The geometry of cyclo-octatetraenes differs fundamentally from the lower homologue benzene in exhibiting slow (nuclear) valence bond isomerism rather than rapid (electronic) bond-equalising resonance. In 1992 Anderson and Kirsch[cite]10.1039/P29920001951[/cite] exploited this property to describe a simple molecular balance for estimating how two alkyl substituents on the ring might interact via the (currently very topical) mechanism of dispersion (induced-dipole-induced-dipole) attractions. These electron correlation effects are exceptionally difficult to model using formal quantum mechanics and are nowadays normally replaced by more empirical functions such as Grimme's D3BJ correction.[cite]10.1002/jcc.21759[/cite] Here I explore aspects of how the small molecule below might be used to investigate the accuracy of such estimates of dispersion energies.
Posts Tagged ‘dispersion’
A molecular balance for dispersion energy?
Sunday, February 7th, 2016Modelling the geometry of unbranched alkanes.
Saturday, March 29th, 2014By about C17H36, the geometry of “cold-isolated” unbranched saturated alkenes is supposed not to contain any fully anti-periplanar conformations. [cite]10.1002/anie.201202894[/cite] Indeed, a (co-crystal) of C16H34 shows it to have two-gauche bends.[cite]10.1002/chem.200801428[/cite]. Surprisingly, the longest linear alkane I was able to find a crystal structure for, C28H58 appears to be fully extended[cite]10.1107/S0108768191011059[/cite],[cite]10.1107/S0567740876005025[/cite] (an early report of a low quality structure for C36H74[cite]10.1107/S0365110X5600111X[/cite] also appears to show it as linear).‡ Here I explore how standard DFT theories cope with these structures.