Intramolecular Charge Transfer in the Gas Phase: Fragmentation of Protonated Sulfonamides in Mass Spectrometry

The fragmentation of protonated molecules (MH+) in mass spectrometry usually results in even-electron product ions, but the MH+ ions of sulfonamides are different as they often produce dominant radical cations of the constituent amines. For a series of benzenesulfonamides of anilines that bear various substituents, we found that the sulfonamides are preferentially protonated at the nitrogen, which is different from the carboxylic amides. Upon N-protonation, the S−N bond dissociates spontaneously to produce an intermediate [sulfonyl cation/aniline] complex. Within the ion−neutral complex, charge transfer between the two partners occurs in the gas phase to give rise to the ionized anilines. A substantial energy barrier was found to govern the reaction, which is consistent with the outer-sphere electron transfer mechanism. This energy barrier prevents the charge transfer when a strong electron-withdrawing substituent is attached to the aniline moiety. In contrast, when the aniline bears an electron-donating group, charge transfer is still more favorable than the dissociation of the intermediate ion−neutral complex, in spite of the existence of the energy barrier, and therefore dominates. A correlation was observed between the intensities of the ionized anilines and the ionization energies of these anilines

Optimized structures of the key species involved in the pyrolysis of trichlorfon at B3LYP/6-311+G(d,p).

<p>Optimized structures of the key species involved in the pyrolysis of trichlorfon at B3LYP/6-311+G(d,p).</p

A series of TIC of trichlorfon with different GC injector temperatures.

<p>A series of TIC of trichlorfon with different GC injector temperatures.</p

The Pyrolytic Reaction of Ketonic Hydrazones from <i>S</i>-Methyl Dithiocarbazate: A Combined Online GC-MS Pyrolysis and DFT Study

The gas-phase pyrolysis of ketonic hydrazones from S-methyl dithiocarbazate R1R2CNNHC(S)SCH3 (R1, R2 = alkyl or aryl) was investigated by online GC-MS pyrolysis and theoretical calculation. Both of these pyrolytic products, ascribed to methanethiol and the corresponding N-isothiocyanate imines, were detected in the total ion chromatography (TIC) of GC-MS. Calculation results exhibit two stable configurational structures for reactants (Re), which can interconvert with relatively low barriers (3SH for syn-Re and a two-step reaction pathway for trans-Re involving tautomer interconversion followed by decomposition of CH3SH, are competitive in the reaction. Both syn-Re and trans-Re exhibit lower critical energies in the propagation step of the radical pyrolysis than that in the unimolecular pyrolysis process (187.76 kJ/mol via 131.91 kJ/mol for syn-Re, and 159.15 kJ/mol via 98.92 kJ/mol for trans-Re). However, much more energy is needed to excite the compound to produce the methylthio radical, with 262.03 and 253.60 kJ/mol for syn-Re and trans-Re, respectively. Therefore, the unimolecular pyrolysis rather than the radical one occurs in the condition of this study

The proposed fragmentation pathways of the deprotonated halofenozide.

<p>The proposed fragmentation pathways of the deprotonated halofenozide.</p

Structures for the deveratives of diacylhydrazine.

<p>Structures for the deveratives of diacylhydrazine.</p

The schematic potential energy diagrams for the pyrolytic reactions of trichlorfon.

<p>The schematic potential energy diagrams for the pyrolytic reactions of trichlorfon.</p

The proposed pyrolysis pathways of trichlorfon.

<p>The proposed pyrolysis pathways of trichlorfon.</p

Investigation on the Gas-Phase Decomposition of Trichlorfon by GC-MS and Theoretical Calculation

<div><p>The gas phase pyrolysis of trichlorfon was investigated by the on-line gas chromatography – mass spectrometry (GC-MS) pyrolysis and theoretical calculations. Two reaction channels were proposed in the pyrolytic reaction, by analyzing the detected pyrolytic products in the total ion chromatography, including 2,2,2-trichloroacetaldehyde, dimethyl phosphite, and dichlorvos. Theoretical calculations showed that there is an intramolecular hydrogen bond between the hydroxyl group and the phosphate O atom in trichlorfon, through which the hydroxyl H atom can be easily transferred to phosphate O atom to trigger two pyrolytic channels. In path-a, migration of H atom results in direct decomposition of trichlorfon to give 2,2,2-trichloroacetaldehyde and dimethyl phosphite in one step. In path-b, migration of H atom in trichlorfon is combined with formation of the O-P bond to give an intermediate, followed by HCl elimination to afford dichlorvos. Path-a is kinetically more favorable than path-b, which is consistent with the GC-MS results.</p></div

Optimized structures of the key species involved in the fragmentation pathway of Path-2 at B3LYP/6-31+G(2d,p).

<p>Optimized structures of the key species involved in the fragmentation pathway of Path-2 at B3LYP/6-31+G(2d,p).</p