Charges in chemistry, like the grin on Lewis Carroll’s cat, can be mysterious creatures. Take for example the following structure, reported by Paul Lickiss and co-workers (DOI: 10.1039/b513203g).
Hunt the charge: the Cheshire cat of chemistry
September 29th, 2011The importance of being complete.
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
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.
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).
Anatomy of a simple reaction: the hydration of an alkene.
September 4th, 2011The hydration of an alkene by an acid is one of those fundamental reactions, taught early on in most chemistry courses. What can quantum mechanics teach us about the mechanism of the reaction?
A stable borylene. An exercise in lateral thinking.
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.
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?
Breakdowns in communication: the two cultures
August 2nd, 2011In his famous lecture in 1959, C. P. Snow wrote about the breakdown in communications between the “two cultures” of modern society — the sciences and the humanities (arts). That was then. This is now, and the occasion of my visit to a spectacular “city of arts and sciences complex” in Europe. An un-missable exhibit representing science and life was the 15m high model of DNA. Now to be fair this is styled an artist’s impression, and one presumes that an artist is allowed license. But how much license? And at how much expense to the science? And is there a counterbalance to the art where the science is fastidiously (but artistically) preserved?