Exploring Ultrafast H-Atom Elimination versus Photofragmentation Pathways in Pyrazole Following 200 nm Excitation

Abstract

The role of ultraviolet photoresistance in many biomolecules (e.g., DNA bases and amino acids) has been extensively researched in recent years. This behavior has largely been attributed to the participation of dissociative (1)πσ* states localized along X-H (X ═ N, O) bonds, which facilitate an efficient means for rapid nonradiative relaxation back to the electronic ground state via conical intersections or ultrafast H-atom elimination. One such species known to exhibit this characteristic photochemistry is the UV chromophore imidazole, a subunit in the biomolecules adenine and histidine. However, the (1)πσ* driven photochemistry of its structural isomer pyrazole has received much less attention, both experimentally and theoretically. Here, we probe the ultrafast excited state dynamics occurring in pyrazole following photoexcitation at 200 nm (6.2 eV) using two experimental methodologies. The first uses time-resolved velocity map ion imaging to investigate the ultrafast H-atom elimination dynamics following direct excitation to the lowest energy (1)πσ* state (1(1)A" ← X(1)A'). These results yield a bimodal distribution of eliminated H-atoms, situated at low and high kinetic energies, the latter of which we attribute to (1)πσ* mediated N-H fission. The time constants extracted for the low and high energy features are ~120 and <50 fs, respectively. We also investigate the role of ring deformation relaxation pathways from the first optically bright (1)ππ* state (2(1)A' ← X(1)A'), by performing time-resolved ion yield measurements. These results are modeled using a (1)ππ* → ring deformation → photofragmentation mechanism (a model based on comparison with theoretical calculations on the structural isomer imidazole) and all photofragments possess appearance time constants of <160 fs. A comparison between time-resolved velocity map ion imaging and time-resolved ion yield measurements suggest that (1)πσ* driven N-H fission gives rise to the dominant kinetic photoproducts, re-enforcing the important role (1)πσ* states have in the excited state dynamics of biological chromophores and related aromatic heterocycles.

Exploring quantum phenomena and vibrational control in σ* mediated photochemistry

“…The above interpretation for 1 ns* mediated S-CH 3 bond scission from 1 pp*(n ¼ 0) also helps to explain the observed dominance of adiabatic dissociation dynamics into channel 2 around CI2, the calculated geometric minimum for which involves f ¼ 0 at R S-CH 3 z 3.4Å, as presented in Fig. 3 21 This limits the possibility for further torsional rearrangement within the dissociating molecule. Further, the shallow topography of the 1 ns*/S 0 CI (see ESI †) means that dissociating ux is unlikely to experience any appreciable force from the adiabatic PES to access the CI2 geometry, effectively avoiding the point of 1 ns*/S 0 degeneracy.…”

“…Both the ns VMI and fs TR-VMI apparatus have been described previously 21,22 and further details are provided in the electronic supplementary information (ESI †) online. Briey, thioanisole ns* singlet electronic PESs of thioanisole (structure inset), shown with respect to the S-CH 3 bond coordinate (R S-CH 3 ) and the torsional angle (f).…”

Ultrafast photodissociation dynamics of pyrazole, imidazole and their deuterated derivatives using ab initio multiple cloning

“…As with imidazole however, this shift cannot be easily seen in the experimental spectra due to "above origin" excitation in the experimental data. Low energy structure in the calculated spectra again does not seem to be as prevalent as is seen in the experimental spectra due to the inability of the AIMC simulations to account for multiphoton processes, which are suggested as the source of the low energy H/D atoms by Williams et al 16 . This is increasingly likely as, like imidazole and imidazole-d 1 , the vast majority of trajectory-guided basis functions undergo dissociation with 88% of pyrazole basis functions dissociating along with 86% of pyrazole-d 1 basis functions, and as such a longer simulation is unlikely to result in increased intensity of the low energy peak.…”

“…Due to this prevalence of imidazole, in recent years there has been significant interest in investigating its ultrafast excited state dynamics following photo-absorption both experimentally [1][2][3][4][5][6][7][8][9][10] and theoretically. 8,[11][12][13][14] Pyrazole, a structural isomer of imidazole rarely found in biomolecules, has been less widely studied both experimentally 3,15,16 and theoretically 17 and thus is, along with imidazole, the subject of a comparative experimental and nonadiabatic quantum dynamics study in the present work. The effect of selective deuteration at the N-H bond has also been studied experimentally, 3,15 but not theoretically, so we also model this for both molecules.…”

“…The gap between the 1 2 A 2 and 1 2 B 1 levels is small in pyrazolyl however, meaning the photoproducts are formed in a mixture of states. 15 Investigations of pyrazole were carried out by Williams et al, 16 who used TR-VMI to investigate N-H dissociation and find H atom appearance lifetimes for pyrazole following λ = 200 nm excitation, finding that the high kinetic energy H atoms had an appearance lifetime of < 50 fs, the low kinetic energy H atoms due to multiphoton processes had an appearance lifetime of 112 ± 34 fs. In the same article, they also considered relaxation through ring deformation and photofragmentation using TR-IY measurements analysed using a model based on calculations of imidazole, finding that only 4-13% of trajectories relax through ring-puckering or ring-opening and concluding that ring deformation was only a minor relaxation pathway in pyrazole, with N-H bond rupture along the 1 πσ * state being the major pathway.…”