Quantum Mechanics of Chemistry

JILA scientists David Nesbitt and J. Mathias Weber investigate the makeup and inner workings of quantum dots, ions, and molecules. The scientists draw upon techniques from chemistry and physics to elucidate the role of quantum mechanics in the structure, dynamics, and energy flow through model chemical systems. Using such techniques as high-resolution laser spectroscopy, slit-jet cooling, and dual molecular beam studies of chemical reactions, Nesbitt performs a variety of investigations of simple ions, molecules, and quantum dots. For his part, Weber studies the flow of energy through molecules.

Investigations of Simple Ions and Molecules

David Nesbitt uses the tools of physics to better understand the structure and dynamics of relatively "simple" ions and molecules. In one major program, Nesbitt and his group use high-resolution slit-jet infrared laser spectroscopy to investigate the dynamics of photon absorption by different entities, including proton-bound complexes, C₃H₃⁺ (abundant in the interstellar medium), H₃O⁺, CH₅⁺ (whose five protons swarm around the central carbon atom such that the very concept of molecular structure is problematic), and vinyl radical (C₂H₃), a precursor of soot formation in the atmosphere. By probing high concentrations of these materials at very low temperatures, Nesbitt and his group seek to understand their spectra well enough to determine precisely how they vibrate and rotate in response to laser light. They then compare their findings with the predictions of quantum mechanics. This research is yielding important insights into the structures and hydrogen-bonding potentials of these radicals and ions.

Investigations of the structure and behavior of atoms and molecules on the quantum level are particularly challenging when the molecule under investigation appears in small amounts or is rapidly transformed into something else during combustion or other rapid chemical reaction. Nesbitt's group has developed an innovative method for studying such elusive chemicals. The method combines infrared laser spectroscopy and slit-jet cooling, which cools the molecules close to 0 K, making them easier to observe. It also uses an electric discharge to create a desired molecule in sufficient quantities for study since short-lived chemicals typically exist in vanishingly small amounts. Using this method, Nesbitt's group recently obtained the first-ever high-resolution IR spectra from phenyl radical, an important, but short-lived, combustion intermediate. The group’s production of another combustion intermediate, vinyl radical, with the relatively gentle electric discharge technique led to another important achievement: Spectroscopic studies of newly minted vinyl radicals revealed, for the first time, that all three of vinyl radical’s hydrogen atoms are equivalent, i.e., the can shuffle between the ion’s three different carbon–hydrogen (CH) bonds, with no net change to the state of the ion itself.

Nesbitt and his group also probe the potential energy of more complex ions such as H₃O⁺. By substituting atoms of deuterium for one or more of the hydrogen atoms in this molecular ion, the researchers can use high-resolution spectroscopy to monitor changes in the vibrational spectrum that correspond to specific quantum mechanical behaviors such as "quantum mechanical tunneling" and "forbidden" states in which the molecule appears to be in two different configurations at the same time. The method is important because quantum mechanical calculations have not yet evolved to the point where they can predict the energy levels of molecules with five or more particles.

Nesbitt conducts a major research program to better understand the fundamental principles that control chemical reactions. The researchers direct molecular beams of highly reactive molecules into a vacuum chamber, so that they intersect. The probability that a chemical reaction will occur at the point where the beams cross is low enough to allow them to study the dynamics of single collisions when they do occur. Experiments using a beam of hydrogen or deuterium molecules and a beam of fluorine atoms have confirmed that their tools are accurate enough to understand single-collision reactions in terms of quantum mechanics. However, both the experiments and the theory to explain even simple chemical reactions are pushing the frontiers of knowledge. In fact, understanding the quantum mechanical behavior of each atom in relatively simple chemical reactions (such as HCl + F → HF + Cl) is one of the major challenges of physics.

For instance, once the Nesbitt group completed two-beam experiments to investigate the formation of HF, the researchers had to develop a new theory to explain the fact that only a small number of collisions resulted in a chemical reaction, but when a reaction did take place, it released an unexpectedly large amount of energy. The theorists found that the formation of HF could only occur if the reactants collided in a very specific orientation: The F atom had to collide with the H end of an HCl molecule such that the two reactants formed a transitory F–H–Cl molecule that was bent at the H, forming an angle of exactly 123.5°! To form this angle, the F atom had to hit the HCl molecule at a slightly wider angle because the collision itself caused the transition state to bend further into the right conformation. However, once the rare chemical reaction did occur it released enough energy to cause other HCl molecules and F atoms to react explosively. Part of the challenge of interpreting this kind of reaction comes from the dynamical complexity that exists at the quantum mechanical level. For instance, a chemical reaction involving only four atoms requires an understanding of what is happening with six internal coordinates in a six-dimensional space!

The Nesbitt group is meeting this challenge with experiments that stretch chemical bonds (making them easier to break), explore the use of induced vibrations (increasing the likelihood of bond breaking), use polarized light to align reactants (allowing the investigation of steric effects on chemical reactivity), and hold one reactant, such as water, in place to see how this affects collision dynamics. The ability to predict factors that increase or decrease the likelihood of a chemical reaction could also shed light on similar processes in biological systems.

Nesbitt and his group have begun efforts to probe chemical reactions (such as HCl + F → HF + Cl) in the liquid phase, which is far more complex and dynamical than reactions in solids and gases. The latter are beginning to be well understood. The group’s investigation of liquid-phase reactions began with experiments that directed molecular beams of F onto the surface of squalene, a low-vapor-pressure hydrocarbon liquid. The F atoms striking the liquid surface "plucked" a hydrogen atom from it and formed a chemical bond, producing HF. The researchers then probed the dynamics of the HF molecules that reflected off the surface of the liquid. This work is expected to lead to a better understanding of atmospheric reactions at the surface of aerosol particles or oceans and industrial processes such as distillation, gas chromatography, and catalysis.

In other studies of gas–liquid-surface interactions, the group has investigated collisions between fast, cold (20 K) carbon monoxide molecules and the surface of an oily liquid (perfluorether). The researchers measured the speed and temperature of a molecular beam of CO₂ created in a supersonic expansion as it hit the liquid surface. About half the CO₂ molecules splashed off the surface as if it were a solid; a second fraction skipped across the surface before being reflected. Only a tiny fraction of the original gas beam entered the liquid and stayed there. The gas molecules that immediately reflected off the surface came off at temperatures 2–3 times hotter than the liquid surface because much of CO₂ molecules’ energy of movement was transformed into heat. Most of these reflected molecules cartwheeled end-over-end off the surface, but a few lifted off like a helicopter, spinning parallel to the surface. In contrast, the molecules that skipped across the surface came off the liquid more uniformly at room temperature, having already dissipated some of their energy of movement.

The Flow of Energy Through Molecules

J. Mathias Weber is intrigued by the flow of energy through molecules during chemical reactions. Such energy flow is the molecular analog of heat conduction in materials. In this context, energy flow through nanoscale systems is of particular interest since technology using progressively smaller electronic devices can encounter the destruction limit of energy density.

On the quantum mechanical level, energy flow occurs via anharmonic coupling of individual vibrational modes of molecules. As a result, intramolecular vibrational relaxation (IVR) has long been a field of study. Until now, most experiments in this field have dealt with the question of how long it takes for energy to drain out of an excited vibrational state. The Weber group uses a different experimental approach to monitor the flow of energy as it drains out of a certain vibrational mode and arrives in a well-defined region in a molecule. The group uses nitroalkanes as model systems for energy-flow studies because their electron affinities are well below the excitation energies for CH-stretching modes, and any excess charge is localized on the nitro group.

The group recently demonstrated that vibrational autodetachment from nitro-hydrocarbon anions occurs when the fundamental CH-stretching modes are excited. This experiment has opened the door to depositing vibrational energy into a specific CH-stretching mode from which it will travel to the nitro group, which is a well-defined region of the molecule. The group can "see" the arrival of energy in the nitro group when it triggers vibrational autodetachment via the excitation of NO₂ vibrational modes. The Weber group is currently using photoelectron imaging spectroscopy to study the quantum mechanical details of the coupling of individual oscillations.