Chemical Education Today
Chemistry in the News
1999 Nobel Prize in Chemistry The Royal Swedish Academy has awarded the 1999 Nobel Prize in Chemistry to Ahmed H. Zewail (California Institute of Technology, Pasadena, CA) “for his studies of the transition states of chemical reactions using femtosecond spectroscopy”. Zewail’s work has taken the study of the rates and mechanisms of chemical reactions to the ultimate degree of detail—the time scale of bond making and bond breaking. How Do Chemical Reactions Take Place? Some chemical reactions take place very slowly, as when a nail rusts. Others occur very rapidly, as when dynamite explodes. After a reaction, the atoms in the product substances are arranged differently from the way they were before the reaction. The chemical bonding and structure are different, which implies that chemical bonds are broken and/or made when a reaction takes place. Knowledge of when and how bonds are broken and formed enables us to control reaction rates and reaction products, thereby also enhancing the industrial processes that create many of the important substances and materials of everyday life. Which bonds are formed or broken and the order in which they are formed or broken are described by a reaction mechanism. A mechanism consists of a sequence of chemical equations, one for each molecular collision. If the rates of individual steps in a mechanism can be predicted or measured, the overall rate of a process can be calculated from them. The mechanism tells us which molecules need to collide and in what order. However, it does not provide much detail about the rate at which each step can occur, about how or why specific bonds are made and broken, or about how temperature affects the reaction rate. The rates of most single-step reactions increase as temperature increases. Higher temperatures correspond to more violent motion of molecules relative to other molecules and of atoms relative to other atoms within the same molecule. More violent motion means that molecules collide more often, and when they collide they hit each other harder. Both of these increase the rate of the reaction. More violent motion also means that each molecule can tumble and vibrate more. (Molecules rotate around their centers of gravity, bonds between atoms stretch and compress, and bond angles enlarge and diminish.) Increased vibrational and rotational motions also contribute to speeding up a reaction. It is difficult to figure out how, and how much, they contribute, because inter-
The information about the 1999 Nobel Prizes has been adapted from the press releases of the Royal Swedish Academy of Sciences at http://www.nobel.se/announcement-99/ Further information is available from the Academy of Sciences, Information Depar tment, PO Box 50005, SE-104 05 Stockholm, Sweden; Web site: www.kva.se; phone: +46-8673 95 95; fax: +46-8-15 56 70; email:
[email protected].
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nal molecular motions are very fast. Most vibrations take between 10 and 100 femtoseconds (1 fs = 1 × 10–15 s). One femtosecond is to a second as a second is to 32 million years! The Transition State Just over 100 years ago, Svante Arrhenius (Nobel laureate in Chemistry 1903), inspired by Jacobus Henricus van’t Hoff (the first Nobel laureate in Chemistry, 1901) presented a simple formula for reaction speed as a function of temperature. According to Arrhenius k = Ae–Ea /RT where k is the rate constant (the rate of the reaction for unit concentrations of the reactants), A is related to the frequency of collisions of the molecules, R is the gas constant, T is the temperature on the Kelvin scale, and Ea is the activation energy. Arrhenius suggested that the activation energy represents a minimum energy that molecules must have for reaction to occur. The activation energy is an energy barrier—those molecules with less than the activation energy will not react when they collide; those with more will. In the 1930s Henry Eyring and Michael Polanyi formulated a theory that examined individual molecular collisions. When two molecules collide, all of the atoms from both molecules are close together for a very brief time. During that time they constitute a larger, extremely short-lived molecule that is called the activated complex or the transition state. The transition state is a supermolecule that has the highest energy of any arrangement of atoms that occurs during the collision. According to Eyring and Polanyi’s theory, the transition state is crossed very rapidly, on the time scale that applies to molecular vibrations—femtoseconds. Complete understanding of the rates and mechanisms of chemical reactions requires understanding the transition state, but this is very difficult because the transition state lasts for such a short time. Other Nobel prizes have been awarded for progress toward complete understanding. Ronald Norrish and George Porter (1967 prize) used a very rapid flash of light to break chemical bonds and initiate reactions (flash photolysis), studying processes that took between 10–3 and 10–6 s. Dudley Herschbach, Yuan Lee, and John Polanyi (son of Michael) received the 1986 prize for the crossed molecular beam technique, in which molecular collisions can be carefully controlled, and processes on the picosecond (10–12 s) time scale can be studied. Zewail’s work extends previous studies to the femtosecond time scale, which provides the ultimate resolution because it corresponds with the time scale of molecular vibrations. Femtochemistry Zewail’s technique uses what may be described as the world’s fastest camera. A strong laser flash of a few femtoseconds duration initiates a reaction. Subsequently, other weaker laser pulses monitor what happens as the reac-
Journal of Chemical Education • Vol. 77 No. 1 January 2000 • JChemEd.chem.wisc.edu
Chemical Education Today
tion occurs. The reactants are introduced as beams of molecules streaming into a vacuum chamber. During the initiating pulse, molecules in a molecular beam are excited by the laser flash. All are excited to the same state—a state in which they are vibrating in unison. The weaker probe pulses are adjusted to wavelengths that allow detection of the original molecule (to find out how much has disappeared) or an altered form of the original molecule (to discover whether and how fast intermediates and products are formed). The time between the initiating laser flash and a probe pulse can be adjusted very precisely. Mirrors cause some of the light from the initiating flash to travel a slightly longer path before it strikes the molecules in the vacuum chamber. (Each 0.03 mm extra distance corresponds to 100 fs time delay.) Each probe pulse occurs at a well-defined time and creates a freeze-frame picture. Putting a series of these pictures together provides a stop-action view of a chemical reaction analogous to Eadweard Muybridge’s famous series of stroboscopic photographs that provided detailed information about the gaits of horses and other animals. The area of physical chemistry that Zewail pioneered has been named femtochemistry, because it involves femtosecond laser pulses and studies processes that take place on a time scale of hundreds of femtoseconds. The principles and experimental techniques involved have been described in one of our Viewpoints articles (JCE 1998, 75, 1105–1118.) Femtochemistry in Practice An example of the detail that can be obtained about short-lived chemical species is provided by Zewail’s study of the ring opening of cyclobutane to form two molecules of ethene. The equations below describe two possible mechanisms. The first is a single step in which two carbon-carbon single bonds break and two carbon-carbon single bonds become double bonds.
H
H
H
C
C
H
C
H
C
C
H
C
H
H
H
C
+
C
H
H
H
H
H
H
H
The second mechanism involves two steps with a tetramethylene intermediate. In this case one of the single bonds breaks before the other. H
H
H
C
C
H
H
C
C
H
H
H
H
H
H C H
C
C
C
H
H
H
H
H
H C
+
C
H
H
H
C
H
H
C H
Zewail’s experimental work showed that the second mechanism is what happens. The tetramethylene intermediate has a lifetime of 700 fs. This corresponds to a slight dip in the top of the energy “hill” over which the system of atoms has to travel. Femtochemistry derived from Zewail’s pioneering work has become a widely used research technique during the 1990s. Reactions are being studied in molecular beams, on surfaces, in clusters of atoms, and even in solutions. Femtochemistry is elucidating processes such as energy conversion in chlorophyll and detection of light by retinal in human eyes. The world’s fastest camera has found myriad applications.
JChemEd.chem.wisc.edu • Vol. 77 No. 1 January 2000 • Journal of Chemical Education
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