We need to set the record straight about something.
It’s now taken for granted that carbon compounds like methane have a tetrahedral structure:
Methane
But I wanted you to know who first determined this extremely important fact, back in 1929. It was not the man who would later take credit for the discovery and who, up to now, has been recognized for it. No, thanks to some recent sleuthing and critical analysis, it has come to light that a major reset is in order. The person actually responsible for our possession of this knowledge was a scientist working in the Royal Institution laboratory of the great Nobel laureate William Henry Bragg.
Her name is Isabel Ellie Knaggs.
Ellie Knaggs as a freshman at Girton College, Cambridge, UK.
This story was finally brought to the attention of the broader scientific community this week in the journal Nature. In its Daily Briefing of January 22, it features an article from two days prior in Chemistry World.
In that piece, Andy Extance points out that this overturning of a historical wrong had been revealed by Bart Kahr within a much larger (and in many ways fascinating) 2015 article in the specialized journal Crystal Growth & Design entitled “Broader Impacts of Women in Crystallography”.
But very few people had read that article (as of now, 141, including me), so Knaggs’ contribution had remained obscure. Kahr explains:
One of the crystallographers profiled, Isabel Ellie Knaggs (1893−1981), made a major contribution to the X-ray analysis of organic compounds by establishing that carbon atoms in isolated molecules adopt tetrahedral coordination geometries. This determination was a capstone on the stereochemistry of the preceding 50 years. Another scientist, working independently, exclusively claimed this discovery. Thus, while studying the general disenfranchisement of women in science, a particular injustice not heretofore recognized came into focus.
The accepted “history” is found in places like the International Union of Crystallography (IUCr)’s own newsletter:
[Isamu Nitta] studied the crystal structure of pentaerythritol C(CH2OH)4 to establish tetrahedral carbon valence bonds. At that time, however, there was a report based on a space group assignment that the central C atom of pentaerythritol in the crystal was (tetragonal) pyramidal. Nitta (1926) found that the reported space group was erroneous and the C atom was actually tetrahedral.
No, he didn’t! He didn’t do that at all! (But he did serve as the IUCr’s Vice President from 1963-69, and so I suppose one gets to influence written history from that position.) Nitta ignored Knaggs’ work completely, and only in 1937 did he finally suggest (and still less forcefully than Knaggs had in 1929) that the bond architecture of the central carbon in methane-like compounds is tetrahedral.
Look at the image at the top of this diary. That is Knaggs’ 1929 structure for the methane derivative pentaerythritol tetraacetate. I mean, she drew you a frickin’ picture!
Knaggs also asserted quite correctly in her 1929 paper that the tetragonal pyramidal geometry reported by others in 1928 was mistaken, and she kindly drew you another picture:
“Such a distribution of the valencies of a carbon atom is quite unsupported by any physical or chemical evidence.” Is that not clear?
I would encourage you to read especially Kahr’s article, but Extance’s January 20 piece summarizes things pretty crisply if you have less time.
That ought to convince you that Knaggs is the one who established this iconic structure. So … let’s see what she actually DID!
I think I have found the most fun and coolest possible introduction to the basic idea behind X-ray crystallography: diffraction art! If you shine a laser through some regularly spaced openings, you get some outrageously amazing interference patterns. If you have a detectable pulse, I think you will really enjoy this:
Woo!
Now, if you didn’t know what the arrangement of the openings was, and you examined one of these series of patterns (plus you were a lot smarter and better at math than I am), you would be able to deduce what that arrangement was.
Knaggs used a very similar principle, but with X-rays instead of visible light, because atomic distances are tiny, and so is the wavelength of an X-ray, leading to more effective diffraction than you would get with visible light.
And, instead of a card with holes in it, she used a crystal of pentaerythritol tetraacetate, the molecule in the main diagram at the top. It was grown out of a dilute alcohol solution in Thorpe and Ingold’s organic chem lab over at Imperial College London (no doubt by grad students — thanks, guys!).
It’s kind of hard to grow a crystal of methane (which melts at -295.6°F) unless you live on Pluto or something, so that’s why she chose a molecule with heavier side groups, one that would be a solid at reasonable temperatures.
She hit the crystal with X-rays from many different angles, more or less like this:
The basic setup of an X-ray crystallography experiment
(She must have been careful with those X-rays, too, because she lived to be 87.)
In a crystal, all the molecules are in a repetitive 3-D pattern, so if you view a crystal from certain angles you see the atoms line up in rows, as in this tetrahedral crystal model that is similar to a diamond’s structure:
When X-rays strike atoms within those planes in the crystal at just the proper angle, you get constructive interference, and thus a spot of X-ray light on the film behind the crystal:
Use Bragg’s Law (2d sin θ = nλ), where λ is the wavelength of the X-rays, to find the best angle for constructive interference, or to find the distance between the planes (d) when you see a spot on the film
Knaggs’ supervisor, W.H. Bragg, enabled the calculation of that angle using the aptly named Bragg’s Law, for which he and his son won the Nobel Prize in Physics in 1915. Knaggs certainly picked a good guy to work with!
Here is just one of the many pictures she generated:
One of Knaggs’ X-ray crystallography photos. That’s iconic s**t right there
So, noting the positions and intensities of the spots in all her diagrams, she had hours of gawd-awful manual calculations to do, as described by J.D. Bernal in 1926. In his paper we are told that the crystallographer must solve equations like these for each spot, with no calculator:
Uh… Can I get an app for this?
Yuck.
So it’s certainly difficult to look at an X-ray diffraction pattern and be able to imagine anything at all about the structure of the crystal, unless you have a boatload of experience.
Fortunately, though, we can get a slightly more intuitive feel for the relationship of these dots to the crystal structure by using as an example the work of Rosalind Franklin, another previously underappreciated female crystallographer (but one you’ve probably heard of). Unfortunately, Franklin may not have been so careful with X-rays, because she died of cancer in her 30s, and that was a major loss to science, because she was truly brilliant.
Franklin and her understudy, Raymond Gosling, generated the now-famous Photograph 51, which gave Watson and Crick the parameters they needed to put together the double-helical structure of DNA.
Photograph 51
They say that Watson knew “X means helix” when he saw the photo, though he needed Maurice Wilkins’ help to get much more out of it than that.
But why does X mean helix?
There are two nice videos out there (one from the University of Hull and one from Steve Mould) that demonstrate this on an understandable scale. Each of them shines a laser through a spring and generates a pattern pretty similar to what Franklin got:
The spring acts as two sets of slits, one behind the other. Behold the X!
Back in Ellie Knaggs’ day, though, not many people had done crystallography at all. There was no intuiting anything. Seriously, look at Bernal's paper to see what she got herself into in order to solve a fundamental puzzle that nature had presented us with.
You know, there’s a rather select group of people we can look back on with a special esteem, because they showed us something important about our world that we didn’t know before. I have such a great respect for that. And for me, it’s time to induct another one into that circle, someone to whom I’d like to reach out across the years and say, “Thank you.”
Her name is Isabel Ellie Knaggs.
Isabel Ellie Knaggs (1893-1980)
