Posts Tagged ‘dispersion’
Sunday, February 7th, 2016
The geometry of cyclooctatetraenes differs fundamentally from the lower homologue benzene in exhibiting slow (nuclear) valence bond isomerism rather than rapid (electronic) bondequalising 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 (induceddipoleinduceddipole) 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.
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References

J.E. Anderson, and P.A. Kirsch, "Structural equilibria determined by attractive steric interactions. 1,6Dialkylcyclooctatetraenes and their bondshift 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

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. 14561465, 2011. http://dx.doi.org/10.1002/jcc.21759
Tags:dispersion, energy, Entropy, lowest energy, lowest energy pose, Physical organic chemistry, Potential theory
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Saturday, March 29th, 2014
By about C_{17}H_{36}, the geometry of “coldisolated” unbranched saturated alkenes is supposed not to contain any fully antiperiplanar conformations. [1] Indeed, a (cocrystal) of C_{16}H_{34} shows it to have twogauche bends.[2]. Surprisingly, the longest linear alkane I was able to find a crystal structure for, C_{28}H_{58} appears to be fully extended[3],[4] (an early report of a low quality structure for C_{36}H_{74}[5] also appears to show it as linear).^{‡} Here I explore how standard DFT theories cope with these structures.
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References

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. 463466, 2012. http://dx.doi.org/10.1002/anie.201202894

N. Cocherel, C. Poriel, J. RaultBerthelot, F. Barrière, N. Audebrand, A. Slawin, and L. Vignau, "New 3π2Spiro LadderType Phenylene Materials: Synthesis, Physicochemical Properties and Applications in OLEDs", Chemistry  A European Journal, vol. 14, pp. 1132811342, 2008. http://dx.doi.org/10.1002/chem.200801428

S.C. Nyburg, and A.R. Gerson, "Crystallography of the even nalkanes: structure of C_{20}H_{42}", Acta Crystallographica Section B Structural Science, vol. 48, pp. 103106, 1992. http://dx.doi.org/10.1107/S0108768191011059

R. Boistelle, B. Simon, and G. Pèpe, "Polytypic structures of nC28H58 (octacosane) and nC36H74 (hexatriacontane)", Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry, vol. 32, pp. 12401243, 1976. http://dx.doi.org/10.1107/S0567740876005025

H.M.M. Shearer, and V. Vand, "The crystal structure of the monoclinic form of nhexatriacontant", Acta Crystallographica, vol. 9, pp. 379384, 1956. http://dx.doi.org/10.1107/S0365110X5600111X
Tags:dispersion, energy, relative energy, relative free energy
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