Dissociation pathways

One important aspect of the kinetics is that ion dissociation may not occur during the time between ion activation near the middle of the octopole, and ion detection in the final quadrupole, ca. 30 ps. The efifect of incomplete dissociation on the experimental time scale (the kinetic shift) is negligible for the triatomic atoms and is 0-1 kJ/mol for a larger molecule like the magnitude depending on whether the transition state for 

The collision gas pressure can influence the observed cross sections because an ion that is not sufficiently energized by one collision with the target gas may gain enough energy in a second collision to be above the dissociation threshold. Such collisions can lead to a measured threshold that is too low. This is accounted for by linearly extrapolating data taken at several pressures to a zero pressure cross section, which is then fit with the method described above. 

Another way of obtaining insight into the bonding of met-cars, nanocrystals, and other cluster systems consists in investigating their mechanisms of decomposition. The method most widely used for met-cars and nanocrystals is photodissociation, in 

---

Lain L, Su C X and Armentrout P B 1992 Collision-induced dissociation ofTi (n = 2-22) with Xe bond energies, geometric structures, and dissociation pathways J. Chem. Rhys. 97 4084... 

Discernible associative character is operative for divalent 3t5 ions through manganese and the trivalent ions through iron, as is evident from the volumes of activation in Table 4. However, deprotonation of a water molecule enhances the reaction rates by utilising a conjugate base 7T- donation dissociative pathway. As can be seen from Table 4, there is a change in sign of the volume of activation AH. Four-coordinate square-planar molecules also show associative behavior in their reactions. 

The extent to which fragmentation takes places for each dissociation pathway of the additives depends on the excess of internal energy imparted to the molecular ion. Because the experimental conditions for product-ion mass spectra are strongly instrument dependent, generally not very well standardised and difficult to exchange between instruments from different... 

Halogen oxide radicals such as CIO and BrO are important reactive intermediates in the catalytic cycles of ozone destruction in the middle and upper stratosphere. The first absorption band CIO(/l211 <— X2 I) starts from 318 nm and has a series of vibronic bands that converge to a broad continuum at wavelengths shorter than 264nm (Fig. 8).98-101 In this continuum region four dissociation pathways are thermodynamically possible,33... 

In the first step, the reaction produces the conjugate acid of the complex by protonation of a coordinated F ion. That conjugate acid then undergoes substitution in a dissociative pathway. 

The rate constant for the k term equals that for reaction of [Ca(parv)] with cydta, consistent with rate-determining dissociation of [Ca(parv)] in both cases the k2 term may be assigned to an associative (adjunctive) process (497). This mechanism parallels that of parallel associative and dissociative pathways established for displacement of edta from Ca(edta)2 by Ttr+ (cf. Section II.D.3 (334)). 

Another piece of mechanistic evidence was reported by Snapper et al. [14], who describe a ruthenium catalyst caught in action . During studies on ring opening metathesis, these authors were able to isolate and characterize carbene 5 in which a tethered alkene group has replaced one of the phosphines originally present in Id. Control experiments have shown that compound 5 by itself is catalytically active, thus making sure that it is a true intermediate of a dissociative pathway rather than a dead-end product of a metathetic process. 

The two different routes, direct reductive elimination and a preliminary ligand dissociation pathway, were similarly investigated for the Pt(IV) complexes, (PR3)2Cl2PtCH3(H) (R = Me, H) (132,133). A part of this study (dealing with one possible isomer) is summarized in Fig. 3. 

---