The Role of Aqueous Solvation on the Intersystem Crossing of Nitrophenols

The photochemistry of nitrophenols is a source of smog as nitrous acid is formed from their photolysis. Nevertheless, computational studies of the photochemistry of these widespread toxic molecules are scarce. In this work, the initial photodeactivation of ortho-nitrophenol and para-nitrophenol is modeled, both in gas phase and in aqueous solution to simulate atmospheric and aerosol environments. A large number of excited states, six for ortho-nitrophenol and 11 for para-nitrophenol, have been included and were all populated during the decay. Moreover, periodic time-dependent density functional theory (TDDFT) is used for both the explicitly included solvent and the solute. A comparison to periodic QM/MM (TDDFT/MM), with electrostatic embedding, is made, showing notable differences between the decays of solvated nitrophenols simulated with QM/MM and full (TD)DFT. A reduced intersystem crossing in aqueous solution could be observed thanks to the surface hopping approach using explicit, periodic TDDFT solvation including spin–orbit couplings.


Figure S1 :Figure S2 :
Figure S1: The TDDFT absorption spectrum of 2-NP in the gas phase (red), and in aqueous solution modelled with QM/MM (purple) and full TDDFT (blue) computed from the first 10 excited states.The dashed curves correspond to experimentally measured absorption spectra of 2-NP in the gas phase and the aqueous solution reproduced from ref. 1 and ref., 2 respectively.

FigureFigure S4 :Figure S5 :
Figure S3: (a) Evolution of ESIPT in the gas phase (red), QM/MM aqueous solution (purple) and full TDDFT (blue) 2-NP trajectories, whereby ESIPT was counted if the O 1 -H bond length was greater than the O H -H bond length.In the graph 0 indicates no proton transfer and 1 denotes only nitronic acid tautomers.(b) Example of a geometry at 250 fs after excitation during which ESIPT was observed.

Figure S9 :
Figure S9: Boxplots showing the bond lengths of the 4-NP gas phase (a), QM/MM (b), and full TDDFT solvation (c) trajectories.The outliers in the O-H bond lengths are caused by the formation of a para-nitrophenolate anion.An analogue narrowing of the distributions after solvation in water is observed for all angles and torsional angles.

Figure S13 :
Figure S13: Top: evolution of the average distance [Å] between the 4-NP hydroxyl oxygen and nitro oxygens in the gas phase (left) and full TDDFT aqueous solution (right).Bottom: evolution of the average ground state energy [eV] during the SH trajectories.All energy values are relative to the minimum energy value of each trajectory set.The coloured bands indicate the standard deviation, while the dotted lines show the minimum and maximum values.

Table S2 :
Comparison of the 2-NP TDDFT excitation energies (in eV) averaged over all initial conformations.

Table S5 :
Overview of the 4-NP occupations 250 fs after excitation.