Experimental and Theoretical Study of the OH-Initiated Degradation of Piperidine under Simulated Atmospheric Conditions.

The OH-initiated photo-oxidation of piperidine and the photolysis of 1-nitrosopiperidine were investigated in a large atmospheric simulation chamber and in theoretical calculations based on CCSD(T*)-F12a/aug-cc-pVTZ//M062X/aug-cc-pVTZ quantum chemistry results and master equation modeling of the pivotal reaction steps. The rate coefficient for the reaction of piperidine with OH radicals was determined by the relative rate method to be kOH-piperidine = (1.19 ± 0.27) × 10-10 cm3 molecule-1 s-1 at 304 ± 2 K and 1014 ± 2 hPa. Product studies show the piperidine + OH reaction to proceed via H-abstraction from both CH2 and NH groups, resulting in the formation of the corresponding imine (2,3,4,5-tetrahydropyridine) as the major product and in the nitramine (1-nitropiperidine) and nitrosamine (1-nitrosopiperidine) as minor products. Analysis of 1-nitrosopiperidine photolysis experiments under natural sunlight conditions gave the relative rates jrel = j1-nitrosoperidine/jNO2 = 0.342 ± 0.007, k3/k4a = 0.53 ± 0.05 and k2/k4a = (7.66 ± 0.18) × 10-8 that were subsequently employed in modeling the piperidine photo-oxidation experiments, from which the initial branchings between H-abstraction from the NH and CH2 groups, kN-H/ktot = 0.38 ± 0.08 and kC2-H/ktot = 0.49 ± 0.19, were derived. All photo-oxidation experiments were accompanied by particle formation that was initiated by the acid-base reaction of piperidine with nitric acid. Primary photo-oxidation products including both 1-nitrosopiperidine and 1-nitropiperidine were detected in the particles formed. Quantum chemistry calculations on the OH initiated atmospheric photo-oxidation of piperidine suggest the branching in the initial H-abstraction routes to be ∼35% N1, ∼50% C2, ∼13% C3, and ∼2% C4. The theoretical study produced an atmospheric photo-oxidation mechanism, according to which H-abstraction from the C2 position predominantly leads to 2,3,4,5-tetrahydropyridine and H-abstraction from the C3 position results in ring opening followed by a complex autoxidation, of which the first few steps are mapped in detail. H-abstraction from the C4 position is shown to result mainly in the formation of piperidin-4-one and 2,3,4,5-tetrahydropyridin-4-ol, whereas H-abstraction from N1 under atmospheric conditions primarily leads to 2,3,4,5-tetrahydropyridine and in minor amounts of 1-nitrosopiperidine and 1-nitropiperidine. The calculated rate coefficient for the piperidine + OH reaction agrees with the experimental value within 35%, and aligning the theoretical numbers to the experimental value results in k(T) = 2.46 × 10-12 × exp(486 K/T) cm3 molecule-1 s-1 (200-400 K).

Experimental and Theoretical Study of the OH-Initiated Degradation of Piperazine under Simulated Atmospheric Conditions.

The OH-initiated photo-oxidation of piperazine and 1-nitropiperazine as well as the photolysis of 1-nitrosopiperazine were investigated in a large atmospheric simulation chamber. The rate coefficient for the reaction of piperazine with OH radicals was determined by the relative rate method to be kOH-piperazine = (2.8 ± 0.6) × 10-10 cm3 molecule-1 s-1 at 307 ± 2 K and 1014 ± 2 hPa. Product studies showed the piperazine + OH reaction to proceed both via C-H and N-H abstraction, resulting in the formation of 1,2,3,6-tetrahydropyrazine as the major product and in 1-nitropiperazine and 1-nitrosopiperazine as minor products. The branching in the piperazinyl radical reactions with NO, NO2, and O2 was obtained from 1-nitrosopiperazine photolysis experiments and employed analyses of the 1-nitropiperazine and 1-nitrosopiperazine temporal profiles observed during piperazine photo-oxidation. The derived initial branching between N-H and C-H abstraction by OH radicals, kN-H/(kN-H + kC-H), was 0.18 ± 0.04. All experiments were accompanied by substantial aerosol formation that was initiated by the reaction of piperazine with nitric acid. Both primary and secondary photo-oxidation products including 1-nitropiperazine and 1,4-dinitropiperazine were detected in the aerosol particles formed. Corroborating atmospheric photo-oxidation schemes for piperazine and 1-nitropiperazine were derived from M06-2X/aug-cc-pVTZ quantum chemistry calculations and master equation modeling of the pivotal reaction steps. The atmospheric chemistry of piperazine is evaluated, and a validated chemical mechanism for implementation in dispersion models is presented.

An instrument to measure fast gas phase radical kinetics at high temperatures and pressures.

Fast radical reactions are central to the chemistry of planetary atmospheres and combustion systems. Laser-induced fluorescence is a highly sensitive and selective technique that can be used to monitor a number of radical species in kinetics experiments, but is typically limited to low pressure systems owing to quenching of fluorescent states at higher pressures. The design and characterisation of an instrument are reported using laser-induced fluorescence detection to monitor fast radical kinetics (up to 25 000 s(-1)) at high temperatures and pressures by sampling from a high pressure reaction region to a low pressure detection region. Kinetics have been characterised at temperatures reaching 740 K and pressures up to 2 atm, with expected maximum operational conditions of up to ∼900 K and ∼5 atm. The distance between the point of sampling from the high pressure region and the point of probing within the low pressure region is critical to the measurement of fast kinetics. The instrumentation described in this work can be applied to the measurement of kinetics relevant to atmospheric and combustion chemistry.

CH2OO Criegee biradical yields following photolysis of CH2I2 in O2.

Yields of CH2OO and CH2IO2 from the reaction of CH2I radicals with O2 are reported as a function of total pressure, [N2] and [O2] at T = 295 K using three complementary methods. Results from the three methods are similar, with no observed additional dependence on [O2]. The CH2I + O2 reaction has a yield of ~18% CH2OO at atmospheric pressure.

Time-of-flight mass spectrometry for time-resolved measurements.

A time-resolved time-of-flight mass spectrometer (TOF-MS) that can simultaneously monitor multiple species on the millisecond time scale has been constructed. A pulsed photolysis laser is used to initiate reaction, and then via a pinhole the reaction mixture is sampled by the TOF-MS. The ions are created by photoionization via either a discharge lamp or a pulsed laser. Comparison between the two ionization sources showed that the laser is at least an order of magnitude more efficient, based on the time to accumulate the data. Also, unlike the continuous lamp the pulsed laser is not mass limited. Frequency tripling the 355 nm output of a Nd:YAG laser provided a convenient laser ionization source. However, using a dye laser provided an equally intense laser ionization source with the ability to tune the vacuum ultraviolet (vuv) light. To show the versatility of the system the kinetics of the reaction of SO and ClSO radicals with NO(2) were simultaneously measured, and using the dye laser the vuv light was tuned to 114 nm in order to observe H(2)CO being formed from the reaction between CH(3)CO and O(2).

Kinetics and product studies of the reaction of Br, Cl, and NO with ClOOCl using discharge-flow mass spectrometry.

The kinetics of the reactions Cl + ClOOCl --> products, Br + ClOOCl --> products (7), and NO + ClOOCl --> products have been studied as a function of temperature at 1 Torr total pressure using a discharge-flow mass spectrometric technique. The measured rate coefficients, expressed in Arrhenius form, are k5 = (7.60 +/- 0.56) x 10(-11)exp((65.4 +/- 17.9)/T) cm3 s(-1) for 217 K < T < 298 K, and k7 = (5.88 +/- 0.50) x 10(-12) exp(-173 +/- 20/T) cm3 s(-1) for 223 K < T< 298 K. The observed temperature dependencies of these rate coefficients indicate a common direct halogen atom abstraction mechanism. Mass spectral product studies indicate that BrCl is the only major Br-containing product from reaction. Extensive product studies were used to estimate that the peroxide form of the ClO-dimer is formed in > 90% yield from the ClO self-reaction. No evidence for the formation of ClOClO and ClClO2 was observed. No reaction between NO and ClOOCl was observed and an upper limit of k8 < 1 x 10(-15) cm3 s(-1) for 220 K < T < 298 K was determined.

Application of a compact all solid-state laser system to the in situ detection of atmospheric OH, HO2, NO and IO by laser-induced fluorescence.

A tuneable, high pulse-repetition-frequency, solid state Nd:YAG pumped titanium sapphire laser capable of generating radiation for the detection of OH, HO2, NO and IO radicals in the atmosphere by laser induced fluorescence (LIF) has been developed. The integration of the laser system operating at 308 nm into a field measurement apparatus for the simultaneous detection of hydroxyl and hydroperoxy radicals is described, with detection limits of 3.1 x 10(5) molecule cm(-3) (0.012 pptv in the boundary layer) and 2.6 x 10(6) molecule cm(-3) (0.09 pptv) achieved for OH and HO2 respectively (30 s signal integration, 30 s background integration, signal-to-noise ratio = 1). The system has been field tested and offers several advantages over copper vapour laser pumped dye laser systems for the detection of atmospheric OH and HO2 radicals by LIF, with benefits of greater tuning range and ease of use coupled with reduced power consumption, instrument footprint and warm-up time. NO has been detected in the atmosphere at approximately 1 ppbv by single photon LIF using the Alpha 2Sigma+ <-- Chi 2Pi1/2 (0,0) transition at 226 nm, with absolute concentrations in good agreement with simultaneous measurements made using a chemiluminescence analyser. With some improvements in performance, particularly with regard to laser power, the theoretical detection limit for NO is projected to be approximately 2 x 10(6) molecule cm(-3) (0.08 pptv). Whilst operating at 445 nm, the laser system has been used to readily detect the IO radical in the laboratory, and although it is difficult to project the sensitivity in the field, an estimate of the detection limit is < 1 x 10(5) molecule cm(-3) (< 0.004 pptv), well below previously measured atmospheric concentrations of IO.