  The relaxation time of several second generation molecular motors is analysed
by calculating the minimum energy path between the metastable and stable states
and evaluating the transition rate within harmonic transition state theory
based on energetics obtained from density functional theory. Comparison with
published experimental data shows remarkably good agreement and demonstrates
the predictive capability of the theoretical approach. While previous
measurements by Feringa and coworkers [Chem.\,Eur.\,J.\,(2017) 23, 6643] have
shown that a replacement of the stereogenic hydrogen by a fluorine atom
increases the relaxation time because of destabilization of the transition
state for the thermal helix inversion, we find that a replacement of CH$_3$ by
a CF$_3$ group at the same site shortens the relaxation time because of
elevated energy of the metastable state without a significant shift in the
transition state energy. Since these two fluorine substitutions have an
opposite effect on the relaxation time, the two combined can provide a way to
fine tune the rotational speed of a molecular motor.
