Research in the Peebles group

Physical Chemistry is not all about Thermodynamics or Electrochemistry - the area of Quantum Chemistry ignores all the rules of the macroscopic world and deals only with the microscopic; the field of Spectroscopy is the experimental application of Quantum Chemistry and is where we are actively engaged in research. If you've had Quantum Mechanics (CHM3920) then you'll have some appreciation of how different this is to any of the Physical Chemistry that you saw in CHM3910. Quantum Chemistry and Spectroscopy allow you to understand what is happening in a chemical system, on the molecular scale. As a Physical Chemist I'm interested in the quantitative aspects of molecular structure and properties. How long is a particular bond? What is the bond angle? Do we see a significant change in the structural or electronic properties of a molecule when it hydrogen bonds to another molecule? Why does a molecule interact with a particular site on another molecule in preference to a different site? How does the formation of van der Waals or hydrogen bonding interactions change physical properties such as barriers to rotation for methyl groups and what effect might this have on reaction rates, binding energies and so on?

Chemical and biological systems are dynamic species and high resolution spectroscopy allows us to probe these systems on a molecular scale and begin to formulate answers to the above questions. By the study of simple chemical systems of small clusters of molecules and the determination of their molecular structure and properties, we provide experimental data, thereby facilitating development of new models and hence allow the improvement of agreement between theoretical predictions and experimental measurements; high resolution spectroscopic studies provide the experimental benchmarks for theorists to test and improve their theoretical calculations.

We're specifically interested in determining the structures of weakly bound complexes (hydrogen bonded complexes or those just held together by van der Waals interactions). We do this in two ways: theoretically (involving computer programs to calculate the structure and properties of these complexes) or experimentally (using pulsed supersonic nozzle Fourier-transform microwave (FTMW) spectroscopy). We have two instruments that we can use for this: the resonant cavity FTMW spectrometer at EIU is shown in the picture to the right, and since June 2009 we have a second operational instrument, known as a chirped-pulse FTMW spectrometer which has significantly increased bandwidth relative to the cavity instrument - with this new CP-FTMW we can look at up to 480 MHz of spectrum at one time, compared to about 1 MHz with the cavity instrument. The CP-FTMW has been constructed in order to look at exotic species (such as ions and radicals) and we are in the process of incorporating a pulsed discharge source and a laser ablation source to generate these species. More details on both instruments can be found here.

Since the complexes we look at are very weakly bound (with binding energies maybe 1000 times smaller than a typical covalent bond), they're not species that we can put in a cell and investigate at our leisure; we have to have a method of generating them and studying them in such a way that they cannot be destroyed by collisions with other molecules. To achieve this we use a supersonic molecular expansion, whereby we expand a gas mixture containing the molecules of interest from a high pressure (typically ~2-3 ×10³ torr) to a low pressure chamber (~10^-7 torr) through a small pinhole. This rapid expansion cools the gas mixture to an effective temperature of a few degrees Kelvin and, in this collisionless expansion, there are no ways for any complexes formed between the molecules of interest to fall apart. We can then probe this expansion with microwaves and measure the microwave (rotational) spectrum of the species of interest. Measurement of the rotational spectrum ultimately allows us to determine, to unprecedented accuracy (often to with a few hundredths or thousandths of an Angstrom) the bond lengths. This information thereby allows us to understand the balance of forces at work between the molecules and can shed light on the way much larger systems interact (drug interactions, what drives carbohydrate or protein structure and so on).

There are numerous ways for students to be actively involved in this research. Since we do both computational and experimental work there are several different ways that you might contribute. Students typically start off doing some computational work since this is relatively straightforward and is a useful way for you to become accustomed to the research area. We use the Gaussian 03W software with the GaussView graphical interface so setting up calculations is as easy as point and click to build your molecule and submit the calculation. Students with computing or mathematical backgrounds may be interested in getting even more involved with this aspect of the research, such as fitting mathematical functions, determining potential energy surfaces and even writing their own simple programs to carry out certain tasks.

Students with a more experimental interest can be involved with the measurement and assignment of the rotational spectra for the complexes (or molecules) that we are working on. This can also involve the prediction and fitting of the spectrum using our array of custom written computer programs as well as fitting the structure, dipole moment and so on.

If you have an interest in electronics or engineering, we can probably accommodate your interests there as well. Physical Chemistry is always pushing forward experimental boundaries to explore new measurement techniques and methods. Different sorts of chemical systems may be studied with minor adjustments to the equipment that we have in the lab. At present, we're in the process of upgrading our microwave spectrometer to a newer design so there is plenty of opportunity to get involved with both the hardware and software aspects of the spectrometer design.

So, if you're interested in finding out about whether you might enjoy working in this area of research, come and talk to me, or email (sapeebles@eiu.edu) me and tell me your interests.

More details on the Fourier-transform microwave (FTMW) spectrometer can be found here as well as details on our computational projects and facilities. There's also a list of the recent students that have been in the group (includes a list of their projects and publications).