Book: Force of Nature: The Life of Linus Pauling

When I read Linus Pauling and the Chemistry of Life, I said it was the best biography of his. I didn't know that the same author wrote even a better one. In fact Linus Pauling and the Chemistry of Life is more like a summary of this book (700 pages vs. 140 pages). 

Tom Hager did a really good job in explaining Pauling's contributions to chemistry and medicine. He was a man full of ideas and a will to try to execute them. A career full of success.

You can read the book on Kindle for free too!

https://www.amazon.com/Force-Nature-Life-Linus-Pauling-ebook/dp/B0052ACCGS

The Magnetic Properties and Structure of Hemoglobin, Oxyhemoglobin and Carbonmonoxyhemoglobin #chempaperaday 273

Although I kept reading papers almost every day, I have been busy with lab work and some life related problems. All these prevented me from posting some of my readings here. I am hoping that I am back on my schedule again.

Hemoglobin was first isolated in 1840 and Faraday studied its magnetic properties. Here is a paper almost 100 years after its discovery, by Pauling, where they keep investigating its paramagnetism. It is an incredibly clear article and they conclude that hemoglobin contains four unpaired electrons per heme, while oxyhemoglobin and carbonmonoxyhemoglobin contain none. Pauling got a few things incorrect in this paper, but everything else later found to be correct.

http://www.pnas.org/content/22/4/210.full

Multiple Metal-Metal Bonds #chempaper 224

This is another paper by F.A. Cotton on (of course) multiple metal-metal bonds. It was basically a lecture he gave in ACS Seattle (1983). Not surprisingly he summarizes multiple M-M bonding starting as early as mid 18th century. So, I got to learn a lot from this paper that I had not known. There is also a really interesting quote and talk by Linus Pauling on metal-metal bonding where he tries to describe a few Mo-Mo, W-W and Ta-Ta bonds. Cotton also confesses that he had heard about Pauling's paper by pure coincidence.

On the famous Re-Re multiple bonding, Cotton says :

As always, it was a pleasure to read the paper. I hope you enjoy it too.

Pauling's Left-Handed alpha-Helix #chempaperaday 209

Left-handed alpha-helices are not common. In fact, I was able to find only one example with my limited search attempts although I am sure there are many more. There are stereochemical issues, bond distance and dihedral angle restrictions if nature or  you want to make one of them. So, when I first saw the title, I did not want to believe that Pauling proposed a left-handed alpha-helix. I thought he just made a mistake in one of his books or papers. If you read the paper, you will see that this is not the case. For some reason unknown to us, he drew this helix. As the paper explains, in fact he made an arbitrary assignment to R groups when he first reported "the structure of proteins."

 http://www.pnas.org/content/37/4/205.full.pdf

But, as the author explains in detail, he should have known the absolute configuration of the amino acids. In fact, his colleagues at his own department were doing research on the subject. So, he should definitely have known much more about his "incorrect" assignment. You can read the article and learn more about the issue. My own understanding is that he was not in close communication with his faculty members in those years due to the political troubles he had had. Maybe the people who really knew the absolute configuration did not correct him on purpose, or he did not want to be corrected by one of those people who were not on his side when he needed support. But, I don't think he was simply not "interested in the problem of absolute configuration." I think this is contrary what we know about his scientific curiosity and genius.

The author is Jack D. Dunitz who is a giant in his field and you may know his name if you can remember Burgi-Dunitz angle.

"Valence Bond Theory in Coordination Chemistry" #chempaperaday 178

This is a very short paper by Linus Pauling. Apparently, he read an article where the author said that valence bond theory was not as popular and useful as it was. Obviously, the response came from Pauling and his response in fact was very short:

"I do not agree with this opinion."

The rest of the article explains how valence bond theory is still can be used in coordination compounds.

The article was published in Journal of Chemical Education, but here you can read it as pdf for free.

http://depa.fquim.unam.mx/amyd/archivero/VBPAULING_26400.pdf

Notes on Linus Pauling and Magnetic Susceptibility (I)

Linus Pauling is such a big name that whatever text I read, I see his name. He is one of the most important chemists and I truly believe that he deserved at least 3 Nobel Prizes in Chemistry. So, instead of writing a single post about him, I decided to write small pieces as often as I can. After all, there is no way I can tell everything I want to tell about him in one single post. 

I was just reading an inorganic chemistry text and I came across his name at least 10 times in the first 27 pages. Consequently, I decided that it is a good idea to write the most interesting part about him so far.

In early 20th century,  chemists were trying to assign oxidation states to some transition metals and trying to find out the electronic structure of them. They were able to measure the spin. magnetic moments and permanent dipoles of the metal complexes but it was not an easy task to find the correct electronic structure. For example, in 1941, Dwyer and Nyholm prepared several Rhodium complexes and they thought that it was Rh(II). But, they could not understand why the complexes were diamagnetic. Note that Rh(II) has a d7 electron configuration therefore should me paramagnetic and Rh(III) has d6. The answer came in 1960 when it was discovered that the complexes had hydrides and the oxidation state of Rh was three.

Linus Pauling predicted that diamagnetic Ni(II) complexes should be 4 coordinate and at that time this was not observed yet. Later, he also successfully predicted several other geometries for Au(III), Ag(II) and Co(II) complexes. 

He had some unsuccessful predictions too and the most well known of these is his ideas about Vitamin C. Anyway, although his Valence Bond theory is a very successful theory to explain bonding, but for transition metals it was not good enough. I will give the example in the book now.

The magnetic moment of [FeF6]3- is 5.9 Bohr magneton and the moment for  [Fe(CN)6]3- is 1.9 . These suggest that there are 5 and 1 unpaired electrons respectively. In order to explain this, he tried to use Valence Bond Theory and also suggested that the bonding in the first complex was ionic whereas it was covalent for the second complex. 

Better explanations came with Molecular Orbital, Crystal Field and Ligand Field theories. Simply, we now say that F- is a weak field ligand and CN- is a strong field ligand. Therefore, for Fe(III) (d5) in octahedral field the electrons occupy the orbitals like these and give rise to the observed moments:

While at it, I should mention that we can simply calculate the spin only moment by

√n(n+2) 

where n is the number of unpaired electrons. But, because there is also the orbital motion of electrons, another moment is also added to the spin only moments. From my undergrad experience in the tests or practice problems, I saw moments like 2 point something for one unpaired electron etc. So, the experimental values are sometimes very close, sometimes a little bit larger.

So, in theory spin only moments can be summarized in this table:

Number of unpaired electrons	Spin only moment (Bohr magneton)
1	1.73
2	2.83
3	3.87
4	4.91
5	5.92