Posts Tagged ‘chemical shifts’

Dispersion “bonds”: a new example with an ultra-short H…H distance.

Monday, June 26th, 2017
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About 18 months ago, there was much discussion on this blog about a system reported by Bob Pascal and co-workers containing a short H…H contact of ~1.5Å[1]. In this system, the hydrogens were both attached to Si as Si-H…H-Si and compressed together by rings. Now a new report[2] and commented upon by Steve Bachrach, claims a similar distance for hydrogens attached to carbon, i.e. C-H…H-C, but without the ring compression.

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References

  1. J. Zong, J.T. Mague, and R.A. Pascal, "Exceptional Steric Congestion in anin,in-Bis(hydrosilane)", Journal of the American Chemical Society, vol. 135, pp. 13235-13237, 2013. http://dx.doi.org/10.1021/ja407398w
  2. S. Rösel, H. Quanz, C. Logemann, J. Becker, E. Mossou, L. Cañadillas-Delgado, E. Caldeweyher, S. Grimme, and P.R. Schreiner, "London Dispersion Enables the Shortest Intermolecular Hydrocarbon H···H Contact", Journal of the American Chemical Society, vol. 139, pp. 7428-7431, 2017. http://dx.doi.org/10.1021/jacs.7b01879

Molecule of the year? “CrN123”, a molecule with three different types of Cr-N bond.

Friday, December 16th, 2016
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Here is a third candidate for the C&EN “molecule of the year” vote. This one was shortlisted because it is the first example of a metal-nitrogen complex exhibiting single, double and triple bonds from different nitrogens to the same metal[1] (XUZLUB has a 3D display available at DOI: 10.5517/CC1JYY6M). Since no calculation of its molecular properties was reported, I annotate some here.

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References

  1. E.P. Beaumier, B.S. Billow, A.K. Singh, S.M. Biros, and A.L. Odom, "A complex with nitrogen single, double, and triple bonds to the same chromium atom: synthesis, structure, and reactivity", Chem. Sci., vol. 7, pp. 2532-2536, 2016. http://dx.doi.org/10.1039/c5sc04608d

Hydrogen bonding to chloroform.

Monday, November 14th, 2016
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Chloroform, often in the deuterated form CDCl3, is a very common solvent for NMR and other types of spectroscopy. Quantum mechanics is increasingly used to calculate such spectra to aid assignment and the solvent is here normally simulated as a continuum rather than by explicit inclusion of one or more chloroform molecules. But what are the features of the hydrogen bonds that form from chloroform to other acceptors? Here I do a quick search for the common characteristics of such interactions.

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The NMR spectra of methano[10]annulene and its dianion. The diatropic/paratropic inversion.

Saturday, October 26th, 2013
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The 1H NMR spectrum of an aromatic molecule such as benzene is iconic; one learns that the unusual chemical shift of the protons (~δ 7-8 ppm) is due to their deshielding by a diatropic ring current resulting from the circulation of six aromatic π-electrons following the Hückel 4n+2 rule. But rather less well-known is the spectacular inversion of these effects as induced by the paratropic circulation of 4n electrons. A 4n+2 rule can be converted to a 4n one by the addition of two electrons, and chemically this can be done by reduction with lithium metal to form a dianion. Fortunately, this experiment has been done for a molecule known as methano[10]annulene. This is a 4n+2 aromatic molecule 1 with ten π-electrons (n=2) that can be reduced with lithium metal to form an ion-pair 2 comprising lithium cations and the twelve π-electron (4n, n=3) methano[10]annulene dianion.[1]

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References

  1. D. Schmalz, and H. Günther, "1,6-Methano[10]annulene Dianion, a Paratropic 12π-Electron Dianion with a C10Perimeter", Angewandte Chemie International Edition in English, vol. 27, pp. 1692-1693, 1988. http://dx.doi.org/10.1002/anie.198816921

The mysterious (aromatic) structure of n-Butyl lithium.

Sunday, March 17th, 2013
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n-Butyl lithium is hexameric in the solid state and in cyclohexane solutions. Why? Here I try to find out some of its secrets.

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Computers 1967-2011: a personal perspective. Part 1. 1967-1985.

Thursday, July 7th, 2011
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Computers and I go back a while (44 years to be precise), and it struck me (with some horror) that I have been around them for ~62% of the modern computing era (Babbage notwithstanding, ~1940 is normally taken as the start of the modern computing era). So indulge me whilst I record this perspective from the viewpoint of the computers I have used over this 62% of the computing era. (more…)

A molecule with an identity crisis: Aromatic or anti-aromatic?

Monday, April 13th, 2009
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In 1988, Wilke (DOI: 10.1002/anie.198801851) reported molecule 1

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