Dissociation from molecular spectra

The dissociation of molecular plasma gases or analyte molecules within the radiation source is an equilibrium reaction. Accordingly, highly stable radicals or molecules are always present in a radiation source. They emit molecular bands which are superimposed on the atomic and ionic line spectra in the emission spectrum. Radicals and molecules may also give rise to cluster ions which may be detected in mass spectra. Common species in plasma gases are CN, NH, NO, OH. and Nt (or N2). From the analytes, highly stable oxides may persist (e.g.. AlO, TiO", YO ). A thorough treatment of molecular spectra is available [17], [18]. 

A molecule often presents several ionization sites. When a site presents an ionization potential far inferior to those of the other sites, there is no ambiguity. The electron is removed exclusively from this site and aU the ions of the mass spectrum can be interpreted as resulting from dissociations from a unique type of molecular ion M+ . For instance, in the case of n-butanamine (Figure 9.14), it is far easier from an energetic view to remove an electron n from the nitrogen atom than an electron a from one of the C-C, C-H, C-N, or N-H bonds. Most of the ions observed in the mass spectrum can be interpreted by postulating that ionization occurred exclusively on the nitrogen atom. 

Irradiation of the molecular radical anion of DESO, which has a yellow color, with light of X = 350-400 nm partially restores the red color and the ESR spectrum of the radical-anion pair. Similarly to the case of DMSO-d6 a comparison of the energetics of the photodissociation of the radical anion and dissociative capture of an electron by a DESO molecule permits an estimation of the energy of the hot electrons which form the radical-anion pair of DESO. This energy is equal to 2eV, similarly to DMSO-d6. The spin density on the ethyl radical in the radical-anion pair of DESO can be estimated from the decrease in hfs in comparison with the free radical to be 0.81, smaller than DMSO-d6. 

However, the comparison of the whole series of experimental facts involving IR-spectroscopy of adsorption of molecular and atomic hydrogen as well as the change in electric conductivity of adsorbent is indicative of a more complex phenomenon. For instance, in paper [97] both the spectra of adsorption of adsorbed molecular hydrogen were studied together with those of hydrogen atoms adsorbed from gaseous phase. In case when H2 are adsorbed in a dissociative manner one would have expected a manifestation of the same bands 3498 and 1708 cm or at least one of them inherent to adsorption of H-atoms in the spectrum of ZnO. 

A third source of error is associated with the fragmentation pattern caused by dissociation of the molecular ions formed in the source region of the spectrometer. Under severe conditions these processes may proceed with substantial isotopic fractionation, and this obscures the measurements of isotopic composition at the collector. To some extent careful standardization of the instrumental conditions may ensure that errors from fragmentation are systematic, and thus cancel (at least to some extent). Alternatively, softer ionization methods can be used to prevent most or all of the fragmentation. The bottom spectrum in Fig. 7.7 illustrates this approach it shows the mass spectrum of chlorobenzene obtained by photoionization. Only the parent molecular ions are observed. It should be kept in mind, however, that softer ionization usually yields smaller ion currents and consequently statistical counting errors increase. 

The molecular ion (M ) of 16 was reported in <1998GHE297> and was shown to be unstable and dissociates in a number of directions. The most stable ion has mjz 262, and high-resolution MS showed that this ion is formed by removal of the isopropyl group and ethyl radicals from M. To determine the structure of this ion, the collision-activation spectrum was recorded and was found to be compound 27 <1997GHE1306>. 

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