George C. Pimentel
1922 - 1989

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Matrix Isolation:
One of George Pimentel’s most fruitful areas of research

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Discovered in the Pimentel lab in 1954, the matrix isolation technique has been widely used in many areas of research. It was an idea whose time had come: George Porter was doing similar work in the U.K. at that time.

In his 1989 ACS Eminent Chemist interview, George Pimentel describes the evolution of the process worked out with post-doc Eric Whittle and grad student David Dows:

[In the 1950s,]
I was engaged in the use of infrared spectroscopy, which Pitzer had recognized as one of the most important techniques that chemists were not using then—an important way to learn about molecular structure.
[This led to studying the hydrogen bond and publishing a landmark book on the subject.]

The hydrogen bond turns out to be one of the most important bonds in biological systems of any, including the carbon/carbon bond, and so this has been a very important field. At the same time I was studying flames. Pitzer's work had tended to lead me to think about molecules that had unusual chemical bonding. They didn't fall into the normal pattern of chemical bonding and one finds this sort of species—so-called free radicals—abundant in flames, so I started trying to do infrared spectroscopy of flames.

This was very frustrating because we couldn't get the concentrations high enough to get characteristic absorptions of them, and one day, with a postdoctoral student of mine, Eric Whittle, sitting at a bag lunch at my office, I came up with this idea that maybe the thing for us to do was to try to trap these transient species of very short lifetime—micro-second lifetime—in solid inert gas at cryogenic temperatures. If we could trap them, then they'd have an environment that was totally inert and we could do leisurely spectroscopic study. That was the so-called matrix isolation technique. 

That was the early fifties and there was no experience in the field—no cell design or anything—so we had to do everything from scratch, but I was used to working with cryogenic liquids like liquid helium and liquid hydrogen so that didn't bother me a bit. We proceeded to develop techniques and equipment with which we could do this cryogenic spectroscopy. That was a nice test of stubbornness, because it took us about six years before we had our first real success. Of course we made progress all along the line, getting better and better at it and finding out what the variables were. But, finally we had our first success and from then on things have blossomed. A group down at Rice University recently compiled a bibliography of all of the papers they could find on using the matrix isolation technique. There were thousands of papers in that bibliography so it was a very successful technique. 

[A high school student taking a chemistry course] 
might think of the technique—matrix isolation—as analogous to what one pictures and hears about in science fiction of organisms being trapped in solid ice and by reason of the cold, low temperatures, all chemistry being stopped, you see, and then one could come back—you know, in this fictional line—years later and warm it up and you'd have your species back again. That's the general idea except instead of ice, the environment, which is what we call the matrix, is solid argon or solid krypton. In order to keep it solid you have to have the temperature 20 degrees kelvin or even lower—4 degrees kelvin with liquid helium—but then if we can get these very reactive molecules embedded in the material, then it's like the dinosaur in the ice. It's at such a low temperature, chemistry stops and you have all the time you want to study it. 

[Even if nothing happens while it's in this frozen state, you can] 
study its characteristic absorptions, and that's the valuable part of the infrared spectrum as far as a chemist is concerned—that every molecule has characteristic vibrational motions and those characteristic vibrational motions are manifested in characteristic frequencies that act like a fingerprint, and you can identify the species that's there by its fingerprint and decide things about its molecular structure. 

Eric Whittle (now retired back in Wales) wrote me a letter in August 2005 that included “a brief story of the original work” with the caveat: “In reading it, you must remember that memories going back 50 years are not always reliable.” However he had phoned Dave Dows, who had confirmed his recollection, with the same warning about memory.

When I joined George in October 1952 he had just received a research grant from ONR (Office of Naval Research). The objective was "to detect free radicals by infra-red spectroscopy." George was the leader and provider of expertise in spectroscopy while I had a background of gas kinetics. Dave Dows was a research student who completed our team of three. During the next two years much of our work involved trying to detect free radicals using methods that achieved little until near the end. In year 2 of the project, we decided that the most promising route lay via the detection of the radicals in the solid state which meant somehow trapping the radicals on a cold surface. Then we would look at their infra-red spectra. This was a tiny step in the right direction. We then embarked on a series experiments that yielded very little. The initial idea was to produce radicals in the gas phase by passing a suitable compound through an oven at a high temperature (600° C or more). The compound would decompose to give radicals and we would spray the exit gases onto a cold window (at 77K), which would trap the radicals so that their infra-red spectra could be obtained. 

I thought that there was some danger in these experiments that involved nasty compounds like HN3. Each experiment took several hours so we did them during the night when no one was around. George was often present and he always brought a watermelon which we put in the ice chest. The lab got pretty warm from our furnace so the melon was very welcome about 2 a.m. 

Again these experiments yielded very little and after several weeks we ended them (much to our wives' relief). At this point, we did something which, with hindsight, we should have done much sooner. We stepped back mentally from the project and decided that we really knew nothing about how we could preserve radicals on cold surface. We were trying to run before we could walk! What we needed to do was some control experiments. We needed to find a readily available radical or pseudo radical, preferably one that we could get out of a bottle.

I suppose that at this point George and Dave turned expectantly to me. I think I eventually suggested that we use N02. You don't normally think of this as a radical but every chemist knows about the equilibrium.

N204    2 N02

At low enough pressures and room temperature we would have mainly N02 but on a surface at 77K we would expect only N204. The i.r. spectra of both N02 and N204 are well-known so we could monitor what was happening. However, if we sprayed just a mixture of N02 and N204 on to a cold window, we would expect to see only N204. Therefore we needed a way to separate the trapped N02 molecules and stop them combining to give N204. In fact we needed a Matrix. One suitable diluent/matrix might be CO2 and so we tried spraying a low-pressure mixture of N02 + N204 with a large excess of CO2 on to our cold window. At the low pressures used there should have been very little N204 present. And it worked, provided the CO2/N02 ratio was large enough to prevent the N02 molecules from meeting each other on the cold surface, i.e. provided that the N02 molecules were trapped in the matrix. 

At last we were moving in the right direction though we still hadn't trapped any what I would call real free radicals. That came later after I had left Berkeley. Very tantalising to have to quit for career reasons just as the wagon began to roll.

Sydney Leach
, a post-doc with George in 1957, puts the work in a wider context in his article "In my Time: Scenes of Scientific Life" in the 1997 Annual Review of Physical Chemistry.


In June 1953 there was an international meeting in Paris on Molecular Spectroscopy, organized by the Société Française de Physique, where I presented the spectra obtained from discharges through N3H, NH3, and their deuterated equivalents. George Pimentel, who was present at the meeting, visited my laboratory in the rue Pierre Curie. He was intent on obtaining IR spectra of free radicals, and we discussed whether the Schuler tube could be adapted to study IR absorption of transient species by a method of discharge modulation. George invited me to spend a postdoctoral year with him to work on the possibility of doing free radical spectroscopy. It was not possible for me to come until 1956 and in the meantime both he and George Porter began work on matrix isolation absorption spectroscopy for the study of free radicals, respectively in the IR and in the visible/UV. Before I left for the United States I started a program in Paris, with a new doctoral student, Eva Migirdicyan, using matrix isolation techniques to study photochemical reactions at low temperatures.

I arrived in Berkeley in December 1956, nominally in time for the delivery of a Cary 20 visible/UV recording spectrophotometer with which I was supposed to set up a program of electronic spectroscopy of free radicals in matrices at 20 K, alongside George Pimentel’s IR program. It was my good fortune to be associated with George Pimentel, who, though only two years older than myself, was already a master scientist and a man of vision and action. His quick mind, analytical approach to practical problems, distrust of “theory,” general hands-on attitude, and truly infectious enthusiasm made me a willing adherent of the Pimentel scientific ethos in its local manifestation. He personified for me the best in American science.

The Department of Chemistry and Chemical Engineering, as it then was in 1956, had a great cast, many of them "retired" but still there in 1996. Ken Pitzer was the Dean, efficient and soft-spoken; Bill Dauben, besides being a fine organic chemist, was a leading light of the pool table sessions after lunch along with Andy Streitweiser; Bill Gwinn, serious but smilingly so, was always ready for a scientific discussion; and George Jura, who impressed me with his high-pressure experimental techniques, was attempting to create metallic hydrogen. Leo Brewer, Bob Connick, and the grand old man Joel Hildebrand were other members of the department with whom I discoursed. George Pimentel's group contained some lively doctoral students, among whom were John Baldeschweiler, Harmon Brown, Bob Curl, George Ewing, Ted Goldfarb, Dick Milligan, and Warren Thompson, whose later destinies were as varied as their personalities. One thing very clear was that US students (at that time) knew relatively little on entering doctoral studies, as compared with European students, but that with their fresh minds they quickly caught up and often surpassed the Europeans.

One of my first jobs in George's lab was to build an asbestos cubicle for each experiment that used liquid hydrogen so as to improve the precarious safety conditions. My Cary spectrophotometer did not actually arrive until May 1957, and in the meantime I advised on photochemical techniques and worked with many of the IR people who prepared samples which were later studied by me in the visible/UV. 

The Free Radicals Program, initiated in 1956 by the US Department of Defense but conceived and beautifully run and coordinated by Herb Broida and Arnold Bass at the National Bureau of Standards, was the beacon of research on free radicals. The raison d'être was the promise of improvement of propellants by using the energy released in the recombination, at the right moment, of trapped free radicals. Thus the Broida/Bass group included over 35 visiting scientists from the United States and Europe, working freely on various aspects of free radical trapping and associated physical and chemical properties. This splendid program was a fine example of "collective" small science, and a lot of good research was done. James Moyer, the "minder" of the program, speaking at the Fourth Free Radicals Meeting held in Washington in 1959, concluded that although "the program did not result in a so-called 'breakthrough' for new rocket propellants, [this] should disappoint only the space merchants. The remainder of the scientific fraternity is grateful for a vigorous novel approach to team research which resulted in the flowering of the state of the art in a field of universal interest and importance." 

This series of meetings initiated by the pioneer Paul Giguère (Monsieur Peroxyde) in Quebec in 1956, still continues, the Twenty-fourth Free Radicals symposium being held in Sweden in August 1997, with the same all-encompassing spirit of Herb Broida (and the Department of Defense of 1956) as to what constitutes valid free radicals research. A fitting testament to a great scientist, prematurely departed in 1978. 

Not long after I arrived in Berkeley, pushed by George Pimentel, who was funded by the USAF, I promised to give a presentation of my (future) Berkeley work at the next (second) Free Radicals conference, to be held in Washington, DC in September 1957. Since my Cary spectrophotometer did not actually arrive until May 1957, I had to work very hard during the summer in order to present a paper on "Matrix Isolation Method. Some Studies by Electronic Spectroscopy." The work I did in Berkeley on the spectroscopy and photochemistry of benzene, cyclooctatraene, and other molecules in matrices at 20 K, and which included an unrecognized anticipation of the Shpolsk'ii effect, was summarized in a talk given in Paris on my return but never written up in more extended form.

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In 1986, in a lecture delivered as part of his Welch Award celebration, George summarized the use of matrix isolation to date. The lecture was subsequently edited and published by the Welch Foundation as a monograph: Hot Chemistry in a Cold Environment—link to come, I hope. (My audio recording of most of the original talk is available on request: )

Hundreds of publications relating to Matrix Isolation have appeared. Current papers are listed on the MIDE web site (see below).

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The International Conference on the Physics and Chemistry of Matrix Isolated Species started in 1973, and since then has taken place every other year, alternating between Europe and the US. The 1983 US conference marked the first time that the meeting was organized under the auspices of the Gordon Research Conference.

At the 2005 international meeting, MATRIX 2005 , in Funchal, Portugal, an award was established in George’s name, marking the 50th anniversary of Matrix Isolation. See the Awards page on this site for subsequent awards.

The most recent conference, in 2007, was held at Bates College, Lewiston, ME 

The next one will be in July 2009, at Magdalen College, Oxford, UK. 

Matrix Isolation research features in many other conferences, including those of the Western Spectroscopy Association, USA. 

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I urge you to visit the Matrix Isolation web site, which includes the MIDE newsletter, for information on current conferences, publications, researchers, jobs, and more. The newsletter was edited for many years by Werner Klotzbücher, and taken over in 2006 by Travis Fridgen, who also established the website. He gave this overview in December 2006:

Dear Colleagues, 

First I would like to introduce myself. My name is Travis Fridgen and I am Assistant Professor of Chemistry at Memorial University of Newfoundland in Newfoundland and Labrador, Canada. I started receiving MIDE immediately after the 1995 Gordon Conference in Plymouth, NH, which I attended as a graduate student with Mark Parnis. In my experience Werner’s newsletter has been an invaluable resource in keeping me up to date with happenings in the matrix isolation community worldwide since its inception in 1991. Werner’s “Editor’s In-Basket” has been priceless in compiling the most recent publications from edition to edition. I don’t think that I am alone in missing this valuable source of information recently, especially since there are currently well over 350 subscribers to MIDE. I am currently setting up labs to spectroscopically probe ions and solvated ions in both the gas and matrix isolated phases. As such, I wanted to ensure that MIDE continues. In consultation with Werner and a few others in the matrix community I decided to set up a website which is dedicated to the matrix isolation community ( and to continue MIDE. I would like to thank Marilyn Jacox for the idea and content for the title webpage. I would also like to invite everyone to explore the website and submit comments and ideas for the website as I consider it to be a work in progress. The more ideas the better it can serve you. There is a “Contact” tab, which you can use to make suggestions. Also, there is a tab containing a list of researchers in the matrix isolation community, which you can sort by name, institution or country. Please ensure that your webpage is listed. If it isn’t there and you would like it linked, please go to the Contact tab and submit your information and I will add it as soon as possible. As well, if any of your students or colleagues would like to receive the MIDE newsletter please use the Contact tab and I will add them to the list for the next mailing. The Editor’s In-Basket will continue and follows in this edition for publications that appeared roughly in the month of October and early November. I will admit it is very likely not complete and is mainly to serve as an example of what the In-Basket will look like. I plan to put out the newsletter roughly monthly and include any news that has been submitted by you. The In-Basket will only include publications that have been submitted, by you! To submit the reference to your paper here is a link on the main webpage “MIDE Submissions” which will allow you to submit, preferably, the “digital object identifier” (DOI) for the paper which will allow people to simply click the link and be connected to your paper directly from the MIDE newsletter. Alternatively, you can submit the full details of the reference but I cannot promise to have time to find the DOI information so there may not be a live link to your paper unless you submit the DOI. I will archive all new editions of MIDE on the webpage in both .pdf and .htm format. There will also be a link to all of the previous editions of MIDE.

I hope that you enjoy the webpage and new format of MIDE. Again, I strongly urge you to submit any ideas you have, submit any news you want published (jobs offers, tips, etc.) in the next issue, and of course submit the references to your papers for the Editor’s In-Basket because 

“If news about YOUR research is missing
it is because YOU did not submit it”

Sincerely Yours, 
Travis D. Fridgen 

A Message from Werner:

Dear Colleagues, 

I have been taking care of MIDE for close to 15 years, and you all have noticed that I have found less and less time for it. Not that I have left the field, but new duties required a shifting of priorities. MIDE has been a source of information for many of us, has spread the news about conferences, job opportunities and our activities - it is dearly needed. And I was sad that I am no longer able to provide the kind of service I would appreciate myself. Thus I am glad, delighted and thankful that Travis has volunteered to pick up the torch and will provide a new and improved version of MIDE - including setting up a website. Please let us all assist him by sending in our references and news. There is an unsent pile of references of matrix work covering the last two years in my computer. I will sort the data over the Christmas break and mail it to Travis for dissemination. With my best wishes to all of you, 

-- Werner Klotzbücher

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Reminiscences, comments, and news about Matrix Isolation are very welcome and will be added to this page. Please e-mail them to me, Jeanne Pimentel .

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Copyright © 2003 - 2018 Jeanne Pimentel

Welcome | Commencement Address | Selected Biographical Summaries
Awards Named for George C. Pimentel | Students and Research Collaborators
Continuing Influence: Mars Research  | Matrix Isolation
Pimentel Archive at UC Berkeley
  Site Map | Contact Us