Molecular products

For some applieations it is desirable that the fluids be free from the volatile low molecular products that result from the randomising equilibration reaction. This operation may be earned out either batchwise or continuously using a vacuum still. Commercial non-volatile fluids have a weight loss of less than 0.5% after 24 hours at 150°C. 

Polymeranalogous reactions considered above may be referred to as intramolecular condensation transformations since they are accompanied by elimination of low-molecular products. On the other hand, PCSs can be obtained via polymeranalogous transformations, principally intramolecular polymerization reactions . Thermal and chemical cyclization of poly(acrilonitrile) (PAN) is an example of processes of this type. It was demonstrated by a number of researchers216-225 that thermal transformations of PAN follow the scheme ... 

The formation of high molecular products during the cationic polymerization depends on whether the propagation reaction, consisting of the interaction of the cationic chain end as a reactive intermediate with the monomer, reproduces the reactive intermediate (see Eq. (1)). For this reason the monomer functions as the agent and as the substrate when in the form of the cation. This means, however, the interaction between the monomer and the cationic chain end is a function of the monomer structure itself when all other conditiones remain the same. 

The only species which apparently cannot lead directly to molecule formation is a highly charged atom with low kinetic energy. (Even these could undergo thermal reactions if they were not scavenged first.) It thus appears that almost anything can under appropriate conditions lead to molecular products of some sort. 

Matrix IR spectra of various silenes are important analytical features and allow detection of these intermediates in very complex reaction mixtures. Thus, the vibrational frequencies of Me2Si=CH2 were used in the study of the pyrolysis mechanism of allyltrimethylsilane [120] (Mal tsev et al., 1983). It was found that two pathways occur simultaneously for this reaction (Scheme 6). On the one hand, thermal destruction of the silane [120] results in formation of propylene and silene [117] (retroene reaction) on the other hand, homolytic cleavage of the Si—C bond leads to the generation of free allyl and trimethylsilyl radicals. While both the silene [117] and allyl radical [115] were stabilized and detected in the argon matrix, the radical SiMc3 was unstable under the pyrolysis conditions and decomposed to form low-molecular products. 

To monitor a possible influence of molecular products of photolysis of ethylene (acetylene, ethane, and butane) on the sensor, a second sensor was positioned at a distance of 50 cm from the photolysis zone. The second sensor was designed to introduce corrections into the readings of the movable sensor. The specified distance was chosen so that atoms and radicals produced in the lower part of the vessel could not reach the... 

Consequently conventional antioxidant mechanisms must be expected to protect against photo-oxidation. Thus hydroperoxide decomposition to inert molecular products will reduce the rate of photoinitiation and scavenging of any of the free radical species will be beneficial, although the effectiveness of conventional antioxidants in photo-oxidations is limited by their own stability and the photo-sensitizing propensity of their products (3,). 

In theory, one assumes the formation of radicals before the chemical stage begins (see Sect. 2.2.3). These radicals interact with each other to give molecular products, or they may diffuse away to be picked up by a scavenger in a homogeneous reaction to give radical yields. The overlap of the reactive radicals is more on the track of a high-LET particle. Therefore, the molecular yields should increase and the radical yields should decrease with LET. This trend is often observed, and it lends support to the diffusion-kinetic model of radiation-chemical reactions. 

Hummel et al. (1966) have used radiations from 37Ar to determine the free-ion yield in n-hexane (see Sect. 9.3.1), but no molecular product has yet been measured with this radiation, which is highly desirable in view of its mono-energetic (2400 eV) character. Mozumder (1971) has developed a diffusion theory for ion recombination for (initially) multiple ion-pair cases, which can be applied to 3H and 37Ar radiations. According to this theory, the track is cylindrically symmetric to start with. As neutralization proceeds, the track... 

Samuel and Magee (1953) employed a 1-radical model to find the relative forward yield in water radiolysis as a function of radiation quality. In such models, no distinction is made between reactive radicals or molecular products. The products of radiolysis are called forward (F) to denote observable molecular yield or radical (R), denoting yield of scavenger reaction at small concentration. The aim of the theory is to calculate the relative forward yield G(F)/[G(F) + G(R)], where the G values refer to the respective yields for 100 eV energy absorbed in... 

In the infinite time limit, no radical survives. They either combine to give molecular products or undergo scavenging reactions. The probability of the latter is given by... 

The difference is clearly seen for a spur initially containing two dissociations of AB molecules into radicals A and B (Pimblott and Green, 1995). Considering the same reaction radii for the reactions A + A, A + B, and B + B and the same initial distributions of radicals, the statistical ratio of the products should be 1 4 1 for A2 AB B2, since there is one each of A-A and B-B distances but there are four A-B distances. For n dissociations in the spur, this combinatorial ratio is n(n - l)/2 n2 n(n - l)/2, whereas deterministic kinetics gives this ratio always as 1 2 1. Thus, deterministic kinetics seriously underestimates cross-recombination and overestimates molecular products, although the difference tends to diminish for bigger spurs. Since smaller spurs dominate water radiolysis (Pimblott and Mozumder, 1991), many authors stress the importance of stochastic kinetics in principle. Stochasticity enters in another form in... 

Clifford et ah (1987a,b) considered acid spurs (primary radicals H and OH) and computed the evolution of radical and molecular products by the master equation (ME) and IRT methods. Reasonable values were assumed for initial yields, diffusion constants, and rate constants, and a distribution of spur size was included. To be consistent with experimental yields at 100 ns, however, they found it necessary that the spur radius be small—for example, the radius of H distribution (standard deviation in a gaussian distribution) for a spur of one dissociation was only in the 0.4—0.75 nm range. Since in acid spurs H atoms inherit the distribution of eh, this is considered too low. This preliminary finding has later been revised in favor of spurs of much greater radius. 

HYDROPEROXIDES AS PRIMARY MOLECULAR PRODUCTS OF HYDROCARBON OXIDATION... 

Hydroperoxides were proved to be the only primary molecular product of the oxidation of aliphatic and alkylaromatic hydrocarbons [79-84], When the hydrocarbon is oxidized under mild conditions, in which the formed hydroperoxide is a stable product, the amount of produced ROOH was found to be nearly equal to the amount of consumed dioxygen [45,80,82],... 

Detailed information about molecular products of hydrocarbon oxidation is given in monographs [45,46,80,82]. The kinetic schemes of the oxidation of alcohols, ketones, aldehydes, and acids are discussed in Chapters 7,8. The thermochemical scale of hydrocarbon oxidation is given in Table 1.7. 

---