Isomers relative abundance

The o-nitrotoluene isomer is easy to identify because of the loss of OH from the molecular ion. All the nitrotoluenes lose 30 Daltons from their molecular ions. The m- and p-isomers can be distinguished from each other by the relative abundances of the m/z 65 ion versus the molecular ion, particularly if both isomers are present (see Figure 22.1). 

Nowadays, the preferred methodology is the separation and quantification of porphyrins by high-performance liquid chromatography (HPLC) with fluorescence detection. Thus, a method that detects the main isomers of specific porphyrins is recommended, as a high number of different isomers exist, especially in pathological conditions, and the relative abundance of the different isomers supports the distinction of the different porphyrias (Table 7.3.1). 

Isomer 60 was calculated to be 27.5 kcalmol-1 and 20.9 kcalmol-1 more stable than 61 and 62, respectively. The relative abundances of fragment ions in the CA mass spectra reflected the predicted energetics for the dissociation processes. While CA mass spectrometry does not allow for a distinction of cyclic versus noncyclic isomers, the fact that the CA mass spectrum contained an Si2+ and Si02+ ion leads to the exclusion of isomers 61 and 62. On the neutral [Si2, 02] surface215 only two energetically low-lying... 

Thiophene hopanoids. Two C35 thiophene hopanoids are present in the A3 fraction of the bitumens. One of these isomers (XXVI, Figure 19) has been previously identified by Valisolalao et al (7) in black shales. The other isomer (XXVII, Figure 19) has been tentatively identified by Sinninghe Damste et al (6). Because of the lack of a suitable standard for this fraction it was impossible to quantify these compounds as precisely as the previously discussed alkylthiophenes. However, the relative abundance of these hopanoid thiophenes (see experimental section) varies considerably throughout the sequence (Figure 19a). 

The question then is whether and in which cases meaningful results can be expected from the computation of naked molecules. It was pointed out that the relative abundance of an isomer may be related to its elution rate from an ion exchange column12331. This is evidently so because both the abundance and elution rate are related to the stability of the solvated and ion-paired species. Aqueous solutions of chloride, nitrate and perchlorate do not lead to any chromatographic separation of the three isomers of [Co(dien)2]3+[2331. These anions may therefore be... 

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