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

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 < λ_phot < 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 λ_phot 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 λ_phot ≥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 λ_phot 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 λ_phot ≤ 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.