Protonated Methane Probed

A new spectroscopic study of protonated 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 work still needs to be done for the ion's structure and dynamics to be fully characterized, but the new study represents a key step toward that goal.

The work was carried out by a collaborative team including Britta Redlich, manager of the Free Electron Laser for Infrared Experiments (FELIX) Facility at the Foundation for Fundamental Research on Matter (FOM) Institute for Plasma Physics, Nieuwegein, the Netherlands; physics professor Stephan Schlemmer of the University of Cologne, Germany; and professor of theoretical chemistry Dominik Marx of Ruhr University, Bochum, Germany (Science, published online June 30, dx.doi.org/10.1126/science.1113729).

Elucidating the fundamental nature of CH₅⁺ and similar carbocations is considered 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 of polyatomic species in cold interstellar clouds," Marx writes in a review.

"Experimental and theoretical colleagues have hunted for about 50 years for the spectrum of CH₅⁺ and about 30 years for an understanding of its structural dynamics," Marx tells C&EN. But success has been limited, primarily because the ion is unconventional. The component atoms of most ions and molecules occupy equilibrium positions. CH₅⁺ instead seems to have a constantly fluctuating "fluxional" or "floppy" structure that belies easy characterization.

"Lots of people worldwide are attacking this species," says chemistry professor Mark A. Johnson of Yale University. "It represents one of the most important unsolved spectroscopy problems involving small molecules." Johnson and a collaborative team recently helped solve a similar problem by obtaining the previously inaccessible low-energy part of the IR spectrum of the hydrated proton, H₃O⁺ (C&EN, July 4, page 26).

Redlich, Schlemmer, Marx, and coworkers have now obtained the IR spectrum of CH₅⁺ over a much wider frequency range than has been accessible previously. And they used theoretical means to assign the ion's IR absorptions to specific motions in the ion.

The new CH₅⁺ spectrum ranges from about 600 to 3,200 cm^-1. This covers not only CH₅⁺'s full C­H stretching band but also its complete C­H bending band. The spectrum does not have sufficiently fine detail to resolve CH₅⁺'s rotational structure, however.

A major conclusion of the study is that CH₅⁺ can be usefully considered to be a CH₃⁺ tripod structure with an added H₂ group linked to the carbon by a three-centered (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, two-electron bonding." Given the low energetic barriers for structural interconversion of the ion, it has not been clear up to now "whether this bonding motif is the dominant one in the cluster," he notes.

"The three-center, two-electron bond coordinates the five protons, which, roughly speaking, move on a sphere about the carbon nucleus," Marx explains.

A separate H₂ group can indeed be identified in the CH₅⁺ spectrum, but its hydrogens continually exchange with the ion's three other hydrogens, a process called scrambling. Hence, the H₂ group constantly rotates about the carbon center as its hydrogens change, as shown in an animation at www.theochem.rub.de/go/ch5p.html. The ion's three-center, two-electron bonding pattern and the rotating motions of its five protons are the keys to understanding its IR spectrum.

This new study and an earlier one by another group, "are likely to be taken as models for thoroughly understanding the IR spectra of highly fluxional molecules by elaborate molecular dynamics techniques," Schreiner says. The earlier study, reported in 2003, was one in which chemistry professor Joel M. Bowman of Emory University and coworkers first calculated the vibrational spectrum of CH₅⁺ by ab initio techniques.

Other notable earlier studies on CH₅⁺ included late-1960s work by several groups, including Olah's, that proposed the possibility of viewing CH₅⁺ as a CH₃⁺ tripod with a three-center, two-electron-bonded H₂ moiety. In the early 1970s, Werner Kutzelnigg of Ruhr University, Bochum, and coworkers conducted theoretical studies on CH₅⁺, one of which was the first "where the claim or conjecture concerning the ion's fluxional nature was spelled out, based on ab initio theory," Marx says.

Accurate ab initio calculations of the ion's potential surface and an explicit expression of its highly fluxional nature were obtained in 1993 by chemistry professors Henry F. Schaefer III at the University of Georgia, Athens; Paul von Ragué Schleyer at the University of Erlangen-Nuremberg, Germany; and coworkers, including Schreiner. The paper caused quite a stir, Schreiner says, by showing that CH₅⁺ is an unusual ion because a single equilibrium structure seemingly could not be assigned to it.

Further progress was made in a 1997 theoretical study led by Kutzelnigg; Jozef Noga of the Slovak Academy of Sciences, Bratislava, Slovakia; and chemistry professor Willem M. Klopper of the University of Karlsruhe, Germany. "They used a novel electron correlation approach to determine the energetics underlying the hydrogen motions around CH₅⁺'s central carbon with ultimate precision," Marx says.

In 1999, the first IR spectrum of CH₅⁺ was obtained after a 16-year effort by now-emeritus professor of chemistry and of astronomy and astrophysics Takeshi Oka of the University of Chicago and coworkers. Their spectrum achieved rotational resolution, but the vibrational part of the spectrum was incomplete. It covered only part of CH₅⁺'s C­H stretching band, from about 2,800 to 3,100 cm^-1. 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 reveal much about CH₅⁺'s structure, but it did help confirm the 1993 Schaefer-Schleyer study's claims about the ion's floppy character. In addition, recent theoretical studies of CH₅⁺--by chemistry professor Anne B. McCoy of Ohio State University, Bowman, and coworkers, and independently by senior lecturer in chemistry Meredith J. T. Jordan of the University of Sydney, Australia, and coworkers--helped confirm the same claims computationally.

This year, professor of chemistry and biochemistry David J. Nesbitt of the University of Colorado, Boulder, and coworkers obtained a high-resolution IR spectrum of jet-cooled CH₅⁺ that, like the Oka group's study, achieved rotational resolution, but it was less congested and easier to understand. Oka calls this low-temperature spectrum "a great breakthrough," in part because it is simpler to analyze.

The Redlich-Schlemmer-Marx study now makes further inroads on CH₅⁺ by greatly extending the range of its IR spectrum. The researchers irradiated CH₅⁺ with IR light from a free-electron laser at the FELIX Facility while the ion was isolated in a low-temperature ion trap, a device developed by physics professor Dieter Gerlich and coworkers at the University of Technology, Chemnitz, Germany. They used laser-induced reaction (LIR), a technique developed by Gerlich, Schlemmer, and coworkers, to generate the ion's IR spectrum. In all previous IR studies of CH₅⁺, Schlemmer says, spectra were recorded using a different technique called vibrational predissociation, which can cause artifacts in the data, whereas LIR does not.

With LIR, the experimentalists "monitored the product of an endothermic reaction with CO₂ that selectively occurs with excited CH₅⁺ ions," Johnson says. Using that to get the CH₅⁺ spectrum was "a real tour de force."

THE RESEARCHERS then assigned the spectrum. They also computed the ion's IR spectrum using ab initio molecular dynamics and found that the theoretical and measured spectra agreed closely.

They plausibly assigned CH₅⁺'s vibrational features, despite the ion's fluxional nature, by using computational tricks to artificially freeze the ion's rotational and structural properties. "The computations significantly helped in analyzing the IR spectrum," Schreiner says.

"The paper is very nice in that they have given for the first time the overall spectrum of this important species, albeit in low resolution," Oka says. "This is clearly a step forward."

The study provides "a survey spectrum that exquisitely shows the overall pattern of vibrational transitions, and that is a great achievement," Johnson says. "Now we need to look at these bands at much higher resolution to decode the energy-level pattern and reveal the intrinsic physics at play in this prototype molecule."

Oka agrees, noting that despite the study's success, "I do not think the problem of CH₅⁺ is close to a solution. It is clear to me that observation of the ion's rotational spectrum will be the key to understanding this beast." Professor of astrophysics and planetary sciences Takayoshi Amano of Ibaraki University, Japan, now moving to the University of Waterloo, Canada, is attempting to observe such a spectrum.

"In 1999 when I published our paper, I believed it would take at least 20 years to completely understand the spectrum of CH₅⁺," Oka says. "Several people challenged me and said that such an understanding would be obtained in a few years. It is already six years since then, and there is still no solution in sight. I personally think that 20 years is a gross underestimate."

"I agree that a full analysis will take some time," Nesbitt says. "CH₅⁺ still has a few tricks up its sleeve."