Ultrafast Dynamics of UV‐Excited Imidazole

Abstract

The ultrafast dynamics of UV-excited imidazole in the gas phase is investigated by theoretical nonadiabatic dynamics simulations and experimental time-resolved photoelectron spectroscopy. The results show that different electronic excited-state relaxation mechanisms occur, depending on the pump wavelength. When imidazole is excited at 239.6 nm, deactivation through the NH-dissociation conical intersection is observed on the sub-50 fs timescale. After 200.8 nm excitation, competition between NH-dissociation and NH-puckering conical intersections is observed. The NH-dissociation to NH-puckering branching ratio is predicted to be 21:4, and the total relaxation time is elongated by a factor of eight. A procedure for simulation of photoelectron spectra based on dynamics results is developed and employed to assign different features in the experimental spectra.

Sigma*-mediated electronic relaxation in 200nm photoexcited ammonia and heteroaromatics

“…[2] From ab initio non-adiabatic dynamics simulations [3] it is known that 83% of the excited state flux proceeds along a non-radiative 1 *→ 1 * pathway which is the focus of the present study. The TRPES spectrum of imidazole is displayed in Figure 3 together with a time-integrated photoelectron spectrum and energy-integrated timetraces.…”

“…Timetraces of the IP1 and IP2 region provide 50 and 74fs decay times, respectively, which are interpreted as evolution of the excited state wavepacket out of the ionization window. [3] Appearance times of H-atom photoproducts from NH-dissociation provide a better measure of these dynamics. A TRPTS spectrum is shown in Figure 4, but not enough information is know about the energetics of the imidazolyl co-fragment to allow conversion from an H-atom kinetic energy to an imidazolyl internal energy axis.…”

“…Alternative sources for slow H-atoms discussed in the literature include multiphoton dissociative ionization and statistical decay of vibrationally hot imidazole following relaxation back to its ground state. [3] The latter is excluded based on the timescale measured here. …”

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

“…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 . Dynamics calculations show that only the first three channels are active upon populated in the 1 ππ * state in the 6.0–6.2 eV spectral region, whereby 83%, 16%, and 1% of excited state trajectories will relax, respectively, along the NH‐stretching, NH‐puckering, and CN‐stretching coordinates .…”

Photochemical Steps in the Prebiotic Synthesis of Purine Precursors from HCN

“…AICN reaction is its robustness. [7] on imidazole photostability) and resistant to hydrolysis (lifetime: 2000 years at pH 8 [8] ). [6] The imidazole derivative 2 is photostable (5 % reduction in absorbance after irradiation at 254 nm for 3 h; [4d] see also Ref.…”