively with recent calculations on C 5 H 5 that assume three active modes with Jahn-Teller coupling parameters, λ 2 of 0.6–1.9.

Reaction rate constants of OH + CHF3 → products and O(3P) + CHF3 → OH + CF3 at 500–750 K

Abstract The reactions OH + CHF 3 → products and O ( 3 P ) + CHF 3 → OH + CF 3 were studied in the temperature range 500–750 K and the pressure range 20–160 Torr using laser-induced-fluorescence detection of OH. For the reaction, OH + CHF 3 , the experimental rate coefficient is k = (1.1 ± 0.3) × 10 −12 exp(−(2300 ± 200)/ T ). For the reaction O ( 3 P + CHF 3 → OH , the rate coefficient is k = (2.49 ± 0.06) × 10 −13 exp(3000 ± 1000)/ T ) in the temperature range studied. For the reaction CF 3 CFHCF 3 + O ( 3 P ) → OH + CF 3 CFCF 3 , an upper limit for the rate coefficient is determined to be k −13 exp(−(2700 ± 400)/ T ).

Einstein transition probabilities for the AlH A 1Π—X 1Σ+ transition

Abstract Ratios of the Einstein transition probabilities, band oscillator strengths and vibrational transition probabilities for the (0,0) and (0,1) bands and the (1,0), (1,1), and (1,2) bands in the AlH A 1 Π—X 1 Σ + transition are measured. These ratios indicate that emission occurs predominantly in the diagonal terms. Using literature values of the lifetimes of the ν=0 and 1 levels, absolute Einstein transition probabilities are derived.

Electronic quenching of the B 2Σ+ state of AlO

Abstract Electronic quenching rate constants for the B 2 Σ + state of AlO were determined using a pulsed-laser photolysis laser-induced fluorescence technique by examining fluorescence decay rates in the presence of seven atomic and molecular collision partners. The two polar molecules CO and NO were found to have rate constants of 4.8 × 10 −12 and 1.8 × 10 −10 cm 3 s −1 , respectively. The nonpolar gases He, H 2 , N 2 , CO 2 and Xe were found to be poor quenchers with a bimolecular quenching rate constant having an upper bound of 1 × 10 −13 cm 3 s −1 for He, H 2 , N 2 and Xe and 1 × 10 −12 cm 3 for CO 2 .