Microsolvation and Excited-State Calculations of Nitrobenzene

For the microsolvation
calculations, nitrobenzene was optimized on the S₀ surface
using the CAM-B3LYP^{70 (link)}/Def2-TZVP^{71 (link)} method
employing empirical dispersion with Becke–Johnson damping (GD3BJ).^{72 (link)} The analysis of the vibrational frequencies
confirmed this geometry as a minimum. Molecules of each solvent system
were positioned by hand around the nitrobenzene molecule to maximize
interactions with the NO₂ group, and the resulting microsolvated
system was reoptimized at the CAM-B3LYP/Def2-SVP level of theory.
The analysis of the vibrational frequencies again confirmed the resulting
microsolvated systems as minima. These calculations were performed
using the Gaussian 16 program.⁷³ Following our earlier work,^{29 (link)} all excited-state
calculations on the various gas-phase nitrobenzene structures were
performed at the CASPT2 level, with the same CAS(14,11) active space
as before, which includes the π-orbitals on the ring and the
lone pairs on the nitro-oxygen atoms.^{29 (link)} The basis set was the atomic natural orbital (ANO) of S-type double-ζ
with polarization (ANO-S-VDZP).^{74 (link)} The geometries
were taken from our earlier work^{29 (link)} and
calculations were performed using the OpenMolcas program 2023.^{75 (link)} The CASSCF wave functions were state averaged
over 6 singlets and 5 triplets, except for the triplet excitations
from the T₁(n_Aπ*) and T₂(π_Oπ*) minima which included 7 triplet states. Solvation
effects were included using a water PCM, with the equilibrium charges
from the ground state used for nonequilibrium PCM calculations of
the excited states. Structure optimizations were all at the CASSCF
level.