An Ab Initio Computational Study of Electronic and Structural Factors in the Isomerization of Donor–Acceptor Stenhouse Adducts

In
this work, the photochemically and thermally induced isomerization
of multiple donor–acceptor Stenhouse adducts (DASAs) of the
first, second, and third generation is studied by means of state-of-the-art
ab initio electronic structure methods leading to new insight into
multiple facets of the reaction mechanism. Importantly, prior to any
studies of the reaction mechanism, a set of test calculations demonstrate
the suitability of the applied ADC(2) and CC2 methods in the present
context. An important aspect in this regard is the availability of
electronic energies and gradients under implicit consideration of
solvent effects. On the basis of calculated reaction energies and
barriers as well as a thorough analysis of the wave function compositions,
interesting features of the reaction mechanism are deduced. For example,
the closed form of second- and third-generation DASAs can be significantly
stabilized by π – π interactions between the donor
and acceptor termini when certain structural requirements are fulfilled.
The central point of this work concerns the delicate balance between
neutral and zwitterionic resonance structures that governs the relative
barrier height for the crucial C2–C3 and
C3–C4 bond rotations. Finally, a set
of calculations on yet unreported derivatives highlights how this
balance and hence the barrier heights can be tuned through variation
of the donor–acceptor strength as well as the solvent polarity.