Tellurium-129m maps

Dear Reader,

It has come to my attention that the Japanese government have published a map of the tellurium-129m contamination levels in the countryside near to Fukushima. Here is a link to the maps from Japan.

Now before we get going we might want to consider what tellurium is.

Tellurium is a heavy version of sulphur (sulfur), it is named after the earth while selenium (the element) above it is named after the moon. Tellurium has some rather interesting chemistry it is more than just a heavy version of sulphur.

Many organisms are able to transform it from one chemical form to another, one of the typical symptoms of tellurium poisoning in humans is that the unlucky person who is stricken with tellurium will start to stink to high heaven of garlic.

This is because inside the human body the tellurium is converted into dimethyl tellurium, while I do not think that the tellurium will leave the reactor in the form of dimethyl tellurium I think it is likely that bacteria or animals will convert the tellurium into this volatile form. It is possible that the ability of living things to form this volatile organometallic may make the environmental chemistry of the tellurium more complex.

Now I hope that it is OK with my readers but right now I do not want to get sucked into a discussion of the environmental chemistry of tellurium, but I am willing to comment on another aspect of tellurium chemistry.

One of my all time favourites in chemistry is VSEPR, or Valence Shell Electron Pair Repulsion theory. For those of you who are not in the know, this is a simple theory which predicts correctly the majority of covalent compounds which lack transition metals. Be careful of the transition metals the electrons in the d orbitals can throw you a bit of a weirdo ball, on the other hand the main group elements tend to just play nicely with a normal over the shoulder delivery of the cricket ball without trying to do things like spin bowling.

The core idea of VSEPR is that the central atom has a series of electron clouds (orbitals) which poke out of the atom to form sigma bonds. The sigma bonds would look like sausages if we could see them. Think it is a nice thing that s is for sausages and also for sigma bond.

These sausage like clouds of electron density will repel each other, this is a plain simple electrostatic effect much like the effects you get if you comb your hair and then wave the comb near some small scraps of paper.

If an atom has two such sausages (opps I mean bonds) then the furthest they can be from each other on a sphere is at 180 degrees (pi radians) from each other, an example of this would be the arrangement of atoms in acetylene.

If an atom has three sausages then the furthest they will be from each other is at 0, 120 and 240 degrees around the equator of a perfect sphere. An example of this type of arrangement would be the atoms in benzene or styrene. While we are at it be thankful for styrene, without styrene there would be no polystyrene or ABS plastic. The chances are that your computer screen has a case made of ABS.

If an atom has four sausages then these bonds will be arranged in a tetrahedral manner, this is the arrangement which makes them the furthest from each other.

If the atom has five sausages then the geometry which puts the bonds the furthest apart would be to stick one at each of the poles, and three spaced at 120 degrees apart on the equator. One alternative would be a square based pyramid but I have done the maths for that
yet.

If the atom has six sausages then the geometry which puts all the bonds in an octahedral arrangement.

When an atom has a lone pair this should be regarded as a sigma bond which goes to nowhere. Treat it as if it almost as it it was just another bond, as the electrons are on average closer to the atom the lone pair is more able to repel other other bonds than a normal sigma bond.

The fun thing about tellurium is that sometimes its lone pair is stereochemically active this means that it repels other electron pairs just like any other electron pair. But sometimes the tellurium lone pairs are stereochemically inactive which means that these lone pairs do not repel other electron pairs (bonds and lone pairs). The non-stereochemically active lone pair does not normally appear in light elements such as sulfur and oxygen, it tends to be something which only happens with heavy elements such as tellurium and lead.

Prussain blue

Dear Reader,

I have explained how cyanide can bind to metals such as iron to form complexes, these complexes have lone pairs poking out which can bind to other metals. Here is a picture of a unit cell of prussian blue.

A unit cell of a cesium nickel iron cyanide

The carbons are black, the nitrogens are blue, the irons are purple, the nickels are gold coloured and the green atoms are the cesium.

Why and how does Prussian Blue form

Dear Reader,

Welcome back and I have to warn you fine folk that I am still thinking about Prussian blue the wonder substance which helps us to manage the radioactive cesium from the Fukushima accident.

While on a boat crossing the north sea I asked myself the question of why does Prussian blue form and how. I think that I have come up with an answer. It is important for us to start with the unfriendly sounding molecule hydrogen cyanide. It goes backward and forwards. It is refined, very much maligned and misunderstood. Go easy on this fellow, he must never be abused. He gets the metals going and you find him fizzing in the corner in the bleach bin.

Some of you may have spotted the reference to 1980s culture, those of you who have not then do not worry. All will become clear soon. It is important to bear in mind that Prussian blue will not give you cyanide poisoning.

HCN is a very refined fellow, the modern and green way to make the dinitrile required for the production of the 1,6-diaminohexane required for nylon-6.6 production is to use hydrogen cyanide (with a nickel catalyst) rather than using sodium cyanide. So the next time some asks you to name a green reagent you can say “hydrogen cyanide” in a truthful way. While it is a toxic reagent it is more green than sodium cyanide as its use forms less toxic solid waste which is hard to deal with.

For a process to be truly green it must satisfy three things.

1. Be economically sustainable (Eg process for making aspirin at £ 10 per gram will not be OK)

2. Be environmentally sustainable, it must not guzzle resources or spew out vast amounts of waste for a small amount of product (Eg if I have to cut down a square mile of rainforest and kill five rare birds to make you an egg sandwich then this method is not an OK egg production system)

3. Be socially sustainable (Eg if a process requires small children to climb up chimneys then it will not be considered morally acceptable. As a result it will be impossible to sustain the process in today’s Soceity)

Next HCN is a very maligned and misunderstood substance; it is a toxic gas but if we want to base our vilification of gases on purely their toxicity then hydrogen sulphide beats it in the top ten worst ever gases. My own view is that carbon monoxide is more of a fright gas as CO has absolutely no smell and is much more common (check your when your gas appliances were last checked by a service engineer). But as a result of the fact that HCN was the poison gas used at some Nazi extermination camps, in the American gas chamber and in many detective stories hydrogen cyanide has acquired a super nasty reputation. It is interesting to note that carbon monoxide was also used by the Nazi murderers (the gas van), but why then has CO not become viewed with equal horror by the public ?

I would say that as a chemist or an industrial worker it is important to avoid breathing in or otherwise absorbing HCN, it is bad for your health. As well as the dire short term effects which are well known it can have some horrible long term effects which are sometimes seen in parts of Africa where people tend to live on a vegetable known as cassava. If you prepare this food wrongly then you will get a dose of cyanide in every meal, this can lead to chronic cyanide poisoning which causes among other things trouble with the nervous system. So my advice is to “go easy on your body” when working with cyanide. Do not abuse your body by forcing it to endure the stress of having to metabolize cyanide, take that bit of extra care to lower your occupational intake of cyanides.

The cyanide anion is a very strong ligand for many transition metals, indeed it does get the metals going. Sometimes in very much the wrong way, some time ago there was a large spill of cyanide waste in eastern Europe. It ended up in a river where it then killed the fish, one of the problems with cyanide it binds to an iron complex in mitochondria which then stops oxygen binding. As a result the fish could no longer use oxygen, as a result they died. But we need to understand why does cyanide bind to metals so well, the binding of cyanide to metals is much stronger than the binding of most simple monodentate ligands.

Monodentate ligands is a fancy term for a molecule or atom which binds through one atom onto a metal.

A snake which grabs you with its mouth is a monodentate animal

A crab which grabs you with both claws is a bidentate animal

A scorpion which grabs you with both claws and applies the stinger to you is a tridentate animal

The reason is the “backwards and forwards”, hydrogen cyanide when deprotonated forms the cyanide anion which uses a lone pair on the carbon to form a sigma bond to a metal. It also uses its empty pi* orbitals to suck electron density off of metals thus forming pi bonds to the metal.

Now we need to look at the orbitals of the hydrogen cyanide, the orbitals of the cyanide anion are almost identical.

Lets start with the HOMO, this is not a sexual term it means Highest Occupied Molecular Orbital in chemistry. Those of you who were expecting something sexual here, I am sorry but I am going to disappoint you, this blog is not about sexual matters. But feel free to carry on reading as you might find the chemistry interesting.

The HOMO of HCN

Here you should be able to see that on the nitrogen atom (blue atom) a lobe of the orbital pokes out into space away from the CH group, this part of the orbital will form the lone pair which allows the nitrogen to bind to things. Around the hydrogen atom is a big blue lobe. When the HCN loses a proton this will form a cloud of electron density which also pokes out into space. Here is another view which may make it more clear, the lone pair on the nitrogen and the blue blob on the carbon will allow it to form the sigma bonds which go to metal atoms.

Alternative view of HCN's HOMO

Here is a view of the HOMO of the cyanide anion, look at how similar it is to the HOMO of hydrogen cyanide.

Next here are two alternative views of the HOMO of the cyanide anion to allow you to have a better idea of the shapes of the orbitals.

The next thing to look at is the p orbitals of HCN, I have calculated these orbitals for the cyanide anion and they are the same shape so I will only show you one set. Here is one of the them.

One of the pi bonding orbitals of HCN

The hydrogen cyanide molecule has two occupied pi orbitals which look like a pair of sausages arranged parallel with the line between the carbon and the nitrogen. Here is a view of the other one.

A view of the other pi orbital

Next we have the pi* antibonding orbitals.

LUMO of HCN

HCN LUMO +1

I guess they looked the same to you, so here is the end view. Note that they are at ninety degrees to each other.

HCN LUMO

HCN LUMO +1

Now to understand antibonding, I want you to think of a nice person. How about St Francis of Assisi, after a wayward youth he grew up to be a man known for being kind to poor people and taking care of animals.

The anti-St Francis would be a nasty man who steals bread from staving single mothers and homeless men, for fun he throws
animals down the well.

The anti-St Francis is the total opposite of St-Francis, everything good about St-Francis has been turned into something horrible in the anti version. In the same way all the energy lowering effects of the bonding orbitals are turned into energy increasing effects in an antibonding orbital. Typically an antibonding orbital is more antibonding than the bonding orbital is bonding. So if you fill up both orbitals with electrons then overall the sum of the two orbitals is antibonding.

In case you want to see some of the other orbitals of HCN then here they are.

LUMO +3

LUMO +2

 

LUMO +1

LUMO

HOMO

HOMO -1

HOMO -2

HCN HOMO -3

HCN HOMO -4

I hope to bring you some more about our new friend (Prussian Blue) soon.

More about prussian blue

Dear Reader,

Now some of my readers have become interested in Prussian blue, this is the miracle drug which removes cesium from human bodies. I was recently reading the work of Peter W. Stephens et. al., Inorganic Chemistry, 2010, 49, pages 1524 to 1534. His paper is about the crystal structures and magnetic properties of mixed oxidation state manganese versions of Prussian blue. This has allowed us to use X-ray technology to look inside a prussian blue crystal.

Here is a chance to look at the potassium version of the prussian blue, it is clear that the potassium atoms are in channels which run through the solid.

K2Mn{Mn(CN)6}

The way in which the prussian blue works is to allow out the potassium, the cesium ions then diffuse in to take the place of the potassium ions. When the potassium ions are replaced in this solid with cesium the larger cesium ions cause the solid to change slightly. Here is a picture of the cesium solid.

Cs2Mn{Mn(CN)6}

I checked the literature and other prussian blue like model compounds have similar structures, one early report was made by R. Rigamonti, Gazzetta Chimica Italiana, 1938, 68, 803-809. Where this italian reported a potassium salt of Co[Fe(CN)6]. I think that due to the ligand field energy effects this was a simple cubic solid which looked just like the cesium one which I showed above.

Now I am going to tell you the story about how I found out about Prussian blue, years ago in 1999 I went on an adventure to eastern europe. I joined Josef Novosad’s research group for a while. While I worked with Josef on phosphorus chemistry (we both share an affinity for this element), between making some interesting compounds which may have improved our understanding of dithioimidodiphosphinates and enjoying the delights of Brno (it is a very wonderful place) we did talk about chemistry.

One of the things which Josef told me about was what he did in his youth, he told me that he was given a job by the communists working on uranium in farmyard animals at a research centre close to Brno. But after the Chernobyl accident he was moved onto cesium in farmyard animals. What used to happen at Josef’s research site was that an animal would be given a dose of 1 MBq of Cs-137, then using additives to the animal’s feed the workers would then try to remove the cesium from the animal.

Before anyone gets worried about the effect of the cesium on the farmyard animal lets do an estimate of the dose which the animal gets. If we assume that the animal is identical to a typical human in size and that it is identical to a human then we can use the data for humans. Using the radiation protection advice from a US university we can get a thing called an ALI for oral exposure to Cs-137. The ALI is the Annual Limit of Intake which for Cs-137 in the US is 100 microcuries. As 1 Ci = 37 GBq this works out as a dose of 3.7 MBq to the animal being the limit. The US limit is worked out based on a 5 rem dose to the body. So the animal will get 1.35 rem. Now some of you might be getting a bit confused with the different radiation units. Here is a look up sheet

100 rem = 1 Sv

100 rad = 1 Gy

1 Curie = 37 GBq = 37000 MBq = 37000000 KBq = 37000000000 Bq

1 milli Curie = 37 MBq

1 microcuries = 37 kBq

So our “animal” will have got a 13.51 mSv dose, this dose is far too small to cause “radiation sickness”. If we repeat the calculation using the ALI value used in Sweden (based on a 20 mSv dose) which is 1.5 MBq then our animal gets a dose of 13.33 mSv. This is not a dose which will make the animals die of radiation sickness and if we use the accepted dose to chance of cancer conversion factor of 5 % for a 1 Sv dose then if we assume that the LNT model is right then if the animal was a human then it would have a 1 in 1500 chance of getting cancer as a result of the cesium intake. As most farmyard animals weigh more than a human the real dose to the animal would be likely to be lower.

To put it in perspective if I got a dose of 13.3 mSv at work then I know that my radiation protection officer would be very very concerned about me but I would not have gone over the yearly limit for a classified radiation worker, but if I got that dose in one month in Sweden then it would trigger an investigation into me. Such a dose is in the range where the national radiation protection authority would want to know what I was doing and how I got the dose. However if a member of the public got that dose at work then I imagine that the national radiation protection would be hopping mad to say the least ! The occupational dose for a non radiation worker is only 1 mSv per year, as a non-radaition worker is unlikely to be wearing a film or TLD badge then it might take quite a lot of extra work to work out the dose compared with the effort needed to estimate a dose for a dosemeter wearing radiation worker.

But lets get back to the prussian blue.

Josef told me that he tried almost every transition metal, I think that he did not try using nickel as nickel is toxic. He then used the batches of the “prussian blues” to try to clean the animals up. What Josef found was that no two batches of prussian blue which he made worked quite the same way in the experiments. So my advice to anyone planning on making prussian blue for medical use in Japan is that the production of the medical grade solid is not a simple matter, I have to confess that I do not know how to reliably make medical grade prussian blue.

If you want to read about Josef’s cesium work then see  H. Prochazka, J. Jandl, J. Novosad, O. Neruda, J. Hejzlar and S. Spelda, Veterinarni Medicina, 1991, 36, 341 to 348. The paper is entitled “Affection of Radiocesium Retention in Miniature Pigs”

The abstract of this paper comments that stable cesium (1 mg per kilo of body weight) is not effective as a means of removing cesium from pigs. Josef and J. Jandl published another paper in which they used a modified zeolite to treat sheep which were contaminated with cesium. This paper can be found at “In-Vivo Reduction of Radiocesium by Modified Clinoptilolite in Sheep”, Veterinarni Medicina, 1995, 40, pages 237-241.