Investigation of Reaction Intermediates and Ion Structures in the Gas-Phase

The present work comprises three independent studies in which novel infrared ion spectroscopy data sets as well as extensive theoretical computational data and EI/ESI mass spectrometry results were acquired. In these studies, we mainly used methods of cryogenic messenger and IRMPD ion spectroscopy at room temperature combined with sophisticated computational chemistry based on ions formed by mass spectrometry to characterize and identify ion structures of important species in the gas-phase. The molecular ions of phenol and aniline as well as the common fragment ion at m/z 66 formed by electron ionization were examined with cryogenic messenger ion spectroscopy. According to our data, aromatic ground state molecular ions of phenol and aniline are clearly identified. However, as reported in the literature, our data set supports the assumption that tautomeric molecular ions of phenol and aniline are only short-lived intermediates on the reaction pathway towards the final loss of CO in case of phenol and the loss of HNC in the case of aniline. The potential energy surfaces of their reaction trajectories further indicate that once they isomerized, they possess more than enough energy to directly decompose, showing that the isomerization is the kinetic bottleneck of these reactions. We further showed that the common fragment ion at m/z 66 is indeed the cyclopentadiene radical cation in both cases as proposed in the literature. The present work also provides independent evidence for the structures of the product ions formed by the intriguing 2H and 3H rearrangement reactions and subsequent fragmentation in EI and ESI-MS with IRMPD ion spectroscopy discovered by Kuck et al. several decades ago. In the early reports it was pointed out that the proposed mechanisms and ion structures remain speculative and need further evidence, which is what motivated us to investigate and clarify these interesting reactions with modern techniques, that have not been available at the time. We found that the 2H rearrangement product C9H14N+ at m/z 136 is N,N-dimethyl-4-toluidine, however, protonated at the position 2 instead of the ipso position 4 as reported earlier. The structure of the 3H rearrangement had to be revised, because our data proofs that the rearranged fragment ion C8H13N• present at m/z 123 is the radical cationic N,N-dimethyl-2,3-dihydro-4- toluidine. Our calculations also showed that this isomer is the most stable one in the ion series tested in this work. We furthermore propose revised mechanisms for the 2H and 3H rearrangement reactions based on the new data sets from computational chemistry and IRMPD ion spectroscopy. The last part of this work reports the investigation of the Claisen and Aza-Claisen rearrangement in the gas-phase with ESI-MS, IRMPD ion spectroscopy and computational chemistry. Our data indicates that before the Claisen rearrangement occurs, the molecular ion [M+H]+ of allyl-2,4,6 trimethylphenylether is protonated in ortho-position to the ether-group, which is an interesting finding and different from most common textbook literature, in which such species are usually protonated at the oxygen. Upon collision activation of this molecular ion the sidechain migrates to the ortho position and is not transferred further to the paraposition as the Claisen-Cope-Tandem reaction would suggest. In addition, we found that the rearrangement already occurred without any activation in case of the Aza-Claisen, reflecting the lower energy needed for the Aza-Claisen rearrangement to be triggered. We also proposed a new mechanism in this case for the Claisen rearrangement in the gas-phase