Thermochemistry of organic and heteroorganic species. Part XIII. Structural aspects and thermochemical approach to isomerization and fragmentation of negative ions. Simple bond cleavage and hydrogen rearrangement processes

Abstract

Many hundreds of isomerization and fragmentation processes for negatively charged ions obtained in electron attachment dissociation, collision induced dissociation and negative chemical ionization have been analyzed to find out certain regularities in gas-phase monomolecular chemistry of negative ions like it was earlier performed for positive ions [Eur. J. Mass Spectrom., 8 (2002) 409]. This analysis combined with systematic application of the data on energies of the processes (appearance energies, isolation of stable resonance states on energetical scale in the range 0–15 eV, enthalpies of formation of even negative ions and electron affinities of corresponding free radicals) allowed to come to quite definite conclusion of validity of Eyring's quasi-equilibrium theory (QET) to mass spectrometry of negative ions. However, it is valid only in the limits of a stable resonance state rather than in a whole range of energies being applied. The validity of QET was illustrated by many diverse simple bond cleavage and rearrangement processes. Those hundreds of known processes for negative ions dissociation were reduced to a few isomerization and fragmentation types for odd- and even-charged negative ions—8 types for simple bond cleavage and 7—for H-migration, all being in certain aspect common with those suggested earlier for positive ions. The isomerization and fragmentation types were assigned to about 370 processes of simple bond cleavage and H-migration with presentation of possible structures for isomeric molecular and fragment ions. Possibility of such specification of the processes for negative ions, partial thermochemical approach for their description, general and specific application of QET make unnecessary the appeal to participation of high-energetical electron excitation, nonergodic fragmentation, to any specifity in charge-remote processes—all these used in literature to explain occurrence of some unusual processes. The latter could be described by the introduced reaction types or their combination, by formation of many (instead of a single one) neutrals in one-step process, H-wandering over ionized benzene ring, ring opening as a routine event (e.g. to explain the appearance of molecular ions in several domains of energies for aromatic compounds.).