The
influence of vibrational motion on electronic excited state
properties is investigated for the organic chromophore 2-nitronaphtalene
in methanol. Specifically, the performance of two vibrational sampling
techniques – Wigner sampling and sampling from an ab initio
molecular dynamics trajectory– is assessed, in combination
with implicit and explicit solvent models. The effects of the different
sampling/solvent combinations on the energy and electronic character
of the absorption bands are analyzed in terms of charge transfer and
exciton size, computed from the electronic transition density. The
absorption spectra obtained using sampling techniques and its underlying
properties are compared to those of the electronic excited states
calculated at the Franck–Condon equilibrium geometry. It is
found that the absorption bands of the vibrational ensembles are red-shifted
compared to the Franck–Condon bright states, and this red-shift
scales with the displacement from the equilibrium geometry. Such displacements
are found larger and better described when using ensembles from the
harmonic Wigner distribution than snapshots from the molecular dynamics
trajectory. Particularly relevant is the torsional motion of the nitro
group that quenches the charge transfer character of some of the absorption
bands. This motion, however, is better described in the molecular
dynamics trajectory. Thus, none of the vibrational sampling approaches
can satisfactorily capture all important aspects of the nuclear motion.
The inclusion of solvent also red-shifts the absorption bands with
respect to the gas phase. This red-shift scales with the charge-transfer
character of the bands and is found larger for the implicit than for
the explicit solvent model. The advantages and drawbacks of the different
sampling and solvent models are discussed to guide future research
on the calculation of UV–vis spectra of nitroaromatic compounds
