High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of pyrrole

Abstract

The fragmentation dynamics of pyrrole molecules following excitation at many wavelengths in the range 193.3 < λ_phot < 254.0 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Excitation at the longer wavelengths within this range results in (vibronically induced) population of the 1¹A₂(πσ*) excited state, but once λ_phot ≤ 225 nm the electric dipole allowed 1¹B₂ ← X¹A₁(π* ← π) transition becomes the dominant absorption. All of the total kinetic energy release (TKER) spectra so derived show a ‘fast’ peak, centred at TKER ∼7000 cm⁻¹. Analysis of the structure evident in this peak, particularly in spectra recorded at the longer excitation wavelengths, reveals selective population of specific vibrational levels of the pyrrolyl co-fragment. These have been assigned by comparison with calculated normal mode vibrational frequencies, leading to a precise determination of the N–H bond strength in pyrrole: D₀ = 32850 ± 40 cm⁻¹, and the enthalpy of formation of the pyrrolyl radical: Δ_fH₀°(C₄H₄N) = 301.9 ± 0.5 kJ mol⁻¹. The recoil anisotropy of the fast H atom photofragments formed following excitation to, and dissociation on, the 1¹A₂(πσ*) potential energy surface (PES) is seen to depend upon the vibrational level of the pyrrolyl co-fragment. This observation, and the finding that the mean TKER associated with these fast H + pyrrolyl fragments is essentially independent of λ_phot, can be explained by assuming that, upon N–H bond fission, the skeletal vibrational motions in pyrrole(1¹A₂) molecules evolve adiabatically into the corresponding modes of the ground state pyrrolyl fragment. A second, ‘slow’ peak is increasingly evident in TKER spectra recorded at shorter photolysis wavelengths, and becomes the dominant feature once λ_phot ≤ 218 nm. This component exhibits no recoil anisotropy; its TKER profile is reminiscent of that observed in many other dissociations that yield H atoms by ‘statistical’ decay of highly vibrationally excited ground state molecules. The form of the TKER spectra observed at these shorter excitation wavelengths is rationalised by assuming two possible decay routes for pyrrole molecules excited to the ¹B₂(ππ*) state. One involves fast 1¹B₂ 1¹A₂ radiationless transfer and subsequent fragmentation on the 1¹A₂ PES, yielding ‘fast’ H atoms (and pyrrolyl co-fragments)–reminiscent of behaviour seen at longer excitation wavelengths where the 1¹A₂ PES is accessed directly. The second is assumed to involve radiationless transfer to the ground state, either by successive 1¹B₂ 1¹A₂ X¹A₁ couplings mediated by conical intersections between the relevant PESs or, possibly, by an as yet unrecognised direct 1¹B₂ X¹A₁ coupling, and subsequent unimolecular decay of the resulting highly vibrationally excited ground state molecules yielding ‘slow’ H atoms (together with, most probably, cyanoallyl co-fragments).