Molecular ions relative stability

An electron impact (El) ion source uses an electron beam, usually generated from a rhenium filament, to ionize gas-phase atoms or molecules. Electrons from the beam (usually 70 eV) knock an electron from a bond of the atoms or molecules creating fragments and molecular ions [366,534,535]. Several factors contribute to the popularity of El ionization in environmental analyses such as stability, ease of operation, simple construction, precise beam intensity control, relatively high efficiency of ionization, and narrow kinetic energy spread of the ions formed. 

A comparison of the relative strength of functional groups to cause a-cleavage is summarized in Table 6.5. [6] This also corresponds to a rough measure of relative charge-stabilizing capability of the respective substituent, e.g., the ratio H2C=0H /H2C=NH2 from 2-amino ethanol molecular ion is 2.3/100 and the ratio of H2C=0H /H2C=SH from 2-thio ethanol molecular ion is 42/70. [20]... 

The thermodynamic stabilities of phenonium ions relative to the parent have been determined in the gas phase by measuring the position of the equilibrium between (46) and (47)7 The results followed a Yukawa-Tsuno relationship with a p value of -12.6 and an r+ value of 0.62, the general behaviour being more like benzenium ions than benzyl cations, with tt-delocalization less effective than in benzyl cations. A theoretical study of the elimination of molecular H2 from the benzenium ion C6H7+ shows that the barrier to this process appears to be very small.The gas-phase Friedel-Crafts alkylation reaction of CF3C6L6+ (L = H or D) with C2L4 is accompanied by isotopic scrambling, which has been used to elucidate the mechanism of this process. A theoretical calculation shows that the lifetime of triplet phenyl cation must be very short. ... 

High-level ab initio calculations have provided more precise structural details, and relative stability estimates, for members of the 7-norbornyl anion series (12-15). Far from being classical carbanions, each of the ions is stabilized by delocalization of the negative charge into accessible LUMOs of anti-parallel C—C bonds of the molecular framework and each is more stable than methyl carbanion. Consequently, it is unlikely that solution studies of the unsaturated systems will reveal any bishomo-antiaromatic character. 

The number of abundant ions in the mass spectrum and their distribution is indicative of the type of molecule. As discussed above, the mass and the relative abundance of the molecular ion gives an indication of the size and general stability of the molecule. An abundant molecular ion is expected, for example, from aromatic and saturated polycyclic molecules, provided that no easily cleaved group is present. A spectrum consisting of a few prominent ions suggests there are only a few favoured decomposition pathways indicating a small number of labile bonds or stable products. 

A further complication associated with the application of molecular mechanics calculations to relative stabilities is that strain energy differences correspond to A(AH) between conformers with similar chromophores (electronic effects) and an innocent environment (counter ions and solvent molecules), whereas relative stabilities are based on A(AG). The entropy term, TAS, may be calculated by partition functions,... 

The estimate of relative stabilities via the comparison of total strain energies is in general limited to a series of conformers and isomers (see for instance Chapter 7 and the relevant chapters in Parts I and III). The determination by molecular mechanics calculations of relative stabilities of a series of complexes with metal ions having differing geometric preferences (electronic effects) and preferences in terms of donor atoms is therefore a questionable approach. A comparative study is only useful if the structural preferences of the different metal ions are similar and/or if the electronic effects may be separated from steric effects. 

The pronounced difference in the relative stability can be understood as a manifestation of molecular orbital interactions. The SOMO of the cyclobutene ion has high orbital coefficients on the internal sp2 hybridized carbons (—CH=), whereas the butadiene SOMO has the highest coefficients in the terminal ( = CH2) carbons. Accordingly, substituents in the l-(and 2-) position will stabilize the closed ion. On the other hand, substituents in the 3-(and 4-) position will favor the ring opened ion and lower the barrier to ring opening. 

The relative stability of the delocalized, non-vertical radical cation relative to a localized, vertical isomer was demonstrated also in gas phase experiments [404]. The molecular ions of m/e 132 obtained by gas phase ionization of the [4 + 2] dimer exhibited a bimodal decay, a result which was interpreted as evidence for the presence of two isomeric ions with different structures. The possibility that the reactive ion is a species with excess internal energy was discounted, when equivalent decay curves were observed in experiments using 10 eV and 70 eV electron impact ionization energy. In dramatic contrast, the molecular ions derived from the [2 + 2] dimer fail to react apparently the ion population resulting in this experiment is homogeneous [404],... 

Molecular orbital calculations of varying sophistication have been used to try to predict the relative stabilities of the W, S, and U conformers. The earliest using the EHMO method predicted that the U conformer is the most stable (70), but MNDO (74) suggests that the W form is of lowest energy for the parent C5H7 anion. Most of these calculations relate to the gaseous ion, but similar conclusions were reached by inclusion of lithium or BeH (71-73). The merits and demerits of the calculations are discussed by Dewar et al. (74). 

The intensity of the molecular ion peak depends on the stability of the molecular ion. The most stable molecular ions are those of purely aromatic systems. If substituents that have favorable modes of cleavage are present, the molecular ion peak will be less intense, and the fragment peaks relatively more intense. In general,... 

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