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 < lambda(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 1(1)A(2)(pisigma*) excited state of 2,5-DMP, but once lambda(phot) decreases to approximately 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 lambda(phot)> or=244 nm show a structured fast component, many of which are dominated by a peak with TKER approximately 5100 cm(-1); analysis of this structure reveals lambda(phot) dependent population of selected vibrational levels of 2,5-DMPyl, and enables determination of the N-H bond strength in 2,5-DMP: D(0) = 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 1(1)A(2) 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 1(1)A(2)-X(1)A(1) absorption appear to evolve adiabatically into the corresponding vibrations of the 2,5-DMPyl products. Methyl torsions can also promote the 1(1)A(2)<-- X(1)A(1) 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 approximately 250 nm, and PTS spectra recorded at lambda(phot)< or = 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 1(1)A(2) state.

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

“…The experimental setup for the H ͑Rydberg͒ atom PTS studies was identical to that described previously. 13,15 These studies required that three laser beams were spatially overlapped, with their intersection point defining the interaction volume through which the jet-cooled molecular beam propagates. The photolysis radiation was provided by a frequency doubled Nd-YAG ͑yttrium aluminum garnet͒ pumped dye laser ͑Spectra Physics GCR-270 plus PDL-2, together with a ␤-barium borate ͑BBO-B͒ frequency doubling crystal͒ or, in the case of phot = 193.3 nm, by an ArF excimer laser ͑Lambda-Physik, Optex͒, with typical UV pulse energies in the range of 200 J -2.5 mJ.…”

“…As in our previous studies of the UV photolysis of pyrroles 13,15,17 and phenol, 25 it is easier to review the product vibrational energy disposal, and its phot dependence, by replotting the TKER spectra on a common internal energy E int scale, as shown in Fig. 5.…”

“…Such agreement encourages confidence in the predicted wave numbers for the radical vibrations, which accord well with those obtained in another recent ab initio study. 19 Three of the 21 parent modes listed in Table I are largely associated with the motion of the N-H hydrogen atom ͑the N-H stretch v 1 and the in-plane and out-of-plane N-H wagging modes v 15 and v 21 ͒ and, as such, will disappear on N-H bond fission. The structure evident in the phot = 234 nm spectrum is assigned in Fig.…”

“…13,15,17,20 We report what appears to be the first published UV absorption spectrum of gas phase imidazole, together with time-of-flight ͑TOF͒ spectra of H atoms formed following the UV excitation of imidazole at 16 different wavelengths in the range of 210 nmഛ phot ഛ 240 nm and at phot = 193.3 nm, obtained using the H ͑Ry-dberg͒ atom photofragment translational spectroscopy ͑PTS͒ technique. As in the case of pyrrole, H atom photofragments are seen to be formed in two distinct velocity subgroups.…”

“…9,10 In the case of pyrrole, at least, this suggestion has now been substantiated by a number of experimental studies demonstrating that prompt N-H bond fission is the major decay channel following excitation to its 1 1 A 2 state. [11][12][13][14][15][16][17] Direct dissociation on this potential energy surface ͑PES͒ results in the formation of fast H atoms, together with pyrrolyl radical cofragments, which are formed in a very limited subset of the available vibrational state density. Such product state selectivity has been rationalized by assuming a high degree of vibrational adiabaticity in the fragmentation process; parent skeletal modes that contribute vibronic transition probability to the 1 1 A 2 ← X 1 A 1 absorption, but are orthogonal to the dissociation coordinate, tend to map into the corresponding product vibrations.…”