Posts Tagged ‘chemical shifts’

(another) WATOC 2017 report.

Tuesday, August 29th, 2017

Another selection (based on my interests, I have to repeat) from WATOC 2017 in Munich.

  1. Odile Eisenstein gave a talk about predicted 13C chemical shifts in transition metal (and often transient) complexes, with the focus on metallacyclobutanes. These calculations include full spin-orbit/relativistic corrections, essential when the carbon is attached to an even slightly relativistic element. She noted that the 13C shifts of the carbons attached to the metal fall into two camps, those with δ ~+80 ppm and those with values around -8 ppm. These clusters are associated with quite different reactivities, and also seem to cluster according to the planarity or non-planarity of the 4-membered ring. There followed some very nice orbital explanations which I cannot reproduce here because my note taking was incomplete, including discussion of the anisotropy of the solid state spectra. A fascinating story, which I add to here in a minor aspect. Here is a plot of the geometries of the 52 metallacyclobutanes found in the Cambridge structure database. The 4-ring can be twisted by up to 60° around either of the C-C bonds in the ring, and rather less about the M-C bonds. There is a clear cluster (red spot) for entirely flat rings, and perhaps another at around 20° for bent ones, but of interest is that it does form something of a continuum. What is needed is to correlate these geometries with the observed 13C chemical shifts to see if the two sets of clusters match. I include this here because in part such a search can be done in “real-time” whilst the speaker is presenting, and can then be offered as part of the discussion afterwards. It did not happen here because I was chairing the meeting, and hence concentrating entirely on proceedings!


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

Monday, June 26th, 2017

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.



  1. J. Zong, J.T. Mague, and R.A. Pascal, "Exceptional Steric Congestion in an in,in-Bis(hydrosilane)", Journal of the American Chemical Society, vol. 135, pp. 13235-13237, 2013.
  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.

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

Friday, December 16th, 2016

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.



  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", Chemical Science, vol. 7, pp. 2532-2536, 2016.

Hydrogen bonding to chloroform.

Monday, November 14th, 2016

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.


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

Saturday, October 26th, 2013

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]



  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.

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

Sunday, March 17th, 2013

n-Butyl lithium is hexameric in the solid state[1] and in cyclohexane solutions. Why? Here I try to find out some of its secrets.



  1. T. Kottke, and D. Stalke, "Structures of Classical Reagents in Chemical Synthesis:(nBuLi)6,(tBuLi)4, and the Metastable(tBuLi· Et2O)2", Angewandte Chemie International Edition in English, vol. 32, pp. 580-582, 1993.

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

Thursday, July 7th, 2011

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

In 1988, Wilke[1] reported molecule 1

A [24] annulene. Click on image for model.

A 24-annulene. Click for 3D.

It was a highly unexpected outcome of a nickel-catalyzed reaction and was described as a 24-annulene with an unusual 3D shape. Little attention has been paid to this molecule since its original report, but the focus has now returned! The reason is that a 24- annulene belongs formally to a class of molecule with 4n (n=6) π-electrons, and which makes it antiaromatic according to the (extended) Hückel rule. This is a select class of molecule, of which the first two members are cyclobutadiene and cyclo-octatetraene. The first of these is exceptionally reactive and unstable and is the archetypal anti-aromatic molecule. The second is not actually unstable, but it is reactive and conventional wisdom has it that it avoids the undesirable antiaromaticity by adopting a highly non-planar tub shape and hence instead adopts reactive non-aromaticity. Both these examples have localized double bonds, a great contrast with the molecule which sandwiches them, cyclo-hexatriene (i.e. benzene). The reason for the resurgent interest is that a number of crystalline, apparently stable, antiaromatic molecules have recently been discovered, and ostensibly, molecule 1 belongs to this select class!



  1. G. Wilke, "Contributions to Organo-Nickel Chemistry", Angewandte Chemie International Edition in English, vol. 27, pp. 185-206, 1988.