Abstract
To explore the effects of ring substitution on dissociation dynamics, the primary photochemistry of 2-ethylpyrrole was explored using ultrafast ion imaging techniques. Photoexcitation to the S1 state, a πσ* state, in the range from 238 to 265 nm results in cleavage of the N-H bond with an H atom appearance lifetime of ca. 70 fs. The insensitivity of this lifetime to photon energy, combined with a small kinetic isotope effect, suggests that tunneling does not play a major role in N-H bond cleavage. Total kinetic energy release spectra reveal modest vibrational excitation in the radical counter-fragment, increasing with photon energy. At wavelengths less than or equal to 248 nm, an additional low kinetic energy H atom loss mechanism becomes available with an appearance lifetime of ∼1.5 ps, possibly due to the population of higher-lying 1ππ* states.
Highlights
The photochemistry and photophysics of biopolymers is of fundamental interest to chemists and biologists seeking to understand the photostability of life’s essential molecules
After photoexcitation to the S1 state below the barrier, the H atom appearance lifetime is ∼130 fs; the same excitation in deuterated pyrrole yields a kinetic isotope effect (KIE) of 11.22 When the photon energy places excited-state population above the barrier, the H atom appearance lifetime decreases to ∼50 fs with a KIE of 3.22 Both theory and experiment have concluded that this dramatic decrease in both H atom appearance lifetime and KIE demonstrate the importance of tunneling dynamics during pyrrole photodissociation.[22,26]
For pump wavelengths longer than 245 nm, the entire total kinetic energy release (TKER) spectra show an average H atom appearance lifetime of ca. 70 fs at 5450 cm−1, ∼800 cm−1 less than the peak TKER value in the corresponding H atom spectrum and in agreement with the trends observed from the H Rydberg atom photofragment translational spectroscopy (HRA-PTS) study of d5-pyrrole.[42]
Summary
The photochemistry and photophysics of biopolymers is of fundamental interest to chemists and biologists seeking to understand the photostability of life’s essential molecules. As the basic subunit of diverse biomolecules such as tryptophan or heme,[11,12] the photodissociation of the aromatic heterocycle pyrrole (c-C4H5N) from the S1 (1πσ*) state has been extensively studied with a majority of studies focusing on the rapid N−H bond fission.[13−16] Utilizing a variety of timeresolved techniques, several groups have investigated the ultrafast excited-state dynamics via time-resolved ion yield (TR-IY) of the pyrrole cation,[17−19] photoelectron spectroscopy,[20,21] or H atom elimination from pyrrole[17,22] with some of the most recent work exploring the excited-state dynamics of pyrrole dimers.[23,24] The electronic structure and dissociation dynamics have been extensively modeled theoretically with the most recent work focusing on the choice of initial conditions[25] and the role of tunneling[26−28] during H atom loss From these efforts, it is clear that the S1 state of pyrrole has an ∼0.2 eV barrier along the N−H stretch coordinate, followed by a repulsive potential leading to the production of photoproducts (predominantly) in the electronic ground state. After photoexcitation to the S1 state below the barrier, the H atom appearance lifetime is ∼130 fs; the same excitation in deuterated pyrrole yields a kinetic isotope effect (KIE) of 11.22 When the photon energy places excited-state population above the barrier, the H atom appearance lifetime decreases to ∼50 fs with a KIE of 3.22 Both theory and experiment have concluded that this dramatic decrease in both H atom appearance lifetime and KIE demonstrate the importance of tunneling dynamics during pyrrole photodissociation.[22,26]
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