For a chemical reaction to occur, atoms and molecules need to collide. However, not every collision produces a reaction. If the atoms and molecules are not favourably aligned with each other, nothing will happen. The “point of impact” of the colliding entities is also important, and it is hard to control because it requires confining the reactants along precise paths.

A team at the University of Graz, Austria, has now shed fresh light on this question by observing molecular coupling reactions on a single-crystal surface. “For the first time, we have been able to directly observe in real space and with single molecules which collision geometry must be met for a successful reaction,” explains Leonhard Grill, a physicist at Graz who co-led the study with post-doctoral researcher Matthew Timm. “Importantly, our experiment covers all geometric aspects of a reaction, namely, the precise location at which the reactants ‘touch’ each other upon collision, the ‘impact parameter’ (that is, how far the collision is from the reactants’ centre of mass) and the orientation of the reactants.”

Based on these observations, the researchers concluded that reactions only occur within a so-called “cone of reaction” that is defined by the colliding particles contacting at a specific point and a certain angle. This finding could lead to a better understanding of chemical reactions at the atomic scale and, ultimately, to ways of controlling and optimizing them.

Control of impact parameters

Grill and Timm used a scanning tunnelling microscope (STM) to project a molecule of difluorocarbene (CF₂) onto a target radical (BTFyl) anchored to a copper surface. This set-up enabled them to control the impact parameter by launching the CF₂ molecules directly and in a straight line along different atomic rows on the surface. They were also able to control the orientation of the BTFyl target by rotating it around its anchor point.

The researchers observed that reactions mainly occur when the CF₂ approaches the BTFyl target on the same copper row to which the target is bound. This didn’t happen often. In fact, only six of the 79 collisions they observed – corresponding to a narrow range of impact parameters – resulted in a chemical reaction, and reactions did not happen at all when the orientation of the CF₂ and the BTFyl deviated by more than 15 °.

“Controlling the impact parameter was for a long time considered to be the ‘forbidden fruit’ of reaction dynamics because of the technical difficulties to control it,” explains Grill. “The main challenge for us was how to do such experiments with relatively large molecules, which can have many adsorption orientations on the surface.”

A new view of collisions

Grill believes that the study’s findings could change the way we view collisions between reactants. “The new picture we have gleaned concerns not only the impact parameter, so where one reactant has to collide with another, but also the orientation of the involved compounds,” he tells Physics World. “If the reactants become larger and also have various side groups, understanding the cone of reaction might become even more important to understand, control and predict reaction rates.”

The researchers, who detail their work in Science, say they are already studying similar systems to assess whether the behaviour they observed can be generalized. In a related article, Jonas Björk of Linköping University, Sweden, suggests that further measurements could reveal how molecular structure, reactive sites and environment shape reaction pathways, enabling more predictive control of surface chemical transformations. “They could also enable single-molecule studies of how chirality influences chemical reactivity by directly controlling the handedness and orientation of reactants,” he adds.

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Interfaces between air and water are omnipresent in nature and in industrial processes, but we understand surprisingly little about what goes on there. Researchers in Germany now report that a new spectroscopy technique could reshape our understanding of the molecular structure of these aqueous interfaces and the dynamical processes at play. Their discoveries could lead to better models of atmospheric processes and improved electrochemical devices such as batteries, to name but two examples.

When air and water meet, the interface between them strongly influences the behaviour of the first four layers of water. This interfacial water, as it is known, is only 7-8 angstroms thick, and the water below it behaves like a bulk liquid. To study interface effects, researchers therefore need to probe only these four layers and characterize the way their H₂O molecules are oriented.

Probing the H-O-H bending vibration

One way to do this is to observe the bending vibration of the H-O-H structure, as this parameter approximately aligns with the water molecule’s dipole. In particular, one can analyse how the anisotropic bending mode changes with the thickness of the interfacial water. This can be calculated using a factor known as the depth-dependent second-order susceptibility, 𝜒(ଶ)(𝑧).

There is a problem with this approach, however, because it requires the H-O-H bending vibration to originate from the electric dipole of H₂O and to contain only an interfacial dipolar signal. This is not always the case because electric quadrupolar signals from the bulk of the sample, as well as magnetic dipolar signals, can also contribute to the spectra.

These other signals do not provide any information on the orientations of the H₂O dipole. Worse, they can mask the structural information that researchers are looking for. This means they either need to be ruled out or have their contributions to the overall signal removed.

A tuneable visible upconversion

The new technique, developed by Martin Thämer and colleagues in the Nonlinear Interfacial Spectroscopy Group of the Fritz-Haber Institute der Max-Planck-Gesellschaft, involves feeding the 800-nm-wavelenght light output from a Ti:sapphire laser into two independent optical parametric amplifiers. The first amplifier produces mid-infrared light through a process called difference frequency generation (DFG) and the second produces a signal beam that is subsequently doubled in frequency to produce a tuneable visible upconversion.

Using these two beams, the researchers irradiated the surface of a water sample and excited nonlinear vibrations in the water molecules. This excitation generates two new light beams at different visible frequencies. By measuring the differences in the phase and amplitude of these beams, the team was able to isolate the vibrational response of the interfacial water layer and separate it from the bulk-water quadrupole term.

Thämer and colleagues then combined their spectra with high level molecular dynamics simulations developed by their colleagues at the Frei Universität Berlin to determine the precise orientations of the water molecules in the interfacial region.

Traditional description is insufficient

Traditionally, the structure of interfacial water is described in terms of water molecules pointing up or down (the “tilt angle”). However, based on the team’s results, Thämer says this description is insufficient. An additional orientation parameter is required, namely the “water twist angle”, or the molecule’s rotation about the axis of its dipole. “The new picture of the water structure we present is layered one with alternating twist and tilt angles that indeed extends over only four molecular water layers,” Thämer tells Physics World.

Looking ahead, the researchers, who detail their work in Science Advances, say they now plan to study other aqueous interfaces, including charged interfaces and biomolecular systems.

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