On the stability of the organic dication of the bisquaternary ammonium salt decamethoxinum under liquid secondary ion mass spectrometry

Abstract

In the course of a liquid secondary ion mass spectrometric (SIMS) investigation on a bisquaternary ammonium antimicrobial agent, decamethoxinum, unusual pathways of fragmentation of the organic dication M2+ of this bisquaternary salt, with preservation of the doubly charged state of the fragments, were observed. To reveal the structural and electronic parameters of decamethoxinum, which are responsible for the stabilization of its organic dication in the gas phase, a comprehensive SIMS study using metastable decay, collision-induced dissociation and kinetic energy release techniques complemented by ab initio quantum chemical calculations was performed. Pathways of fragmentation of two main precursors originating from decamethoxinum-organic dication M2+ and its cluster with a Cl- counterion [M.Cl]+-and a number of their primary fragments were established and systematized. Differences in the pathways of fragmentation of M2+ and [M.Cl]+ were revealed: the main directions of [M.Cl]+ decay involve dequaternization similar to thermal degradation of this compound, while in M2+ fragmentation via loss of one and two terminal radicals with preservation of the doubly charged state of the fragments dominates over charge separation processes. It was shown that pairing of the dication with a Cl- anion does not preserve the complex from fragmentation via separation of two positively charged centers or neutralization (dequaternization) of one such center. At the same time the low abundance of M2+ in the SIMS spectra is to a larger extent controlled by a probability of M2+ association with an anion than by the decay of the dication per se. Quantum chemical calculations of the structural and electronic parameters of the decamethoxinum dication have revealed at least three features which can provide stabilization of the doubly charged state. Firstly, in the most energetically favorable stretch conformation the distance between the quaternary nitrogens (rN1-N2=1.39 nm) is relatively large. Secondly, an intramolecular solvation of quaternary groups by carbonyl oxygens of the adjacent groups of the dication occurs, which contribute to structural stabilization. Thirdly, an important feature of the electronic structure of the dication is the presence of a partial negative charge on the nitrogen atoms and smearing of a positive charge mainly over the hydrogens of alkyl groups attached to the quaternary nitrogens, which reduces the net repulsion between the quaternary groups. The possible influence of charge smearing on the kinetic energy released on the dication fragmentation is discussed.

‘Wet chemistry’ and crystallochemistry reasons for acidic matrix suppression by quaternary ammonium salts under matrix‐assisted laser desorption/ionization conditions

“…In the FAB mass spectra of surface-active decamethoxinum and 2,5-DHB solution in glycerol two characteristic groups of ions (with appropriate isotopic distribution) with comparable abundance corresponding to Cat 2þ Á Cl À (reported earlier in FAB/ SIMS studies on decamethoxinum 22,23 ) and Cat 2þ Á (DHB-H) À were recorded, pointing to the occurrence of partial anion exchange. It is important here that we probe not some separate 'ion pair' associates in the liquid, but a continuous two-dimensional surface layer formed by the cations and anions of the surface-active organic salt.…”

“…formation of tertiary amines via loss of neutral or alkyl-anion molecules. 10,22,57 It is observed on thermal evaporation of (alkyl)ammonium salts for analysis in the gas phase (e.g. by electron ionization), or as an abundant pathway of in-source, metastable and collision-induced decay in desorption techniques.…”

The effect of cone voltage on electrospray mass spectra of the bisquaternary ammonium salt decamethoxinum

“…The side groups R 1 contain menthyl rings R m attached by an ester bond. Ab initio quantum chemical calculations on decamethoxinum11 show that the distance between the quaternary nitrogen atoms for the extended conformation of the R 2 chain is approximately 14 Å. A significant feature of the electronic structure of the organic dication11 is the delocalization of the positive charge over a number of hydrogen atoms of the quaternary groups.…”

“…Quaternary ammonium salts are popular test compounds that have been studied by various soft mass spectrometric techniques,1–10 since the presence of fixed ‘preformed’ charges in their organic cations facilitates investigations on the fundamentals of ion generation and on fragmentation pathways. In our previous studies11–17 we selected the bisquaternary ammonium compound decamethoxinum as a structure and energy probe for soft ionization methods, because it demonstrated a strong dependence of the types of ions recorded in the mass spectra on the mass spectrometric technique and experimental conditions applied. Data on the fragmentation of decamethoxinum obtained with liquid secondary ion mass spectrometry (SIMS)11 and matrix‐assisted laser desorption/ionization (MALDI)12 techniques were reported.…”

“…In our previous studies11–17 we selected the bisquaternary ammonium compound decamethoxinum as a structure and energy probe for soft ionization methods, because it demonstrated a strong dependence of the types of ions recorded in the mass spectra on the mass spectrometric technique and experimental conditions applied. Data on the fragmentation of decamethoxinum obtained with liquid secondary ion mass spectrometry (SIMS)11 and matrix‐assisted laser desorption/ionization (MALDI)12 techniques were reported. In the present study we focus on the behavior of decamethoxinum in the electrospray ionization (ESI) mode with a view to obtaining further insights into the mechanisms of ion formation.…”