Rate Constants for the Thermal Dissociation of N2O and the O(3P) + N2O Reaction

Re-Examination of the N2O + O Reaction

The reaction of N2O with O is a key step in consumption of nitrous oxide in thermal processes. It has two product channels, NO + NO (R2) and N2 + O2 (R3). The rate constant for R2 has been measured both in the forward and the reverse direction at elevated temperature and is well established. However, the rate constant for the N2 + O2 channel (R3) has been difficult to quantify and has significant error limits. The direct reaction on the triplet surface has a barrier of around 40 kcal mol-1, and it is too slow for the N2 + O2 channel to have any practical significance. Recently, Pham and Lin (2022) suggested an alternative low activation energy reaction path that involves intersystem crossing and reaction on the singlet surface. In the present work, we re-examined a wide range of experiments relevant for the N2O + O reaction through kinetic modeling, paying attention to the impact of artifacts such as impurities and surface reactions. Experimental results from shock tubes and batch reactors on the final NO yield in N2O decomposition, covering temperatures of 973-2200 K and pressures of 0.013-11.5 atm, support k3 ∼ 0, consistent with the high activation energy for reaction on the triplet surface and a low probability of ISC.

Theoretical study on the gas-phase reaction mechanism of ammonia with nitrous oxide

Applications of nitrous oxide (N₂O) as an oxidant in green propellants and propulsion systems have attracted a lot of attention. In this study, the reaction pathways for the oxidation of ammonia (NH₃) with N₂O were studied using the B3LYP/6-31++G** method of density functional theory (DFT). The results reveal that the reaction between N₂O and NH₃ proceeds through a chain reaction mechanism. N₂O reacts with NH₃ to form N₂ and NH₃O first and then NH₃O decomposes into NH₃ and O. This process corresponds to the apparent reaction N₂O+M=N₂+O+M (M=NH₃), but the energy barrier of the process (183.49 kJ/mol) is much lower than the direct decomposition reaction of N₂O=N₂+O (279.05 kJ/mol). The O radical produced in this process reacts subsequently with NH₃ and N₂O to produce more radicals such as NH₂, OH, and NO, which will take part in further reactions like NH₃+OH=NH₂+H₂O and NH₂+NO=N₂+H₂O until the reactants are consumed.

Critical ignition conditions in exothermically reacting systems for arbitrary reaction kinetics

In this work, a universal method for determination of the critical ignition conditions taking into account the reactant consumption is proposed. Based on the analysis of the phase trajectories equation, the equation for maximal temperatures of exothermic reactions was obtained. In this case, the asymptotic criterion of ignition is determined by the impossibility of slow reaction mode realization with low value of maximum temperature. The method allows demarcating the regions of low- and high-temperature modes of exothermic reactions and to establish the criteria of transition to the region of high-temperature modes. The corresponding parametric diagrams can be characterized as the bifurcation ones (bistability). It was found that the region of thermal explosion (TE) existence is bounded by the classical TE conditions from below and by the degeneration conditions from above. The comparison of analytical calculation results with the results of numerical calculation gives a satisfactory agreement.

The reaction of N2O with phenylium ions C6(H,D)5(+): an integrated experimental and theoretical mechanistic study

The reaction of N(2)O (known to be an O atom donor under several conditions) with the phenyl cation is studied by experimental and theoretical methods. Phenyl cation (or phenylium), C(6)H(5)(+), and its perdeuterated derivative C(6)D(5)(+) are produced either by electron impact or by atmospheric pressure chemical ionization of adequate neutral precursors, and product mass spectra are measured in a guided ion beam tandem mass spectrometer. The ions C(5)(H,D)(5)(+), C(6)(H,D)(5)O(+), and C(3)(H,D)(3)(+) are experimentally detected as the most relevant reaction products. In addition, the detection of the adduct (C(6)H(5)N(2)O)(+), which is collisionally stabilized in the scattering cell of the mass spectrometer, is reported here for the first time. The reaction pathways, which could bring about the formation of the mentioned ions, are then explored extensively by density functional theory and, for the more promising pathways, by CASPT2/CASSCF calculations. The two reacting species (1) form initially a phenoxydiazonium adduct, C(6)H(5)ON(2)(+) (2a), by involving the empty in-plane hybrid C orbital of phenylium. The alternative attack to the ring pi system to produce an epoxidic adduct 2c is ruled out on the basis of the energetics. Then, 2a loses N(2) quite easily, thus affording the phenoxyl cation 3. This is only the first of several C(6)H(5)O(+) isomers (4-6 and 8-12), which can stem from 3 upon different cleavages and formations of C-C bond and/or H shifts. As regards the formation of C(5)H(5)(+), among several conceivable pathways, a direct CO extrusion from 3 is discarded, while others appear to be viable to different extents, depending on the initial energy of the system. The easiest CO loss is from 4, with formation of the cyclopentadienyl cation 7. Formation of C(3)H(3)(+) is generally hindered and its detection depends again on the availability of some extra initial energy.

Experimental and Theoretical Studies of Rate Coefficients for the Reaction O(3P) + C2H5OH at High Temperatures

Rate coefficients of the reaction O(3P)+C2H5OH in the temperature range 782-1410 K were determined using a diaphragmless shock tube. O atoms were generated by photolysis of SO2 at 193 nm with an ArF excimer laser; their concentrations were monitored via atomic resonance absorption. Our data in the range 886-1410 K are new. Combined with previous measurements at low temperature, rate coefficients determined for the temperature range 297-1410 K are represented by the following equation: k(T)=(2.89+/-0.09)x10(-16)T1.62 exp[-(1210+/-90)/T] cm3 molecule(-1) s(-1); listed errors represent one standard deviation in fitting. Theoretical calculations at the CCSD(T)/6-311+G(3df, 2p)//B3LYP/6-311+G(3df) level predict potential energies of various reaction paths. Rate coefficients are predicted with the canonical variational transition state (CVT) theory with the small curvature tunneling correction (SCT) method. Reaction paths associated with trans and gauche conformations are both identified. Predicted total rate coefficients, 1.60 x 10(-22)T3.50 exp(16/T) cm3 molecule(-1) s(-1) for the range 300-3000 K, agree satisfactorily with experimental observations. The branching ratios of three accessible reaction channels forming CH3CHOH+OH (1a), CH2CH2OH+OH (1b), and CH3CH2O+OH (1c) are predicted to vary distinctively with temperature. Below 500 K, reaction 1a is the predominant path; the branching ratios of reactions 1b,c become approximately 40% and approximately 11%, respectively, at 2000 K.

Experimental and theoretical studies of rate coefficients for the reaction O(3P)+CH3OH at high temperatures

Rate coefficients of the reaction O((3)P) + CH(3)OH in the temperature range of 835-1777 K were determined using a diaphragmless shock tube. O atoms were generated by photolysis of SO(2) with a KrF excimer laser at 248 nm or an ArF excimer laser at 193 nm; their concentrations were monitored via atomic resonance absorption excited by emission from a microwave-discharged mixture of O(2) and He. The rate coefficients determined for the temperature range can be represented by the Arrhenius equation, k(T) = (2.29 +/- 0.18) x 10(-10) exp[-(4210 +/- 100)T] cm(3) molecule(-1) s(-1); unless otherwise noted, all the listed errors represent one standard deviation in fitting. Combination of these and previous data at lower temperature shows a non-Arrhenius behavior described as the three-parameter equation, k(T) = (2.74 +/- 0.07) x 10(-18)T(2.25 +/- 0.13) exp[-(1500 +/- 90)T] cm(3)molecule(-1) s(-1). Theoretical calculations at the Becke-3-Lee-Yang-Parr (B3LYP)6-311 + G(3df,2p) level locate three transition states. Based on the energies computed with coupled clusters singles, doubles (triples) [CCSD(T)]/6-311 + G(3df,2p)B3LYP6-311 + G(3df,2p), the rate coefficients predicted with canonical variational transition state theory with small curvature tunneling corrections agree satisfactorily with the experimental observations. The branching ratios of two accessible reaction channels forming OH + CH(2)OH (1a) and OH + CH(3)O (1b) are predicted to vary strongly with temperature. At 300 K, reaction (1a) dominates, whereas reaction (1b) becomes more important than reaction (1a) above 1700 K.

The collimation angle shift of desorbing product N2 in a steady-state N2O+CO reaction on Rh(110)

The angular distribution of desorbing product N2 was studied in N2O decompositions on Rh(110) in the temperature range of 60-700 K. The N2 desorption collimates along 62 degrees -68 degrees off normal toward either the [001] or [001] direction in a transient N2O decomposition below ca. 470 K or in the steady-state N2O+CO reaction above 540 K. In the steady-state reaction at the temperature from ca. 470 to 540 K, however, the collimation angle shifts from 62 degrees to 45 degrees with decreasing surface temperature. This angle shift is ascribed to the steric hindrance by coadsorbed CO because the N2 collimation in transient N2O decomposition at around 65 degrees is recovered in the range of 380-500 K by an abrupt CO pressure drop followed by the decrease in CO coverage. N2O is oriented along the [001] direction before dissociation. A scattering model of the nascent N2 by adsorbed CO is proposed, yielding smaller collimation angles.

An evanescent-field-driven self-assembled molecular photoswitch for macrocycle coordination and release

Two self-assembled monolayer (SAM) films containing the photoswitchable 4-pyridylazophenoxy chromophore have been deposited onto a gold-coated glass substrate. One film contains the chromophore as a single component, 1 SAM, and the other is doped with a nonphotoactive component as a 1:1 mixture, 2 SAM. The reversible photoswitching performances of 1 SAM and 2 SAM via the evanescence field using light of appropriate wavelengths have been investigated by UV spectroscopic and electrochemical monitoring. In principle, the trans-form SAMs present a coordinating surface, the "on" state, that can be switched "off" in the cis form. This has been illustrated by immersing both the as-deposited (trans form) SAMs and the photoswitched (predominantly cis form) SAMs into solutions of cobalt and zinc tetraphenylporphyrin (CoTPP and ZnTPP, respectively) and an octaoctyl-substituted cobalt phthalocyanine. In a further phase of this study, the remote control of binding events at the surface of the SAMs has been demonstrated through evanescent-field-driven photoswitching of trans-form SAMs coordinated at the surfaces with examples of these metallomacrocycles. This photoswitching was undertaken with the constructs immersed in neat toluene, and the macrocycles were released from the surface into the solvent. The release was measured by spectroscopic monitoring of the material remaining on the constructs. The study was extended to develop an in situ release/coordination cycle. Thus, irradiation of a construct of ZnTPP bound to the surface of trans-form 2 SAM using waveguided light at 365 nm releases the macrocycle into a toluene solution of ZnTPP. Further irradiation of the SAM, now in its cis form, with waveguided 439 nm light regenerates the trans form, which recoordinates ZnTPP from the solution. The results demonstrate the potential for using waveguided light to control molecular events within and at the surfaces of SAM constructs.