Molecular designs for expanding the limits of ultralong C-C bonds and ultrashort HH non-bonded contacts.

Recent experiments have reported the formation of very long C-C bonds (d > 1.80 Å) and very short HH non-bonded contacts (d < 1.5 Å) in several sets of molecules. Both these rare phenomena arise due to specific donor-acceptor interactions and London dispersion interactions respectively. Favorable negative hyperconjugation, namely HN(lone-pair) →σ*(C-C), creates an ultralong C-C bond in diamino-o-carborane with d > 1.829 Å and a planar amine reminiscent of a transition-state like structure for ammonia inversion. The small and narrow barrier favours rapid inversion through quantum mechanical tunnelling (QMT) and produces a translationally averaged planar amine as observed in the experiments. On the other hand, designing specific confined molecular cavities or chambers like in,in-bis(hydrosilane) or its germanane analogs furnishes an ultrashort HH distance = 1.47 Å and 1.38 Å respectively. The predisposition of such closely placed HH contacts arises from the rather effective attractive dispersion interactions between them. Controlling the strength of the dispersion interactions provides a rich landscape for realizing such close HH distances. Molecular design ably assisted by computational modeling to further tune these interactions provides new avenues to break the glass-ceilings of ultralong C-C bonds or ultrashort HH contacts. Dispersion-corrected DFT calculations and ab initio molecular dynamics simulations generate a large library of such unique features in a diverse class of molecules. This feature article highlights the design principles to realize hitherto longest C-C bonds/shortest HH contacts.