Posts Tagged ‘dispersion’

A molecular balance for dispersion energy?

Sunday, February 7th, 2016

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[1] 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.[2] Here I explore aspects of how the small molecule below might be used to investigate the accuracy of such estimates of dispersion energies.

(more…)

References

  1. J.E. Anderson, and P.A. Kirsch, "Structural equilibria determined by attractive steric interactions. 1,6-Dialkylcyclooctatetraenes and their bond-shift and ring inversion investigated by dynamic NMR spectroscopy and molecular mechanics calculations", Journal of the Chemical Society, Perkin Transactions 2, pp. 1951, 1992. http://dx.doi.org/10.1039/P29920001951
  2. S. Grimme, S. Ehrlich, and L. Goerigk, "Effect of the damping function in dispersion corrected density functional theory", Journal of Computational Chemistry, vol. 32, pp. 1456-1465, 2011. http://dx.doi.org/10.1002/jcc.21759

Tags:, , , , , , ,
Posted in Uncategorised | No Comments »

Modelling the geometry of unbranched alkanes.

Saturday, March 29th, 2014

By about C17H36, the geometry of “cold-isolated” unbranched saturated alkenes is supposed not to contain any fully anti-periplanar conformations. [1] Indeed, a (co-crystal) of C16H34 shows it to have two-gauche bends.[2]. Surprisingly, the longest linear alkane I was able to find a crystal structure for, C28H58 appears to be fully extended[3],[4] (an early report of a low quality structure for C36H74[5] also appears to show it as linear).‡ Here I explore how standard DFT theories cope with these structures.

(more…)

References

  1. N.O.B. Lüttschwager, T.N. Wassermann, R.A. Mata, and M.A. Suhm, "The Last Globally Stable Extended Alkane", Angewandte Chemie International Edition, vol. 52, pp. 463-466, 2012. http://dx.doi.org/10.1002/anie.201202894
  2. N. Cocherel, C. Poriel, J. Rault‐Berthelot, F. Barrière, N. Audebrand, A. Slawin, and L. Vignau, "New 3π‐2Spiro Ladder‐Type Phenylene Materials: Synthesis, Physicochemical Properties and Applications in OLEDs", Chemistry – A European Journal, vol. 14, pp. 11328-11342, 2008. http://dx.doi.org/10.1002/chem.200801428
  3. S.C. Nyburg, and A.R. Gerson, "Crystallography of the even n-alkanes: structure of C20H42", Acta Crystallographica Section B Structural Science, vol. 48, pp. 103-106, 1992. http://dx.doi.org/10.1107/S0108768191011059
  4. R. Boistelle, B. Simon, and G. Pèpe, "Polytypic structures of n-C28H58 (octacosane) and n-C36H74 (hexatriacontane)", Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry, vol. 32, pp. 1240-1243, 1976. http://dx.doi.org/10.1107/S0567740876005025
  5. H.M.M. Shearer, and V. Vand, "The crystal structure of the monoclinic form of n-hexatriacontant", Acta Crystallographica, vol. 9, pp. 379-384, 1956. http://dx.doi.org/10.1107/S0365110X5600111X

Tags:, , , ,
Posted in Uncategorised | No Comments »