Should the Past Define the Future of Interpretation of Infrared and Raman Spectra?

When I was invited to contribute a short article about a current challenge or subject of concern, I paused. The problem I faced was a matter of how close I was to what is going on in the molecular spectroscopy world currently. I retired some time ago, and my primary contact since then has been with participants in and lecture staff for the infrared (IR) and Raman spectroscopy short courses at Bowdoin college, where I have been working as an instructor.  

During my early days, I was fortunate to learn with the course from the staff at that time, Prof. Richard Lord of Massachusetts Institute of Technology (MIT), Lionel Bellamy of the Ministry of Supply in the United Kingdom, Prof. Foil Miller of the University of Pittsburgh, and Prof. Dana Mayo of Bowdoin College. As time went by, guest lecturers were invited and sometimes became permanent staff members. These included over the years Prof. Ellis Lippincott of the University of Maryland, Dr. Jeannette Grasselli Brown of Sohio Research, Norman Colthup of American Cyanamid, Prof. Peter Griffiths of Idaho University, Dr. Harry Willis of Imperial Chemical, Prof. James DeHaseth of the University of Georgia, Dr. Richard Nyquist of Dow Chemical, and so many more (I apologize to those whose names are not included here). In addition, I participated as a laboratory instructor in courses at the University of Minnesota (with Prof. Bryce Crawford and Prof. John Overend), and at Fisk University (with Prof. Nelson Fuson). It was a good thing there were so many courses because I sure had a lot to learn, and indeed I did learn from every one of them.

What became obvious over the years was that outside the colleges and universities mentioned above (plus a few others I was not aware of), there was a lack of good instruction at the university level in interpretive IR and Raman spectroscopy-that is, in the assignment of bands in the spectrum to functional groups and, at times, to an entire molecular structure. The needs of industrial chemists were being overlooked and in some cases the need for good interpretive skills in the university environment was also overlooked. This situation was true regardless of which subdivision of chemistry one considered, whether it was analytical, organic, biochemistry, inorganic, forensic, environmental, chemical engineering, polymer science, or another area. The poor understanding of how to use and assign absorption bands in the IR spectrum to their origin in functional groups was astounding.

At first, I wondered why this topic wasn’t being included in a university curriculum. The excuses often given were that IR and Raman spectroscopy are old disciplines, the literature is complete, there is nothing new to discover, and we have computers that can perform identifications using library search programs. New techniques are so much more interesting, finding applications is easier, and getting grants to support work in these new areas is easier. So why should I waste my time and money teaching these older topics like IR interpretation?

There are several somewhat obvious answers to this question, and I stated one of them before, which is simply that industry has spectroscopy instruments and needs trained people to apply them to their problems. Furthermore, industrial facilities may not have the newest technologies. In the absence of instruction in universities, where should one turn? Expecting texts to accomplish the training without a trained and critical eye being present is too optimistic. To start with, texts on IR and Raman spectroscopy are generally suspect in terms of their accuracy, confused in their approach to interpretation, and make little effort to couple the chemistry of a molecule to its spectrum. Computer library searches must be viewed critically for what they are able to do and not do. Expecting a computer search of a library having 50,000 spectra to find a compound, when there are over 16,000,000 organic compounds in the literature, is not very realistic. 

I have considered whether my exposure to interpretive analysis was incomplete and misleading, albeit recently; maybe there was more out there that I was missing. But as I think back to my own undergraduate and graduate level chemistry lectures, I don’t recall ever hearing IR and Raman interpretation discussed in any detail, or even mentioned-not even in organic chemistry where there is so much to be learned from the IR spectrum about the material being studied, and even more if the Raman spectrum is included. It was only because I chose a thesis advisor-Prof. Walter Edgell at Purdue-who was directing projects in molecular vibrational spectroscopy that I began to become acquainted with and interested in IR and Raman spectroscopy and their application.

When I joined Alcoa Research after completing my degree, I soon found out I was woefully unprepared to contribute to understanding the inorganic, organic, and polymer chemistries associated with their business. There were not many texts to learn from. I was on my own and a fast learning path was necessary. Fortunately, in spite of the lack of training, I guess I was successful; the laboratory grew from just one technician plus me, to three technicians, and from one single-beam instrument to two double-beam dispersive instruments. We went from a couple hundred analyses a year to several thousand per year, very quickly.

Have things changed? Certainly, they have. Instrumentation for IR and Raman spectroscopy has gotten orders of magnitude better. The first spectrum I obtained on a single-beam dispersive spectrometer in the late 1950s took over an hour, and to obtain an IR microscopy spectrum was an overnight adventure. Instruments today take microseconds to make similar measurements and might even take only a few seconds to make an energy starved measurement like IR microscopy. In Raman spectroscopy, the first measurements I made took hours using mercury arc excitation and a triple monochromator. Now using laser excitation and a Fourier-transform instrument, obtaining the same spectrum takes only a minute or less.

But in terms of interpretation, things have not changed much over the years; interpretation is still a weak point. In the literature, there are band assignment errors, a failure to understand the part frequency shifts play in making group frequency assignments, a lack of good data on intensity of an absorption band and how that information might play a role in assignments.

There is a lack of understanding of the part least squares and similar applied mathematics routines could play to isolate better frequency and intensity data for group frequencies. Also, there are few new investigators who are excited about finding answers to why functional groups don’t have any intensity under some symmetry situations when the selection rules say a band should be observed. Why, for example, does the group frequency symmetric deformation for CH₃ near 1375 cm^-1 require that the group be attached to an unsaturated carbon if it is to be seen in the Raman, but the weak overtone is seen near 2750 cm^-1 in the Raman regardless of structure?

There are a great many unanswered questions of these kinds which would assist in the interpretation problem. Everything has not been done. Of course, good instruction would be helpful, too.  In the absence of university instruction, the short courses at Bowdoin, Minnesota, Fisk, Canisius, and Arizona used to provide help to the field. To my knowledge, only one of these remains: the course at Bowdoin College. IR and Raman spectroscopy are still probably the most useful tools to study organic groups and interpretation skills are still required.

My suggestion would be for some newer science practitioners to scratch the surface of what they can find in the literature, and critically study the old bones they uncover to see what has been left unsaid or is wrong. Who knows, they may find some exciting, amusing things to discuss with their students that would prepare them to be successful in the industrial environment-like IR interpretation.

Robert Hannah is retired from an industrial spectroscopy career that included roles as the IR laboratory manager at Alcoa Research and as the director of research for the instrument group at PerkinElmer. Direct correspondence to robert.hannah.att@gmail.com