Hannover 2013 – scientific programme
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MO: Fachverband Molekülphysik
MO 21: Femtosecond Spectroscopy V
MO 21.1: Talk
Thursday, March 21, 2013, 11:00–11:15, F 102
Just arrived and already gone: Tracking the ultrafast reaction of carbocations with femtosecond spectroscopy — •Roland Wilcken1, Julia Benedikter1, Christian Sailer1, Herbert Mayr2, and Eberhard Riedle1 — 1LS für BioMolekulare Optik, LMU München — 2Department Chemie, LMU München
Biomolecular chemical reactions in solution are generally believed to be limited in their speed by the diffusive approach. We perform ultrafast measurements where a highly reactive carbocation is immersed in a solution of the nucleophilic reaction partner. In this way the diffusion limit can readily be surpassed. The carbocations are generated from selected benzhydryle chlorides by 30 fs UV pulses. The heterolytic bond cleavage takes 125 fs and is followed by a sp3 to sp2 hybridization within hundreds of fs. The signals of the intermediates are recorded by broadband femtosecond spectroscopy up to 2 ns with a 100 fs time resolution. In the reactive solvents the cation signal decreases rapidly on the sub-ps to 100 ps time scale. This is the direct signature of the chemical reaction.
We measure the reaction of carbocations with alcohols and amines to drive reaction times to the extreme. The reaction times decrease with the increasing reactivity of the carbocation and the nucleophile. In the alcohols times approximately down to the rotational diffusion time are found, still varying with the partners. For the amines the selectivity decreases and the times are well below the rotational time, as low as 250 fs. These are the fastest bimolecular reactions of reactive ground state species ever reported.