Pyridine and other molecules containing five carbon atoms

6 PYRIDINE AND OTHER MOLECULES CONTAINING FIVE CARBON ATOMS 

Rate coefficients in the range 10s—106 s-1 have been determined by PIPECO for the decomposition of (CSHSN) from pyridine to give (C4H4)t [274], These metastable ions had internal energies of about 4eV and the rate coefficients were readily reproduced in QET calculations. There appeared to be a kinetic shift of 0.5—0.7 eV. Unlike most other determinations of rate coefficients by PIPECO, photons of a fixed wavelength were used. 

Pyridine has also recently been studied in the special type of threshold PIPECO experiment (see Sect. 4.1.1), in which the residence time of the ion in the source is varied [719]. In terms of eqn. (9), the limit t2 on the sampling interval or observation window was variable. Breakdown diagrams for formation of (C4H4)t were determined at two residence times (t2 = 0.805 and 5.925 ps). Detailed analysis of the results suggested that the k(E) vs. E curve for this decomposition was somewhat steeper than indicated by the earlier PIPECO study [274], It was concluded that the transition state for 

If it is considered that the charge exchange data is broadly reliable [4, 5], there are two sets of non-competing decomposition channels of the molecular ion of benzene. One hypothesis, which constitutes a basis 

The decompositions of bromobenzene [717] and chlorobenzene ions [716] have been studied by the special PIPECO experiment using variable source residence times. In the case of chlorobenzene, increasing the residence time from 0.7 to 8.9 ps resulted in a shift (kinetic shift) in the breakdown curves by 0.4 eV. Detailed analysis of the effects of varying residence time provided information on the k(E) vs. E curve in the vicinity of 104—106 s-1. The k(E) vs. E curve obtained differed significantly [by almost an order of magnitude in k(E) at some energies] from the curve reported in the earlier PIPECO study of metastable ions [22], The initial analysis [716] placed the critical energy for chlorine loss at 3.40 0.05 eV, but this has subsequently been revised to 3.19 0.02 eV [717]. The transition state was found to be loose . 

7 BENZENE, HALO BENZENES AND OTHER MOLECULES CONTAINING SIX 

Benzene has been investigated rather thoroughly by PIPECO [269, 277, 278, 758, see also 715] however, its unimolecular ion chemistry remains somewhat enigmatic. It was suggested, on the basis of charge exchange results, that the decompositions of the molecular ion to form (CaHa) and (04114) did not compete with those forming and 

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