Calculation of Free-Energy Barriers with TD-DFT: A Case Study on Excited-State Proton Transfer in Indigo

• The performance of time-dependent density functional theory (TD-DFT) for the calculation of excited states of molecular systems has been the subject of many benchmark studies. Often, these studies focus on excitation energies or, more recently, excited-state equilibrium geometries. In this work, we take a different angle by instead exploring how well TD-DFT reproduces experimental free-energy barriers of a well-known photochemical reaction: the excited-state proton transfer (ESPT) in indigo. Specifically, by exploiting the possibility of using TD-DFT to locate and compute free energies of first-order saddle points in excited states, we test the performance of several popular density functionals in reproducing recently determined experimental free-energy barriers for ESPT in indigo and in an N-hexyl substituted derivative thereof. Through the calculations, it is found that all of the tested functionals perform quite well, uniformly overestimating the experimental values by 1.4–3.5 (mean error) and 2.5–5.5 kcal mol–1 (maximum error) only. Given that these errors are not larger than those typically observed when barriers for ground-state proton transfer reactions are calculated in ground-state DFT, the results highlight the potential of TD-DFT to enable accurate modeling of ESPT reactions based on free energies and explicit localization of transition states.

Ämnesord

NATURVETENSKAP  -- Kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences (hsv//eng)

Nyckelord

Excited states

Time-dependent density functional theory

Proton transfer

Free-energy barriers

Indigo

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