Chemists are fairly adept at finding out what molecules are present in a sample, but what happens when the sample is light years away? Astronomers have worked out how to do some spectroscopy, but it is not exactly easy to do. One of the interesting reports recently was the announcement of the measurement of an exoplanet's atmosphere (Nature 526: 526 – 529). When starlight passes through the atmosphere, various absorption lines can be seen as long as the atmosphere is basically transparent. While the star is a strong source of light (somewhat too strong, since most of the light does not go through the planetary atmosphere, as the star is very much bigger) the path length of a giant planet's atmosphere is also somewhat longer than the average laboratory cell! In this case, the main signal detected was water, and it was noted that the level of heavier elements in the atmosphere relative to hydrogen was no greater than 700 times that of the star, as would be expected if the planet was a giant that accreted gas from an accretion disk. Not that there would be many other ways of making a gas giant.
 
Another study (Icarus 243: 39 – 47) considered the chemistry of cometary methanol during impacts. Impacts cause methanol to dissociate into CO and CH₄, however the energies are such that methanol should survive accretion onto the large icy satellites, including Callisto, Ganymede, Titan, Ceres and Pluto, although cometary impacts following accretion will produce dissociation. Asteroid impacts onto Ceres would dissociate methanol. However, Callisto could have produced up to 10^-2 bar during the late heavy bombardment, while Titan would have acquired 0.1 bar. Since it did not, the authors imply that the methanol concentration in the Saturnian system was much lower than that of comets, or alternatively, some unspecified conversion of CO to CH₄ occurred. This supports my mechanism of planetary formation, in which comets were not the source of methanol or other carbonaceous material on the icy bodies. Titan would have contained methanol, but this would be converted to methane by geochemical processes. These authors also show that CH₃OH and CH₄ abundances on a persistently shadowed part of the moon cannot be of cometary origin.
 
One of the more difficult questions is what the original earth was like. The standard theory has it that the planet formed as a consequence of giant collisions that led to a magma ocean, but a recent publication (arXiv:1403.0806) throws up interesting constraints. The authors propose at least two giant impacts to generate a global magma ocean based on the ratios of 3He to 22Ne. The depleted mantle has a ratio at least 10, while a more primitive mantle has a ratio of 2.3 - 3. The solar ratio is 1.5.  In-gassing of gravitationally accreted nebular atmosphere will explain the 3, but to get to 10 it requires at least two episodes of atmospheric blow-off and magma ocean outgasssing. The preservation of the low ratio in a primitive reservoir sampled by plumes suggests that later giant impacts, including the moon-forming impact, did not generate a whole mantle magma ocean. Atmospheric loss episodes with giant impacts provide an explanation for Earth's subchondritic C/H, N/H and Cl/F elemental ratios, while preserving chondritic isotope ratios, but if so, a significant proportion of terrestrial water and other volatiles were accreted prior to the last giant impact, otherwise the fractionated elemental ratios would have been overprinted by the late veneer. What is most surprising here is that the collision that caused the moon to form was insufficient, yet the carbon, nitrogen and halogens were determined relative to hydrogen prior to the moon-forming event. That would require the current volatiles were degassed from the earth at a later date.
 
There were two big events in November. The first involved Philae landing on a comet, and apparently it has made a lot of measurements, and sent the data back to Earth. However, as yet we have no idea what was discovered. The fact it landed and ended up in the shade was bad news because the solar cells will not recharge the batteries adequately. For me the big disappointment was that the device that bored into the comet apparently struck something hard, and when the drill was withdrawn, apparently there was no sample. This is one of the difficulties with robots; whoever designs them has to know what the conditions would be. Why would there be no sample? One possibility is that the ice has clathrated or adsorbed gas in it, and the heat of the drill vaporized the gas, the pressure of which blew out the sample, however I guess we shall never know because "no sample" cannot be analysed.
 
The second big event involved the European Space Agency, who have studied the star, HL Tauri and found an accretion disk around it. The star is about 1 million years old, and the disk has rings in it, with dark gaps between them. The most obvious cause for such rings would be the formation of planets, although that does not mean there is a planet in every gap, because while a planet will clear out dust on its path, gravitational resonance will also clear out material. One problem is we cannot see the planets. Why would we? We can see four giant planets around the star HR 8799. These are newly-formed giants, and the gravitational energy of the gas falling onto the planet heats it to a yellow-white heat, hence they glow. These are all very much bigger than Jupiter. Similarly, there is a star LkCa 15 that is 3 million years old, and we see a planet much bigger than Jupiter, and significantly further from the star. Planetary growth should be faster the closer to the star, at least for the same sort of planet, because the density of matter increases as it falls into the star. Since we only see one giant, my theory requires there to be three other giants we cannot see, presumably because they are yet of insufficient size to glow sufficiently brightly for us to image them. So, if I am right, 1 My gets you giants of the size we have, and the longer the disk lasts, the bigger the giants get.

It seems to me there are two purposes for theory: to enable the calculation of things of interest so that predictions can be made, or to lead to understanding so that even if calculations are not practical, at least educated guesses can be made to guide further action. At the risk of drawing flak from the computational chemists, I think the second purpose is of more importance to chemists. The problem is, chemistry is based on a partial differential equation that cannot in general be solved, if for no other reason than the equations relating to a three-body problem involving a central field cannot be solved exactly. That leaves the question, if you cannot solve that, what can you do? What chemists have done is to take solutions from what can be solved (the hydrogen atom) and base models on those. Thus we have orbitals that correspond to the excited state solutions of the hydrogen atom. The perceptive reader of my previous posts will realize I have argued that the actual orbitals do not exactly correspond, nevertheless the wave functions I argue for (essentially superpositions of waves with fewer nodes based on the principle that separation is possible provided all components have quantized action) are essentially the same in terms of angular distributions, so that issue is irrelevant to the present issue, which is what to do with these orbitals relating to dative bonds? Most chemists are familiar with one answer relating to dative bonds: models based on arrows, etc.
 
Recently, we have seen a debate about dative bonds in Angew. Chem. In. Ed. (2014, 53, 370 – 374; 2014, 53, 6040 – 6046.). There seem to be several points being made, but they tend to boil down to the use of arrows, what the dative bond is, and what model is worth following. This discussion attracted the heretic in me!
 
First, why models? One of the protagonists (Frenking) used this quote: Bonding models are not right or wrong but they are more or less useful. This raises the issue, what do we mean by "right or wrong", and when can a model that is known to be wrong continue to be used? In the first case a model can be seen to give useful outputs and can be used while there are no known examples of it being wrong, and, of course, there is nothing wrong with using a model that you know to be an approximation, as long as everyone accepts that it is an approximation. Another time when the model is strictly wrong but can still be used (in my opinion anyway) is when it is only wrong when a given external condition is imposed that gives a known effect, in which case it can be used when that effect is absent. The most obvious example is Newtonian mechanics. Newton assumed action at a distance was immediate. It is not, and when that is relevant we have to resort to Einstein's mechanics, but when motion is such that the effects of light speed can be considered as effectively instantaneous, you would be mad not to use Newtonian mechanics.
 
However, back to the dative bond. What is it? Seemingly Haaland (Angew. Chem. Int. Ed. 1989, 28, 992 – 1007.) considered: The basic characteristics of a dative bond, depicted with an arrow “→”, are its weakness, the substantially longer bond length compared to typical single bonds, and a rather small charge transfer. My personal view is this does not help much. What does " substantially longer bond length compared to typical single bonds" mean? In this sense, it must be recalled that bond lengths vary, and the dative bond does not have a non-dative counterpart. Both parties to this discussion used the example of borazane (NH₃→BH₃). Right – what is the length of a non-dative nitrogen-boron single bond free of other complications, including lone-pair interactions? The next question, though, is, if we write it like that, what does the arrow mean? What I was taught as an undergrad, and it seems reasonable enough, is that a two-electron bond forms using both electrons from the nitrogen lone pair. Now, part of this discussion then focused on, what does that mean?
 
A lot of people seem to think that what happens is that the nitrogen transfers an electron to the boron atom, then the two electrons pair. The net result is that the molecule is a zwitterion, with N and B^- charges on the relevant atoms, with a little subsequent polarization of the hydrogen atoms. That would seem to contradict Haaland in that such a distribution would give a very strong dipole moment, but note now what Frenking says: "Writing ammonia borane H3N-BH3 as a zwitterion yields a negative charge at boron and a positive charge at nitrogen, while the partial charges exhibit the opposite polarity." What exactly does that mean? From what I can make out from a cursory glance at the literature, borazane has a dipole moment of 5.2 D. Now, which way is that likely to go? I cannot see a sufficient electron transfer to get that dipole moment from boron to nitrogen, so it seems reasonable to me to assign the direction of flow to be from the lone pair of nitrogen towards boron, as the arrow indicates. Accordingly, I find this discussion just a little misleading. However, I also do not feel that the concept of the nitrogen transferring an electron to boron, and the two pairing is very helpful either.
 
So, how do I see the dative bond? In my picture, the nitrogen atom has a lone pair, and those electrons are described by a wave function that has a barrier at infinity, while boron, if it hybridizes, can create an sp³ configuration with an empty wave function, which I shall describe as a hole. If the nitrogen atom approaches such that the lone pair wave function is directed towards the hole on the boron atom, the boron atom now provides a barrier to the lone pair wave function, perhaps described as the vacant sp³ orbital "capturing" the lone pair and reducing the range over which the electrons can roam by the boron atom providing a turning point. As positional uncertainty is lowered, momentum increases, kinetic energy increases, and by the virial theorem, total energy is lowered. In that picture, the arrow is a great way of describing it, and the lone pair mechanics are now determined both by the nitrogen atom and the boron atom, and to maintain the sp³ hybridization, the lone pair has to spend increased time away from the nitrogen atom, hence the high dipole moment. Note that that is also more valence-bond type thinking than molecular orbital thinking. As to why I put that here, apart from highlighting the debate, the sort of thinking of this last model, which is essentially that the dative bond forms as a cosnequence of the change to boundary conditions applied to a lone pair helps me; whether it helps anyone else is, I suppose, a more interesting question.