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High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of 2,5-dimethylpyrrole

Content Provider	Royal Society of Chemistry (RSC)
Author	Devine, Adam L. Nix, Michael G. D. Ashfold, Michael N. R. Cronin, Bríd Dixon, Richard N.
Copyright Year	2006
Abstract	The photodissociation dynamics of 2,5-dimethylpyrrole (2,5-DMP) has been investigated following excitation at 193.3 nm and at many near ultraviolet (UV) wavelengths in the range 244 < λphot < 282 nm using H Rydberg atom photofragment translational spectroscopy (PTS). Complementary UV absorption and, at the longest excitation wavelengths, one photon resonant multiphoton ionisation spectra of 2,5-DMP are reported also; analysis of the latter highlights the role of methyl torsional motions in promoting the parent absorption. The deduced fragmentation dynamics show parallels with that reported recently (B. Cronin, M. G. D. Nix, R. H. Qadiri and M. N. R. Ashfold, Phys. Chem. Chem. Phys., 2004, 6, 5031) for the bare pyrrole molecule. Excitation at the longer wavelengths leads to (vibronically induced) population of the 11A2(πσ*) excited state of 2,5-DMP, but once λphot decreases to ∼250 nm stronger, dipole allowed transitions start to become apparent in the parent absorption. All total kinetic energy release (TKER) spectra of the H + 2,5-dimethylpyrrolyl (2,5-DMPyl) fragments measured at λphot ≥244 nm show a structured fast component, many of which are dominated by a peak with TKER ∼ 5100 cm−1; analysis of this structure reveals λphot dependent population of selected vibrational levels of 2,5-DMPyl, and enables determination of the N–H bond strength in 2,5-DMP: D0 = 30 530 ± 100 cm−1. Two classes of behaviour are proposed to account for details of the observed energy partitioning. Both assume that N−H bond fission involves passage over (or tunnelling through) a small exit channel barrier on the 11A2 potential energy surface, but differ according to the vibrational energy content of the photo-prepared molecules. Specific parent out-of-plane skeletal modes that promote the 11A2–X1A1 absorption appear to evolve adiabatically into the corresponding vibrations of the 2,5-DMPyl products. Methyl torsions can also promote the 11A2 ← X1A1 absorption in 2,5-DMP, and provide a means of populating a much higher density of excited vibrational levels than in pyrrole. Such excited levels are deduced to dissociate by redistributing the minimum amount of internal energy necessary to overcome the exit channel barrier in the N–H dissociation coordinate. Coupling with the ground state surface via a conical intersection at extended N–H bond lengths is proposed as a further mechanism for modest translational → vibrational energy transfer within the separating products. The parent absorption cross-section increases considerably at wavelengths ∼250 nm, and PTS spectra recorded at λphot ≤ 254 nm display a second, unstructured, peak at lower TKER. As in pyrrole, this slower component is attributed to H atoms from the unimolecular decay of highly vibrationally excited ground state molecules formed via radiationless decay from photo-excited states lying above the 11A2 state.
Starting Page	599
Ending Page	612
Page Count	14
File Format	HTM / HTML PDF
ISSN	14639076
Volume Number	8
Issue Number	5
Journal	Physical Chemistry Chemical Physics
DOI	10.1039/b513949j
Language	English
Publisher	Royal Society of Chemistry
Access Restriction	Open
Subject Keyword	Spectroscopy Chemical bond Photodissociation Dissociative Ashfold Ultraviolet Photon Rydberg atom Internal energy Potential energy surface Conical intersection Qadiri Dublin Metropolitan Police Pyrrole Qadiriyya Dipole Cronin Nix Kinetic energy Nuclear fission
Content Type	Text
Resource Type	Article
Subject	Physics and Astronomy Physical and Theoretical Chemistry

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