Molecules Spin Free in a Drop of Liquid Helium

Molecules Spin Free in a Drop of Liquid Helium

by John Emsley

WHAT'S HOT IN CHEMISTRY

Rank	Paper	Citations This Period Nov-Dec 01	Rank Last Period Sep/Oct 01
1	G. Schaftenaar, J.H. Noordik, "Molden: a pre- and post-processing program for molecular and electronic structures," J. Comp.-Aid. Molec. Des., 14(2):123-34, February 2000. [Nijmegen U., Netherlands] *279EG	22	1
2	A.F. Littke, C. Dai, G.C. Fu, "Versatile catalysts for the Suzuki cross- coupling of arylboronic acids with aryl and vinyl halides and triflates under mild conditions," J. Amer. Chem. Soc., 122(17):4020-8, 3 May 2000. [MIT, Cambridge] *312NZ	17	6
3	T.-Q. Nguyen, et al., "Controlling interchain interactions in conjugated polymers: The effects of chain morphology on exciton-exciton annihilation and aggregation in MEH-PPV films," J. Phys. Chem., 104(2):237-55, 20 January 2000. [U. Calif., Los Angeles] *278TT	13	8
4	J.P. Wolfe, et al., "Simple, efficient catalyst system for the palladium-catalyzed amination of aryl chlorides, bromides, and triflates," J. Org. Chem., 65(4):1158-74, 25 February 2000. [MIT, Cambridge, MA] *289DH	12	7
5	J. Gräfenstein, et al., "An accurate description of the Bergman reaction using restricted and unrestricted DFT: Stability test, spin density, and on-top pair density," J. Phys. Chem., 104(8):1748-61, 2 March 2000. [U. Goteborg, Sweden] *289VF	11	9
6	S. Grebenev, et al., "The rotational spectrum of single OCS molecules in liquid ⁴He droplets," J. Chem. Phys., 112(10):4485-95, 8 March 2000. [Max Planck Inst. Fluid Dynamics, Gottingen, Germany] *288LG	11	†
7	A.K. Chatterjee, et al., "Synthesis of functionalized olefins by cross and ring-closing metatheses," J. Amer. Chem. Soc., 122(15):3783-4, 19 April 2000. [Caltech, Pasadena] *308TT	10	4
8	S. Takenaka, et al., "DNA sensing on a DNA probe-modified electrode using ferrocenylnaphthalene diimide as the electrochemically active ligand," Anal. Chem., 72(6):1334-41, 15 March 2000. [Kyushu U., Fukuoka, Japan; Kyushu Inst. Technol., Izuka, Japan] *295NR	9
9	H. Benedict, et al., "Nuclear scalar spin-spin couplings and geometrics of hydrogen bonds," J. Amer. Chem. Soc., 122(9):1979-88, 8 March 2000. [Free U. Berlin, Germany; Slovak Acad. Sci, Bratislava, Slovakia; St. Petersburg St. U., Russia] *294ME	9	†
10	S. Noro, et al., "A new methane adsorbent, porous coordination polymer [{CuSiF₆(4,4’-bipyridine)₂}_n]," Angew. Chem.-Int. Ed., 39(12):2082, 16 June 2000. [Kyoto U., Japan; Tokyo Metropol. U., Japan; Osaka Gas Co., Japan] *328LA	9	†
SOURCE: ISI’s Hot Papers Database. the Legend.

erhaps the most surprising paper in the current Hot Ten is #6. Why should a paper about the infrared spectrum of the simple molecule, carbonyl sulfide (OCS), generate a lot of interest? However, it is not so much interest in the molecule, as the medium in which it sits. That medium is a droplet of liquid helium, the only substance that can exist as a liquid down to absolute zero. Moreover, as the isotope helium-4, it has superfluid properties, which means it does not interfere with the motions of materials dissolved in it. Paper #6 is all about OCS in minute droplets of liquid helium-4 at temperatures of 0.38 K.

The lead author of #6 is the eminent molecular physicist J. Peter Toennies, who is based at the Max Planck Institute for Fluid Dynamics, Gottingen, Germany. His work is attracting attention because of the sophistication with which it allows molecular motions to be analyzed. Toennies began working on helium droplets in 1990 and, in 1994, a member of his group, Andrei F. Vilesov, found that he could observe fine detail in the rotational spectrum of sulfur hexafluoride (SF₆), thereby proving that the molecules were rotating unimpeded inside the helium droplets. (Giacinto Scoles of Princeton University had examined SF₆ this way in 1992 and detected infrared absorptions, which he interpreted in terms of the molecule being on the surface of the droplets.)

Rotational spectra cannot be observed in normal liquids because these impede such molecular motions, but in liquid helium there is no such interference. Helium nano-droplets are also proving to be an ideal medium for other kinds of spectroscopy; the infrared spectra of more 30 compounds have so far been examined this way, as have the visible spectra of 20 others, including relatively large molecules such as complex aromatic hydrocarbons and amino acids.

Toennies first published the infrared spectrum of OCS in liquid helium in 1998 in Science (S. Grebenev, et al., 279[5359]:2083-6, 1998), the molecule being chosen because it is both simple and linear. In paper #6 he analyzes it in greater detail. The earlier paper showed that the free rotations were not evident when OCS was embedded in droplets of helium-3. The remarkably fine spectra are only observed in helium-4, proving that superfluidity is the key because only helium-4 has this property. As a result, free rotation is now seen as a new microscopic manifestation of superfluidity, a property that has previously only been observed at the macroscopic level, the best example being the uphill flow of liquid helium against the pull of gravity.

Paper #6 also reveals that the OCS molecule has a ring of six helium-4 atoms about its waist and that these are carried along as the molecule spins end-over-end.

According to Toennies, the advantage of the droplet technique, compared to that of other spectroscopic matrices, is threefold: first, it holds a sample at very low temperatures of either 0.38 K (helium-4) or 0.15 K (helium-3) and this greatly simplifies the spectra. Second, it allows gentler molecular motions, such as rotations, to be observed and the spectral lines are much sharper. Thirdly, new species can assemble inside the droplets, and molecules will stick together when they would not otherwise do so. This effect was reported in the Journal of Chemical Physics (114:617, 2000) for OCS with hydrogen (H₂) and deuterium (D₂). Even larger "complexes" of OCS have been observed with several H₂ molecules clustering around it.

Toennies believes the helium drop method will continue to attract attention: "The technique has proven to be a new universal matrix with unique properties and many potential applications for high resolution spectroscopy," he tells Science Watch. "Paper #6 has probably received attention because it provides the first in-depth analysis of the infrared rotational spectrum of a simple linear molecule."

Readers who would like to know more about helium nano-droplet techniques should consult the review by Toennies and Vilesov in Annual Review of Physical Chemistry (49:1-41, 1998) or the several reviews in Journal of Chemical Physics (115:special issue, 8 December 2001).

Dr. John Emsley is Science Writer in Residence
at the Department of Chemistry, University of Cambridge, U.K.

Science Watch^®, May/June 2002, Vol. 13, No. 3
Citing URL: http://www.sciencewatch.com/may-june2002/sw_may-june2002_page7.htm