Is the simplest chemical reaction really that simple?

May 14, 2020

Most people think that quantum theory, which describes the motion of molecules and atomic and subatomic particles, is counterintuitive, since quantum mechanics describes behavior at odds with classical mechanics. Even Albert Einstein, who never accepted quantum mechanics, famously said that "He (God or Nature) does not play dice" - meaning that the laws of physics do not surrender to uncertainty or chance as implied by quantum theory.

A chemical reaction sometimes occurs in an odd way, since in microscopic view the progress of a reaction is governed by the quantum theory.

New research by scientists at the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) has shown, surprisingly, in the simplest, well-studied reaction, there is still uncovered mechanism. It leads to clear quantum interference and verifies again that Nature does "play dice".

The reaction in question is H + HD → H2 + D. In the study, published in Science on May 15, groups led by Profs. YANG Xueming, ZHANG Donghui, SUN Zhigang and XIAO Chunlei of DICP discovered a new kind of quantum interference in this simple reaction.

In physics, interference is the combination of two or more waveforms to form a resultant wave, in which the displacement is either reinforced or canceled. Quantum interference can happen between particles that arrive at the same position or quantum state but by different paths.

Since a chemical reaction is essentially a collision and scattering process involving atoms and/or molecules, we can expect interference phenomena to occur in a chemical reaction.

Among all chemical reactions, the H + H2 reaction and its isotopologues are the simplest ones. This reaction only involves three electrons; thus it is convenient to deal with accurate quantum chemistry to calculate the interaction energy involving the three atoms.

Last year, DICP researchers found strong and regular oscillations as a function of energy at certain scattering angle of the product H2 during the H + HD reaction in particular rovibrational states.

Actually, similar oscillations have been observed in other reactions, but they are not as regular as those in the H + HD reaction. The physical origin of such oscillations remains unclear.

To understand this interesting phenomenon, the researchers conducted a combined theoretical and experimental study of the H + HD reaction.

Experimentally, by improving the crossed molecular beam apparatus, they recorded reactive scattering signals at certain scattering angle as a function of relative high energy.

They further developed quantum dynamics methods by applying topological theory to analyze the paths through which the reaction proceeded. Topological theory revealed that the observed regular oscillations resulted from interference between products generated via two different paths.

The researchers analyzed the reaction dynamics mechanisms using quasi-classical trajectory (QCT) theory. The results showed that the reaction proceeded via one path using the traditional direct extraction mechanism, i.e., the incoming H atom collided with the H atom in the diatomic reactant HD molecule and extracted it to form a new chemical bond of H2.

The reaction also proceeded via another path using a new roaming mechanism. The snapshots from the QCT theory for the roaming mechanism show that the incoming H atom initially approached the HD molecule via the conical intersection (Cl) region in the direction of the D atom end, and then roamed around the D atom in HD. When the incoming H atom approached the CI region, the HD bond started to stretch, making it possible for the roaming H atom to insert itself into the stretched HD molecule. The incoming H atom then formed a new chemical bond with the H atom in HD.

The products (H2) from these two paths were scattered into the same scattering angle, where quantum interference occurred.

Moreover, the probability for such an unusual roaming mechanism to occur is quite low - only about 0.3% of all reactions.

This work once again demonstrates the quantum nature of a chemical reaction at the microscopic level. It also reveals that chemical reactions are complicated.

Even the simple reaction H + HD → H2 + D, which has been studied for decades, has a small probability of employing unexpected mechanisms.

In life, many big events are triggered by small-probability events. Who can guarantee that a reaction mechanism of such small probability will not lead to surprising results?
-end-


Chinese Academy of Sciences Headquarters

Related Chemical Reactions Articles from Brightsurf:

Shedding light on how urban grime affects chemical reactions in cities
Many city surfaces are coated with a layer of soot, pollutants, metals, organic compounds and other molecules known as ''urban grime.'' Chemical reactions that occur in this complex milieu can affect air and water quality.

Seeing chemical reactions with music
Audible sound enables chemical coloring and the coexistence of different chemical reactions in a solution.

Nanocatalysts that remotely control chemical reactions inside living cells
POSTECH professor In Su Lee's research team develops a magnetic field-induced heating 'hollow nanoreactors'.

New NMR method enables monitoring of chemical reactions in metal containers
Scientists have developed a new method of observing chemical reactions in metal containers.

Levitating droplets allow scientists to perform 'touchless' chemical reactions
Levitation has long been a staple of magic tricks and movies.

Predicting unpredictable reactions
New research from the University of Pittsburgh's Swanson School of Engineering, in collaboration with the Laboratory of Catalysis and Catalytic Processes (Department of Energy) at Politecnico di Milano in Milan, Italy, advances the field of computational catalysis by paving the way for the simulation of realistic catalysts under reaction conditions.

First-time direct proof of chemical reactions in particulates
Researchers at the Paul Scherrer Institute PSI have developed a new method to analyse particulate matter more precisely than ever before.

Finding the source of chemical reactions
In a collaborative project with MIT and other universities, scientists at Argonne National Laboratory have experimentally detected the fleeting transition state that occurs at the origin of a chemical reaction.

Accelerating chemical reactions without direct contact with a catalyst
Northwestern University researchers demonstrate a chemical reaction produced through an intermediary created by a separate chemical reaction, findings that could impact environmental remediation and fuel production.

Visualizing chemical reactions, e.g. from H2 and CO2 to synthetic natural gas
Scientists at EPFL have designed a reactor that can use IR thermography to visualize dynamic surface reactions and correlate it with other rapid gas analysis methods to obtain a holistic understanding of the reaction in rapidly changing conditions.

Read More: Chemical Reactions News and Chemical Reactions Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.