The Benzyl Radical-Ethylene Molecular Cluster: An Example of Electronic-State Mediation of Chemical Reactivity

Abstract

Three types of experimental data are presented for the benzyl radical/ethylene molecular cluster: massresolved excitation spectra ( M R S ) , ionization energy threshold determination, and excited-state lifetime measurements at different excitation energies. MRES of benzyl radical (C2H4)1,2 exhibit broad features in the D1 -Do benzyl radical absorption region that extend beyond 11 810 cm-1 of vibrational energy in D1. The ionization threshold for the one-to-one cluster is shifted by -1 160 cm-l relative to that of the bare benzyl radical. Cluster excited-state lifetime measurements indicate a shortened lifetime at higher excitation energy. This collection of benzyl radical/ethylene data differs greatly from that of the previously studied benzyl radical/ ethane cluster system. These latter results consist of well-resolved spectroscopic structure with low-energy van der Waals modes and molecular vibrations, a small shift in ionization energy relative to the bare benzyl radical of --50 cm-l, and vibrational predissociation at roughly 700 cm-1 of vibrational energy in DI. The anomalous benzyl radical/ethylene data can be explained if one postulates that an excited-state bimolecular addition chemical reaction occurs between benzyl radical and ethylene upon optical excitation of the benzyl radical (D2, DI -Do). Ab initio calculational results are presented which support the assertion of an apparent excited-state chemical reaction. Finally, unimolecular dissociation rate theories are used to extract an excited-state "binding energy" for the benzyl radical/ethylene "cluster" from excited-state lifetimes.

Solvation of the methoxy radical in small clusters

“…42 The same type of potential energy calculations have been carried out for the benzyl radical ͑CH 4 ͒ 1 cluster. 22 In this latter instance the agreement between calculated and experiment shifts are excellent; the calculated shift is Ϫ35 cm Ϫ1 and the observed shifts are between Ϫ29 cm Ϫ1 and Ϫ39 cm Ϫ1 , depending on the vibronic feature accessed. The most remarkable difference between the benzyl radical ͑CH 4 ͒ 1 and CH 3 O͑CH 4 ͒ 1 clusters is that in the first case a reaction is not anticipated while in the second case one is anticipated.…”

“…Otherwise, the cluster spectrum should be broad, as found for benzyl radical/ethylene system. 22 A comparison between the simulated and experimental rotational contours ͑Fig. 10͒ shows that the experimental spectrum is broader and less well resolved than the calculated one for isomer 2; nonetheless, the calculated isomer 2 is clearly the proper structure based on the shift and rotational contour calculations.…”

“…21,22 Briefly, the experiments have been carried out in a stainless steel vacuum chamber at a pressure of ϳ10 Ϫ4 Torr. To produce the clusters, CH 3 OH vapor is mixed with He carrier gas and the clustering agent.…”

Solvation of cyclopentadienyl and substituted cyclopentadienyl radicals in small clusters. I. Nonpolar solvents

“…[14][15][16] In the fluorescence excitation experiments, a general valve pulsed nozzle is used to generate an adiabatic expansion of the appropriate mixture of molecular precursor for Xcpd ͑Ͻ1% of the total pressure͒, the solvent gas ͑0.1%-10% of the total pressure͒, and 50-400 psi He expansion gas. The stainless steel vacuum chamber into which the mixture is expanded remains at ϳ10 Ϫ4 Torr during the FE experiment.…”

“…Of particular note for cluster studies of radicals are those involving OH-Ne, N 2 , H 2 , 9 NH, CH-rare gases, 10 CN-Ne, 11 C 2 H 3 O-Ar, 12 C 5 H 5 -He, Ne, N 2 , 13 C 5 H 4 CH 3 -He, Ne, 13 and CH 3 O, 14 NCO, 15 and C 6 H 5 CH 2 /Ar, N 2 , CH 4 , C 2 H 6 , and C 3 H 8 . 16 Reports involving vdW clusters of aromatic radicals are limited to the benzyl, cyclopentadienyl, and methylcyclopentadienyl radicals to the best of our knowledge.…”