Probing the potential energy landscape for dissociation of protonated indole via threshold collision-induced dissociation and theoretical studies

Collision-induced dissociation of protonated indole with Xe is studied as a function of kinetic energy using guided ion beam tandem mass spectrometry techniques. Activated dissociation resulting in endothermic loss of neutral HCN (or HNC) is the only pathway observed over the range of collision energies examined. The cross-section threshold for this activated dissociation pathway is interpreted to yield 0 and 298 K activation energies for this process after accounting for the effects of multiple ion-neutral collisions, the internal energy distribution of the protonated indole cations, and their lifetimes for dissociation. Density functional theory (DFT) calculations at the MPW1PW91/6-31G* level of theory are used to determine the structures of indole, the protonated indole tautomers, and the transition states, intermediates, and products involved in the activated dissociation of protonated indole. Four distinct pathways between the reactant ion and dissociation products are computed. In all cases, the ionic product formed is C6H5CH2+, while the neutral product is HCN in three of the pathways and HNC in the fourth. The vibrational frequencies and rotational constants of the ground state tautomer of protonated indole and the rate-determining transition state along each of the pathways computed are used for the thermodynamic analysis of the experimental data. The theoretical activation energies and potential energy landscapes for activated dissociation of protonated indole are determined from single point calculations at the MPW1PW91/6-311+G(2d,2p) and MP2(full)/6-311+G(2d,2p) levels of theory, using the MPW1PW91/6-31G* optimized geometries. Both theories produce similar potential energy landscapes for elimination of HCN (or HNC) from protonated indole. Theory suggests that elimination of HNC is favored over HCN. However, our threshold measurements probe the lowest-energy pathway available and are in much better agreement with the higher-energy HCN elimination pathways, suggesting that theory underestimates the activation energy for loss of HNC.