Investigation On The Gas Phase Ion Chemistry Of Thiosemicarbazones In ESI-MS~n

Protonation is the major ionization method in ESI, and dissociation reactions of the protonated molecules are the base of structural analysis in electrospray ionization tandem mass spectrometry (ESI-MSn). Therefore, it is very important at all times to carry out mechanistic investigation and pattern induction for the fragmentation reactions in the gas phase. In this thesis, the gas-phase chemical behaviors of thiosemicarbazones have been investigated by using ESI-MSn techniques in combination with quantum calculation, including the following four parts:1. Coordinated dissociative proton transfers:S-methyl benzenylmethylenehydrazine dithiocarboxylate, PhCH=N-NHC(=S)-SCH3, was utilized as a model compound, and investigations have been carried out for the dissociation reactions of the protonated molecule upon collisional activation. Eliminations of (NSC)SCH3, H2S and CH3SH are the three dissociation reactions of the protonated molecules observed in the tandem mass spectrometry, witnessed by the MS/MS analysis of the native 34S-isotopic ion and of the D-labeling molecular ion. Of the three fragmentations, both the external proton and the thiocarbamide hydrogen transfer to the fragment ion (m/z 106) in the dissociation of (NSC)SCH3 elimination, while both migrate to the neutral fragment H2S to generate a minor product ion at m/z 177. In the case of the most feasible reaction process of CH3SH elimination, either the added proton or the thiocarbamide hydrogen (charge-remote fragmentation) migrates to the fragment ion at m/z 163, and the other to the neutral fragment. These results indicated that fragmentation of the protonated molecule was viewed as a result of the coordinated migration of both the added proton and the thiocarbamide hydrogen, which leads to the "coordinated dissociative proton transfers".2. Decomposition of the neutral molecules resulting from transfer of the thioamide hydrogen (the gas phase pyrolysis):To further investigate on the above charge-remote fragmentation (CH3SH elimination), ketonic hydrazones from S-methyl dithiocarbazate, R1R2C=N-NH-C(=S)SCH3 (R1, R2=alkyl or aryl) have been selected as model compounds to perform online GC-MS pyrolysis and theoretical calculation. The online GC-MS pyrolysis experiment results indicate that these neutral compounds can also undergo CH3SH elimination when heated. The unimolecular reaction channel rather than the radical one occurs in the gas phase elimination by comparison of the potential reaction profile of the two mechanistic channels. Therefore, both the gas phase pyrolysis and the charge-remote fragmentation are the unimolecular reaction with CH3SH elimination, induced by the migration of the thioamide hydrogen, although there is some difference in the detailed mechanism on the migaration of the thioamide hydrogen.3. Positional effects of the methoxyl substituent on the fragmentation behaviorThe dissociation chemistry of the ortho-, meta-or para-isomers of protonated S-methyl methoxyl-(or chloro-) benzenylmethylenehydrazine dithiocarboxylate, RPhCH=N-NHC(=S)-CH3 (R=CH3O- or Cl-), has been investigated for mechanistic exploration of the positional effects of the substituent. The reaction center of losing CH3SH is far from the phenyl group, and the dissociation reaction is immune to the positional effects for both the methoxyl and the chrolo- substituted isomers. The three methoxyl-substituted isomers, however, were easily differentiated according to the different abundance of the characteristic ion at m/z 136, resulting from the different reactivity of the (NSC)SCH3 elimination. This fragmentation reaction is triggered by the positive charge upon protonation on the imine N2, which is significantly affected by the positional effects of the methoxyl group, such as electron donating resonance, intramolecular hydrogen bond. However, the chloro-substituted isomers share the similar energy barrair and the CID-MS spetrom, due to its weak positional effect.4. Gas-phase ligand exchange reaction and intracomplex proton transferDue to the mixed hard-soft donor character and versatile coordination behavior, S-methyl benzenylmethylenehydrazine dithiocarboxylate (TSC) was selected as a model compound to investigate the chemical behavior of its nickel complex ion. The complex ion [Ni2+(TSC) (TSC-H)-]+(m/z 477), formed by ESI of a solvent containing TSC and Ni2+, preforms dissociation upon collisional activation and mainly yield the product ion m/z 315. The active product ion m/z 315, upon collisional activation, undergoes ligand exchange reaction with the residue ESI solvent molecules in the ion trap. Witnessed by CID-MS of the phenyl-deuterated TSC-Ni2+ complex ion, in which two kinds eliminations of CH3SH and CH3SD were observed simultaneously in the CID-MS, the above ion m/z 315 is proved to be a mixture of [Ni2+(TSC) (SCH3)-]+(CI-R1) and [Ni2+(TSC-H)-(CH3SH)]+(CI-R2). To our interest, ligand exchange of CI-R1 involves CAr-H activation and intracomplex proton transfer. In addition, Pd2+ shows more capability for CAr-H activation than that of Ni2+, by comparison of the abundance of the product ions resulted from the two ligand exchange reactions.