Discharge-flow systems

Oxidation rate constant k for gas-phase second order rate constants, kOH for reaction with OH radical, kNQ3 with N03 radical and ko3 with 03 or as indicated, data at other temperatures see reference kOH = 4.25 x 10 1 cm3 molecule-1 s 1 at 298 K (discharge flow system-MS, Morris Niki 1971)... 

Morris, E.D., Jr., Stedman, D.H., and Niki, H. Mass spectrometric study of the reactions of the hydroxyl radical with ethylene, propylene, and acetaldehyde in a discharge-flow system, J. Am. Chem. Soc., 93(15) 3570-3572, 1971. 

Elias et al.43 observed that the heat liberated on a silver-coated platinum wire coil was about 10% higher than that expected from the known concentration of atomic oxygen in a discharge flow system, and they ascribed the effect to appreciable concentrations of 02(1A5) in the discharge products. It was then shown that some heat was released even after all the atomic oxygen had been removed by reaction with ethylene. 

The results make it clear that no great specificity for 02(1Ag) may be expected, and where O and 02(1E9+) are present they can also contribute to the heat liberated. Nevertheless, as will be seen in Section IV-A, it is possible to remove most of the atomic oxygen from a discharge-flow system and retain O A,) since it is believed that the concentration of 02(1Ss+) is much less than that of 02(1A9), the calorimetric method works well under these conditions, and a value of AH = 23 kcal/mole may be adopted. 

More recent studies have made possible direct determination of the rate constants k7a and k7b. McNeal and Cook47 have followed the concentration of 02(1A9) in a discharge-flow system by the photoionization technique (Sect. III-E). In the presence of ozone, 02(1A,) decays by a predominantly first-order mechanism, so that, presumably, the second-order pooling process (11) does not contribute significantly to the loss of 02(1A9). If it is assumed that the only loss process for Oa(1A9) is reaction with ozone in reaction (7a), then k7a lies between 1 x 106 and 2 x 106 liter mole-1 sec-1. This value is considerably lower than the rate constant measured by Mathias and Schiff87 if Oa in reaction (7) is largely O Aj). McNeal and Cook consider the possibility that their photoionization current decreases by less than that expected on the basis of [02(1A9)J decay, as a result of the formation of vibrationally excited oxygen in reaction (15d)... 

This conclusion is borne out by kinetic evidence which shows that the intensity of emission is proportional to [02(1A9)]3. Since [02(1H9+)] oc [Oz(1A9)]a in the discharge-flow system, the result indicates that [N02 ] oc [02(1A9)][02(1S9+)]. The experimental evidence does not allow description of the detailed mechanism for reaction (32). Two possibilities are (a) that a low-lying excited state of N02 is excited from one or other of the excited 02 species before a second energy-transfer reaction produces the emitting state of N02, or (b) that direct transfer to N02 takes place from an 02(1Afl) 02(1S9+) dimol. Although emission from this latter dimol is not observed in the gas phase, since [02(1S9+)] is normally very small, it has been seen in condensed phase systems.20... 

The reaction now affords investigators another valuable tool because the absolute intensity of the chemiluminescence has been carefully measured. It can thus be used as a standard light source against which other chemiluminescent reactions can be measured without the need of detector calibrations and geometry corrections. This convenience is due largely to the fine work of Fontijn, Meyer, and Schiff,145,147 who used chemical actinometry to measure the emission from a discharge-flow system. In their first report on this reaction, they determined the value of k12 as 1.0 x 104 M x sec-1 for emission in the... 

Table I. Chemical Mechanism Used to Describe Chemistry Occurring in the Discharge Flow System...

In their study, Martin et al. (12) obtained from 13 experiments the best agreement between experimental and computed values with the rate constant ki = (1.5 0.5) x 10 11 cm3 molecule V1 for the reaction 10 + DMS. Moreover, the mass spectrometric analysis of the products of reaction (1) led to the detection of a peak at mass 78, which corresponds to the parent peak of DMSO. Although a quantitative analysis of DMSO was not possible in the discharge flow system, this observation supports the mechanism proposed by Barnes et al. (16). 

The mechanism of the hydroxyl radical-initiated oxidation of /i-pincnc in the presence of NO has been investigated using a discharge-flow system. Propagation of hydroxyl radicals was observed after the addition of O2 and NO, and the measured concentration profiles were compared with simulations based on both the master chemical mechanism and the regional atmospheric chemistry mechanism for /i-pinene oxidation.228... 

In this section, we introduce the theories of moving bed flows in hoppers and general standpipes. The hopper flow theory may be extended to the cases of moving bed flows through a discharge valve. A unique hopper-standpipe-discharger flow system is then discussed as an example of the application of these theories. Multiplicity of steady flows in a standpipe system is also illustrated. 

To take one example, let us consider the effects of rotational relaxation in BrF. The excited 53FI(0+) state in BrF is crossed by another 0+ state which leads to predissociation of the B state in vibrational levels 7 and 6. The initial study of the dynamics of the B state was carried out in a discharge flow system where the minimum operating pressure was 50 m Torr. The gas-kinetic collision rate coefficient at 298 K for He + BrF(B) collisions is 4.4 x 10-10 cm3 molecule-1 s-1. Thus, at the minimum pressure of 50 m Torr, the average time between collisions of excited BrF molecules and helium buffer gas is 1.5/us. This time is short compared with the radiative lifetime of BrF (42—56/ns [43]) and therefore significant redistribution in the excited state can occur before it radiates. 

Many reactive radicals have been generated by reaction, usually in discharge flow systems where the reaction generally involves an atom produced by microwave or radio frequency discharge. Clyne and Nip [17] present an account of such methods. One feature of the radicals produced by reaction is that they may be formed in excited states, usually vibrational or electronic. For example, the OH radical may be generated [27] in low vibrational levels (v < 3) by the reaction... 

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