Thermal electron attachment to aromatic halides and nitro-compounds

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1973

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Abstract

The combination of thermal electron attachment (TEA), electron beam (EB) and plasma chromatograph-mass spectrometry (PCMS) techniques is shown to be a most powerful method to study electron capture phenomena. The proposed thermal electron attachment mechanisms for halo-aromatics are that upon electron attachment, aromatic fluorides show only the electron attachment-detachment mechanism, while both aromatic chlorides and aromatic bromides show a dissociative mechanism with a parent negative ion intermediate. However, in chloro-aromatics, the intermediate ion, ArCl[raised -] has a longer lifetime, whereas, in bromo-aromatics, ArBr[raised -] dissociates within a period of a vibration (10[raised -12] sec) along the C-Br bond (89). These electron attachment mechanisms for aromatic halides are properly demonstrated in this study employing the TEA, EB and PCMS techniques. In the TEA technique, the residual electron concentration in a electron capture detector is monitored to obtain the electron capture coefficient, K. The electron attachment mechanism for aromatic fluorides is revealed by the fact that the temperature dependence of the electron capture coefficient (K) shows only one linear region with a positive slope in a LnKT[raised 3/2] vs 1/T graph; for aromatic chlorides, two linear regions with a positive and a negative slope are generally observed; while for aromatic bromides, only one linear region with a negative slope is observed. However, whenever the capture coefficient, K, of a compound is too large, then a LnKT[raised 3/2] vs 1/T graph shows only a linear region with a slightly negative slope which is referred to as the region where the recombination rate between the negative ion and positive ion and/or radicals is large. This phenomenon is demonstrated by compounds such as chloropentafluorobenzene, pentafluorobenzene etc. Electron affinities of the capture species and their activation energies for dissociation are the molecular parameters that can be obtained from the slopes of LnKT[raised 3/2] vs 1/T graphs in the TEA study. The electron affinities obtained this way in the aromatic chlorides show an increase whenever the number of highly electronegative substituents such as CF3, Cl, F is increased. For example, the EA of p-dichlorobenzene is 4,10 Kcal/mole, 1,3,5-trichlorobenzene is 7.74 Kcal/mole, 2,4-dichlorobenzotrifluoride is 7,16 Kcal/mole, 1,2,4,5-tetrachlorobenzene is 11.0 Kcal/mole and 3'-chloroacetophenone is 13.45 Kcal/mole. However, the activation energy for dissociation in the capture species do not show such a dependence but in general fall in the range from 7 ~ 12 Kcal/mole for aromatic chlorides, 2 ~ 6 Kcal/mole for aromatic bromides. In the EB technique, the vertical transition processes of electron attachment to aromatic halides involves two temporary negative ion resonance states (TNIR). The first vertical transition involves the [pi]* M. 0. of the berrzene ring. The peak energy of this transition is generally less than 1 eV (33,45). The second vertical transition involves the [sigma]* M. 0. of the C-X bond. This transition is the only transition that appears in the EB study of aliphatic halides (6); but, until this study,had not been found for the aromatic halides. The electron beam energy corresponding to this transition was found to be in the range of 3.5 eV to 4.5 eV for aromatic chlorides and is consistently observed for all of the aromatic chlorides in this study. However, it appears that these second peaks, generally, are much weaker and broader than the first peak. This is attributed to the highly repulsive nature of the [sigma]* M. 0. of the C-Cl bond. The vertical transition processes occuring in the EB technique and the general kinetic processes in the TEA technique have been correlated by means of potential energy curves. Generally, two of the molecular parameters: electron affinity of the molecule or radical (TEA), activation energy for dissociation (TEA), or vertical transition energy (EB) are used to establish each negative ion potential energy curve. Since only 2 of these parameters need to be used to establish the negative ion potential energy curve, the remaining experimental parameters can be used to test the agreement with the value of these parameters calculated from the potential energy curves. Potential energy curves have been constructed for every compound investigated in this study. The agreement in the comparisons are quite good and deviation-H's less than 4 Kcal/mole made on the EA as well as the activation energies for dissociation...

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