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Probing ultrafast dynamics in photoexcited pyrrole: timescales for pi sigma* mediated H-atom elimination.

Authors: Gareth M, Roberts; Craig A, Williams; Hui, Yu; Adam S, Chatterley; Jamie D, Young; Susanne, Ullrich; Vasilios G, Stavros;

Probing ultrafast dynamics in photoexcited pyrrole: timescales for pi sigma* mediated H-atom elimination.

Abstract

The heteroaromatic ultraviolet chromophore pyrrole is found as a subunit in a number of important biomolecules: it is present in heme, the non-protein component of hemoglobin, and in the amino acid tryptophan. To date there have been several experimental studies, in both the time- and frequency-domains, which have interrogated the excited state dynamics of pyrrole. In this work, we specifically aim to unravel any differences in the H-atom elimination dynamics from pyrrole across an excitation wavelength range of 250-200 nm, which encompasses: (i) direct excitation to the (formally electric dipole forbidden) 1(1)pisigma* (1A2) state; and (ii) initial photoexcitation to the higher energy 1 pipi* (1B2) state. This is achieved by using a combination of ultrafast time-resolved ion yield and time-resolved velocity map ion imaging techniques in the gas phase. Following direct excitation to 1(1)pisigma* (1A2) at 250 nm, we observe a single time-constant of 126 +/- 28 fs for N-H bond fission. We assign this to tunnelling out of the quasi-bound 3s Rydberg component of the 1(1)pisigma* (1A2) surface in the vertical Franck-Condon region, followed by non-adiabatic coupling through a 1(1)pisigma*/S(0) conical intersection to yield pyrrolyl radicals in their electronic ground state (C4H4N(X)) together with H-atoms. At 238 nm, direct excitation to, and N-H dissociation along, the 1(1)pisigma* (1A2) surface is observed to occur with a time-constant of 46 +/- 22 fs. Upon initial population of the 1pipi* (1B2) state at 200 nm, a rapid 1pipi* (1B2) --> 1(1)pisigma* (1A2) --> N-H fission process takes place within 52 +/- 12 fs. In addition to ultrafast N-H bond cleavage at 200 nm, we also observe the onset of statistical unimolecular H-atom elimination from vibrationally hot S(0) ground state species, formed after the relaxation of excited electronic states, with a time-constant of 1.0 +/- 0.4 ns. Analogous measurements on pyrrole-d1 reveal that these statistical H-atoms are released only through C-H bond cleavage.

Related Organizations
Keywords

Pyrroles, Molecular Dynamics Simulation, Photochemical Processes, Hydrogen

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citations
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
77
Top 10%
Top 10%
Top 10%
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