Cation radical cycloadditions and related sigmatropic reactions

Ketene chemistry: the second golden age

In this review, we present a brief summary of the theoretical methods most frequently used in gas-phase ion chemistry. In subsequent sections, the performance of these methods is analyzed, paying attention to the reliability of geometries, vibrational frequencies, energies, and entropies. The possible pathologies of the different methods, in the form of instabilities of the wave function or spin contamination problems, are discussed. Several examples are presented to illustrate the usefulness of ab initio or density functional theory (DFT) methods to predict the existence of elusive molecules and/or to characterize non-conventional structures, and to rationalize the charge redistributions normally associated with ion-molecule interactions and which result in bond-weakening or bond-reinforcement effects. Finally, the role of non-classical structures in ion-molecule interactions is also illustrated with different examples.

Computational chemistry: a useful (sometimes mandatory) tool in mass spectrometry studies.

We investigate the nature of the hydrogen bonding in the gas-phase halide−water clusters (X-···H2O), with special emphasis on how the hydrogen bonding affects the frequency of the hydrogen-bonded OH stretch. We present two models for describing the electronic structure of the hydrogen bond. The first model (non-charge-transfer, or non-CT) includes only electrostatic interactions between the halide ion and the water molecule. The second is a two-valence-bond (VB) state model in which the first VB state has the charge character X-···H2O and the second is a charge-transfer VB state with electronic structure XH···OH-. We find that the non-CT model is inadequate for describing the frequency shifts in the hydrogen-bonded OH stretch for the halide−water clusters as compared with both experimental and ab initio results. Further, analysis of the charge distributions of the clusters obtained from ab initio calculations indicates significant contribution of charge transfer in the electronic structure. This analysis ...

Frequency Shifts in the Hydrogen-Bonded OH Stretch in Halide−Water Clusters. The Importance of Charge Transfer

PET processes between ground and excited states of donor-acceptor pairs lead to the formation of radical ion pairs which upon disproportionation gives neutral radicals and/or ions required for synthetic purposes. A host of new reactions has been uncovered, the nature of which is governed by the chemical properties and reactivity profiles of these reactive intermediates. Our objective in this chapter will be to enlist critical examples from the last ten years dealing with the radical ion reactions of synthetic importance initiated by photoexcitation of donor-acceptor pairs.

J. C. Morrow

Papers

A theoretical study of the reaction of ketene radical cation with ethylene: nucleophilic addition or concerted [2 + 1] cycloaddition?

Unimolecular Chemistry of Methyl Formate Cation Radical. A Combined Theoretical and Experimental Study