Abstract
This paper presents an investigation of proton and charge transfer reactions to 2-, 3- and 4-nitroanilines (C6H6N2O2) involving the reagent ions H3O+·(H2O)n (n = 0, 1 and 2) and O2+, respectively, as a function of reduced electric field (60–240 Td), using Selective Reagent Ion–Time-of-Flight–Mass Spectrometry (SRI–ToF–MS). To aid in the interpretation of the H3O+·(H2O)n experimental data, the proton affinities and gas-phase basicities for the three nitroaniline isomers have been determined using density functional theory. These calculations show that proton transfer from both the H3O+ and H3O+·H2O reagent ions to the nitroanilines will be exoergic and hence efficient, with the reactions proceeding at the collisional rate. For proton transfer from H3O+ to the NO2 sites, the exoergicities are 171 kJ mol−1 (1.8 eV), 147 kJ mol−1 (1.5 eV) and 194 kJ mol−1 (2.0 eV) for 2-, 3- and 4-nitroanilines, respectively. Electron transfer from all three of the nitroanilines is also significantly exothermic by approximately 4 eV. Although a substantial transfer of energy occurs during the ion/molecule reactions, the processes are found to predominantly proceed via non-dissociative pathways over a large reduced electric field range. Only at relatively high reduced electric fields (> 180 Td) is dissociative proton and charge transfer observed. Differences in fragment product ions and their intensities provide a means to distinguish the isomers, with proton transfer distinguishing 2-nitroaniline (2–NA) from 3- and 4-NA, and charge transfer distinguishing 4-NA from 2- and 3-NA, thereby providing a means to enhance selectivity using SRI–ToF–MS.
Highlights
Selective Reagent Ion–Mass Spectrometry (SRI–MS) is a commonly used soft chemical ionisation technique used in a broad range of analytical fields and applications [1, 2]
For the product ion distribution (PID) plots, the voltage applied to the drift tube is shown in the main x-axis, and the reduced electric field E/N achieved for that particular voltage is showed in the secondary x-axis
This work reports the product ions from the reaction of 2, 3and 4-nitroaniline isomers with H3O+ and O2+ as a function of the reduced electric field in a SRI–ToF–MS
Summary
Selective Reagent Ion–Mass Spectrometry (SRI–MS) is a commonly used soft chemical ionisation technique used in a broad range of analytical fields and applications [1, 2]. To aid in the interpretation of the experimental measurements involving the reagent ions H3O+·(H2O)n (n = 0, 1 and 2), quantum mechanical calculations have been undertaken to determine proton affinities and gas-phase basicities For this investigation, a Kore Technology Ltd. Series I Selective Reagent Ion–Time of Flight–Mass Spectrometer (SRI– ToF–MS) instrument was used, details of which been given elsewhere [1, 28], and only brief and pertinent details will be presented in this paper. H3O+ (and associated protonated water clusters—depending on the value of the reduced electric field) dominates the reagent ion signal, other reagent ions are always present in the drift tube. These “impurity” reagent ions result from back diffusion of the buffer gas in the drift tube into the ion source. This can be ignored owing to its signal intensity being approximately 0.1% of the O2+ signal for the experimental conditions used throughout our measurements
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More From: Journal of the American Society for Mass Spectrometry
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