In two previous posts, I have looked at why cis-butene adopts conformation (a) rather than (b). I suggested it boiled down to electronic interactions between the methyl groups and the central alkene resulting in the formation of a H…H “topological” bond, rather than attraction between the H…H region to form a weak chemical “bond“. Here I take a look at what happens when that central C=C bond is gradually removed.
Archive for the ‘Interesting chemistry’ Category
cis-Butene: a reaction coordinate dissected and methyl flags.
Wednesday, October 12th, 2011Are close H…H contacts bonds? The dénouement!
Monday, October 10th, 2011I wrote earlier about the strangely close contact between two hydrogen atoms in cis-butene. The topology of the electron density showed characteristics of a bond, but is it a consensual union? The two hydrogens approach closer than their van der Waals radii would suggest is normal, so something is happening, but that something need not be what chemists might choose to call a “bond“. An NCI (non-covalent analysis) hinted that any stability due to the electron topologic characteristics of a bond (the BCP) might be more than offset by the repulsive nature of the adjacent ring critical point (RCP). Here I offer an alternative explanation for why the two hydrogens approach so closely.
Are close H…H contacts bonds?
Friday, October 7th, 2011The properties of electrons are studied by both chemists and physicists. At the boundaries of these two disciplines, sometimes interesting differences in interpretation emerge. One of the most controversial is that due to Bader (for a recent review, see DOI: 10.1021/jp102748b) a physicist who brought the mathematical rigor of electronic topology to bear upon molecules. The title of his review is revealing: “Definition of Molecular Structure: By Choice or by Appeal to Observation?”. He argues that electron density is observable, and that what chemists call a bond should be defined by that observable (with the implication that chemists instead often resort to arbitrary choice). Here I explore one molecule which could be said to be the focus of the differences between physics and chemistry; cis-but-2-ene.
The importance of being complete.
Monday, September 26th, 2011To (mis)quote Oscar Wilde again, ““To lose one methyl group may be regarded as a misfortune; to lose both looks like carelessness.” Here, I refer to the (past) tendency of molecular modellers to simplify molecular structures. Thus in 1977, quantum molecular modelling, even at the semi-empirical level, was beset by lost groups. One of my early efforts (DOI: 10.1021/ja00465a005) was selected for study because it had nothing left to lose; the mass spectrometric fragmentation of the radical cations of methane and ethane. Methyl, phenyl and other “large” groups were routinely replaced by hydrogen in order to enable the study. Cations indeed were always of interest to modellers; the relative lack of electrons almost always meant unusual or interesting structures and reactions (including this controversial species, DOI: 10.1021/ja00444a012). Inured to such functional loss, we modellers forgot that (unless in a mass spectrometer), cations have to have a counter anion. Here I explore one example of the model being complete(d).
Molecular Matryoshka dolls
Tuesday, September 20th, 2011A Matryoshka doll is better known as a Russian nesting doll. They can have up to eight layers. Molecules can only emulate two layers, although see here for a good candidate for making a three-layered example (the inside layer is C60, which itself might encapsulate a small molecule. See also DOI: 10.1021/ja991747w). These molecular dolls can be created out of quite simple molecules. Here I explore just one, and focus on what is happening inside!
Some fun with no-go areas of chemistry: cyclobutadiene.
Sunday, September 18th, 2011Organic chemistry has some no-go areas, where few molecules dare venture. One of them is described by a concept known as anti-aromaticity. Whereas aromatic molecules are favoured species, their anti-equivalent is avoided. I previously illustrated this (Hückel rule) with cyclopropenium anion. Now I take a look at cyclobutadiene, for which the π-system is said to be iso-electronic (where two electrons in a double bond have replaced the carbanion lone pair).
A stable borylene. An exercise in lateral thinking.
Sunday, August 7th, 2011I have often heard the question posed “how much of chemistry has been discovered?” Another might be “has most of chemistry, like low-hanging fruit, already been picked?“. Well, time and time again, one comes across examples which are only a simple diagram or so away from what might be found in any introductory chemistry text, and which would tend to indicate the answers to these questions is a resounding no. Take for example the three reactions shown below.
Extreme chemical intimacy: the Xe2@C60 ion-pair.
Wednesday, August 3rd, 2011Unusual bonds are always intriguing, and the Xe-Xe bond is no exception. It was first reported (10.1002/anie.199702731) for the species Xe2+. Sb4F21– and its length (3.09Å) was claimed as “unsurpassed in length in main group chemistry by any other element -element bond”. Krapp and Frenking then creatively tweaked the bond (in a computer). The counterion was replaced by C60, and the two xenon atoms placed inside! Buckyballs have a fascinating ability to absorb electrons, up to six in fact, from whatever is placed inside the cavity, and so this assembly acts as a rather intriguing ion-pair. So the issue reduces to how many electrons does C60 manage to scavenge from two Xenon atoms, and what is the nature of any resulting bonding formed between these two atoms?