Why Chemical Reactions Occur So Much Faster Than Expected: Quantum Tunneling
Quantum mechanics often seems to be of interest to physicists and mathematicians due to the Ideas that are difficult to make sense of because of complex equations, and subatomic particles that seem to have nothing to do with chemistry. A chemist, who takes any quantum mechanics course, asks the question, “Where am I going to use this information?”. Because Quantum mechanics contains a structure and formalism that can make you say this kind of stuff.
On the other hand, if we think that the effects of quantum mechanics increase as we go down to nanometers and smaller levels, and if we remember that the size of an average atom is around 0.1 nanometer, we can roughly understand how important quantum mechanics is on the cornerstones on which chemists work. In this article, We will talk about a dramatic effect of quantum mechanics on chemical reaction rates; quantum tunneling.
The Universe of Possibilities: Quantum Mechanics
The biggest feature that distinguishes quantum mechanics from Newtonian (classical) mechanics, which we are accustomed to in daily life, is that quantum mechanics is a probabilistic theory. Let us try to explain it like this; classical mechanics will basically formulate precisely the entire motion of the tennis ball, so you will know exactly where the ball will be at any given moment and what its speed will be. On the other hand, when you do the same calculation with quantum mechanics, you will get a probability distribution with the position and velocity of the ball, that is, you will have an idea of the position and velocity of the ball only with certain probability values.; There’s a 30% chance it’s here, a 10% chance it’s here. So, what should we do to know exactly where the ball is?
For this, we need to observe the system; Your observation will allow the probability distribution to “condensation” on one point and let you know where the ball is. The important thing here is that the “condensation” work is purely dependent on probability distribution and not reversible. So quantum mechanics, in general, is based on determining and calculating this probability distribution, or “wave function” (actually the square of the absolute value of the wave function).
Overcoming its Potential: Quantum Tunneling
Perfect. Now let’s talk about the effects of this probability distribution collapse (condensation) event on chemistry. If the quantum gives us a probability distribution of mechanically found wave function and if this probability distribution collapses into one of the possible states with the observation made, it means that it can move our particle to “unexpected” places. So much so that even if the energy of our particle is not enough, we can see it overcoming potential obstacles greater than its energy. Let’s bring it to life for better understanding; if you throw the ball 2 meters high, you do not expect it to go over the 3 meters high wall; However, in quantum mechanics, the ball thrown 2 meters high has a chance to climb the wall 3 meters above. We call this strange but observed phenomenon quantum tunneling.
Drilling Through Mountains Instead of Crossing Over
When we see it this way, it becomes inevitable that quantum tunneling will have a dramatic effect on the rates of chemical reactions. According to the transition state theory, which we use today to understand how and at what rates chemical reactions take place, our particles must come together in certain geometries and certain energies, and their energies must exceed the potential energy required for the reaction. In other words, if we consider that there is a mountain between the reactants and the products, the transition theory orders us to go “over” this mountain. Of course, since the number of particles with this energy is inversely proportional to height, the higher this height, the lower the rate of our reaction. However, since quantum mechanics allows us to tunnel “through” this mountain, our particles can react without having to climb all that “height”, thus increasing our speed.
Let’s give a few examples; the hydrogenation of alcohols in an acidic medium. When we calculate it in a classical way, our speed will be 5-10 times below the true value. The reason for this is as I explained above; The protons in the hydronium cation can sometimes tunnel through the region in between and bind to the alcohol, even though their energy is not enough. Another example is electron transfer in proteins, that is, the transfer of one or more of the electrons in the protein molecule to another location on the same protein or to another molecule. Here, quantum tunneling plays a very fundamental role; Since it is very difficult for two protein molecules to collide with the exact geometry, quantum tunneling is one of the only mechanisms that enable electron transfer to take place.
How Does It Cross the Potential Barrier Without Enough Energy?
Before closing the issue, I would like to answer two questions that may come to mind. First, how do our particles cross the potential barrier without enough energy?
Mathematically, they have a chance to pass; but if you want a physical picture, you can think of the particle as “borrowing necessary” energy from the environment and energy fluctuations in space. The strange and difficult thing left to understand is how this energy is taken, unfortunately, it is not possible for us to answer it in this article.
Does the Observer Have an Effect in Tunneling?
Our second question is if the probability distribution is collapsed on the basis of tunneling, and this collapse happens by observation, would tunneling occur if we did not observe the system?
The result is that we start the reaction and run it for a long time without taking any measurements, but the rates still turn out to be higher than we expected. This is a question that can be very difficult to answer, and it will push us to understand and explore what measurement is in quantum mechanics. However, let me briefly say: Vacuum/void fluctuations that occur constantly in the systems we examine create the effect of being observed in the system, and thus the tunneling process takes place even if “we” do not observe it; It is as if nature is constantly observing itself. In order to better understand and make sense of this subject, I suggest you consult quantum electrodynamics resources.
Quantum Mechanics Is Essential To Understand Reaction Rates!
In summary, although quantum mechanics is mathematical, it leaves a significant impact on the reactions and rates that chemists deal with, and therefore every chemist must have a basic understanding/infrastructure of quantum mechanics. Quantum mechanics, on the other hand, shows its effect in the form of increasing reaction rates with the tunneling effect, as it can be in many different and fundamental subjects.
References and Further Reading
KRÄMER, K. (2021, May 11). Call for chemists to stop ignoring quantum tunnelling. Chemistry World. Retrieved October 29, 2021, from https://www.chemistryworld.com/news/call-for-chemists-to-stop-ignoring-quantum-tunnelling/3008527.article.
McMahon, R. J. (n.d.). Chemistry. Chemical reactions involving quantum tunneling. Meta. Retrieved October 29, 2021, from
https://khepri-node.dev.meta-infra.org/papers/chemistry-chemical-reactions-involving-quantum/12574607.
Staff. (2013, September 10). Quantum tunneling allows “impossible” chemical reactions to occur in space. SciTechDaily. Retrieved October 29, 2021, from https://scitechdaily.com/quantum-tunneling-allows-impossible-chemical-reactions-occur-space/.
Images not cited are used through Canva Pro with a royalty payment.
The proofreading has been done by Asu Pelin Akköse and Mete Esencan.
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