Product distributions reaction channels

A second feature of the Y + CH3OH reaction that is common to many metal reactions is the presence of competing reaction channels, in this case YH2+H2CO and YOCH3+H. Time-of-flight spectra for both these products are shown in Fig. 11. The corresponding lab angular distributions and CM distributions used to fit the TOF spectra are shown in Fig. 12. 

Indeed, by using soft El ionization, we have been able to unambiguously detect products from all five reaction pathways (2a)-(2e), determine their branching ratio and characterize their dynamics.34 Here we discuss some of the results that we have obtained on this reaction, which well exemplify the power of soft El ionization. First of all, from measurements of the El efficiency curves at various to/e ratios (15, 42, and 43), we have found that the parent ion at m/e = 43 (CH2CHO+, corresponding to one of the main reaction channels, the vinoxy radical,) is not stable, so measurements of angular and TOF distributions were carried out at m/e = 42. Incidentally, from the El ionization efficiency curve at m/e = 42 we have obtained some direct information on the IE of the vinoxy radical, for which no such information was available till now. The IE should be <11 eV. 

In these reactivity studies, reactions 22a and b were studied and the rate coefficient and product distribution determined as reported above. This product distribution is at variance with a much earlier study where only an association channel was reported, although with a similar rate coefficient 1(—26) cm6 s-1, equivalent to a binary rate coefficient of 2(—10) cm3 s-1 at 0.5 torr.61 The CHsO+, produced in this way and by reaction 23, was reacted with a series of molecules with proton affinities varying from 166 to 193 kcal mol-1 and encompassing that of CH3OH see Table 3. For the production of CH50+ in the association reaction 22a, sufficient water was... 

The catalytic activity of hierarchical and conventional Beta zeolites for acylation of 2-MN is displayed in Figure 2(a) The Beta (PHAPTMS) sample shows a superior catalytic activity than the conventional one, due to its enhanced textural properties. In this case, the bulky nature of both substrate and products may cause the existence of diffusional problems inside the zeolitic channels, which are attenuated in the modified Beta sample due to the presence of the hierarchical porosity. Regarding the product distribution (Figure 2(b)), two main products are observed and a third isomer, 8-A,2-MN isomer is produced just in minor amounts. Interestingly, the selectivity towards the desired isomer increases in the material obtained from silanized seeds, reaching values around 75%. Probably, the active sites located on the surface of the secondary porosity are able to catalyze also the formation of 6-A,2-MN by transacylation. However, this reaction is expected to be strongly hindered in the conventional Beta zeolite since it requires the participation of two bulky molecules as reactants. 

The situation changes if one of the oxygen atoms is replaced by a chlorine. Limberg could show that the product of the addition of Mn03Cl to ethylene is more stable in the triplet state and that the product distribution can be explained in terms of reaction channels [47]. 

Catalyses of HY and HL are not controlled by sterlc circumstances of pore and channel. HY and HL have enough spaces for transition state to allow the formation of all IPBP Isomers. Product distribution changes markedly by increasing reaction temperature. Catalysis at low temperatures is determined by the reactivity of each position of biphenyl molecule to yield 2- and 4-IPBP as principal isomers. However, the selectivity of 3-IPBP increases extensively with decrease of 2-IPBP with rising temperature, and an equimolar mixture of 3-and 4-IPBP is produced at high temperatures. These changes in product distribution are ascribed to the isomerization of 2-IPBP to the more stable 3-IPBP by a de-alkylatlon and alkylation mechanism. Catalysis of HZSM-5 at 300°C is nonselective with low activity. The reaction occurs at external surface because the pore is too small to allow the entrance of biphenyl molecule. 

The rate of reaction (2) is significant in ambient air, even though its rate constant is only 1.5 x 10-15cm3 molecule-1 s-1 at 298K [5,6]. Also, the self-reaction of the radical product H02 [6], i.e., 2H02 - H202 + 02, could be kept negligible in these experiments, so that the relative importance of the three primary reaction channels could be derived from the product distribution... 

It is interesting to note that the earlier FA experiments only recognised the two major product channels in reaction (17) whereas the SIFT, which is especially valuable for the determination of product distributions, identified the minor 0+ product channel... 

Whilst the rate coefficients for many binary and ternary negative ion-molecule reactions have been acquired recently, predominantly using the FA technique (see the data compilation of Albritton115 ), many more are required if the important paths in the synthesis of the observed negative ion clusters are to be identified. Product distributions have been studied even less for negative ion-molecule reactions, principally because of experimental difficulties, yet more than one product channel is accessible in several atmospherically important reactions73, for example,... 

Microscopic branching refers to the observation of a bimodal product energy distribution from one reaction channel (HX) and a normal product energy distribution from the other reaction channel. It appears that the bimodal distribution occurs for the product, HX, containing the most electronegative atom. Figure 9 shows the triangle plots for the detailed vibrational state distributions for the HBr and HC1 products from H + BrCl. Bimodality is seen in the HC1 distribution. The HBr distribution from H + BrCl closely resembles the HBr distribution from H + Br2 and results from a direct reaction with the Br end of BrCl. 

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