Electron ionization labeled molecules

Electron ionization is a perfect method for the analysis of labeled molecules as in this case ion-molecular reactions are suppressed. It is better to use for the calculations the most intense spectral peaks with the highest m/z values. Molecular ion is the best choice. However, if notable [M + H]+ or [M — H]+ peaks are present in the spectrum of the unlabeled compound the correct calculation will be problematic. To eliminate [M + H]+ peaks it is helpful to record a spectrum with the minimum quantity of sample. To consider interference with [M — H]+ ions one should know from what position the hydrogen atom is lost and whether deuterium could be in this position. 

Fig. 11.3. Electron ionization and methane Cl mass spectra of toluene. The key features of the respective mass spectra are labeled. Spectral interpretation is based on recognition and understanding of these key features and how they correlate with structural elements of the analyte molecule of interest. The signal representing the most abundant ion in a mass spectrum is referred to as the base peak, and may or may not be the molecular ion peak (which carries the molecular mass information). Cl spectra provide confirmation of molecular mass in situations where the El signal for the molecular ion (M+ ) is weak or absent. The Cl mass spectrum provides reliable molecular mass information, but relatively little structural information (low abundance of the fragment ions). Compare with Fig. 11.4.

The resulting spectrum [35] (Figure 3) is dramatically simplified by comparison to that obtained following electron bombardment ionization [36]. The vibrational excitation in the ion can be varied by changing the wavelength of the first, ionization laser to be resonant with a intermediate Rydberg level of differing v. This results in spectra with different vibrational activity and these results combined with the same spectra obtained for isotopically labeled molecules can be used to provide a detailed vibrational analyis [37] for this band system which was previously only subject to qualitative interpretation. 

Since electron impiact (El) mass spectra of Dns derivatives usually indude the molecular ions, these derivatives are quite useful for the mass spectrometric identification of unknown compounds, although only a few typical fragments characteristic of the labelled molecule are formed (Figure 3). The main fragment ion (m/z 170 or 171) is that formed by cleavage of the S-C bond of the sulphonyl group [20,83-87J. Ionization methods other than El, although useful for the identification of Dns derivatives [88,89], have not been systematically studied. 

A phenylcarbene anion (22) has been generated in the gas phase by deprotonation of the parent phenyldiazirine by hydroxide, which led to a mixture of the diazirine anion and the phenylcarbene anion. This carbene anion has been calculated to be isoenergetic with the cycloheptatetraene anion (23). Experimentally, an equilibrium between these two forms has been confirmed by reaction with small molecules (such as CO2) which showed a scrambling of the label in benzylic-labelled compounds. The same group synthesized a phenylcarbene anion (24) in the gas phase by electron ionization. " This carbene anion was characterised after reaction with small... 

We have recently pointed out that 7(r) affords a means of avoiding the need to identify and count valence electrons [68], If 7(r) is computed on an outer surface of the atom, then it is fully consistent with Allen s focus upon the average ionization energy of the valence electrons but without requiring that these be specified. We chose to compute 7(r) on the surface defined by the 0.001 au (electrons/bohr3) contour of p(r) for molecules, this typically encompasses at least 98% of the electronic charge [69], The 7(r), now labelled 7s(p = 0.001), was obtained for H-Kr with dementi s extended-basis-set HF wave functions [70], except for hydrogen, for which we took the exact e, and p (r). 

Radiolytic spin labeling of molecules adsorbed in zeolites occurs by ionization to form radical cations and by formation of H-adduct radicals by H atom addition. Ionization of adsorbed molecules is a two-step process, equations (1) and (2). Because the adsorbate loading used in experiments is low (typically one percent or less by weight), energy is absorbed by the matrix and not directly by the adsorbate. Holes (Z" ) created in the zeolite lattice migrate to adsorbate (A) by charge transfer. Stabilization of radical cations is made possible at low temperature by sequestration in the zeolite pores and by trapping of electrons by the matrix. 

Solvated electrons. Electrons produced hy ionizing radiation are deloca-lized in a molecular orbital over a number of solvent molecules (e.g., water, ammonia). (a) Label the conjugate acid-base pairs in the reaction e (aq) H,0+ H 0 -I- H. (b) The rate equation for the reaction... 

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