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 d e t e r m i n i n g 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 travelingwave 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 Electromagnet
Heat sink YIG sphere
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 yttriumiron 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
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 t h r o u g h 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 radiofrequency. It lies roughly between 1 and 100 GHz (30 cm to 3 mm wavelength; 0.033 to 3.33 c m - 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.
22
August 24, 1987 C&EN
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 solidstate devices as Gunn diodes or fieldeffect 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 travelingwave 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
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 tem peratures. Disadvantages of traveling-wave instruments are the high, severalwatt power levels of stimulating mi crowave radiation needed to achieve broad irradiating bandwidths and the limitation of samples to gases at equilibrium. High power levels ne cessitate costly electronic modula tion equipment. And static gas sam ples must be compounds or com plexes that are stable at the lowest temperatures achievable. Advantages of the standing-wave technique include low, milliwattpower 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 forc es. They also let workers see transi tions from rotational levels as low as J = 0 for many species. Use of a molecular beam reduces peak broad ening caused by finite gas pressures, collisions among molecules, and col lisions of molecules with wave guide surfaces. Disadvantages of the standingwave approach are the costly, highvolume 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 sec ond. The narrow spectral range is related to the separation distances available between mirrors for cavi ty 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 impres sive results from both traveling- and standing-wave instruments. For ex
Microwave emission reveals structural data Nozzle
u I 1
,
Frequency = f Stimulating oscillator
PIN1
PIN 2
h
Fabry-Perot cavity
Frequency = fm
Frequency = f- 20 MHz Local oscillator
Mixer 2
Mixer 1
Frequency — 20 MHz Mixer 3
Amplifier Frequency = f - f m + 2 0 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 tempera ture 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 rotation al 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 result ing signal is sent to a computer for Fourier transformation to spectral data
ample, Bauder of Zurich has record ed microwave spectra of monodeuterobenzene and methane in his traveling-wave spectrometer. To show rotational transitions, mole cules must have permanent electric dipoles. So if the spectrum of benzene-di was difficult (dipole mo ment μ = 0.008 debye), that of spher ically symmetrical, nonpolar meth ane should have been impossible. Bauder used the centrifugal dis tortion of methane to determine its rotational spectrum. As the mole cule rotates at high speeds about
one carbon-hydrogen bond, the oth er three carbon-hydrogen bonds "fan out," flattening it and giving it a permanent electric dipole mo ment. The Swiss chemist recorded 27 transitions from 8 to 18 GHz among rotational states between J = 11 and J = 19. Meanwhile, in a routine demon stration of the sensitivity of their standing-wave spectrometer, Lovas and Suenram of NBS have recorded the microwave spectrum of carbonyl 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 carboxylic 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 - 1 9 6 °C, allowed the cell to warm, and scanned the microwave spectrum from 26 to 40 GHz. At - 1 3 0 °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 Ar3HC1, 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. 24
August 24, 1987 C&EN
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 - 1 1 0 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-dideuteroethylene 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 phosphorusboron 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 S0 2 -H 2 S 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 S0 2 -H 2 S complex. As increasing amounts of data on hydrogen-bonded complexes have become available, Legon of University College, London, has correlated these with gas-phase nucleophilicities 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
