Structure and Magnetism in Metal Coordination Complexes of Nitroxyl Free Radicals

General Information, science
Published April 3, 2017
Introduction

This article is based on one I wrote in the 1990s and submitted to the Journal of the American Chemical Society. It was rejected; more on that later. I no longer have the original paper but there are certain facts about this research that are very fresh in my mind, despite the lapse of about twenty years. This story might be interesting to only a few people yet there are some fascinating results of more general interest that I am afraid will get lost if I do not set them down here.

I attended the University of California at Irvine in the late 1970s to early 1980s, earning my Ph.D. in Chemistry in 1984. I learned X-ray crystallography from my research advisor, Professor Robert J. Doedens, who had learned it from Larry Dahl, who learned it from Rundle, who learned it from Pauling, who learned it from Dickinson, who learned it from Burdick, who learned it from William Bragg. I wanted to learn it because it was mysterious to me: how one could start with a crystal of a compound and end up, through some arcane process, with a picture of the atoms inside the crystal. It is a beautiful discipline at the intersection of physics, mathematics, chemistry, and biology.

The diffractometer at Irvine was in the basement of the Physical Sciences building and had no low-temperature capabilities. All data collection was performed at room temperature. This is an important fact in the story.

Nitroxyl Radicals and Metal Complexes

Stable organic nitroxyl radicals (or nitroxides) can be considered organic derivatives of nitric oxide. One of the simplest stable nitroxyl radicals, di-tert-butyl-N-oxyl, has two tert-butyl groups attached to the nitrogen atom. The bond order of nitric oxide (2.5, a triple bond plus one anti-bonding electron) is lowered to 1.5 in nitroxyl radicals, with the other two bonding electrons now participating in the nitrogen-carbon bonds.

Russell Drago, in the exploration of his acid-base parameters, found that one stable radical, 2,2,6,6-tetramethylpiperidine-1-oxyl, or TEMPO, formed a 1:1 complex with the copper-based Lewis acid bis(1,1,1,5,5,5-hexafluoroacetylacetonato)copper(II) (henceforth Cu-bis-hfac). The structure of this complex was unknown at the time but Drago, in an edition of his classic textbook Physical Methods in Inorganic Chemistry, in a problem set at the end of a chapter, proposed a structure in which a lone pair of electrons on the nitroxyl oxygen is coordinated to the copper atom through a sigma bond.

Drago found that the copper-nitroxyl complex also exhibited strong magnetic coupling which is why Bob Doedens was interested in it. Doedens had published a well-received paper on the magnetic properties of copper acetate dimers and wanted me to determine the structure of Drago’s free-radical complex. Which is why, as his graduate student, I grew crystals of the complex and performed a series of structure analyses on them. Normally one structure would have been enough; but at room temperature the CF3 groups exhibited a lot of rotational disorder, leading to an R-factor of about 10%, at the limit of acceptability. The structural result was reproducible, however, and also surprising(1).

The angles around the Cu atom of the Cu-bis-hfac:TEMPO complex, in the crystalline state, were halfway between square pyramidal and trigonal bipyramidal. The pattern of four short and one long bond led us to describe it as distorted square pyramidal. The first surprise was that the long bond was between the Cu and an hfac oxygen atom, rather than to the TEMPO oxygen as one might guess at first.

The next surprise was that the coordination geometry of the TEMPO molecule was not as Drago expected, through a lone pair, but apparently through a TEMPO antibonding orbital. In retrospect this makes perfect sense because the antibonding orbital, a SOMO (semi-occupied molecular orbital) is also the HOMO (highest occupied molecular orbital), and the HOMO is the expected choice for metal coordination. However, since the complex is diamagnetic, meaning the spins are paired, one might also expect the unpaired electron on the copper atom to spend some time in that antibonding orbital, which should lead to a lengthening of the N-O bond. No such lengthening is observed. Rather than conclude that the coordination might be through a one-electron bond with only through-space coupling accounting for the diamagnetism, our paper reporting the structure simply avoids any detailed discussion of the bonding. There is, of course, the possibility that the Cu electron occupies the antibonding orbital but the Lewis acid nature of the Cu complex attracts the nitroxyl antibonding electron just enough to cancel out any structural effect. This seemed unlikely, but without any certain knowledge of the complex’s molecular orbitals, it appeared safer to simply say nothing.

Subsequently Leigh Porter joined the group and was working on copper acetate dimer derivatives, when I suggested he try using TEMPO to coordinate to some of them. The result(2) was an unusual copper dimer type where the TEMPO-Cu interaction superseded the normal Cu-Cu interaction.

Meanwhile Leigh and I started looking at other first-row transition-metal-hfac complexes, and produced structures and magnetic results for the corresponding Mn TEMPO adduct, and the Mn, Co, and Ni PROXYL adducts(3,4) where PROXYL is the five-membered ring analog of TEMPO. All these structures are trans-octahedrally coordinated with the metal atom on an inversion center in space group P21/c.

The magnetic results for the Mn(II) complexes showed linear Curie law behavior at low temperatures with increasing paramagnetism above a certain temperature. This was treated using the usual magnetic modeling techniques with a least-squares fit to find coupling parameters.

One of the structural features of the TEMPO complex was a fairly large N–O–M angle and a relatively large thermal ellipsoid for the oxygen. Modeling the oxygen as disordered, however, did not produce stable refinements using the room-temperature data we had, and we concluded that the oxygen was not significantly disordered.

I remember talking about this a couple of years later during a presentation to another crystallography group and the head crystallographer there made a disapproving noise at that point but did not say anything in particular—he presumably knew that it was my research advisor’s conclusion rather than my own. In fact I had not questioned this in my own mind. A few years of study and about ten structures were not enough experience for me to form my own opinions about such matters.

This turned out to be a mistake, but one can’t really blame Professor Doedens, as there were no low-temperature capabilities at Irvine at the time.

Much Later. . .

Now we fast-forward a few years to find me at Georgetown University working for Professor Michael T. Pope and studying tungsten oxide cluster anions (polyoxometalates) using the latest CCD diffractometer with a low-temperature attachment. Using a CCD detector transformed the data collection process from one that took several days or a week or more, into an overnight process for most structures. And that was even allowing for collecting data with multiple redundancy to calculate an absorption correction. This allowed us to collect data routinely on structures that would have been simply impossible otherwise.

This allowed me to gain a lot of experience quickly. In addition, Professor Geoffrey Jameson was a valuable on-site resource until he moved back to New Zealand to explore protein crystallography.

One of the best things about the Pope group was that I was able to do some independent research. Purely out of curiosity I decided to look at the Mn(II)-TEMPO complex again but at low temperature since the starting materials were now commercially available. Using liquid nitrogen to cool the crystals resulted in slowing down and stopping the rotation of the CF3 groups, yielding much more accurate data.

Comparing the low-temperature (LN2) structure with the published room-temperature structure gave me a shock. The nitroxyl ligand appeared to have moved, and had a smaller Mn-O-N angle. I did a few more structures, and summarized the results in a preliminary talk at an ACS meeting(5). Perhaps, I thought, there was some connection between the moving ligand and the magnetic results. I contacted Bob Doedens and he arranged for me to send crystals to UCLA for data collection at liquid helium temperatures. I fully expected the liquid helium data to show that the ligand motion continued as the temperature decreased. But when I got the results, the LHe data showed the same structure as the LN2 data—even worse, the errors for the LHe data set were higher, although not high enough to throw doubt on the structure. It seemed to me at the time that publishing their data would be more of an embarrassment for UCLA without really contributing anything new. In hindsight I should have included the structure from their data anyway.

At this point I had several sets of data: room temperature, LN2, a few in-between, and LHe. The two sets at low temperature showed essentially identical structures, both quite different from the room temperature data set. So I started collecting data at more temperatures intermediate between LN2 and room temperature, as well as another at room temperature to reproduce the previous result.

The room temperature results confirmed the (by then) fifteen-year-old structural study from Irvine discussed above, while the intermediate temperature results showed a fascinating pattern. Starting at the temperature where the magnetism becomes nonlinear, the nitroxyl oxygen begins to show disorder between two positions. It wasn’t the entire ligand that was moving, contrary to my earlier hypothesis; it was  just the nitroxyl oxygen that moved. The motion of the oxygen is like a dog’s tail wagging, back and forth.

I went to extreme lengths to make sure this temperature-dependent disorder was not some aberration caused by random disorder that seemed systematic due to chance. First I used different crystals at each temperature. Then, I repeated the experiment using the same crystal at different temperatures. I performed about twenty structural experiments. All the results were consistent. Nitroxyl oxygen disorder started at about the same temperature where the magnetic results changed from linear to non-linear; increased with increasing temperature until the disorder was 50%; then remained constant. As before, the room-temperature refinement was not stable with a disorder model, but this may be due at least in part to the influence of the highly disordered CF3 groups.

(About this time I published a structure in Acta Cryst. for the Co(II)TEMPO complex(6) which noted there is disorder in this structure, and that I was exploring whether there was also disorder in the Mn(II) complex, and whether it was correlated with the magnetic results.)

After a while I had a set of temperature-dependent disorder data for the nitroxyl oxygen and I wanted to see if I could somehow relate it to the magnetic data. To do that I had to make some assumptions. I assumed that as the oxygen became disordered, there were three vibrational isomers, or “vibromers”. These were: 1) both oxygens in the ground state (low-temperature configuration, AA); 2) one oxygen in the ground state and the other in the higher-energy position (AB); and 3) both oxygens in the higher-energy position (BB). I assumed that the statistical distribution of these vibromers followed ordinary statistical laws, of the A(squared), 2AB, B(squared) type where A is the low-temperature position of the oxygen atom and B is the higher-energy position, and so 10% disorder of the nitroxyl oxygen in the structure means a distribution of 81% of the AA vibromer, 18% AB, and 1% of the BB vibromer at that particular temperature.

I then assumed that all three vibromers follow the Curie law, as one might expect for manganese complexes, and calculated what the magnetic susceptibility would be for various spin values of the different vibromers as the vibromer composition varied with temperature. I should note at this point that I had the help of Dr. Andrew McDaniel, a mathematician I have known for decades, and who is proficient in using Wolfram’s Mathematica program, to do the calculations for this project.

The results were stunning, as the susceptibility curve was reproduced almost exactly by using spin values of 3/2, 3/2, and 5/2 for the AA, AB, and BB vibromers respectively. No other combination of spin values was even close. No curve fitting was used to obtain the spin values or the curve calculated from them. This was clearly an improvement over the curve-fitting method used in the original paper. I had no explanation as to why those particular spin values should be the correct ones, but they fit.

I was also able to use elementary thermodynamics to calculate the enthalpy and entropy of this first-order process. I wrote up the results and sent it off to JACS, the Journal of the American Chemical Society. Since there were numerous structures in the paper, and JACS at that time required a table of printed structure factors for each structure, I printed them out and sent the resulting thirty-pound crate by parcel post along with the manuscript.

I had high hopes for this paper, as it was the first one to model a magnetic susceptibility using only multiple-temperature X-ray structural data and a few assumptions without any curve-fitting.

Hopes Dashed

When the reviews came back, I was bewildered. One of the reviewers said the paper was remarkable but he was certain something was wrong with it, he just didn’t know what. A second one said the paper would be worthy of JACS as soon as I had an explanation of why the vibromers had those particular spin values (another twenty years have passed and density functional theory still doesn’t have an answer for that one). The third reviewer said the whole thing looked fine but it belonged in Inorganic Chemistry instead of JACS.

The editor was somewhat apologetic but had to reject the paper given the referees’ comments. At that time papers submitted to JACS were not automatically re-routed to another journal (here, it would have been Inorganic Chemistry) as they were a year or so later if a referee had suggested it. Disheartened, I went back to my regular research which, after all, was going quite well. I couldn’t stomach mailing another thirty-pound box of structure factors only to be told the results weren’t believable.

A bit later I gave a poster on this subject at an American Crystallographic Association (ACA) meeting(7). The posters were up for awards, and I got grilled by the judges who explained to me they couldn’t believe I had found such a nice example of temperature-dependent disorder that correlated with magnetism, and much like the first reviewer, suspected something was wrong but they couldn’t decide what it was. I didn’t get the award, which didn’t bother me, but the judges remained unconvinced, which did.

So What Does it Mean?

There are plenty of systems, compounds or molecules, that display higher spin states with increasing temperature. Presumably as the temperature increases, electrons are bumped into higher energy states as thermal energy overcomes pairing energy. Normally one doesn’t see a specific change in molecular or atomic positions as this occurs. However, in this case, the N–O “wagging” motion of the nitroxyl oxygen needs very little energy, and so occurs at relatively low temperatures, and the motion presumably causes a change in orbital interaction between the nitroxyl oxygen and the metal center, which correlates with a change in magnetic behavior. This is an unusual case of a specific low-energy observable atomic motion that correlates with a change in magnetic behavior. Not earth-shaking,  but certainly of general interest to chemists.

It’s Just Waiting for Someone

It’s curious to note that if the original structures had been all performed at low temperature, as many are routinely now, none of this would likely have occurred.

In the meantime I have done a lot more X-ray structures, including some very nice ones due to having had some excellent co-workers, and it no longer bothers me that this work has been mostly ignored. Like most basic research there is no obvious practical use for these compounds or the observed magnetic behavior. Still, if anyone is interested, the compounds are easy to make, the crystals easy to grow, and if you have the capability of measuring structures at multiple temperatures, and can deal with the magnetic data that is already published (or can generate your own), there are still some quite interesting tidbits to be explained. For example, in the corresponding Ni(II) complex, the disordered positions of the nitroxyl oxygens are reversed with respect to the manganese complex. That is, the ‘A’ low-temperature oxygen position in the Mn(II) complex corresponds to the high-temperature ‘B’ position in the Ni(II) complex. This must be due to switching of the energy levels with respect to the two metal complexes, but it certainly would be nice to explore further.

Anyone seriously interested in any of this is encouraged to contact me.

– Michael H. Dickman

References

1. “Structure of Bis(hexafluoroacetylacetonato)(2,2,6,6-tetramethylpiperidinyl-1-oxy)-copper(II), a Copper(II)-Nitroxyl Radical Complex with Substantial Magnetic Coupling” Dickman, M. H.; Doedens, R. J. Inorganic Chemistry 1981, 20, 2677-2681.

2. “A Novel Variation on a Classical Dimeric Structure Type. Preparation and Structure of the Metal-Nitroxyl Complex [Cu(O2CCCl3)2(Tempo)]2” Porter, L. C.; Dickman, M. H.; Doedens, R. J. Inorganic Chemistry 1983, 22, 1962-1964.

3. “Bis(nitroxyl) Adducts of Bis(hexafluoroacetylacetonato)manganese(II). Preparation, Structures and Magnetic Properties” Dickman, M. H.; Porter, L. C.; Doedens, R. J., Inorganic Chemistry 1986, 25, 2595-2599.

4. “Bis(nitroxyl) Adducts of Cobalt and Nickel Hexafluoroacetylacetonates. Preparation, Structures, and Magnetic Properties of M(F6acac)2(proxyl)2 (M = Co2+, Ni2+)” Porter, L. C.; Dickman, M. H.; Doedens, R. J., Inorganic Chemistry 1988, 27, 1548-1552.

5. “Variable Temperature X-ray Crystal Structure Analysis of a Nitroxyl Radical Adduct of Manganese(II): Motion of a Ligand and Correlation with Magnetic Behavior” Dickman, M. H., Inorganic Division Abstract #676, 211th ACS Meeting, New Orleans 1996.

6. “Bis(2,2,6,6-tetramethylpiperidinyl-1-oxy-O)-bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato-O,O’)cobalt(II)” Dickman, M. H., Acta Crystallographica 1997, C53, 1192-1195.

7. “Thermodynamics and Magnetism of Nitroxyl Complexes from Variable–Temperature X–ray Crystallography” Dickman, M. H., American Crystallographic Association Abstract #P248, 1998 ACA Meeting, Arlington, VA.

A Potter’s Field

etymology, General Information

It’s easy to imagine how the technology of pottery might have begun in the distant past. Once fire was used routinely, it would be natural to build fires in the open, maybe dig out a depression in the dirt to hold the wood. Then it wouldn’t take long in the scheme of things to notice that after a rain, some of these depressions drained relatively quickly, while a depression dug into clay became hardened after a fire and would hold the rainwater for a long time. It’s a small step from there to fashioning bowls out of clay and putting them in a fire so they could be used to collect, hold, or carry water.

Something like this could have happened more than once and in more than one place. Pottery is one of humankind’s oldest technologies, naturally not as old as fire but perhaps before agriculture. But this isn’t about the origin of pottery; this is about the origin of the term “Potter’s Field”.

A Potter’s Field, as the term is used today, is a burial ground for the poor or indigent who could not afford to buy a plot in a private cemetery. People often wonder where the term originated. If you look it up you will quickly find a reference to the New Testament. Specifically, to Matthew 27:7—“And they took counsel, and bought with them the potter’s field, to bury strangers in” (King James Version). In this passage “they” are priests and the “them” happens to be the thirty pieces of silver, blood money paid to Judas Iscariot, who in Matthew’s version, gives the coins to the temple priests before hanging himself. So in Matthew the term “potter’s field” appears as a place to bury—not the indigent, but rather strangers. This may not be an important distinction because strangers might well be indigent. But curiously, in the next verse we learn “Wherefore that field was called the Field of Blood unto this day.”

So the original “Potter’s Field”, if you wish to believe this is the first, had a completely different name. Perhaps we shouldn’t be surprised that modern versions are not called Fields of Blood because that is rather unappealing compared with Potter’s Field. However, I would suggest there may be a different reason to call such a burial ground a Potter’s Field, perhaps the same reason the priests would buy a (lowercase) potter’s field as a burial ground rather than some other piece of land.

First, though, I should point out that the other Gospels in the New Testament don’t mention a potter’s field at all. They don’t even agree on how Judas Iscariot died. The death of Judas is one of those problematic points in scripture, but my interest here is not to delve into biblical scholarship. Whoever wrote the Gospel of Matthew told about priests who bought a field, once used for digging clay, as a burial ground, which was then called the Field of Blood. The term Potter’s Field may have originated in this story, either at the time or later; it isn’t clear. That, in fact, is my point: it isn’t a cut-and-dried case for the origin of the term “Potter’s Field”. It is, of course, a possibility.

There is another possibility. Perhaps people have used potter’s fields as burial grounds for a long time before the Gospel of Matthew was written. The main feature of burial is that a hole must be dug. It’s not always easy to dig a hole in the ground. Before iron was mined or steel was invented and shovels were manufactured using it, the job of digging was an even more onerous one. The profession of grave-digging was one for strong, younger men who would need to be paid for their labor. Who else had to dig holes for a living? Potters did. People who made things out of clay had to dig up the clay from fields where it was close to the surface. And why would they fill in the holes? They wouldn’t, because there was no reason to do so. They might come back later and want to dig there again.

So the main features of an actual potter’s field are first, the clay is near the surface so the ground is probably not well-suited for agriculture; and second, there are holes dug by the potter where clay was extracted. Holes that are not filled in, just waiting for someone to dump a body there.

Such a place would be quite natural to use, hopefully after the potter has left or sold it, for burial of the poor who could not even afford to pay the grave-digger’s fee. And since that’s the case, it would be strange indeed if during the ten thousand years or so that pottery has been made, that potter’s fields were not used as convenient burial grounds, and likely earlier rather than later in that history. I would suggest (without evidence, of course) that the priests in Matthew were not doing something unprecedented or even unusual by using a potter’s field for a cemetery, but instead something that had probably happened many times in the past.

As for the exact phrase “Potter’s Field,” I don’t know of any evidence that it was used before the Gospel of Matthew in connection with burial. So this might be the technical start of the phrase, even if the concept was not new. As often happens, as with pottery itself, the mists of time obscure the origin of ancient things.

The Moon’s Not a Balloon: the Moon Illusion

General Information, science,

Recently, a discussion of the “size of the moon” illusion has once again surfaced (see http://nautil.us/issue/24/error/your-brain-cant-handle-the-moon-rp).

(Caution: snarkiness ahead) There has been a lot of drivel written on this common phenomenon, mostly by people who don’t know what they are talking about. At the risk of adding even more drivel, I’m going to explain the effect so nobody else need ever write about it again.

First, some definitions and comments, so we all know what is being discussed.

1. The moon looks larger when it is on the horizon, rising or setting, than it does when it is higher in the sky.

2. It’s not just the moon, of course: the sun provides the same effect, but with the moon you are less likely to go blind staring at it.

3. The effect can be seen by people with only one eye (that is, it has nothing to do with binocular vision).

4. The effect can be seen even if you are on a ship in the middle of the ocean, with no trees, mountains, or buildings at the horizon for comparison.

Now for the explanation: it’s due to perspective.

That’s it. No need for fancy psychological/physiological theories.

“But, wait! Isn’t there more? Don’t statements numbers three and four completely rule out the possibility of perspective as the controlling factor?”

Um, no. No, they don’t. Humans perceive distance in a couple of ways. One is, to be sure, by using binocular vision. When we look at something nearby with both eyes, things farther away can’t be seen clearly because the retinal images of the distant objects can’t be fused, and vice versa when looking at distant objects. But the moon at the horizon, as well as the horizon itself, are effectively at infinite distance as far as our eyes are concerned. Binocular vision is irrelevant when considering the moon illusion.

Another way we judge distance is by the vanishing point. There is a well-known optical illusion of two lines that converge in the “distance,” and one object at the front of these lines, and the same object at the farther end. Even though the objects are identical, the one at the far end looks larger. This is the Ponzo illusion (see http://en.wikipedia.org/wiki/Ponzo_illusion). This is the one, essentially, that produces the illusion of the changing size of the moon. We compare the moon with other things at the horizon, and they look really, really small like the lines in the distance of the Ponzo illusion. The moon looks very big by comparison. The vanishing point causes us to estimate the size of the moon as much larger than anything we see on the Earth. When the moon is higher, there is no vanishing point to compare it with, and instead we compare it with the width of the sky. The sky is very wide, so the moon looks smaller.

“Ah,” comes the reply, “but I’ve got you now. Statement number four completely contradicts what you just said. There are no buildings, mountains, or trees on the ocean! You totally overlooked that small detail.”

Well, no, I totally didn’t. While it is true that there are no objects on the horizon at sea, there is still a vanishing point. On the ocean, the vanishing point is made of ocean waves and the reflections of the moon from those waves. The reflections form an isosceles triangle with you at the bottom, the short side, and the other two long sides pointing toward the moon, at the vanishing point, just as in the Ponzo illusion. It isn’t necessary to have mountains, trees, or buildings at the horizon, just a vanishing point. The ocean waves also look smaller in the distance, of course.

Even on “a featureless plain on a dark night” there is enough light furnished by the moon to reflect off surfaces and provide you with a vanishing point.

One more thing. It is true that the moon looks large at the horizon in comparison with the vanishing point, and smaller when it is high in the sky when there is no vanishing point. But the moon really is very large compared with any object on Earth. So maybe the real illusion is the fact that it appears so much smaller when it is higher in the sky.

You’re welcome.

The Alchemy of Seasoning Cast-Iron Skillets

science

I like using seasoned cast-iron skillets. Cast iron, before it is seasoned, is a metallic gray color and can rust quickly. Cooking food in an unseasoned cast-iron skillet will likely lead to a massive stuck-on clean-up job. But a glossy black seasoned cast-iron skillet is practically non-stick and resists rusting by comparison. So what’s going on? What is this mysterious “seasoning” thing, anyway, and how do you transform a cast-iron skillet into a seasoned one?

Hey, I know! Let’s look at the internet to find out!

Okay… (Googling madly) Hmmm… heat the skillet, coated with a thin layer of fat or oil, everyone says that. But then the disagreements start. The trick is to use animal fat, that’s how your great-great-Grandma did it. Or wait, not animal fat, but vegetable oil — any vegetable oil. But no, in particular, flaxseed oil, because it polymerizes. Yeah, that’s the ticket, polymerization. And don’t heat it to the smoking point, heat it to 200° F, or maybe 350°, or no, 400–500°.

Okay, got it? Yeah, me neither.

Being a chemist myself, this situation seemed rather unsatisfying. There is no consensus about exactly how to season a cast-iron skillet or what precisely it is that seasoning does. Maybe there are scientific papers out there on the subject but they haven’t appeared in my internet searches yet. So what can my chemistry training bring to this discussion?

First, let’s start with some observations, either from my own experience, or things that everyone seems to agree on. As mentioned above, cast iron starts out gray. Seasoning, which involves heating some oil or fat in the skillet, produces a black, glossy, hard, non-stick surface. This surface can be removed, it is said, by harsh lye soap or cooking acidic foods like tomatoes in the skillet. Leaving water in the seasoned skillet for an extended period leads to rust, which then must be scraped off with sandpaper or salt, and then the skillet must be re-seasoned. Repeated seasoning is reputed to build up a layer of seasoning that works better.

What does all this mean? To a chemist, one key word in the paragraph above is “surface”. Many strange things happen at the surface of a metal, especially when organic compounds are involved. All sorts of chemical reactions, like hydrogenation, de-hydrogenation, carbon-carbon bond splitting, and carbon-carbon bond formation can occur. And yes, polymerization.

Which brings me to the subject of flaxseed oil and polymerization. As much as I would like to congratulate the people who came up with this hypothesis on their impulse to use a scientific explanation to address the issue of seasoning, I would even more strongly like to point out the inconsistencies in their position. First (and don’t take my word for it, look up “seasoning cast iron flaxseed oil” on the internet), the idea is basically that using an unsaturated oil like flaxseed is best to season cast iron because unsaturated oil polymerizes on the surface and that is what makes the surface non-stick.

Unfortunately for this hypothesis it runs into several problems. First, if it were that simple, then using a thick layer of flaxseed oil would be best, because after heating it produces a detectable plastic-like layer of polymerized oil. Which, however, is sticky, soft, and not a suitable cooking surface. Second, practically any fat or oil, saturated or unsaturated, is reputed to result in a good layer of seasoning, provided it is thinly applied, so unsaturated oils producing polymerization may not be the essential thing. Third, seasoning is a black, hard, glossy layer, and polymerized flaxseed oil is none of these.

Which brings us back to the main question about seasoning: What is it? If it’s not a polymer, what could it be?

Well, I’m not 100% certain that polymerization isn’t part of the answer. But the description of a hard, black, glossy layer does suggest something: carbon. More precisely, carbon and iron, or “iron carbide” which isn’t a single compound but rather a solid solution of carbon in iron: an interstitial compound of varying composition. Cementite, epsilon carbide, and Hågg carbide are names of interstitial iron carbides of various Fe/C ratios. Cementite for example, according to our good friend Mr. Wikipedia, is a hard, brittle ceramic material. Ceramics tend to be good non-stick cooking surfaces but usually don’t conduct heat very well. In this case, it would be a thin ceramic layer on top of metal, fine for cooking.

Okay, what does that mean for the process of seasoning? It could mean that when oils or fats are heated on cast iron they decompose to elemental carbon which dissolves at the iron surface to make a thin ceramic non-stick coating of “iron carbide”. This would imply that the composition of the starting oil or fat isn’t important, but a high enough temperature to decompose the oil or fat is crucial. Since the iron might act as a catalyst for the decomposition, that temperature might be below the measured smoking point of the oil or fat; without measurements I have no specific ideas about that. Subsequent seasonings and continued use might allow this carbon layer to penetrate a bit farther into the metal, producing a thicker non-stick surface.

And that’s it. Let me be the first person to emphasize that the foregoing is a hypothesis, a guess that fits the facts as I see them, and could turn out to be wrong in light of more facts or measurements. As such, I encourage anyone to point out any faulty reasoning, or facts that would tend to show where it is incorrect. That’s how science works. Is there anyone with LEED (Low Energy Electron Diffraction) measurements of seasoned cast iron?

Now, people may swear by their particular method of seasoning (whatever it is) and claim that their method gives much better results than other methods. To which I would say, provide evidence. Make objective measurements of some kind, to show that flaxseed oil, or bacon grease, or coconut oil, or doing it during a full moon, or whatever, gives the best results. Because without objective measurements, it’s just a lot of opinion about what’s best, and it’s not science but alchemy. However, objective measurements are difficult to come by in this game. So maybe we’re stuck with alchemy for now.

AnimaCrostics by Cynthia Morris

Acrostica

This post has been updated (February 2016) for Cynthia Morris’ second volume and name change of the series. The original post was from December, 2014.

A marvelous new collection of acrostic puzzles for the younger set!

Sometimes, things just seem to work out. Back in 2014 I was working on improving AcrostiComposer, the software I use to write acrostic word puzzles. I was thinking about publishing a book of puzzles, and found out that I needed to be able to adjust the margins in the PDF file of the finished puzzle.

As I was doing the programming for that, I got email from Cynthia Morris, asking me if it would be possible to add adjustable margins to AcrostiComposer, because she wanted to publish a book of her puzzles! Cynthia has the cool website AmericanAcrostics, with acrostic puzzles she writes having an American history theme. The book she wanted to publish turned out to be a book of acrostic puzzles written specifically for young people.

I’m unaware of any other book of acrostic puzzles written for kids. It’s difficult enough to write them for adults without taking into consideration the vocabulary, interests, and understanding of younger students. But Cynthia has done it, and what an amazing success it is! The two volumes are titled AnimaCrostics and is written for eight-year-olds and up — but five-year-olds might be able to work on it with help, and there’s no real upper age limit. The focus of this volume of puzzles is animals and there is a friendly, jolly penguin on the cover.

Cynthia didn’t just write the puzzles and publish them. She has clear step-by-step instructions on how to solve the puzzles, little hints for more difficult words, and she even took the book to a classroom for battle-testing. The kids (and their parents) loved it.

The best kind of learning is the kind that doesn’t seem like learning at all. That’s what these puzzles are. They’re fun and yet they are crammed full of information. Solving them means getting the spelling correct, as well as perhaps learning a new word or two per puzzle. The quotes are fun and educational, too. In short, if you need gifts for kids, I can wholeheartedly recommend these puzzles. Cynthia Morris has done a fantastic job with AnimaCrostics and this book deserves a place on every young person’s bookshelf.

My New Book

Acrostica

They say any idiot can publish a book these days, and to prove that’s true my book “Acrostica I” is now available on Amazon. “What’s an Acrostica?” you ask. Well, funny you should ask because I’m here to tell you.

Everybody knows about crossword puzzles but acrostic puzzles are a bit more obscure. Invented by Elizabeth Kingsley in 1934, there has been a small but devout following ever since. Acrostic puzzles today can be found periodically in the New York Times and the Wall Street Journal. I began writing them over ten years ago, and five years ago started my own website acrostica.com to display them and allow visitors to solve them. Check them out!

I’ve put fifty puzzles into my first volume of acrostic puzzles, titled “Acrostica I” published by Pocamug Press.

Update (December 2018): There are now six volumes and more coming in 2019 if all goes well! Thank you if you’ve purchased one (or more)!

Who?

General Information

I grew up thinking that “Michael Dickman” was an unusual name. So in the late 1970s when it was time to publish my first paper, I went with my first name and last name only. A short time later the professor who was submitting the paper called me into the office. “There’s another chemist named Michael Dickman,” he informed me, “an organic chemist in Canada, who has published papers before you. We need to change your name.” So I added my middle initial, and from then on published as Michael H. Dickman. But, what an odd coincidence, I thought. Both of us chemists, publishing papers. It gave me an undeserved feeling of pride in my name, a feeling that us Michael Dickmans weren’t just nobodies, we were making a difference in the world.

Fast forward a couple of decades to the advent of the internet and searching with Google or various predecessors. Another Michael H. Dickman author popped up, involved with learning or leadership or both. And then there was a Michael Dickman who published poetry, and one who was in Minority Report, the movie based on a Philip K. Dick story. Oh, wait, those last two are the same person, and he has a twin brother Matthew.

Well, at least the twin wasn’t also called Michael. So now I’m like, “Get off my lawn, you kids!” Because middle initial.