| Mass spectrometry was used in combination with theoretical calculations and statistical rate theories to probe both the unimolecular dissociation and isomerization characteristics of proton-bound molecular pairs. These proton-bound pairs have been found to dissociate by simple hydrogen bond cleavage or rearrange via a unimolecular SN2 mechanism to form an isomeric complex that loses water. The proton-bound pair rearranges to an intermediate ion complex (IC) via TSa followed by the formation of the thermodynamically stable isomer that loses water via TSb. Empirical relationships based on the number of stabilizing alkyl groups on the central SN2 carbon and differences in proton affinities have been developed for estimating the energies of the two transition states (TSa and TSb) and IC in the isomerization reaction.; The entropy of activation, DeltaS‡, for the dissociation of the proton-bound pairs to CH3CNH+ and ROH was found to change systematically with molecular functionality, with the entropy decreasing from 70 J K-1 mol-1 in (CH3 CN)(CH3OH)H+ to 6 J K-1 mol -1 in (CH3CN)((CH3)2CHOH)H +. This systematic change was not observed in the dissociation to CH3CN and ROH2+ or in the thermodynamic entropy change for the dissociation. The entropies of activation for the competing dissociation channels for (CH3CN)(CH3OH)H+ and (CH3CN)(CH3CH2OH)H+ were the same within error and thus Delta(DeltaS‡) ≈ 0 while this value ranged from 40-45 J K-1 mol -1 for the propanol-containing pairs. The Delta(DeltaS) for all four proton-bound pairs was zero.; The unimolecular dissociation of the series of (NO)(A)+ cluster ions (where A = benzene, pyridine, furan, thiophene and benzonitrile) was examined using tandem mass spectrometry and evidence was found for the participation of excited states in their dissociation. The unimolecular dissociation of metastable ionic complexes generated by NO chemical ionization of benzonitrile changes as a function of ion source pressure. As the NO concentration in the source increases, the metastable ion (MI) mass spectrum goes from being dominated by NO+ to one dominated by ionized benzonitrile and its fragmentation products. A similar result is observed when A = pyridine. When A = benzene, the MI mass spectrum is dominated by the ionized aromatic, a result that is consistent with the adiabatic dissociation of the ground singlet-state of the complex. The MI mass spectra of the complexes involving furan and thiophene are consistent with the dissociation of the lowest energy singlet-state. |
