Fourier-transform Microwave (FT-MW) Spectroscopy
at Eastern Illinois University

(this page is always under construction)

The FT-MW technique provides high accuracy structural information, allowing determination of bond angles and bond lengths from measurement of the rotational spectra for a particular molecule or weakly bound complex to unparalleled accuracy (eight or more significant figures in bond lengths can be determined in some cases!). The technique of FT-MW spectroscopy, originally developed by T.J. Balle and W.H. Flygare at the University of Illinois Urbana-Champaign in the late 1970's, involves pulsing a gas mixture containing the species to be studied, into an evacuated cavity where it is probed by microwave radiation. The gas mixture, typically a 1-3% mixture of the molecules of interest diluted in 97-99% rare gas (such as Ar or He/Ne) is expanded supersonically through a pulsed valve into the evacuated cavity. This expansion makes it possible to effectively 'freeze out' the weakly bound complexes, providing them with no means by which to fall apart so their rotational spectra can then be measured. The current spectrometer hardware/software design described on these pages is based very closely on the 1991 University of Kiel design. This machine uses software developed at Kiel by Jens-Uwe Grabow which allows the spectrometer to automatically scan large regions of the spectrum unassisted.

Measurement of the rotational spectra of molecules and complexes allows a determination of the parameters known as rotational constants. These rotational constants are inversely proportional to the moments of inertia of the species under study and hence allow a structural determination to be carried out. (The moment of inertia is a property that depends on the mass and the 3-D coordinates of the individual atoms in the molecule, hence you can see the source of the structural information that is available from the measurement of the rotational spectrum). The high resolution afforded by the technique allows hyperfine structure due to nuclear quadrupole splittings, spin-rotation or even spin-spin interactions to be resolved, whereas the high sensitivity often makes observation of isotopes in their natural abundance possible. FT-MW spectrometers are sensitive enough to observe transitions from species with very low abundance in the expansion, for instance rotational transtions due to the ¹⁸O¹³C³⁴S isotopomer of OCS (which has a natural abundance of about 0.00009%) may be measured.

So, in summary, microwave spectroscopy provides us with the structures of molecule to very high precision. We can also obtain information on the environments of certain nuclei, for instance, determination of the electric field gradient at nuclei can provide data on the electronic environments and identify the degree of charge transfer upon complexation. Information on energy barriers to internal rotation of methyl groups or motions of monomers within a weak complex may also be available. This sort of information, both the structural parameters and molecular properties, is of importance in understanding intermolecular forces and how molecules interact with one another. Such interactions have important consequences in understanding interactions in real-life, larger scale systems, such as drug interactions, protein folding, even in nanotechnology.

The picture above shows the inside of the vacuum chamber. At the bottom left of this picture is the stepper motor (the thing with the wires coming out of it) that moves the mirror in and out to tune the cavity to the particular frequency of microwave radiation being used. The bottom mirror is the only one that moves, the upper one is fixed. The region between the mirrors is the region into which the gas pulse expands through the solenoid valve (which would extend into the chamber by about 8-12 inches through the port on the left of the picture). Between the mirrors, above and below where the nozzle expansion would be) are the Stark plates, two steel mesh plates to which we can apply high voltages of opposite polarity when doing Stark effect measurements (as described above). The surface of the far mirror can be seen in this picture - it is a smooth polished aluminium face with a microwave antenna located at the center (it is reflecting the Stark plate in this picture and so looks strange!) The box of Kimwipes sitting on the Stark plate is not an essential element of the spectrometer :-) It is clearly important to remove any objects from the chamber before sealing it up and pumping it down and also to avoid dropping anything down into the diffusion pumps (which are ideally placed in just the position you need for this to happen). Incidentally, views like this (i.e. the chamber with the lid off) are usually bad news since it (a) means there is a problem or (b) I am in the chamber tinkering which usually results in (a). (The spectrometer chamber is always kept under vacuum even when the pumps are shut off). As you can imagine, taking a lid that weighs a few hundred pounds off the vacuum chamber is not a trivial matter and we have an engine hoist (pictured below) that we use to perform this task.

Representative Projects

The earlier projects are systems that I worked on as a postdoc at the University of Michigan. The work carried out at EIU is described below and at the end of this page):

• HCCH-OCS dimer. This is a rare example of two different isomers being isolated in a supersonic expansion. The first, with the HCCH and the OCS monomer axes rougly parallel to one another, had the most intense spectrum and was predicted by theory to be the most stable energetically. (J. Phys. Chem. A, 103, (1999), 3884). The surprise was finding a second, less stable T-shaped isomer where the sulfur end of the OCS molecule was found to interact with the triple bond of the acetylene molecule. (Chem. Phys. Lett., 312, (1999), 357).

• OCS-OCS-HCCH and OCS-HCCH-HCCH trimers (J. Chem. Phys., 111, (1999), 10511) and (J. Mol. Struct. (Theochem), 500, (2000), 391).

• Several trimeric systems have recently been studied including HCCH-OCS-OCS, CO₂-OCS-OCS, OCS-CO₂-CO₂, N₂O-CO₂-CO₂. These systems are of particular interest since they allow the exploration of third body effects (i.e. the changes in a dimer structure upon the addition of a third body). This gives information on the relative importance of the various types of intermolecular force that exist between the monomers.

• 1,2-dichlorotetrafluorocyclobutene - Assignment of the rotational spectra of the normal species (both ³⁵Cl atoms) and the various ³⁷Cl isotopomers (³⁷Cl, ³⁷Cl) and (³⁷Cl, ³⁵Cl). The two unique ¹³C species were also assigned allowing a determination of structure of the heavy atom framework. (J. Phys. Chem. A, 104, (2000), 8702).

• Maleonitrile and (Z)-Pent-2-ene-4-ynenitrile - These closely related molecules were synthesized by the Robert McMahon group at the University of Wisconsin Madison and shipped to Michigan for the spectroscopic studies. John Stanton at the University of Texas provided ab initio corrections to the ground state spectroscopic structure to enable us to present the equilibrium structure of these molecules. In addition to the structural information, the laboratory measurement of the microwave spectra provides astronomers with rotational frequencies thereby allowing searches for these important molecules in the interstellar medium. (J. Am. Chem. Soc., 123, (2001), 12353).

• Bromobenzene - Assignment of the rotational spectra of the ⁷⁹Br and ⁸¹Br isotopomers as well as eight ¹³C singly substituted species (four for each of the Br isotopomers) allowed a determination of structure of the heavy atom framework. (J. Mol. Struct., in press, 2003).

• 3-butyn-1-ol - Assignment of the rotational spectra of numerous isotopomers of the lowest energy conformer of numerous isotopomers of the 3-butyn-1-ol molecule. As with bromobenzene this will allow a determination of structure of the heavy atom framework. This molecule is of particular interest since it contains an intramolecular O-H...&pi hydrogen bond. More details below in the Summary of Past and Present Research Students)

Current Status (last updated 10/19/2008)
(Scroll down, it might have been updated a lot more recently than the date above suggests since I have a tendency to forget to update that date)

March 2003: We recently finished the assignment of numerous isotopomers of the bromobenzene monomer and this work is due to appear in Volume 257 of the Journal of Molecular Structure. Currently we're assigning isotopic species for 3-butyn-1-ol (including the ¹⁸O species in natural abundance, approximately 0.2%) to determine the structure of the conformer that is present in our supersonic expansion. Theoretical calculations to determine the barrier to rotation (and hence the relative stability of other possible conformers) are also underway using the Chemistry department Alpha Tru64 Unix workstation as well as my own 2.8 GHz Pentium 4 PC.

August 2003: We're making our last frantic attempts to get some research done before the Fall classes start again. Currently we're attempting to assign the rotational spectrum of the OCS-CS₂ weakly bound dimer.

October 2003: We finally have our Glassman High Voltage high voltage power supplies (EL Series) that will allow us to do Stark effect measurements on some of our more intense OCS/CS₂ transitions. This will hopefully give us valuable information on what the quantum numbers are for these transitions and allow us to make some attempts at assigning this spectrum. Using our new cylinder of He/Ne carrier gas (to replace the Ar we have up to now been using) we have managed to improve the intensity of several OCS/CS₂ transitions to the point where a Stark effect is possible.

The Ar-bromobenzene weak complex continues to give us headaches - we have made some progress in recent weeks in assigning the hyperfine structure but the quality of the fit indicates something is not right (bad quantum number assignments, wrong transition frequencies or something else weird that we have yet to think of). It seems like we are on the right track with this assignment but we're still awaiting the breakthrough :-)

December 2003: Preliminary searches for the DME-OCS complex have been carried out in recent weeks leading to several promising transitions having been located. Mixing tests are underway (to test that these lines require both DME and OCS!) and the high voltage connectors (that will enable us (eventually) to carry out Stark effects on these transitions) have been ordered (Reynolds Industries Series 521 plugs, P/N 167-3516). Once we have the power supplies hooked up and can do some Stark effects we're hoping this will give us some hints as to the quantum numbers of the transitions we have found and will aid in the spectral assignment.

During the Christmas break, we measured the rotational spectra of the normal isotopic species and six isotopically substituted species of cyclopropyl carbinol (a cyclopropane with a CH₂OH group on one of the ring carbons). This allowed a heavy atom structure determination, and that, coupled with the calculations that Josh carried out on this molecule will eventually be put into a manuscript. Additional work on dipole moment measurement and searches for higher energy conformers is underway. In addition to the experimental work, Josh is keeping both the lab PC and the departmental workstation busy on DME-OCS, DME-HCCH and DME-HCN calculations.

February 2004 : The power supplies are finally working and have already assisted in the assignment of a weakly bound complex spectrum! Thanks to Jim Wentz of the Computer Electronics Electrical Services at the School of Chemical Sciences at the University of Illinois, Urbana-Champaign who fitted the connectors and helped us with getting "standard" connectors to fit in "standard" feedthroughs (which initially did not fit). We got the OCS electric field calibration carried out with our new power supplies (on Jan. 29th) and then did a Stark effect on two of the most intense lines from the DME/OCS search. As soon as those were done and plotted, we had the information that we needed to assign that dimer spectrum and the normal isotopic species spectrum is now measured :-) We're now working on getting the dipole moment measured properly and then we're on to thinking about isotopes.

April 2004 : The measurement of the DME-OCS isotopomer spectra is finished and we have completed the determination of the structure of this complex. High on our list of things to do is the preparation of our talks for the Columbus spectroscopy meeting in June 2004. We have also started upon the upgrade of the spectrometer to a newer (and hopefully more efficient) design. This involves a major upgrade of the hardware and software used to drive the spectrometer. This project will continue for the summer (and beyond) although the current configuration of the spectrometer will be used to continue our experimental studies until the time comes for us to switch over and test the new design. Currently we are experiencing serious problems with our Dell Precision 650 workstation, which is freezing up right at the start of the boot up process and giving us error messages about possible graphics card failure. A replacement ATI Fire GL X1 128 MB card from Dell would not work at all; even a replacement of the entire motherboard and a new ATI card gives us similar problems. It is looking likely that there is a serious conflict with the graphics card somewhere. Current hardware count: 3 motherboards, 2 CPU's, 2 sets of memory, 2 power supplies, 4 video cards (the last one an NVIDIA QuadroFX500).

June 2004 : We're working on more DME complexes - DME-CO₂ has been assigned and is currently being written up and at present we're working on assigning the DME-CS₂ complex. This system is particularly interesting (and difficult) since we seem to have a tunneling motion that interconverts between two equivalent structures of the complex. Spectroscopically this complicates matters significantly because some of the transitions are perturbed considerably with the lines being split into a doublet and those components being displaced to high and low frequency. We also managed to get some Stark effects carried out (finally!) on the OCS-CS₂ transitions that we had (see the entry above for August and October 2003) and we assigned this spectrum. We've now measured the normal and four additional isotopic spectra in natural abundance and have added this one to the list to write up. Of course, the main highlight this month was the week we spent at the 59th Ohio State University International Symposium on Molecular Spectroscopy. The official picture archive for the conference is online here. A few pictures of the EIU representation at the meeting are here.

August 2004 : Currently we're working on the DME-HCCH complex (the DME-CS₂ is on hold while we try to figure out how to obtain the barrier to the internal motion from the inversion splittings and the OCS-CS₂ is being written up). We have found several transitions for the DME-HCCH complex and have a tentative assignment (we have a-type K = 0 and K = 1 lines for the 2-1, 3-2, 4-3 and 5-4 transitions but no K = 2 lines and no c-type transitions yet. We're expecting some internal motion problems just as we saw in the DME-CS₂ complex although haven't yet identified the tunneling doublets (if they exist :-) Prof. Walther Caminati at the University of Bologna in Italy very kindly supplied us with the Fortran code for a flexible model program which he has used in the past to fit the barrier to inversion for similar tunneling motions in DME complexes with rare gas atoms. This gave us the excuse we've been looking for to justify the purchase of the Absoft ProFortran Compiler Suite v.8.2. This will allow to compile and run this program and determine the barrier to the inversion motion for DME-CS₂ (once we have figured out a tunneling pathway and potential energy function for the motion!)

Newby finally left this month to go to graduate school at Purdue University (and the new Wal Mart in West Lafayette), leaving behind several hundred megabytes of output files from his numerous Gaussian jobs.

Michal Serafin joined the group at the start of the Fall 2004 semester. Michal has begun work on modeling the dimethyl sulfide-CO₂ complex using Gaussian 98.

September 2004: Michal continues to work on calculating the possible structures for the DMS-CO₂ complex and has also been spending time predicting rotational spectra.

October 2004: Michal is preparing his poster on DMS-CO₂ for the upcoming Third Annual Undergraduate Research Celebration (to be held in the Chemistry Department, 3rd floor, 5.30pm on November 1st). We're also in the process of systematically applying the ORIENT semi-empirical model to several complexes of DME to see how well it is capable of predicting their structures. The DME-CO₂ paper was accepted this month for publication in J. Phys. Chem. and should appear in a January 2005 issue of this journal.

November 2004: We're in the process of measuring the ¹³C isotopomers for the DME-HCCH species (in natural abundance).

January 2005: We spent some time over the Christmas break searching for the DMS-CO₂ spectrum although this was made difficult by some hardware problems that affected the autoscan facility. Mike will be returning to the lab this semester and one of his first jobs will be to do the basis set superposition error (BSSE) calculations on this complex. The OCS-CS₂ work that we submitted to the Royal Society of Chemistry journal Phys. Chem. Chem. Phys. in September was accepted for publication in December and was published as an Advance Article on the web January 10th 2005 (article number b414897e).

April 2005: We're still proceeding slowly with the spectrometer upgrade :-) Mike is continuing his calculations on the DMS systems and is involved in the modeling of ionic complexes. The DME-HCCH manuscript was published in J. Phys. Chem. A. 109, (2005), 5316.

May 2005: Summer at last! It looks as though we have an assignment for the DME-HCF₃ weakly bound complex (dimethyl ether complexed with fluoroform (trifluoromethane)). We've found a number of a-type transitions but they are all split to varying degrees (possibly by internal motions of the DME methyl groups in addition to the internal rotation of the trifluoromethane subunit) e.g. the K = 0 lines are apparently split into quartets while the K = 1 lines all seem to be doublets. In talking to Prof. Caminati at Columbus in June 2005, we found that he has assigned this complex and (fortunately) has already figured out the internal motion splittings in the spectrum :-)

June/July 2005: In searching for the DMS-OCS spectrum, we have an assignment for the DMS-Ne complex (we've so far got the normal isotopic species and the ²²Ne species). Also this month was the 60th International Symposium on Molecular Spectroscopy at the Ohio State University, meaning we had far too many doughnuts and coffee during the week of June 20-24.

We have also recently assigned a couple of isotopic species for the DME-CS₂ complex (in the hope that we can sort out unambiguously the structure and tunneling pathway) and have a tentative assignment for HCCH-HCF₃ (the acetylene-fluoroform (trifluoromethane) complex); there are lots of lines observed here so clearly some more internal motion to sort out.

August-October 2005: Michal measured the DCCD-HCF₃ spectrum. This is also complicated by significant amounts of internal motion splitting but the ease with which we were able to locate the spectrum for the deuterated species tells us that our predicted structure is quantitatively close to the true structure. Mike also started searching for the less symmetric OCS-HCF₃ complex, which we hope will give us some insight into the nature of the splittings in the HCCH-HCF₃ spectrum.

November 2005: The spectrum for the normal isotopomer of the OCS-HCF₃ species is assigned and considerably less complex than the spectrum for the HCCH-HCF₃ species.

January 2006: We're working on dipole moment measurements of the OCS-HCF₃ species and also on the measurement of conformers of some silane samples supplied to us by Prof. Gamil Guirgis of the College of Charleston, South Carolina.

May 2006: We're continuing measurements on the silanes sent to us by Prof. Guirgis. Once that's done Mike will be working on measuring isotopic spectra for the OCS-HCF₃ complex (assuming the O¹³CS isotopically enriched sample that went missing somewhere between Icon Isotopes and EIU in late March is located).

June 2006: One of our projects early this month involved us trying (three times) to synthesize some deuterated fluoroform (DCF₃). Unfortunately we failed to get significant (probably any) deuteration and ended up buying some commercial isotopically enriched DCF₃. Here is a rare picture of physical chemists in the lab doing real chemistry (Mike's stirring the hexanes/liquid nitrogen slush bath that we used to condense and keep the fluoroform a liquid during the course of the reaction - trying to do chemistry with substances that boil below -80°C is not a trivial matter!). The other main event in early June was that the O¹³CS isotope finally arrived (some two months after it was shipped). Apparently its journey from New Jersey to EIU involved stops in Indianapolis and Decatur before it was finally located in North Carolina (although quite what it was doing there we never really quite got to the bottom of). Mike managed to get this spectrum measured in time for his talk at the Columbus meeting - his first talk at a meeting! Here's a picture of us at the Columbus picnic - from left to right: Newby, Dr. S. Peebles, Larry Keniley, Mike Serafin. Here's another with Dr. S. Peebles replaced by Dr. R. Peebles.

July 2006: Mike's gone for a week's vacation and before we knew what was happening Rebecca had laid claim to the spectrometer for a week to search for some complexes of CHCl₂F (dichlorofluoromethane). Pictures of the current state of construction of her own cavity ring-down spectrometer can be found here.

In late July (a few hours after Mike finished for the Summer) we assigned the CO₂-HCF₃ complex; this spectrum is complicated by the existence of A and E states (arising from internal rotation of the HCF₃ subunit), but also exhibits b-type transitions. These transitions should be forbidden due to symmetry but occur because of a mixing of the K_-1 asymmetry doublets (which is permitted by the internal rotation motion). There is evidence for this sort of behavior in the literature and we are working to tighten up the quality of our fit of the observed transitions to within our measurement accuracy.

September 2006: We're currently measuring the rotational spectra of the gauche conformer of the H₂Ge(CH₃)(c-C₃H₅) cyclopropyl methyl germane (CMG) compound supplied to us by Prof. Guirgis. This has five Ge isotopes (⁷⁰Ge, ⁷²Ge, ⁷³Ge, ⁷⁴Ge and ⁷⁶Ge). We apparently have A and E states for all the observed transitions (arising from the internal motion of the methyl group) as well as nuclear quadrupole hyperfine structure in the quadrupolar ⁷³Ge isotopomer. Unfortunately we had very little sample and exhausted it before we could complete all the measurements.

November 2006: Just before the Thanksgiving break we got an a-type assignment on the methyl fluoride-OCS complex. This means we have a series of three fluorinated methane complexes with OCS (CHF₃, CH₂F₂ and CH₃F). All we need now is to find some b-type transitions.

March 2007: Mike's working on trying to find more methyl fluoride-OCS transitions while Jon's been working on doing Morokuma decomposition calculations using GAMESS and NBO analyses using Gaussian. There should also be some more measurements to do on H₂Ge(CH₃)(c-C₃H₅) since we are expecting to receive another sample soon so Jon's predicting the quadrupole coupling constants for the ⁷³Ge isotope to aid in deconvolution of the internal rotation and nuclear quadrupole hyperfine components.

April 2007: Mike found the b-type lines for the MeF-OCS complex finally! Now we have to see if we can find any additional (E-state) lines that can be attributed to the internal rotation of the MeF subunit about its symmetry axis. With the arrival of more of the CMG sample from Dr. Guirgis we were able to finish measurements of the nuclear quadrupole hyperfine structure for the ⁷³Ge species. The quadrupole coupling constants calculated by Jon Murray for this species proved to be in perfect agreement with the observed values and hence made assignment of this quadrupole structure quite straightforward.

May-August 2007: We're visiting Dr. Steve Cooke's lab at the University of North Texas for the summer where we're hoping to look at some Pt and Pd containing species, with a break in mid-June to go to the 62nd International Symposium on Molecular Spectroscopy at Ohio State University (where Mike will be presenting a paper titled "Characterization of Weakly Bound HCF₃-OCS, H₂CF₂-OCS, H₃CF-OCS, and HCF₃-CO₂ Van Der Waals Complexes by Ab Initio Calculations and Microwave Spectroscopy"). Some pictures of the EIU gang at the Columbus picnic: Dr. S. Peebles with Dr. Steve Cooke, Dr. R. Peebles, Amanda Steber, Mike Serafin, Mike listens intently to the introductions, Newby joins the EIU table at the picnic, Amanda enjoying her ice-cream.

November 2007: It's been a busy semester so far, we managed to get the ¹³C-DFM-OCS spectrum assigned in natural abundance (finally, after several attempts) and were able to get a decent structure on this complex. We're also in the process of putting together a pulsed-discharge nozzle.

May 2008: Mike defended his Masters thesis.

June 2008: It's time for the Columbus spectroscopy meeting again. Pictures are here.

August 2008: We're currently testing a new pulsed discharge nozzle (PDN) which we hope to use to generate some new radical and cationic species. And since our NSF-RUI proposal was also funded last month, we're also working on the details of a new instrument with which to pursue additional studies of ions and ionic complexes.

October 2008: I'm on sabbatical for this academic year and I am in the process of building a new instrument. We have been very busy this semester getting things ordered as well as continuing our research efforts and also trying to make progress on writing up some of the collaborative projects that we have been working on in the last couple years.

Vacancies: We're always looking for undergraduate research students so if you want to come by and see what we do then feel free to drop by my office (Room 3420 in the Physical Science Building) or my lab (Room 2443), or you can email me at sapeebles@eiu.edu or call me at (217) 581-2679. Below is a list of undergraduate researchers and an idea of the projects that they have worked on.

Summary of Past and Present Undergraduate/Graduate Research Students (and Their Projects)

Elizabeth D. Slagle (Summer 2003): ab initio calculations of conformers of 3-butyn-1-ol, o-anisaldehyde, hydrocinnamaldehyde.
Josh J. Newby (Fall 2003 - Summer 2004): Josh was involved in some of the early work that got us moving into our first experimental studies of dimethyl ether (DME) complexes. He did ab initio and experimental work on DME-OCS, DME-CO₂, DME-CS₂, DME-HCCH, OCS-CS₂, cyclopropyl carbinol, DMS-OCS. As of Fall 2008, Newby's already entering his final year as a member of Tim Zwier's group at Purdue University. Newby gave his first talks (WI06 and FD02) at the International Symposium on Molecular Spectroscopy at Ohio State University in Columbus, OH in June 2007.
Michal M. Serafin (Fall 2004 - Summer 2008): Mike started with some ab initio calculations on DMS-HCCH and DMS-CO₂, and moved on to carry out experimental measurements on DME-DCCD and DCCD-HCF₃. In Fall 2005 he searched for and assigned the spectrum of his own complex (the OCS-HCF₃ dimer). Additional measurements on several isotopomers of OCS-HCF₃ and the assignment of the spectra for some of the silane samples provided by Prof. Gamil Guirgis were carried out in Spring 2006. In the Summer of 2006 he extended his experimental studies to difluoromethane (DFM) complexes, assigning DFM-OCS before coming back to CO₂-HCF₃ (which turned out to be quite interesting - more details are given under the "July 2006" entry in the previous section). Mike continued on his research in the group as a Graduate Student starting in Spring 2007 and graduated with his Masters degree in May 2008. He's currently studying at the Medical Academy of Gdansk, Poland.
Jon M. Murray (Fall 2006, Spring 2007) - worked on measurements on the germanium compound obtained from Prof. Guirgis; also carried out calculations of the electric field gradient at the Ge nucleus to provide predictions of the nuclear quadrupole hyperfine structure in the ⁷³Ge spectrum; Natural Bond Order analysis and Morokuma decomposition analysis calculations on the fluorinated methane series of complexes. Jon's currently attending Midwestern University.

Further details of their research projects, publications and presentations, as well as what these students are doing now can be found here.

References

T.J. Balle and W.H. Flygare, Rev. Sci. Instrumen., 52, (1981), 33. Back to text

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