nicholas r. walker- new opportunities and emerging themes of research in microwave spectroscopy

8/3/2019 Nicholas R. Walker- New opportunities and emerging themes of research in microwave spectroscopy

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doi: 10.1098/rsta.2007.0001, 2813-28283652007Phil. Trans. R. Soc. A

Nicholas R Walkerresearch in microwave spectroscopyNew opportunities and emerging themes of

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New opportunities and emerging themes ofresearch in microwave spectroscopy

BY NICHOLAS R. WALKER*

School of Chemistry, University of Bristol, Bristol BS8 1TS, UK

It is easy to set a frisbee spinning but hard to flip a javelin end-over-end. The properties of arotating body are determined by its moment of inertia. Changes in the energy associatedwith the rotation of a single molecule are incremental, or quantized, in contrast with theeveryday examples of the frisbee and the javelin. Only photons with energies that

correspond to specific discrete frequencies of electromagnetic radiation can be absorbed oremitted to cause transitions between different rotational energy levels. Photons that causerotational transitions generally have microwave or millimetre-wave frequencies. Micro-wave spectroscopy thus provides a basis for exploring molecular structure through studiesof molecular rotation. The first experiments involving microwave spectroscopy exploitedtechnology developed for radio detection and ranging during the Second World War.Microwave spectroscopy is now being applied to study chemical reactions significant inatmospheric chemistry and to probe superfluidity in Hen clusters. This article reviews theresearch themes that were the focus of the past 50 years and surveys new opportunities.

Keywords: microwave spectroscopy; spectroscopic techniques; helium clusters;

atmospheric chemistry; chirped pulse

1. Introduction

Spectroscopy exploits the emission, absorption and scattering of electromagneticradiation for the study of molecules and materials. Emission takes place when amolecule loses energy by radiating light. Absorption is the reverse process. In eachcase, the energy of the photon involved is coincident or resonant with the differencebetween two energy levels predicted by quantum mechanics. In general terms,

photons from visible and ultraviolet regions of the electromagnetic spectrum inducetransitions between electronic energy levels and infrared light excites transitionsbetween vibrational levels. Microwave frequencies (between 3 and 300 GHz) areassociated with changes in the rotational energy of molecules. These are intimatelyrelated to the geometrical structure of molecules. Microwave spectroscopy thusprovides the opportunity to measure precisely the structure of isolated, gas-phasespecies. This article will provide a brief introduction to theexperimental methods andessential concepts that underpin this powerful technique. Several themes of currentresearch will be explained and new emerging opportunities will be discussed. Anoverview of the historical development of the field is provided to set the scene.

Phil. Trans. R. Soc. A (2007) 365, 28132828

doi:10.1098/rsta.2007.0001

Published online 21 September 2007

One contribution of 20 to a Triennial Issue Chemistry and engineering.

*[email protected]

2813 This journal is q 2007 The Royal Society

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(a) Landmarks in microwave spectroscopy

The earliest experiment where microwave frequencies were used to interrogatea molecule was performed by Cleeton & Williams (1934). This group constructedcustom-made magnetron oscillators to perform a study of the inversion

vibrational mode of NH3. The construction of more powerful sources ofmicrowave radiation was essential to the development of radio detection andranging (RADAR) equipment during the Second World War. Early equipmenttests illustrated starkly the significance of molecular resonances at microwavefrequencies. Townes (2002) recalls how atmospheric water vapour completelyabsorbed the microwave radiation from one source limiting the range to eightmiles in RADAR applications. Gordy (1983) describes how a boat transportinggarbage released ammonia that absorbed the radiation from another device.Experiments were necessarily halted when the boat was passing. Thedevelopment of the klystron source had been intended to allow the constructionof superior RADAR equipment but it had proved unsuited to the purpose.Instead, it provided a boost to the new field of microwave spectroscopy.

The initial post-war surge of experiments, in 1946, all involved studies of theinversion vibration of ammonia. Microwave emission from a klystron source wascoupled into a cell using a waveguide and the molecular absorption of microwaveswas measured. Bleaney & Penrose (1946) identified rotational structure in thevibrational band of NH3. This was evidence of transitions between differentrotational energy levels. Good (1946) confirmed the presence of hyperfinestructure, later attributed to coupling between the electric quadrupole moment ofa nucleus and the electric field gradient within the molecule (Dailey et al. 1946).Townes (1946) reported on the effects of power saturation on line shapes. Dakin

et al. (1946) reported the splitting of a spectral transition by an external electricfield. This first measurement of the Stark effect allowed the group to determinethe dipole moment of a molecule from spectroscopic data for the first time. Gordy(1948) was able to prepare a review of the results contained in approximately 100research papers. The results obtained from approximately 25 different moleculeswere included. These ranged in size from a single diatomic, ICl, to species as largeas CF3CH3. Microwave spectroscopy had become a powerful means of determiningthe molecular structure of isolated gas-phase molecules.

The difficulty involved with using microwave spectra in the determination ofgeometrical structure increases with the size and complexity of a molecule. It was

logical for the first microwave spectroscopists to perform experiments on thosesmall molecules that were most amenable to study. Later experiments extendedthe range of systems. High-temperature ovens (Stitch et al. 1952) were used togenerate gas-phase species containing refractory metals. Organic molecules ofincreasing complexity were studied as researchers became interested inmeasuring the energetic barriers to internal rotation about the axes of CCbonds (Wilson 1951). Rotational transition frequencies could be measured withhigh precision but researchers were unable to provide structural information ofcorresponding accuracy. This situation was improved through advances in thenumerical methods used to obtain molecular structure from rotational constants

(Costain 1958). Even today, rotational constants are determined experimentallyto higher precision than the molecular structures that are calculated from them.Microwave amplification by stimulated emission of radiation (MASER) was

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reported by Gordon et al. (1955). Their experiment proved that microwaveamplification by stimulated emission could be achieved and principles wereestablished which later guided the design and construction of laser instruments.The invention of the MASER ultimately stimulated the development of themodern lasers used in applications as diverse as compact disc recorders, surgical

instruments and bar code readers.Research into molecular structure remained the central theme in microwave

spectroscopy between 1960 and 1970. The related technique of molecularbeam electric resonance was providing new details of molecular structure,dipole moments and hyperfine interactions ( Dyke et al. 1972). Research wasgradually evolving towards experiments that probed the intermediates ofchemical reactions and their mechanisms. Researchers began to explorereactive complexes and free radicals. Dixon & Woods (1975) completed thefirst assignment of the microwave spectrum of a molecular ion, COC. Theidentification of molecules in the interstellar medium was another highlight of

the 1960s. Microwave transitions were measured in the laboratory and matchedto emissions from the interstellar medium using a radiotelescope. NH3 was thefirst stable polyatomic molecule to be identified in the interstellar medium(Cheung et al. 1968). H2O (Cheung et al. 1969) and H2CO (Snyder et al. 1969)were detected during the following year. Many molecules of varying sizeand complexity have since been identified through radioastronomy. Recentstudies confirm that molecules as large as HC11N are present in interstellarnebulae (Thaddeus 2006). Studies that investigate the extent of prebioticchemistry in the interstellar medium have become the sustaining activity ofmany researchers.

Improvements to electronic instrumentation and vacuum equipment broughtnew opportunities after 1970. It was found that molecules generated or stabilizedwithin a supersonically expanding gas sample could be effectively probed throughspectroscopic experiments. Ekkers & Flygare (1976) reported the use of high-power pulse trains of microwaves of single selected frequency to excite transitionsin preference to the more conventional, continuously swept sources. Thetransient emission signal was collected and Fourier transformed using acomputer. A major advantage of this method was that individual measurements(taken after each pulse) could be readily summed to improve the signal/noiseratio of the final result. Pulsed spectroscopic techniques were increasing inpopularity at this time owing to the increased availability of pulsed lasers, ability

to control and synchronize experiments through fast electronics and the growingimportance of experiments performed under high vacuum.

Supersonic expansion maximizes the population of low energy rotational levelssuch that transitions between those levels can be observed with greatest intensity.The cold temperatures in a supersonic jet allow the formation of molecules andcomplexes that are unstable against decomposition at room temperature andatmospheric pressure. A pulsed injection of sample gas offers several advantages inthe context of spectroscopic experiments: (i) the necessary experimental conditionscan be established with lower total gas throughput and smaller, slower, lessexpensive vacuum pumps, and (ii) pulsed sources are highly compatible with

methods that involve the pulsed irradiation of a sample. Balle et al. (1979).implemented these advantages in their design of the cavity Fourier transformmicrowave spectrometer. The development of a spectrometer employing a cavity

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and free-space cell opened up a vast range of weakly bound molecules andcomplexes to first study by microwave spectroscopy (Legon 1983). It provided thebasis for an instrument that was ultimately exploited throughout the world. Adescription of this design of spectrometer will provide a suitable introduction to theexperimental methods used by the microwave spectroscopists whose work is

featured in 4.

2. The BalleFlygare Fourier transform microwave spectrometer

The parallel configuration of the BalleFlygare Fourier transform microwave(FTMW) spectrometer is shown in figure 1. Antennae allow introduction of themicrowave excitation pulse and detection of the molecular emission. A pulsednozzle is used for introduction of the gas sample. The two concave, aluminiummirrors comprise a FabryPerot cavity that can be tuned into resonance with the

microwave excitation pulse.Each individual experiment involves the same basic sequence of activities: (i) agas sample is introduced through the orifice of the pulsed nozzle. Opening times

tuning

mirror

pulsed valveAr/OCS supersonic

expansion

stationary

mirror

antenna

free induction decay frequency domain

spectrum

Figure 1. The parallel configuration of BalleFlygare Fourier transform microwave spectrometer isshown. A more complete description of the instrument is provided by Grabow et al. (1996).Microwaves are absorbed with a frequency slightly higher than that of the molecular absorption

when propagating in the same direction as the supersonic jet. A corresponding shift to lowerfrequency exists where the absorbed microwaves are travelling in the opposite direction. The beatof these two components is seen in the free induction decay, which can be Fourier transformed toyield the frequency domain spectrum. The correct frequency for the rotational transition can beaccurately determined by taking the average of the two Doppler components.

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of 200400 ms are typical, (ii) on resonance with a rotational transition, a pumpmicrowave excitation pulse of 0.33 ms in duration rotationally excites theabsorbing species, (iii) the pump pulse is deactivated and the molecular emissionis measured, and (iv) the molecular emission is Fourier transformed to yield aplot of transition intensity against frequency. The above scheme describes a

single resonance experiment (i.e. one microwave pulse). Double resonanceexperiments are particularly useful in assigning complex spectra. One example ofa double resonance experiment is described by Suma et al. (2004). An initialmicrowave pulse was used to increase the population of a selected rotationalenergy level. The molecular microwave emission from that state was monitoredwhile a millimetre-wave source was scanned. On resonance with a millimetre-wave transition, the state excited initially was depopulated and the depletion ofthe microwave emission signal was measured. Many other schemes that exploitdouble resonance are described in the wider literature. Regardless of whethersingle or double resonance is used for any given experiment, the four stages of the

cycle described above can be completed in less than one-tenth of a second. Theresults obtained over many cycles can be accumulated and averaged in order toimprove the signal/noise ratio of measured transitions.

3. Spectroscopic analysis

Transitions between rotational energy levels can be observed in isolation throughpure rotational spectroscopy. Alternatively, transitions can accompany changesin vibrational and/or electronic energy levels. Microwave spectroscopy is generallyconcerned with pure rotational spectra and these will be the focus of this work.Generating a particular target molecule for study often requires a gas sample thatcontains a mixture of different precursors. Many of these will have microwavespectra. A single measurement is therefore insufficient to determine which moleculeis responsible for a given rotational transition. Empirical tests can inform thecorrect assignment. For example, is a transition observed despite the fact that animportant precursor has been removed from the gas sample? Ultimately,unambiguous assignment to the correct molecule depends on fitting all observedtransitions to a model Hamiltonian for the molecular rotational motion.

The quantum mechanical Hamiltonian of a molecule can be constructed fromknowledge of anticipated coupling interactions. Observed transition frequencies

are assigned quantum numbers and fit to the predictions of the modelHamiltonian. The value of each parameter in the Hamiltonian is adjusted toobtain better agreement. A spectroscopist can be reasonably confident of successwhen a fit yields values for parameters that are physically reasonable and withinthe experimental precision. Assignment is most often challenging wheremolecules are geometrically complex and non-symmetrical, contain extensivefine or hyperfine structure or yield only weak rotational transitions. The detailsin such spectra usually allow formidable insight once the assignment has beencompleted. Simpler molecules can be assigned more readily but the evaluatedparameters provide less information about the physics of the species. The

methodology necessary to make assignments of the correct transitions to thecorrect molecule is now well established. There will be very few examples ofpublished rotational spectra that have been assigned to the wrong molecule.





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(a) The rigid rotor

Quantum mechanics provides the description of molecular rotational motionnecessary to interpret microwave spectra. Only species that possess a permanentdipole moment absorb or emit microwave radiation during pure rotational

transitions. The rotational constants of a molecule are related to the principalmoments of inertia (Ix,y,z) by

BxZh2

8p2Ix;

where h is the Planck constant, and Bx is the rotational constant associated withthe x-axis. Corresponding expressions apply to the y- and z-axes. Microwavespectroscopy involves the experimental measurement of the transitionfrequencies. The rotational constants can then be used to determine momentsof inertia and hence the molecular structure. The determination is simplest for a

diatomic molecule for which the moment of inertia, Ix, is related to theinternuclear distance, R, by

IxZm1m2

m1Cm2

R2;

where m1 and m2 are the masses of the constituent atoms; Ix and Iy are equalwhile Iz is infinitesimal. A rigid rotor model assumes that the diatomic can berepresented as two hard spheres connected by an inextensible, incompressiblebond. The frequency of light absorbed or emitted during a transition betweenrotational levels is given by

DEJZ 2BeJC1;

where Be is the equilibrium rotational constant associated with an axisperpendicular to the molecular axis of symmetry; J is the quantum numberassociated with the lower energy rotational state involved in the transition;DE(J) is measured in the laboratory through measurements of microwaveemission or absorption and then used to determine molecular structure using theexpressions provided above. The rigid rotor model provides a useful foundationfor studies but does not fully reflect the true nature of molecules. It is possible toassign a distinct rotational constant to every vibrational state within the

molecule because bonds extend in response to vibrational excitation. Therotational constant, Bv, is the expectation value of the quantity for a givenvibrational state with quantum number, v. It turns out that the energy can stillbe expressed in a form identical to that of the rigid rotor but with Be replaced byBv. This model is referred to as the effective rigid rotor.

The above equation is often sufficient to define an effective rigid rotor for apolyatomic linear molecule. More sophisticated equations connect the transitionfrequencies with rotational constants for polyatomic symmetric and asymmetrictop molecules. The methodology involved with assigning spectra is qualitativelythe same as for a diatomic, but the assignment is generally more challenging. Data

acquired from a series of different isotopologues allow bond lengths and bondangles to be determined. The accuracy of structural determinations is dependenton whether samples containing appropriate isotopic substitutions are available.

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(b ) Centrifugal distortion

The effective rigid rotor model does not account for the extension of bonds inresponse to centrifugal force. In reality, all molecules experience centrifugaldistortion and rapidly rotating molecules are the most severely distorted. This

effect can be measured through microwave spectroscopy and used to infer trendsin vibrational frequency and bond strength. The effective rigid rotor expressionfor a diatomic molecule is revised to include a term in Dv quantifying the effectsof centrifugal distortion,

DEJZ 2BvJC1 4DvJC13:

The second term is small at the low values of J that correspond with lowrotational energy; Dv can be measured accurately when higher energy states areprobed in millimetre-wave spectroscopy.

(c) Fine and hyperfine structures

Electrons and nucleons can possess spin that couples to the rotational angularmomentum of a molecule. This has the effect of splitting rotational energy levelsinto a greater number of components leading to a corresponding increase in thenumber of observed transitions. The fine structure observed in spectra arisesfrom the interaction of individual unpaired electrons with the spins of otherelectrons, the molecular rotational angular momentum and/or the molecularorbital angular momentum. Hyperfine structure is the result of interactions thatcouple the spin of one or more nuclei with the framework angular momentum.The relative magnitudes of these effects depend on the molecule. The presence ofmany coupling interactions within a molecule yields spectra that are morechallenging to assign, but the effects can provide great insight into chargedistribution where they can be thoroughly characterized.

4. Emerging themes

Sources of millimetre wavelengths (approx. 30300 GHz in frequency) weredeveloped shortly after tuneable microwave generators and are now widelyexploited for spectroscopy. Until recently, instruments capable of generatingradiation at terahertz frequencies had been unavailable. Sources are now being

manufactured and applications appear likely to proliferate (Davies et al. 2004).Descriptions of experiments that exploit millimetre wavelengths and terahertzfrequencies are outside the focus of this article. This work will focus instead on resultsrecently achieved by contemporaries using microwave spectroscopy. The purpose isnot to provide a complete description of the experiments and motivations of thegreat number of groups that now use microwave spectroscopy worldwide. Theselected examples are only intended to be illustrative of a few themes and theirconnections with other areas of research. They are intended to demonstrate howworkers have adapted to new imperatives and are exploiting emerging opportunities.It is hoped that this snapshot will encourage the reader to seek out further details of

research in microwave spectroscopy in the broadest possible sense. Sections 4acdescribe experiments conducted using BalleFlygare cavity FTMW spectrometers.Section 4ddescribes the development of an entirely new tool for research.





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(a) Atmospheric chemistry and remote sensing

A central theme of modern research in chemistry is the study of chemicalreactions and processes in the Earths atmosphere. The upper stratosphere iscold and sparse compared with conditions in the troposphere near the Earthssurface. Supersonic expansion of a gas sample provides a local environmentwhere cold, weakly bound molecules can be prepared for study by microwavespectroscopy. Suma et al. (2004) recently probed ClO2, a radical which maycontribute to the depletion of ozone within the polar stratosphere. Studies ofHOOO, HOOOH, H2OO2H (Suma et al. 2005a,b, 2006) and H2OOH (Ohshimaet al. 2004) were also completed by the group. These species are of interest owingto their roles as intermediates in reaction pathways that involve OH radicals.The structure of HOOO, reprinted with permission from Suma et al. (2005a,b) isshown in figure 2. Details of geometric and electronic structure gained throughmicrowave spectroscopy are used to refine our models of atmospheric chemicalreactions and processes.

Studies are further useful in providing the spectral signatures of moleculesthat may be detected through remote sensing (Alder 1990). Mass spectrometersare unable to distinguish between different components that have the sameatomic mass. Microwave spectra are unambiguous with respect to the identityof a molecule and its structure. A further advantage of microwave spectroscopyis that neutral (rather than ionic) molecules can be probed directly. There is no

requirement that sample molecules are ionized before analysis. Successfulapplication of microwave spectroscopy to trace gas detection would requirethat reliable assignments of transition frequencies were available fromlaboratory studies beforehand. Bousquet et al. (2005) have described effortsto adapt the techniques of microwave spectroscopy for the detection ofchemical warfare agents.

(b ) Structure and solvation of amino acids

Microwave spectroscopy can also provide information about the structure and

function of different molecules significant in biology. The chemistry of biologicalsystems is fundamentally dependent on the properties of comparatively largeorganic molecules. These adopt different conformations which may change in

1.688

0.972 90.04

111.021.225

Figure 2. The molecular structure of the HO3 radical obtained from the experimentally determinedrotational constants. The OHO2 radical is proposed to be an intermediate for the HCO3/OHCO2reaction (adapted from Suma et al. (2005b)). Reprinted with permission from AAAS.

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response to chemical environment or temperature. Species that contain both acarboxylate group and an amine group belong to the class of molecules termedamino acids. The amine group on one amino acid undergoes a condensationreaction with the carboxylate group on the second during the formation ofpeptide bonds. The linkage of a large number of amino acid residues leads to

the formation of polypeptides and proteins. Amino acids can adopt manydifferent conformations and the number of different possibilities increases withthe size of the acid. In biological environments, the propensity of an amino acidto adopt a given conformation can depend on the strength and extent ofintramolecular hydrogen bonding and intermolecular bonds that it shares withneighbouring molecules.

Studies by Brown et al. (1978) and Suenram et al. (1980) used ovens toevaporate material and succeeded in characterizing several conformers of glycine,NH2CH2CO2H by microwave spectroscopy. Low vapour pressures and atendency for amino acids to decompose on heating meant that many larger

examples could not be prepared for study in this way. Laser vaporization hassince provided a more versatile and controllable method of preparing moleculesfor study. J. L. Alonso and co-workers have applied this technique to generate awide range of amino acids for study by microwave spectroscopy. In several cases,the molecules studied are possible constituents of the interstellar medium. Theobtained transition frequencies provide important benchmarks for astronomicalmeasurements. Leucine (Cocinero et al. 2007) and valine (Lesarri et al. 2004) areamong the largest and most conformationally flexible residues studied.Objectives are evolving from the exploration of relatively small species to morecomplex and flexible molecules. Formamide contains the same functional groupsthat are present within amino acids. The microwave spectrum of formamidecoordinated by H2O provided a first insight into the structures of solvated aminoacids and their coordinating molecules (Blanco et al. 2006). Microwave spectra ofglycine attached to water molecules have since been reported (Alonso et al.2006). Proteins exist as solvated species within the human body and coordinationmay profoundly affect their shape and function. Studies of more complex andextensively coordinated amino acids can be expected in the future.

(c) Superfluidity in helium clusters

The cold environment in a supersonic expansion allows extensive aggregation

and clustering provided strong intermolecular forces exist between moleculeswithin the expanding gas sample. The intermolecular forces between heliumatoms are comparatively weak. The generation of helium nanodropletscontaining more than 1000 atoms therefore requires a particularly coldexpansion. Nanodroplets provide an ideal environment in which to performmatrix isolation experiments. Embedded molecules can be probed usingspectroscopy and experience minimal perturbations from the surrounding atoms.

The ro-vibrational transitions of OCS embedded in a helium nanodroplet areobserved as sharp lines. This suggests that the molecule rotates comparativelyfreely within the superfluid core of the nanodroplet (Grebenev et al. 1998). Jager

and co-workers performed experiments to explore the transition from theproperties of the isolated OCS molecule to those of the species embedded in thenanodroplet. Their first report (Tang et al. 2002) described the microwave





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spectra of HenOCS clusters for nZ18. The rotational constant was observed todecrease with increasing size of cluster. This trend is consistent with semi-rigidHenOCS units where each helium atom tracks the motion of the OCS absorber.It was expected to reverse at some higher value for n where the superfluidityevidenced in the nanodroplet would become important. The data provided infigure 3 have been reprinted with permission from McKellar et al. (2006).Rotational constants of HenOCS were obtained through a combined study that

yielded microwave data for clusters from nZ

139 and infrared measurements ofro-vibrational transitions for clusters from nZ1272.The addition of a helium atom resulted in an increase in rotational constant

where 9!n!23. This trend is a consequence of the motion of the helium atomsbecoming decoupled from that of the OCS unit and is associated with the onset ofsuperfluidity. At larger values of n, the magnitude of the measured rotationalconstant is convergent with that observed for the nanodroplet-embedded OCS.Oscillation around this value is seen throughout the size range from nZ1072.This is explained by the formation of solvation shells within the helium clusterand is in agreement with theory. These works have furthered our understanding

of the character and extent of superfluidity in finite, microscopic systems. Theyhave provided valuable insight into the localized interactions that give way tothe superfluidity observed in bulk samples.

cluster size, N0 10 20 30 40 50 60 70

rotationalconstant,B"(MHz)

1500

2000

2500

3000

nanodroplet value+

Figure 3. Variation of the rotational constant, B00, of4HenOCS with n. Reprinted with permissionfrom McKellar et al. (2006). Copyright q American Physical Society. Filled triangles and filledsquares indicate the results of theoretical calculations that are described in more detail in the

earlier work. The experimental values ofB00

are represented by filled circles. The value observed for3He clusters with nz60 is shown as a cross.

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(d) Chirped pulse Fourier transform microwave spectroscopy

Microwave spectroscopists have exploited Fourier transform techniques eversince computers able to perform the operation rapidly became available. Theburgeoning market for improved telecommunications equipment has helped to

ensure that electronic devices and instruments for microwave detection andgeneration have been continually improved since the advent of the BalleFlygaredesign of FTMW spectrometer. New technology has continuously provided newopportunities to innovate.

Over the last 20 years, improvements to the BalleFlygare spectrometer haveincreased the frequency range and sensitivity of the device. Automation hasbrought increased efficiency in data collection. Instruments constructed with thedescribed adaptations continued to employ pulse trains at fixed, selectedfrequencies to allow the rotational polarization of the sample. The new chirped-pulse technique of FTMW spectroscopy (CP-FTMW) avoids this requirementby exploiting a pulse which sweeps over an 11 GHz range of frequencies in a few

microseconds (Brown et al. 2006). An oscilloscope with bandwidth greater than11 GHz is used to resolve the coherent emission signal which is then Fouriertransformed to yield the broadband microwave spectrum. All transitions in thespectrum can be measured simultaneously. The performance of the instrumentwas tested through measurements of lines in the spectrum of epifluorohydrin.These data are reprinted with permission (Brown et al. 2006) in figure 4.

Consistent with the increased bandwidth, the time required to accuratelymeasure all transitions in the spectrum of the molecule was reduced from14 hours (BalleFlygare design) to 48 min (CP-FTMW). The line width (fullwidth half maximum) of transitions was 5 kHz using the BalleFlygare

spectrometer and 125 kHz using the CP-FTMW spectrometer. The lowerprecision achieved in the CP-FTMW spectrometer is much compensated bythe increased bandwidth available. The instrument provides data that allowsprecise structural determinations and the evaluation of hyperfine parameters.Significantly, the need to perform an iterative search for transitions at the outsetof an investigation is eliminated. This instrument might be used to greatadvantage in future studies such as those described in 5.

5. The future

Townes (2002) did not anticipate the applications of todays laser systems whendesigning and constructing the first maser. Another microwave spectroscopist,H. W. Kroto, was pleasantly surprised when his interest in the microwavespectra of carbon-containing molecules in space led to his role in the discovery ofC60 (Kroto et al. 1985). Not all paths will bring research success. Sometimes themost interesting results are obtained unexpectedly or by accident. Attempting tosuggest the future of research involves providing a few hostages to fortune. Thereader is encouraged to consider a few possibilities with an open mind.

Molecular units where a metal ion is completely enclosed and coordinated atone or more sites that are all coordinated by a single, much larger molecule are

of considerable significance in biochemistry. There is a wide community ofresearchers currently working to understand molecular recognition in biologicallysignificant species through mass spectrometry. A future role for microwave





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spectroscopy may be to provide precise structural information that is relevant to

molecular recognition but which cannot be obtained by mass spectrometry. Forexample, how is the geometry of 12-crown-4 affected by attachment to a metal-containing molecule? How does the conformational landscape change? Crownethers are cyclic species able to attach to other molecules, particularly metalions, through coordination of oxygen atoms that are oriented towards the centreof a cyclic structure. As phase transfer catalysts, these molecules facilitatesolution-phase chemical reactions even where individual reactants are solubleonly in different kinds of solvent. The interface between biology and the work ofmicrowave spectroscopists appears certain to grow. Other opportunities willderive from the increasing need to understand how topology and the nature of

surfaces affect the structures of adsorbed species.The efficiency of industrial platinum catalysts is known to be adverselyaffected by carbon monoxide poisoning. The reduction in efficiency results from

intensity(mV)

1

0

1

0

1

0

0.02

011 350 11 352

11 350 11 352

11 350 11 352

0.02

0

0.02

0

8000 10000 12000 14000 16000 18000

frequency (MHz)

11350.93 MHz

212 202 Conf. I

(a)

(b)

(c)

Figure 4. The 11 GHz rotational spectrum of epifluorohydrin measured by CP-FTMWspectroscopy. (a) A single gas pulse allows the rotational spectrum of one conformer to beobserved. (b) Thirty seconds of averaging (100 pulses) are necessary to observe a further twoconformers. The inset in each spectrum shows the effect of signal averaging on an expanded scale.The frequency of the 212202 rotational transition of the first conformer is indicated in the full-scalespectrum. The transition is also shown in the expanded scale. (c) Approximately 48 min arerequired to accumulate data from 10 000 gas pulses. This figure was published in Brown et al.(2006). Copyright q Elsevier.

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CO binding to surface sites on the metal and reducing the number available toadsorb H2. There is consequently a fundamental need for a greater understandingof the interactions between CO adsorbates and metal catalyst surfaces. Metal-containing clusters provide an excellent opportunity to study these interactions.Their small size permits the acquisition of data that is tractable with theory.

Their properties are a strongly varying function of size suggesting that size-selected clusters may ultimately provide promising alternatives to continuoussurface catalysts. Ionic clusters can be investigated through mass spectrometrybut other forms of spectroscopy are required to explore the properties of neutralspecies. The assignment of microwave spectra obtained from NinCO clusterswould surely present a significant challenge to the experimentalist but Ziurys andco-workers have demonstrated that microwave spectra can be assigned formolecules in high-spin states ( Halfen & Ziurys 2005). The prospects for researchin this area are encouraging.

The author wishes to thank the Royal Society for the award of a Royal Society University Research

Fellowship and Prof. A. C. Legon (FRS) for a critical reading of the manuscript.

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AUTHOR PROFILE

Nicholas R. Walker

Nick Walker applies spectroscopy and mass spectrometry to characterize metal-containing species in the gas phase. He did both undergraduate and postgraduatedegrees at the University of Sussex before commencing postdoctoral research in2000. He completed a postdoctoral appointment at the University of BritishColumbia with Prof. Michael Gerry and then another at the University of

Georgia with Prof. Michael A. Duncan. He started a Royal Society UniversityResearch Fellowship at the University of Bristol in 2003 and was awarded theMeldola Medal and Prize of the Royal Society of Chemistry in the same year. Hemade four short TV programmes in association with ITV (West) in 2004. He alsoenjoys voluntary work. Acting through Business in the Community, heorganized a team of 32 Bristol postgraduates to redecorate the interior ofWeston Park primary school after fund-raising the costs of the project in 2006.

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nicholas r. walker- new opportunities and emerging themes of research in microwave spectroscopy

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