Secondary reactions after absorption

In 1995, the author s group also measured CIDEP spectra for the photodissociation of triphenyl-phosphine (PhsP) in some fluid homogeneous solutions at 293 K [7], Fig. 10-11 shows a typical spectrum among them. In this figure, absorptive signals due to the 2-hydroxy-2-propyl and diphenylphosphinyl radicals ((CH3) 2C OH and Ph2P ) were observed at a delay time of 0.25 ps after laser excitation. The latter radical is produced by Reaction (10-20e), but the former one by the secondary reaction from the phenyl radical generated from Reaction (10-20e) as follows ... 

Under their conditions more than 80% of the decomposition was effected by wavelengths between 170 and 140 nm. Absorption by ground state C2O was only observed after addition of CO to the photolysis mixture. This is presumably due to formation of C2O in a three-body recombination process. Through spectroscopic comparison of the yield of CO and the consumption of C3O2 at times shorter than collision times, Braun et al. found that 2 molecules of CO were formed for each C3O2 removed. This indicates that primary process 2 dominates process 1 in this wavelength interval. An upper limit of 25% is placed on the yield of C2O. The dependence of carbon atom yields on flash intensity suggest that C( P) and C( D) are formed in a primary photolytic process, while C( S) is the result of some secondary reaction. The relative yields of C( P), C( D) and C( S) were determined to be 1.00, 0.25, <0.025, respectively. 

The threshold wavelength for (Vll-46) is about 3500 A. McQuigg and Calvert (686) have measured H2, HD, and Dj products produced from the high intensity photolysis of HjCO-D CO mixtures. They have concluded that the primary quantum yield, (/>4b -I- ( >47, is near unity over the entire absorption region in the near ultraviolet. They have also found that CO and Hj are formed in nearly equal amounts. The results may be explained on the basis of the following secondary reactions occurring after (Vll-46) and (Vll-47)... 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

The effect of antioxidants such as hindered phenohcs, secondary amine, and thioester on the radiation cross-linking efficiency of LDPE has been reported [260]. Amount of cross-linking at a given dose decreases with aU the antioxidants, the thioester being the most effective. IR absorption spectroscopy has been used to demonstrate dose-rate dependence of trani -vinylene unsaturation in irradiated Marlex 50 PE [261]. When the irradiated polymer is stored in vacuum a decrease is observed in trani-vinylene absorbance over a period of several weeks. After high dose-rate irradiation the decay is preceded by an initial increase. These phenomena have been ascribed to the reaction of trapped radicals. 

NO Reactions. The most informative derivitization reaction of oxidized polyolefins that we have found for product identification is that with NO. The details of NO reactions with alcohols and hydroperoxides to give nitrites and nitrates respectively have been reported previously, and only the salient features are discussed here (23). The IR absorption bands of primary, secondary and tertiary nitrites and nitrates are shown in Table I. After NO treatment, y-oxidized LLDPE shows a sharp sym.-nitrate stretch at 1276 cm-1 and an antisym. stretch at 1631 cm-1 (Fig. 1), consistent with the IR spectra of model secondary nitrates. Only a small secondary or primary nitrite peak was formed at 778 cm-1. NO treatment of y-oxidized LLDPE which had been treated by iodometry (all -OOH converted to -OH) showed strong secondary nitrite absorptions, but only traces of primary nitrite, from primary alcohol groups (distinctive 1657 cm-1 absorption). However, primary products were more prominent in LLDPE after photo-oxidation. 

The cage effect described above is also referred to as the Franck-Rabinowitch effect (5). It has one other major influence on reaction rates that is particularly noteworthy. In many photochemical reactions there is often an initiatioh step in which the absorption of a photon leads to homolytic cleavage of a reactant molecule with concomitant production of two free radicals. In gas phase systems these radicals are readily able to diffuse away from one another. In liquid solutions, however, the pair of radicals formed initially are caged in by surrounding solvent molecules and often will recombine before they can diffuse away from one another. This phenomenon is referred to as primary recombination, as opposed to secondary recombination, which occurs when free radicals combine after having previously been separated from one another. The net effect of primary recombination processes is to reduce the photochemical yield of radicals formed in the initiation step for the reaction. 

Detection of amino acids is typically by UV absorption after postcolumn reaction with nin-hydrin. Precolumn derivatization with ninhydrin is not possible, because the amino acids do not actually form an adduct with the ninhydrin. Rather, the reaction of all primary amino acids results in the formation of a chromophoric compound named Ruhemann s purple. This chro-mophore has an absorption maximum at 570 nm. The secondary amino acid, proline, is not able to react in the same fashion and results in an intermediate reaction product with an absorption maximum at 440 nm. See Fig. 5. Detection limits afforded by postcolumn reaction with ninhydrin are typically in the range of over 100 picomoles injected. Lower detection limits can be realized with postcolumn reaction with fluorescamine (115) or o-phthalaldehyde (OPA) (116). Detection limits down to 5 picomoles are possible. However, the detection limits afforded by ninhydrin are sufficient for the overwhelming majority of applications in food analysis. 

The structure elucidation of 34 was based, in principle, on two facts color reactions and hydrolysis. Compound 34 does not react with ninhydrin (therefore, no primary amino group is present) but it does react with l-fluoro-2,4-dinitrobenzene. By exact determination of the absorption ratio E35O/E390 after the reaction with the latter reagent, Tait concluded that 34 contains a secondary amino group (50). Hydrolysis (6 N HC1, 110°C, 24 hr) of the so-called compound II afforded 2,3-dihydroxybenzoic acid and spermidine (50). The presence of two 2,3-dihydroxybenzoyl residues in 34 was demonstrated by its enzymatic (50) and chemical synthesis (51-54). 

IR data from the model compounds indicate that the amino end groups of the ATBN can react with the double bond in the polyester. In fact, the 1645 cm-1 absorption due to the double bond almost disappeared after 40 min at 100 °C, and the absorption due to the amino groups (N-H) also diminished considerably (Figure 1). A Michael-type addition reaction between the secondary amino hydrogen of piperazine derivatives and the activated double bonds of diethylfumarate in the polyester is known (5, 6). 

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