SCIENCE & TECHNOLOGY GOTCHA This ion trap, developed by Gerlich and coworkers, was the heart of the new experiments on CH5+. The trap is composed of 22 small rods, which are visible in the photo. A radiofrequency voltage applied to the rods causes cooled CH5+ ions to be confined in the trap during the experiments.
PROTONATED METHANE PROBED Researchers obtain the first broad-frequency infrared spectrum of the CH5+ carbocation STU B0RMAN, C&EN WASHINGTON
A
NEW SPECTROSCOPIC STUDY
ofprotonated methane, CH5+, is allowing long-standing questions about the mysterious ion's structure and dynamics to be answered with greater clarity than ever before. CH5+, also called methonium ion, is a carbocation formed in the gas phase by a chemical reaction that adds a proton to methane, and it has also been reported to be present in very acidic solution. It has resisted complete spectroscopic characterization since it was discovered in the early 1950s, but researchers have now obtained the first broad-frequency infrared (IR) spectrum of the ion. The spectrum reveals new details about the structure and dynamics of CH5+. More
sidered important because the ions serve as highly reactive intermediates in hydrocarbon reactions catalyzed by very strong "magic acids" and also play a key role in electrophilic substitution reactions of aliphatic hydrocarbons. The field of carbocation chemistry was developed by organic chemistry professor George A. Olah of the University of Southern California, Los Angeles, who received the 1994 Nobel Prize in Chemistry for his pioneering work in the area, including studies of CH5+. Asked to comment on the new study, Olah says: "Professor Marx and his coworkers have made a significant new contribution to the study of protonated methane. CH5+ has long continued to elude definitive experimental structural assignment {and has} challenged both theory and experiment." The study "gives a solid experimental and theoretical foundation to the structural question of CH5+."
ANOTHER REASON for studying CH5+ is the substantial astrochemical interest in the ion. It is "implicated in reactions that form part of the intricate synthesis ofpolywork still needs to be done for the ion's atomic species in cold interstellar clouds," structure and dynamics to be fully charac- Marx writes in a review. "Experimental and theoretical colterized, but the new study represents a key leagues have hunted for about 50 years for step toward that goal. + The work was carried out by a collabo- the spectrum of CH5 and about 30 years rative team including Britta Redlich, man- for an understanding of its structural dyager of the Free Electron Laser for Infrared namics," Marx tells C&EN. But success Experiments (FELIX) Facility at the Foun- has been limited, primarily because the dation for Fundamental Research on Mat- ion is unconventional. The component ter (FOM) Institute for Plasma Physics, atoms of most ions and molecules occu+ Nieuwegein, the Netherlands; physics py equilibrium positions. CH5 instead professor Stephan Schlemmer of the Uni- seems to have a constantly fluctuating versity of Cologne, Germany; and profes- "fluxional" or "floppy" structure that besor of theoretical chemistry Dominik lies easy characterization. Marx of Ruhr University, Bochum, Ger"Lots ofpeople worldwide are attacking many (Science, published online June 30, this species," says chemistry professor dx.doi.org/10.1126/science.1113729). Mark AJohnson ofTfele University "It repElucidating the fundamental nature resents one of the most important unof CH5+ and similar carbocations is con- solved spectroscopy problems involving
Lots of people worldwide are attacking this species. It represents one of the most important unsolved spectroscopy problems involving small molecules." WWW.CEN-0NLINE.ORG
C & E N / JULY 2 5 . 2005
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small molecules." Johnson and a collabo- at www.theochem.rub.de/go/ch5p.html. rative team recently helped solve a similar The ion's three-center, two-electron bonding pattern and the rotating motions of its problem by obtaining the previously inacfive protons are the keys to understanding cessible low-energy part of the IR specits IR spectrum. trum of the hydrated proton, H 3 0 + (C&ENJuly4,page26). This new study and an earlier one by anRedlich, Schlemmer, Marx, and cowork- other group, "are likely to be taken as models for thoroughly understanding the IR ers have now obtained the IR spectrum of spectra of highly fluxional molecules by CH5+ over a much wider frequency range elaborate molecular dynamics techniques," than has been accessible previously And Schreiner says. The earlier study, reported they used theoretical means to assign the in 2003, was one in which chemistry proion's IR absorptions to specific motions in fessorJoel M. Bowman of Emory Univerthe ion. sity and coworkers first calculated the viThe new CH5+ spectrum ranges from brational spectrum of CH5+ by ab initio about 600 to 3,200 cm 1 . This covers not techniques. only CH5+'s full C-H stretching band but Other notable earlier studies on CH5+ also its complete C-H bending band. The included late-1960s work by several spectrum does not have sufficiently fine groups, including Olah's, that proposed detail to resolve CH5+,s rotational structhe possibility of viewing CH5+ as a CH 3 + ture, however. tripod with a three-center, two-electronA major conclusion of the study is that bonded H 2 moiety. In the early 1970s, CH5+ can be usefully considered to be a Werner Kutzelnigg of Ruhr University, CH3+ tripod structure with an added H 2 Bochum, and coworkers conducted theogroup linked to the carbon by a three-cenretical studies on CH5+, one ofwhich was tered (C-H-H), two-electron bond, notes chemistry professor Peter R. Schreiner of Justus Liebig University, Giessen, Germany. "But this does not imply that its overall, time-averaged structure can be statically depicted" that way, he says. Chemistry professor Kenneth D. Jordan of the University of Pittsburgh, a collaborator of Johnson's on the recent hydrated proton study, agrees that the new study indicates that "the CH5+ ion, at least when sufficiently cold, can be viewed as having three-center, twoelectron bonding." Given the low energetic barriers for structural interconversion of the ion, it has not been clear up to now "whether this bonding mo- RAD SOURCE This free-electron laser at the FELIX tif is the dominant one in Facility (www.rijnh.nl/felix) was used to generate IR radiation for experiments on CH5+. the cluster," he notes. "The three-center, twothe first "where the claim or conjecture electron bond coordinates the five proconcerning the ion's fluxional nature was tons, which, roughly speaking, move on a spelled out, based on ab initio theory," sphere about the carbon nucleus," Marx Marx says. explains. Accurate ab initio calculations of the A separate H 2 group can indeed be idenion's potential surface and an explicit extified in the CH5+ spectrum, but its hydropression of its highlyfluxionalnature were gens continually exchange with the ion's obtained in 1993 by chemistry professors three other hydrogens, a process called scrambling. Hence, the H 2 group constandy Henry F. Schaefer III at the University of Georgia, Athens; Paul von Ragué rotates about the carbon center as its hySchleyer at the University of Erlangendrogens change, as shown in an animation WWW.CEN-0NLINE.ORG
GATEWAY CHEMICAL· TECHNOLOGY Nuremberg, Germany; and coworkers, including Schreiner. The paper caused quite a stir, Schreiner says, by showing that CH5+ is an unusual ion because a sin gle equilibrium structure seemingly could not be assigned to it.
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PROCESS DEVELOPMENT ELUSIVE CH5+ model, showing CH3 tripod structure (black and gray) and H2 moiety (blue).
Further progress was made in a 1997 theoretical study led by Kutzelnigg; Jozef Noga of the Slovak Academy of Sciences, Bratislava, Slovakia; and chemistry pro fessor Willem M. Klopper of the Univer sity of Karlsruhe, Germany "They used a novel electron correlation approach to de termine the energetics underlying the hy drogen motions around CH5+,s central car bon with ultimate precision," Marx says. In 1999, thefirstIR spectrum of CH5+ was obtained after a 16 -year effort by nowemeritus professor of chemistry and of as tronomy and astrophysics Takeshi Oka of the University of Chicago and coworkers. Their spectrum achieved rotational reso lution, but the vibrational part of the spec trum was incomplete. It covered only part of CH5+,s C-H stretching band,fromabout 2,800 to 3,100 cm1. The spectral bands were so complex that "I did not even know where to begin" to assign them, Oka says. The Oka group's study thus did not re veal much about CH5+,s structure, but it did help confirm the 1993 Schaefer-Schleyer study's claims about the ion'sfloppychar acter. In addition, recent theoretical stud ies of CH5+—by chemistryprofessor Anne B. McCoy of Ohio State University, Bow man, and coworkers, and independently by senior lecturer in chemistry Meredith J.T.Jordan ofthe University of Sydney Aus tralia, and coworkers—helped confirm the same claims computationally. This year, professor of chemistry and biochemistry DavidJ. Nesbitt of the Uni versity of Colorado, Boulder, and cowork ers obtained a high-resolution IR spectrum of jet-cooled CH5+ that, like the Oka group's study, achieved rotational resolu tion, but it was less congested and easier to understand. Oka calls this low-temperaWWW.CEN-0NLINE.ORG
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SCIENCE & TECHNOLOGY ture spectrum aa great breaklow résolution," Oka says. 'This is through," in part because it is clearly a step forward." simpler to analyze. The study provides "a survey spectrum that exquisitely shows The Redlich-Schlemmerthe overall pattern of vibrational Marx study now makes further transitions, and that is a great inroads on CH5+ by greatly exachievement," Johnson says. tending the range of its IR "Now we need to look at these spectrum. The researchers irbands at much higher resolution radiated CH5+ with IR light to decode the energy-level patfrom afree-electronlaser at the tern and reveal the intrinsic FELIX Facility while the ion physics at play in this prototype was isolated in a low-temperamolecule." ture ion trap, a device developed by physics professor DiOka agrees, noting that despite eter Gerlich and coworkers at the study's success, "I do not think the University of Technology, the problem of CH5+ is close to a Chemnitz, Germany. They solution. It is clear to me that obused laser-induced reaction servation of the ion's rotational (LIR), a technique developed spectrum will be the key to unby Gerlich, Schlemmer, and derstanding this beast." Profescoworkers, to generate the ion's TOUR DE FORCE The LIR setup used to obtain CH5+'s IR sor of astrophysics and planetary IR spectrum. In all previous IR spectrum is a bench including an ultra-high-vacuum sciences Takayoshi Amano of studies of CH5+, Schlemmer chamber and ion source (left), a 22-pole ion trap mounted Ibaraki University, Japan, now says, spectra were recorded us- on a helium refrigerator (center), and an ion-counting moving to the University of Waing a different technique called detector (right). The setup is axially transparent, so laser terloo, Canada, is attempting to vibrational predissociation, light from the FELIX free-electron laser can reach the observe such a spectrum. which can cause artifacts in the trap after being introduced on the right side. "In 1999 when I published our data, whereas LIR does not. paper, I believed it would take at With LIR, the experimentalists "mon- cal and measured spectra agreed closely. least 20 years to completely understand + itored the product of an endothermic reThey plausibly assigned CH5+'s vibra- the spectrum of CH5 ," Oka says. "Severaction with C 0 2 that selectively occurs tional features, despite the ion's fluxional al people challenged me and said that such with excited CH5+ ions,"Johnson says. Us- nature, by using computational tricks to an understanding would be obtained in a ing that to get the CH5+ spectrum was "a artificially freeze the ion's rotational and fewyears. It is already six years since then, real tour de force." structural properties. "The computations and there is still no solution in sight. I significantly helped in analyzing the IR personally think that 20 years is a gross underestimate." THE RESEARCHERS then assigned the spectrum," Schreiner says. "I agree that a full analysis will take some spectrum. They also computed the ion's 'The paper is very nice in that they have + IR spectrum using ab initio molecular given for the first time the overall spec- time," Nesbitt says. "CH5 still has a few dynamics and found that the theoreti- trum of this important species, albeit in tricks up its sleeve." •
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