Precursor detection

Knickmeier, M. Erker, G Fox, T. Conformational analysis of nonbridged bent metallocene Ziegjer-catalyst precursors-detection of the third torsional isomer. J. Am. Chem. Soc. 1996, 118, 9623-9630. 

Laser photolysis of a precursor may also be used to generate a reagent. In a crossed-beam study of the D + FI2 reaction [24], a hypertliennal beam of deuterium atoms (0.5 to 1 eV translational energy) was prepared by 248 mn photolysis of DI. This preparation method has been widely used for the preparation of molecular free radicals, both in beams and in experiments in a cell, with laser detection of the products. Laser photolysis as a method to prepare reagents in experiments in which the products are optically detected is fiirtlier discussed below. 

Many optical studies have employed a quasi-static cell, through which the photolytic precursor of one of the reagents and the stable molecular reagent are slowly flowed. The reaction is then initiated by laser photolysis of the precursor, and the products are detected a short time after the photolysis event. To avoid collisional relaxation of the internal degrees of freedom of the product, the products must be detected in a shorter time when compared to the time between gas-kinetic collisions, that depends inversely upon the total pressure in the cell. In some cases, for example in case of the stable NO product from the H + NO2 reaction discussed in section B2.3.3.2. the products are not removed by collisions with the walls and may have long residence times in the apparatus. Study of such reactions are better carried out with pulsed introduction of the reagents into the cell or under crossed-beam conditions. 

The search in the Theilhcimer reaetion database [62] provides 161 reactions for this query. If the search is performed without any additional bond spheres (covering only atoms of the inner sphere with a dark gray bac kgroiind in Figure 10.3-42 as well as the added atom groups on the precursor side), 705 reactions arc obtained in the Theilhcimer database. The result of this search is less precise than that of the first search. Additionally, reactions forming any kind of C-0 bonds (c.g., making an ether bond instead of an ester bond) arc found. However, in both searches too many hits arc obtained in order to detect suitable reactions in a reasonable... 

In order to continue the design of a synthesis, 2 is now analysed further by the disconnection strategy for aliphatic bonds. WODCA detects two strategic bonds which are rated 100 and 67, respectively. The disconnection of the bond with the highest rating forms two precursor compounds, 3 and 4 (Figure 10,3-52),... 

Precursor ions are selected by Ql and passed into the collision cell (Q2 orq2 of Figure 33.5). Here, collision with an inert gas (argon or helium) causes dissociation to occur, and the resulting fragment (product) ions are detected by scanning Q3 (Figure 33.6). 

A precursor ion scan. Source ions f,. . .., f, ) are all passed successively by Q1 into the collision cell, Q2, where a selected fragment (i ) is produced and detected by Q3. Only the ions (m, f,. fj) give f, fragment ions in this example. 

Biosynthesis. Two closely related genes encode the three mammalian tachykinins. The preprotachykinin A gene encodes both substance P and substance K, while the preprotachykinin B gene encodes neuromedin K (45—47). The active sequences are flanked by the usual double-basic amino acid residues, and the carboxy-terrninal amino acid is a glycine residue which is decarboxylated to an amide. As with most neuropeptide precursors, intermediates in peptide processing can be detected, but their biological activities are not clear (ca 1994). 

CCK has been detected in two principal forms, ie, the traditional 33-amino acid peptide, and an octapeptide CCK-8. The intestine produces mainly CCK-33 (133) and the brain produces mainly CCK-8 (132). The CCK precursor contains one copy of CCK-33 (133,134) this peptide is flanked on both ends with double basic residues, whereas CCK-8 is formed from CCK-33 by cleavage of a single basic residue. 

A newer hybrid system available commercially is the magnetic sector—TOF hybrid (38). The precursor ions can be selected with better than unit—mass resolution by msl and the product ion ions detected at high sensitivity by the TOF ms2 (39). 

It is applicable to plastic packaging materials, where purities with respect to mobile ions, such as Cl and Na, can be checked. In addition, a-particle precursors, such as U and Th, can be determined in solid plastics with sub-ppb detection limits. 

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