SCIENCE

Fourier-Transform Techniques Enhance Microwave Spectroscopy

Method's higher sensitivity, resolution stimulate study of structures, bond strengths, other properties of single molecules and complexes

Stephen C. Stlnson, C&EN New York

Instrumental and theoretical ad­vances to apply Fourier-transform methods to microwave spectrosco­py over the past 15 years have sparked a small revolution in this once seemingly mature form of spec­troscopy. These advances have im­proved sensitivity and resolution of microwave spectroscopy by orders of magnitude.

Increased resolution has led chem­ists and physicists to new horizons in determining structures, bond strengths and force constants, charge distributions, and electrical dynam­ics of single molecules and com­plexes. Increased sensitivity has en­abled workers to study complexes at vanishingly small concentrations and to see effects of isotopic substi­tution by using natural abundances of isotopes.

The use of gas pulses expanding at supersonic speeds has further ex­tended Fourier-transform microwave spectroscopy (FT-MW) to the study of weakly bound complexes. In par­ticular, some researchers are amass­ing data on hydrogen bonding, im­portant to understand protein fold­ing, base pairing in nucleic acids, and gaseous reactions in the atmo­sphere. And other investigators are looking at complexes of rare-gas atoms, which may lead to better un­derstanding of nucleation of crys­tallization and liquid condensation.

The late physical chemistry pro­fessor Willis H. Flygare of the Uni­

versity of Illinois, Urbana, first de­scribed FT-MW in 1974, but only six laboratories in the world prac­tice it today. Physical chemistry pro­fessors Alfred Bauder of the Feder­al Institute of Technology, Zurich, Switzerland, and Helmut Dreizler of the University of Kiel, West Ger­many, use the original traveling-wave design.

Standing-wave instruments, which Flygare described in 1979, are in use by physical chemistry profes­sors Herbert S. Gutowsky at Illi­nois; Robert L. Kuczkowski (with research assistant Kurt W. Hillig II) at the University of Michigan, Ann Arbor; Stephen G. Kukolich at the University of Arizona, Tucson; An­thony C. Legon at University Col­lege, London; and physicists Frank J. Lovas and Richard D. Suenram at the National Bureau of Standards, Gaithersburg, Md. Bauder of Zurich has also built a standing-wave spec­trometer recently.

This community seems about to expand rapidly. The five academic FT-MW centers are educating stu­dents in the technique. And Lovas and Suenram are attracting others to learn it through NBS postdoctor­al and visiting scholar programs.

In the traveling-wave instrument, the gaseous sample is confined in a rectangular metal wave guide 18 to 60 feet long. A pulse of microwave radiation enters the wave guide through a mica window. The pow­er and duration of the microwave pulse raises about half the mole­cules in the rotational J = 0 state to the J = 1 state, and the resulting mixed state relaxes with the emis­sion of microwave radiation.

As this secondary microwave ra­diation dies away, the time-depen­dent signal is acquired by a com­puter in digital form. Workers al­ternate the phase of stimulating

Oscillating currents generate microwave energy

GaAs oscillators

Microwave output

Source: Watkins-Johnson Co.

Potential applied across a gallium arsenide chip mounted on an electrically conducting heat sink causes current to flow through it and oscillate on a broad band of microwave frequencies. This oscillating current flows through a loop around a sphere of yttrium-iron garnet (YIG), inducing an oscillating magnetic field. The oscillating magnetic field adds to the magnetic field exerted on the YIG sphere by an electromagnet. When the total magnetic field strength and fre­quency applied to the YIG sphere reach a specific value and frequency, ferrite mag­netic dipoles in the sphere process at that frequency. This resonant precession in­duces a current in the output circuit that oscillates at the specific frequency. The thin wire of the output circuit serves as the center conductor of a coaxial cable, which connects to an antenna, which in turn broadcasts microwave electromagnetic ra­diation into a wave guide or between the mirrors of a Fabry-Perot cavity. The opera­tor tunes the microwave oscillator to differ­ent frequencies by varying the current ap­plied to the electromagnet, which varies the resonance frequency of the YIG sphere

August 24, 1987 C&EN 21

Heat sink

YIG sphere

Electromagnet

Science

Standing wave is generated between mirrors (above) in microwave spectrometer ofHillig and Kuczkowski (right)

pulses by 180° and add hundreds or thousands of the resulting digi­tized signals from secondary radia­tion. This allows accumulation of data from signals and cancellation of noise. Fourier transformation of the enhanced signal yields the mi­crowave absorption band.

In standing-wave spectrometers, the gas enters a vacuum chamber in a 1-millisecond burst through a

1-mm pinhole. The gas expands into the chamber at supersonic speeds, which cools it to less than 5 K. This resulting molecular beam passes be­tween two spherical aluminum mir­rors 30 cm in diameter with 60-cm radii of curvature.

The gas for the molecular beam consists of a few percent of the com­pound to be studied in an inert gas such as argon. The extreme chilling

Principles of microwave spectroscopy Microwave radiation is the part of the electromagnetic spectrum lower in en­ergy than infrared and higher than ra-diofrequency. It lies roughly between 1 and 100 GHz (30 cm to 3 mm wave­length; 0.033 to 3.33 cm - 1 ) . For ex­ample, household microwave ovens operate at 3 GHz and several hundred watts of power.

Chemists traditionally have done mi­crowave spectroscopy between about 4 and 40 GHz, measuring absorption as they slowly scanned the spectral range. Traveling-wave Fourier-trans­form microwave spectroscopy (FT-MW) has been carried out between 4 and 26 GHz, with the spectral range de­pendent on the length and cross sec­tional area of the wave guide sample cells used. These range from 2 inches X 2 inches X 60 feet for lower fre­quencies to 1/2 inch X 1/2 inch X 18 feet for the higher frequencies. Power levels range from about 5 to 60 watts.

Most investigators do standing-wave FT-MW from 8 to 18 GHz, limited by the diameters of and distances avail­able between the spherical mirrors that form the resonant cavity. Power levels are a few tenths of a watt.

Spectroscopic microwave sources are klystron tubes or back-wave oscil­lators for traveling-wave spectrome­ters and such gallium arsenide solid-state devices as Gunn diodes or field-effect transistors for standing-wave instruments. All of these sources work by modulating flows of electrons. Elec­trons are made to lose kinetic energy such that they flow in bunches at GHz rates. Lost kinetic energy is radiated as microwave radiation.

Operators tune frequencies of kly­strons or back-wave oscillators by var­ying applied voltages. Solid-state de­vices are tuned by changing the mag­netic field applied to a tiny yttrium-iron garnet sphere coupled to them.

of the gas also causes the molecules under examination to form com­plexes with one or more argon atoms. One example is Ar3-HCl.

While the molecular beam is pass­ing between the two mirrors, a pulse of microwave radiation is broad­cast into the cavity between them through an antenna mounted at the center of one mirror. The curvature of and distance between the mir­rors are set to form what is called a Fabry-Perot cavity. This arrange­ment sets up a standing wave of microwave radiation within the cav­ity. The cavity is tuned throughout the spectral range by moving one mirror.

As in the traveling-wave instru­ment, the stimulating microwave pulse creates a mixed rotational state. When the stimulating pulse ceases, the relaxing mass of molecules emits secondary microwave radiation. To cancel out noise, digitized data from runs with no gas present are sub­tracted from signals obtained in the presence of molecular beams. En­hanced data from hundreds or thou­sands of runs is subjected to Fourier transformation.

The advantages of the traveling-wave device are speed and a broad irradiating bandwidth. Because the gas sample is always present in the wave guide sample cell, experiment­ers can pulse at up to 50,000 times per second, accumulating spectral peaks in seconds. The 5- to 50-MHz irradiating bandwidth means that

22 August 24, 1987 C&EN

Microwave emission reveals structural data Nozzle

u Frequency = f

Stimulating oscillator PIN1

I 1 ,

h Fabry-Perot cavity

Frequency = f- 20 MHz

PIN 2

Mixer 2 Local oscillator

Frequency = fm

Mixer 1

Frequency — 20 MHz

Mixer 3 Amplifier Frequency = f - f m +20 MHz

Frequency = f - f n

Amplifier Analog-to-digital converter Computer I Spectral

peak(s)

PIN = p-doped/intrinsic/n-doped layers

A burst of sample gas enters the cavity between two spherical aluminum mirrors. The gas expands at supersonic speed to form a molecular beam whose tempera­ture is less than 5 K. A pulse of stimulating microwave radiation of frequency f enters the cavity through PIN diode 1 and sets up a standing wave of radiation between the mirrors. The stimulating radiation ceases, PIN diode 2 opens, and the excited gas molecules emit radiation of frequency fm, corresponding to a rotation­al transition. Because microwave radiation of such high frequency as fm is so hard to detect and to measure, this radiation is mixed by electronic means with that from a local oscillator whose frequency is f — 20 MHz. This mixing produces a new signal of frequency f - fm ± 20 MHz. After amplification, this signal is further mixed electronically with a signal from the stimulating oscillator and the local oscillator of 20 MHz to produce an enhanced signal of frequency f - fm. After further amplification, digitization, and averaging of many such signals, the result­ing signal is sent to a computer for Fourier transformation to spectral data

at any one stimulating frequency, rotational transitions can be detected that occur at 2.5 to 25 MHz higher or lower. This allows coverage of the spectral range.

Wave guide sample cells can be cooled to —65 °C This lets workers see relatively low-lying rotational transitions and study complexes that would be unstable at higher tem­peratures.

Disadvantages of traveling-wave instruments are the high, several-watt power levels of stimulating mi­crowave radiation needed to achieve broad irradiating bandwidths and the limitation of samples to gases at equilibrium. High power levels ne­cessitate costly electronic modula­tion equipment. And static gas sam­ples must be compounds or com­plexes that are stable at the lowest temperatures achievable.

Advantages of the standing-wave technique include low, milliwatt-power levels and the extremely low temperatures of the molecular beam. These low temperatures allow study of complexes of low stability, often bonded only by van der Waals forc­es. They also let workers see transi­tions from rotational levels as low as J = 0 for many species. Use of a molecular beam reduces peak broad­ening caused by finite gas pressures, collisions among molecules, and col­lisions of molecules with wave guide surfaces.

Disadvantages of the standing-wave approach are the costly, high-volume pumping system needed to evacuate the cavity between gas bursts, a narrow, 0.1- to 1-MHz irradiating bandwidth, a relatively narrow 8- to 18-GHz spectral range, and limitations of 10 runs per sec­ond. The narrow spectral range is related to the separation distances available between mirrors for cavi­ty tuning. The low repetition rate results from waiting for sample gas to be pumped from the cavity.

Gutowski of Illinois has achieved rates as high as 20 runs per second, however, by using multiple bursts of stimulating radiation with the same molecular beam. And he has extended his spectral range to 2 GHz at the lower end.

Investigators have gotten impres­sive results from both traveling- and standing-wave instruments. For ex­

ample, Bauder of Zurich has record­ed microwave spectra of mono-deuterobenzene and methane in his traveling-wave spectrometer. To show rotational transitions, mole­cules must have permanent electric dipoles. So if the spectrum of ben-zene-di was difficult (dipole mo­ment μ = 0.008 debye), that of spher­ically symmetrical, nonpolar meth­ane should have been impossible.

Bauder used the centrifugal dis­tortion of methane to determine its rotational spectrum. As the mole­cule rotates at high speeds about

one carbon-hydrogen bond, the oth­er three carbon-hydrogen bonds "fan out," flattening it and giving it a permanent electric dipole mo­ment. The Swiss chemist recorded 27 transitions from 8 to 18 GHz among rotational states between J = 11 and J = 19.

Meanwhile, in a routine demon­stration of the sensitivity of their standing-wave spectrometer, Lovas and Suenram of NBS have recorded the microwave spectrum of carbon-yl sulfide containing carbon-13, oxygen-18, and sulfur-32, using only

August 24, 1987 C&EN 23

Science

Ethylene primary ozonide detected

Scientists at the National Bureau of Standards in Gaithersburg, Md., and Rensselaer Polytechnic Institute in Troy, N.Y., have detected and determined the structure of the primary ozonide of ethylene. This feat adds to the evi­dence that the long-suspected mecha­nism of ozonolysis is correct. They used millimeter-wave spectroscopy, which is a branch of microwave spectroscopy.

When ozone reacts with olefins, the first isolable product is the ozonide, which is a 1,3,4-trioxolane. Synthetic chemists go on to decompose this to alcohols, aldehydes, ketones, or car-boxylic acids. But chemists have long wondered how the three connected atoms of oxygen become separated in the ozonide.

A theory advanced by Rudolf Criegee postulated that the first product of the reaction of, for example, ethylene is the so-called primary ozonide, which is 1,2,3-trioxolane. This cleaves to form formaldehyde and dioxymethylene, which recombine to give 1,3,4-tri­oxolane.

But dioxymethylene also can form dioxirane. In 1977, physicists Francis J. Lovas and Richard D. Suenram of NBS condensed ethylene and ozone on the walls of a wave guide sample cell at -196 °C, allowed the cell to warm, and scanned the microwave spectrum from 26 to 40 GHz. At -130 °C, they recorded the spectrum of formaldehyde, and at —100 °C a spec­trum of what they showed was diox­irane.

Now with more sensitive equipment,

the natural abundances of those iso­topes. Though sulfur-32 is 95.1% nat­ural sulfur, the natural abundance of carbon-13 is only 1.1% and that of oxygen-18 only 0.2%. Thus the carbonyl sulfide molecules they sought were only 0.002% of their sample.

Gutowsky has detected complexes of HF and HCl with as many as three atoms of argon, such as Ar3-HC1, a trigonal pyramid with a base of three argon atoms, a chlorine atom at the apex, and an interior hydrogen atom that "points" on av­erage at one of the argon atoms.

Ethylene primary ozonide

Lovas and Suenram collaborated with physical chemistry professor Charles W. Gillies and graduate student Jennifer Gillies of Rensselaer to search for the primary ozonide. Scanning a range from 79 to 113 GHz, they recorded a spec­trum between -110 and - 9 0 °C that they could ascribe to 1,2,3-trioxolane.

Comparing the experimental spec­trum with one calculated for this pri­mary ozonide, they were able both to identify it and visualize its conforma­tion. For the mathematical analysis, they also used spectra from the reac­tion of cis- and frans-1,2-dideutero-ethylene and ethylene labeled with carbon-13. This is because rotational spectra reveal only distances from atoms to the molecule's center .of mass. Isotopic substitution changes the location of the center of mass of a molecule but not its bond distances or angles. Inclusion of different atomic masses in calculations of moments of inertia allows computation of bond lengths and angles.

The methods used by Gutowsky to determine the structure of this complex are typical of analyses of microwave spectra. He was aided in that natural chlorine is 75.4% chlorine-35 and 24.6% chlorine-37. This together with substitution of DF and DC1 in complexes gave him the isotopic variation needed to de­termine bond angles and distances.

In addition, both chlorine isotopes have a spin of 3/2, which gives them a nuclear quadrupole moment. In­teraction of quadrupolar and rota­tional states splits the peaks in the microwave spectrum in a character­

istic manner. And relative abun­dances of the two isotopes meant that signals from the chlorine-35 complex should be three times as intense as those from the chlorine-37 one, which aided in sorting out peaks belonging to each.

Kuczkowski and Hillig at Michi­gan have used FT-MW to investi­gate the nature of the phosphorus-boron bond in the complex of trimethylphosphine and borane. Natural boron is 19.6% boron-10 and 80.4% boron-11. Thus isotopic sub­stitution was ready-made as in Gutowsky's work. And boron-10 has a spin of 3, whereas that of boron-11 is 3/2. Each atom thus has a nuclear quadrupole moment.

In work supported by NSF, the Michigan chemists related nuclear quadrupole coupling with rotational states to the number of electrons contributed by phosphorus to bo­ron. This was found to be 0.61. Com­parison of experimental and calcu­lated spectra showed the bond length to be 1.901 À.

Kukolich of Arizona has worked mainly with hydrogen bonding. In particular, he has studied the com­plex of sulfur dioxide and hydro­gen sulfide with support from the National Science Foundation and the Petroleum Research Fund. Prelimi­nary complex formation may affect reaction rates of pollutant gases in the atmosphere.

Supplemented by spectra with HDS and D2S, data from S02-H2S indicated a stacked structure with hydrogen bonds of each hydrogen of H2S to one oxygen of sulfur di­oxide. Kukolich concludes that there is a dipole-dipole attraction between the two sulfur atoms that adds to the strength of the S02-H2S com­plex.

As increasing amounts of data on hydrogen-bonded complexes have become available, Legon of Univer­sity College, London, has correlat­ed these with gas-phase nucleophil-icities and electrophilicities to pre­dict strengths of hydrogen-bonded complexes still unknown. He gauges strengths of hydrogen bonds by the stretching force constants of bonds between the hydrogen donor HX and the acceptor B. Force constants are available from microwave as well as from infrared spectra. D

24 August 24, 1987 C&EN