Pick the Bond You'd Like to Break, Then Break It

Pick the Bond You'd Like to Break, Then Break It

by John Emsley

WHAT'S HOT IN CHEMISTRY...

Rank	Paper	Citations This Period Jan-Feb 00	Rank Last Period Nov-Dec 99
1	A.T. Brunger, et al., *"Crystallography & NMR System: a new software suite for macromolecular structure determination,"* Acta Cryst. D, 54:905-21, 1 September 1998. [10 institutions worldwide] *120JA	67	1
2	J.W.G. Wildoer, et al., "Electronic structure of atomically resolved carbon nanotubes," Nature, 391(6662):59-62, 1 January 1998. [Delft U. Technol., Netherlands; Rice U., Houston, TX] *YP888	20	2
3	A. Assion, et al., "Control of chemical reactions by feedback-optimized phase-shaped femtosecond pulses," Science, 282(5390):919-22, 30 October 1998. [U. Wurzburg, Germany] *134DH	15	†
4	S.J. Tans, A.R.M. Verschueren, C. Dekker, "Room-temperature transistor based on a single carbon nanotube," Nature, 393(6680):49-52, 7 May 1998. [Delft U. Technol., Netherlands] *ZM028	14	†
5	E. Meggers, M.E. Michel-Beyerle, B. Giese, "Sequence dependent long range hole transport in DNA," J. Amer. Chem. Soc., 120(49):12950-5, 16 December 1998. [U. Basel, Switzerland; Tech. U. Munich, Garching, Germany] *149MW	13	†
6	T.C. Terwilliger, J. Berendzen, "Automated MAD and MIR structure solution," Acta Crystallograph. D - Biol. Crystallograph., 55:849-61, April 1999. [Los Alamos Natl. Lab., NM] *188NW	13	†
7	L.A. Curtiss, et al., "Assessment of Gaussian-2 and density functional theories for the computation of ionization potentials and electron affinities," J. Chem. Phys., 109(1):7764-76, 1 July 1998. [Argonne Natl. Lab., IL; Lucent Technol., Murray Hill, NJ; Northwestern U., Evanston, IL] *132ZZ	12	†
8	S.S. Brown, R.K. Talukdar, A.R. Ravishankara, "Reconsideration of the rate constant for the reaction of hydroxyl radicals with nitric acid," J. Phys. Chem. A, 103(16):3031-7, 22 April 1999. [NOAA, Boulder, CO] *189YK	11	†
9	H. Wang, X. Sun, W.H. Miller, "Semiclassical approximations for the calculation of thermal rate constants for chemical reactions in complex molecular systems," J. Chem. Phys., 108(23):9726-36, 15 June 1998. [U. Calif., Berkeley; Lawrence Berkeley Natl. Lab., Berkeley, CA] *108FU	9	†
10	A. Blyr, et al., "Self-discharge of LiMn₂O₄/C Li-lon cells in their discharged state: Understanding by means of three-electrode measurements," J. Electrochem. Soc., 145(1):194-209, 1 January 1998. [U. Picardie, Amiens, France; Bellcore, Red Bank, NJ; IMN, Nantes, France; UJF-Grenoble, France] *YQ399	9	†
SOURCE: ISI’s Hot Papers Database. the Legend.

hemists generally assume that the weakest bond in a molecule is the one which first undergoes cleavage in a chemical reaction–and that is often the case. But what if it were possible to choose which bond you would like to break? Then a whole new field of chemistry opens up. Now it looks at though this might be possible, using a computer-controlled laser to zap the molecule with photons of just the right energy to break the chosen bond. But could you break a strong bond while leaving weaker bonds intact?

For more than 30 years the precision with which lasers can provide energy has transformed many sciences, and tantalized chemists with the possibility of selectively breaking chemical bonds. Now a group of German chemists, headed by Gustav Gerber at the University of Wurzberg, have finally shown it to be possible. Their work, described in paper #3 in the current Hot Ten, reports more than just selective bond breaking. Via a quick analysis of the products of the reaction, the information is fed back to the femtosecond (10^-15) laser pulse generator, so that it can optimize the energy output and cause only the desired reaction.

Gerber, in collaboration with Thomas Baumert (now at the University of Kassel) and Volker Seyfried, has provided the first clear demonstration of laser control of chemical-reaction dynamics within complex molecules.

The molecules studied by the German scientists were iron pentacarbonyl and dicarbonylchloro (h^-15-cyclopenadienyl) iron. The former consists simply of an iron atom to which is bonded five carbon monoxides; the latter is also iron but with a five-membered cyclopentadienyl ring, attached face-on, at one side of the metal with two carbon monoxides and a chlorine at the other. The former molecule was used to test the method and it showed that feedback of information could be used to refine the process. This information came from the reflectron time-of-flight mass spectrometer which identified the molecular fragments produced, and the signal from this was sent to a computer whose evolutionary algorithm then fine-tuned the laser pulse.

The second molecule was the one that proved it was possible to fine-tune the system to selectively break one of the chemical bonds, either the Fe-ring, Fe-CO, or Fe-Cl. The two alternative pathways turned out to be either breaking of one of the Fe-CO bonds, or the loss of both the ring and the two COs leaving the iron with only the chlorine still attached. Although the authors could have gone for other combinations of bond-breaking they chose these two because the products of the process are so radically different that it made it easier to distinguish between them by mass spectrometric analysis.

The result was all that they could have hoped for, and demonstrated that automated control of photodissociation is possible using tailored femto-second pulses. Indeed they say it is not even necessary to input data about the likely energy needed; the optimization procedure can start with a randomly chosen energy and then selectively direct the process that gives a particular bond cleavage. "It's the equivalent of the biological process of survival of the fittest," says Gerber, "leading to optimum pulse shapes after sufficiently many cycles of the evolutionary process, resulting in the desired products being produced with maximum efficiency."

Earlier work by the group can be found in Applied Physics B (see T. Baumert, et al., 65:779-82, 1997)–a paper that deals with the technicality of shaping femtosecond laser pulses and the evolutionary feedback process, and in Ultrafast Phenomena (XI:471, 1998). Their most recent paper, "Controlling the femtochemistry of Fe(CO)₅," is in the Journal of Physical Chemistry A (see M. Bergt. et al., 103:10381-7, 1999).

"These experiments represent a step towards synthesizing chemical substances with high efficiencies while at the same time reducing unwanted by-products," says Gerber, who believes that they will lead to industrial applications, and who is already cooperating with chemical companies to apply the technique to chemical synthesis.

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