Hybrid QM/QM Simulations
of Excited-State Intramolecular
Proton Transfer in the Molecular Crystal 7‑(2-Pyridyl)-indole
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Abstract
A subtractive implementation of the QM/QM hybrid method
for the
description of photochemical reactions occurring in molecular crystals
is presented and tested by applying it in a simulation study of the
ultrafast intramolecular excited-state proton transfer reaction in
the crystal form of 7-(2-pyridyl)-indole, an organic compound featuring
an intramolecular hydrogen bond within a six-membered ring. By propagating
molecular dynamics on the excited-state potential energy surface,
a mean proton transfer time was calculated as 80 fs. The reaction
mechanism is discussed in terms of three-dimensional reaction coordinate
diagrams. Proton transfer was found to be barrierless and to be strongly
coupled to vibrational modes of the photoexcited molecule that modulate
the proton donor–acceptor distance. Some 300 fs after the initial
photoexcitation, the excited state molecule reached an S<sub>1</sub>/S<sub>0</sub> conical intersection through the mutual twist of the
pyridyl and indolyl moieties