Fragmentation product

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). 

Prior separation of mixtures into individual components may not be needed. If the mass spectrometer is capable of MS/MS operation, one of the mass spectrometers is used to isolate individual ions according to m/z value (mass-to-charge ratio), and the other is used to examine their fragmentation products to obtain structural information. 

The stereochemistry at C-20 does not affect the reaction. However, the 16j5-mesyloxy analogs give poor yields of fragmentation product, as would be predicted from the syn periplanar arrangement of the bonds involved. 

The other ions are products of the further decomposition of the diphenylacetylene ion (m/e 178), or the fragmentation products of the monothiobenzyl92 ion as depicted in equation 493. 

Evans, J., Maccabee, M., Hatahet, Z., Courcelle, H., Bockrath, R., Ide, H. and Wallace, S. (1993). Thymine ring saturation and fragmentation products, lesion bypass, misinsertion and implications for mutagenesis. Mut. Res. 299, 147-156. 

Several procedures that intercept the intermediates have been developed. When ozonolysis is done in alcoholic solvents, the carbonyl oxide fragmentation product can be trapped as an a-hydroperoxy ether.202 Recombination to the ozonide is then prevented, and the carbonyl compound formed in the fragmentation step can also be... 

Ion trap MS is particularly suited for chemical structure elucidation, as it allows for simultaneous ion storage, ion activation and fragmentation, and product ion analysis. The fragmentation pathway of selected ions and the fragmentation products provide information on the molecular structure. Compared with triple-quadrupole and especially with sector instruments, the ion trap instrument provides more efficient conversion of precursor ion into product ions. However, the CID process via resonance excitation, although quite efficient in terms of conversion yield, generally results in only one (major) product ion in the product-ion mass spectrum. MS/MS with a quadrupole ion trap offers a number of advantages ... 

Dispiro[2.2.2.0]octane 22 was obtained in good yield along with a fragmentation product 23 by treating 1,1,2,2-tetrakis-bromomethyl-cyclobutane 21 with zinc dust in aqueous ethanol4g). The cyclobutene analog of 22 has also been reported 4h). 

A Wurtz reaction, using sodium, of the same tetrabromide gave a wealth of fragmentation products whose formation may be reasonably explained mechanistically 4g),... 

When the diazirine was decomposed thermally, avoiding its electronically excited state, the yield of fragmentation products dropped to 1-2%. Further analysis revealed that, under photolytic conditions, cyclobutenes 27 and 28 were formed from both the carbene (63%) and directly from the excited diazirine (17%) fragmentation accounted for the remainder of the material balance. LFP studies by the pyridine ylide method gave rate constants for 19 —> 27 (1.3 x 106 s-1) and 19 — 28 (2.5 x 105 s-1), with the 5-fold preference for CH2 migration to 27 over CMe2 migration to 28 attributed to differential steric effects.45... 

Related observations were reported for frans-r-butylcyclopropylcarbene, 20.46 Photolysis of diazirine 29 in freon-113 gave f-butylcyclobutene, 30 (46-49%), as well as 20% of f-butylethene (and ethene) fragmentation products Scheme 5. 

The fragmentation is stereospecifically anti as shown by complementary geometry obtained in the cleavage of the epimeric pair of epoxycyclobutanones 91 and 92 (Eq. 110). The fragmentation product 93 of cyclobutanone 91 is transformable into the dimethyl ester of the pheromone of the Monarch butterfly. Considering the availability of the starting epoxy ketones from enones, the oxasecoalkylation serves to reorient the oxidation pattern with chain extension as summarized in Eq. 111. 

In fact, Ci2H27Sn+ was identified as one of the fragmentation products even at the low voltage (15v) used, the Sn-Cl bond undergoes scission. Other fragments are produced due to the successive loss of butyl groups. 

Brimble and coworkers172 reported the asymmetric Diels-Alder reactions between quinones 265 bearing a menthol chiral auxiliary and cyclopentadiene (equation 73). When zinc dichloride or zinc dibromide was employed as the Lewis acid catalyst, the reaction proceeded with complete endo selectivity, but with only moderate diastereofacial selectivity affording 3 1 and 2 1 mixtures of 266 and 267 (dominant diastereomer unknown), respectively. The use of stronger Lewis acids, such as titanium tetrachloride, led to the formation of fragmentation products. Due to the inseparability of the two diastereomeric adducts, it proved impossible to determine which one had been formed in excess. 

The QET is not the only theory in the field indeed, several apparently competitive statistical theories to describe the rate constant of a unimolecular reaction have been formulated. [10,14] Unfortunately, none of these theories has been able to quantitatively describe all reactions of a given ion. Nonetheless, QET is well established and even the simplified form allows sufficient insight into the behavior of isolated ions. Thus, we start out the chapter from the basic assumptions of QET. Following this trail will lead us from the neutral molecule to ions, and over transition states and reaction rates to fragmentation products and thus, through the basic concepts and definitions of gas phase ion chemistry. 

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