Pancake π-stacking produces shorter contacts than van der Waals bonding but it has strongly preferred configurations. By high-level multireference average quadratic coupled cluster theory for the singlet and triplet, we identify the specific orbital component and the nonspecific vdW contributions in the prototypical pancake-bonded dimer of phenalenyl thereby explaining the configurational preferences.
The
concept of a double-bonded pancake bonding mechanism is introduced
to explain the extremely short π–π stacking contacts
in dimers of dithiatriazines. While ordinary single pancake bonds
occur between radicals and already display significantly shorter interatomic
distances in comparison to van der Waals (vdW) contacts, the double-bonded
pancake dimer is based on diradicaloid or antiaromatic molecules and
exhibits even shorter and stronger intermolecular bonds that breach
into the range of extremely stretched single bonds in terms of bond
distances and binding energies. These properties give rise to promising
possibilities in the design of new materials with high electrical
conductivity and for the field of spintronics. The analysis of the
double pancake bond is based on cutting edge electron correlation
theory combining multireference (nondynamical) effects and dispersion
(dynamical) contributions in a balanced way providing accurate interaction
energies and distributions of unpaired spins. It is also shown that
the present examples do not stand isolated but that similar mechanisms
operate in several analogous nonradical molecular systems to form
double-bonded π-stacking pancake dimers. We report on the amazing
properties of a new type of stacking interaction mechanism between
π conjugated molecules in the form of a “double pancake
bond” which breaks the record for short intermolecular distances
and provides formidable strength for some π–π stacking
interactions.
Unusually long bonds or short intermolecular contacts occur in the title compounds reminiscent of pancake bonding. Pancake bonding interactions seem analogous to π-stacking interactions, but they display much shorter contact distances than normally seen in van der Waals (vdW) dimers. The interpretation of these SN and SeN containing structures has been an outstanding challenge for some time. The antibonding (π*) singly occupied molecular orbital (SOMO) of the radical is the source of two-electron multicenter bonding (2e/mc). Preferred conformations thus can be traced back to SOMO-SOMO overlap. We used several computational methods to understand the nature of pancake bonding in the title compounds including four wave function methods (WFT) and a dozen density functional theories (DFT) including empirical dispersion corrections. We used experimental data and high level CCSD(T)/6-311++G(d,p) and MRPT2/6-311++G(d,p) calculations for comparison. The analysis provided the interpretation a wealth of experimental data including conformational preferences of these SN and SeN containing radical dimers leading to a better overall understanding of pancake bonding. Analysis of the various components of the inter-radical interactions showed that SOMO-SOMO bonding interaction and dispersion interaction contribute to the binding energy and neither of these interactions alone is sufficient to bind the dimer. The dimer is predicted to show weak diradical character.
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