C 4 H 4 radical cation isomers: Generation, structure, stability, and isomerization reactions

Unimolecular isomerizations of C6H6•+ radical cations: a computational study

CONCEPT

Eighteen concerted isomerization reactions of various C6H6•+ radical cation (RC) species are studied and found to proceed via well-defined transition states, whose relative positions along the reaction pathway generally agree with Hammond's postulate. From the barrier heights, the rate coefficients of these reactions are estimated by using transition state theory, and the activation energies are computed. Through combination among themselves, these 18 isomerizations yielded 15 multi-step conversion routes of various C6H6•+ species to the lowest energy benzene radical cation isomer 1, which routes are compared.

METHODS

Use is made of DFT with the B3LYP and M06-2X functionals, along with the CBS-QB3 approach to arrive at better energies. From the barrier heights for each of the concerted reactions, canonical transition state theory was applied to evaluate rate coefficients k over the temperature range 200-500 K. The Arrhenius activation energies were computed using the plot of ln k vs. 1/T.

Fingerprinting fragments of fragile interstellar molecules: dissociation chemistry of pyridine and benzonitrile revealed by infrared spectroscopy and theory

The cationic fragmentation products in the dissociative ionization of pyridine and benzonitrile have been studied by infrared action spectroscopy in a cryogenic ion trap instrument at the Free-Electron Lasers for Infrared eXperiments (FELIX) Laboratory. A comparison of the experimental vibrational fingerprints of the dominant cationic fragments with those from quantum chemical calculations revealed a diversity of molecular fragment structures. The loss of HCN/HNC is shown to be the major fragmentation channel for both pyridine and benzonitrile. Using the determined structures of the cationic fragments, potential energy surfaces have been calculated to elucidate the nature of the neutral fragment partner. In the fragmentation chemistry of pyridine, multiple non-cyclic structures are formed, whereas the fragmentation of benzonitrile dominantly leads to the formation of cyclic structures. Among the fragments are linear cyano-(di)acetylene˙+, methylene-cyclopropene˙+ and o- and m-benzyne˙+ structures, the latter possible building blocks in interstellar polycyclic aromatic hydrocarbon (PAH) formation chemistry. Molecular dynamics simulations using density functional based tight binding (MD/DFTB) were performed and used to benchmark and elucidate the different fragmentation pathways based on the experimentally determined structures. The implications of the difference in fragments observed for pyridine and benzonitrile are discussed in an astrochemical context.

Generation, structures, relative energies, and isomerization reactions of C5H5+ cations

The B3LYP, MP2, and CBS-QB3 quantum chemical methods are used to study the relative energy and isomerization reactions of C₅H₅⁺ cations. Ease of generation of 14 C₅H₅⁺ isomers by ionic dissociation of halide precursors does not correlate well with carbocation stability. The reaction profiles of concerted isomerization of various C₅H₅⁺ cations to six select cations are established along with the respective transition states. The rate coefficients of these processes are estimated by using transition state theory and activation energies computed. The transition states for these six reactions are characterized with regard to position along the isomerization pathway as per Hammond's postulate. The 6 isomerization reactions are combined to yield multi-step conversions of various C₅H₅⁺ species to the lowest energy vinylcyclopropenyl cation 1. Finally, three different routes for obtaining the select cations from C₅H₅Br precursors are profiled and the most favored pathways predicted.