Time resolved velocity map imaging of H-atom elimination from photoexcited imidazole and its methyl substituted derivatives

Abstract

The photoresistive properties of DNA bases, amino acids and corresponding subunits have received considerable attention through spectroscopic studies in recent years. One photoresistive property implicates the participation of (1)πσ* states, allowing electronically excited states to evolve either back to the electronic ground state or undergo direct dissociation along a heteroatom-hydride (X-H) coordinate. To this effect, time-resolved velocity map imaging (TR-VMI) studies of imidazole (a subunit of both adenine and histidine) and methylated derivatives thereof have been undertaken, with the goal of understanding the effects of increasing molecular complexity, through methylation, on the dynamics following photoexcitation at 200 nm. The results of these measurements clearly show that H-atom elimination along the N-H coordinate results in a bimodal distribution in the total kinetic energy release (TKER) spectra in both imidazole and it's methylated derivatives: 2-methyl, 4-methyl and 2,4-dimethylimidazole. The associated time constants for H-atoms eliminated with both high and low kinetic energies are all less than 500 fs. A noticeable increase in the time constants for the methylated derivatives is also observed. This could be attributed to either: ring methylation hindering in-plane and out-of-plane ring distortions which have been implicated as mediating excited state dynamics of these molecules or; an increase in the density of vibrational states at 200 nm causing an increased sampling of orthogonal modes, as opposed to modes which drive any dynamics that cause subsequent H-atom elimination. The results of these findings once again serve to illustrate the seemingly ubiquitous nature of (1)πσ* states in the photoexcited state dynamics of biomolecules and their subunits.

Ultrafast Dynamics of UV‐Excited Imidazole

“…[26,39] For excitation at 200 nm (window B), the time constants associated with the fast and slow hydrogen atoms are 78 AE 37 and 163 AE 50 fs, respectively. [26] The fast hydrogen release was related with direct dissociation along the 1 ps * NH state, while the slow hydrogen release was associated with dissociation along the first excited state of the imidazolyl radical.…”

“…[26,39] For excitation at 200 nm (window B), the time constants associated with the fast and slow hydrogen atoms are 78 AE 37 and 163 AE 50 fs, respectively. [26] The fast hydrogen release was related with direct dissociation along the 1 ps * NH state, while the slow hydrogen release was associated with dissociation along the first excited state of the imidazolyl radical. Our dynamics simulations did not follow the dissociation process taking place after the conical intersection region, but, in principle, imidazole can follow both dissociation pathways, as well as return to the vibrationally hot ground state (from where the dissociation could still occur but on a much longer timescale).…”

“…[18,19] As with the closely related heterocycles pyrrole (C 4 H 5 N) [20][21][22][23][24] and furan (C 4 H 4 O), [25] UV-excited imidazole is known to return to the ground state by internal conversion. [26] Four conical intersections between the first excited and ground states have been previously described for imidazole. [27] In structural terms, they are characterized by NH dissociation, CN dissociation, N puckering, and NH puckering.…”

Recent advances in experimental techniques to probe fast excited-state dynamics in biological molecules in the gas phase: dynamics in nucleotides, amino acids and beyond

“…By combining the time, energy and angular recoil information, one is able to glean detailed insights into the excited-state dynamics that precede photodissociation. TR-VMI has been successfully used to study excited-state dynamics in adenine [17], phenol [19,45], indole [46], pyrrole [36] and imidazole [47]. …”

Structural dynamics of 4‐formaldehyde imidazole and imidazole in light absorbing S2(ππ*) state

“…Direct excitation to the 1 πσ * state ( 1 A″ symmetry) leads to the formation of H atoms and the ground state imidazolyl radicals with vibrational excitation exclusively in ring framework stretch modes (with a 1 symmetry in the C 2 v point group of the radical). Photoexcitation to the lowest 1 ππ * state results in the ultrafast internal conversion to return to the ground state . Four conical intersections (CIs) and the corresponding reaction paths initiated from the FC region to CIs were respectively determined by nonadiabatic dynamics simulations: (1) NH‐stretching ( 1 πσ * NH /S 0 ), (2) CN‐stretching ( 1 πσ * CN /S 0 ), (3) NH‐puckering, and (4) N‐puckering .…”