The 1986 Nobel Prize in Chemistry recognized Dudley R. Herschbach, Yuan T. Lee, and John C. Polanyi for developing methods that let us watch the intricate dance of molecules during chemical reactions.
For centuries, chemistry happened in a black box. Scientists could measure what went into a reaction and what came out, but the crucial moments of transformation—the actual breaking and forming of chemical bonds—remained too fast, too small, and too complex to observe. This fundamental mystery persisted until 1986, when three visionaries transformed our understanding of chemical events.
Pioneered crossed molecular beams to study single collision events under controlled conditions.
Developed advanced crossed beam apparatus with universal detection for complex reactions.
Created infrared chemiluminescence to measure energy distribution in reaction products.
"Their pioneering work, which gave birth to the field of reaction dynamics, allowed scientists to observe and understand the intricate steps molecules take as they transform into new substances." 1
Before this revolutionary work, chemists largely studied reactions through the lens of chemical kinetics. This approach could tell you how fast a reaction proceeded and what the end products were, but it was like trying to understand a soccer match by only knowing the final score. The critical events—the passes, the tackles, the strategic moves that led to the goal—remained hidden.
Animation: Random molecular motion in traditional chemistry experiments
In a test tube, countless molecules zoom in every direction with random velocities and orientations 2 . When they collide, the details of these encounters become blurred in a statistical average.
"The details of the reaction thus become blurred and cannot be observed precisely enough." 2
Dudley R. Herschbach pioneered an elegant solution to the problem of random molecular collisions: crossed molecular beams. The concept was brilliant in its simplicity—instead of studying random collisions in a container, he would create two directed, well-defined beams of molecules and cross them in a vacuum chamber 2 .
Imagine two streams of specialized particles crossing like laser pointers in a dark room. By controlling their directions and velocities, scientists could precisely determine the energy of each collision.
While Herschbach and Lee were perfecting their molecular crossfire, John C. Polanyi developed a complementary approach: infrared chemiluminescence 2 . His method relied on a fascinating phenomenon—when newly formed molecules are born with excess energy, they eventually release it as extremely weak infrared light.
Polanyi's insight was that the specific wavelengths of this infrared emission would reveal exactly how the product molecules were vibrating and rotating.
| Research Tool | Function in Reaction Dynamics |
|---|---|
| Crossed Molecular Beam Apparatus 2 | Creates directed beams of molecules that cross in a vacuum chamber, allowing study of single collision events under well-defined conditions. |
| Mass Spectrometric Detector 2 | Identifies and measures the masses of product molecules formed during reactions in crossed beam experiments. |
| Infrared Chemiluminescence 2 | Measures weak infrared light emitted by newly formed product molecules to determine their internal energy distribution. |
| Gold-coated Mirrors | Collects and focuses faint infrared emissions in chemiluminescence experiments toward the detector. |
| Infrared Spectrometer | Separates infrared light by wavelength, allowing scientists to identify specific vibrational and rotational states of molecules. |
The experiment that Polanyi and Cashion conducted in 1958 provides a perfect case study of the infrared chemiluminescence method in action.
The researchers created a reaction chamber with gold, D-shaped mirrors positioned at either end. Gold was chosen for its exceptional reflectivity of infrared light .
They introduced atomic hydrogen and molecular chlorine into the chamber as crossed beams of reagents .
The hydrogen and chlorine reacted to form hydrogen chloride (HCl) molecules: H + Cl₂ → HCl + Cl.
The newly formed HCl molecules carried excess energy from the reaction, causing them to vibrate intensely before eventually releasing this energy as infrared radiation .
The curved mirrors collected the extremely weak infrared emissions and directed them toward the detection system .
A lithium fluoride prism within an infrared spectrometer dispersed the light, and sensitive infrared detectors captured the signal, creating a spectrum that revealed the energy states of the product molecules .
Exclaimed Ken Cashion, a newly ordained priest, upon seeing the results of their experiment in a janitorial closet at the University of Toronto's Wallberg Building .
Hydrogen atom reacts with chlorine molecule to form hydrogen chloride and chlorine atom.
The infrared chemiluminescence method allowed Polanyi to observe how the energy from chemical reactions distributed itself among the product molecules. He discovered that product molecules could belong to distinct classes with respect to their internal energy distribution 2 .
How energy released in a chemical reaction typically distributes itself among different forms of energy in the product molecules, based on findings from chemiluminescence studies.
| Type of Energy | Description | Significance |
|---|---|---|
| Vibrational Energy | Energy associated with the vibrating motion of atoms within a molecule. | Often the predominant form of energy release; reveals how chemical potential energy converts to atomic motion. |
| Rotational Energy | Energy associated with the spinning motion of the entire molecule. | Provides information about the geometry and forces at play during the collision. |
| Translational Energy | Energy associated with the straight-line motion of the molecule through space. | Indicates the recoil dynamics of the reaction and the repulsive forces between products. |
The collective work of Herschbach, Lee, and Polanyi created an entirely new field—reaction dynamics—that has profoundly influenced both theoretical and applied chemistry 2 3 .
Understanding reaction dynamics has enabled the design of more efficient catalysts, which are crucial for industrial processes and environmental protection 3 .
It has informed the development of chemical lasers, which have become indispensable tools in medicine and manufacturing .
Their methods provided the foundation for subsequent technological advances, including laser-based techniques that can track reactions with even greater time resolution.
They equipped chemistry with new eyes to observe the unseeable and new language to describe the indescribable, forever changing how we understand the molecular events that shape our world.
"The great epics of reaction dynamics remain to be written." — John C. Polanyi, 1986 Nobel Lecture 4