Quantum chemistry Facts for Kids

Quantum chemistry is a special part of physical chemistry. It uses the rules of quantum mechanics to understand how chemicals work, especially at the tiny level of molecules and atoms. Think of it as using super-advanced physics to figure out how electrons behave in molecules.

Scientists use quantum chemistry to calculate things like how molecules are shaped, what colors they absorb (their spectra), and how much energy they have. These calculations often use clever shortcuts or "approximations" to make them possible on computers. Quantum chemistry also helps us understand how molecules move and how fast chemical reactions happen.

Chemists often use tools like infra-red (IR) spectroscopy or nuclear magnetic resonance (NMR) spectroscopy to learn about molecules. Quantum chemistry can help predict what these tools will show, or help explain the results. Many studies look at the normal state of electrons in a molecule (the ground state) and also when they get extra energy (the excited state). It also helps understand the steps molecules take during a chemical reaction.

One big goal of quantum chemistry is to understand how electrons are arranged and how molecules move. This often means solving a complex math problem called the Schrödinger equation. It's a challenge to make these calculations super accurate for small molecules and also to be able to study very large molecules, because the calculations get much harder as molecules get bigger.

History of Quantum Chemistry
Electronic Structure
Chemical Dynamics
- Adiabatic Chemical Dynamics
- Non-adiabatic Chemical Dynamics
See also

History of Quantum Chemistry

Many people think quantum chemistry really started when the Schrödinger equation was discovered and first used to understand the hydrogen atom. But a very important step happened in 1927. Two scientists, Walter Heitler and Fritz London, used quantum mechanics to explain the chemical bond in a hydrogen molecule. This was a huge breakthrough!

Even before that, in 1916, Gilbert N. Lewis came up with the idea of valence electrons, which are the electrons involved in forming chemical bonds. This idea was very important for understanding how atoms connect.

Later, in the 1930s, Linus Pauling brought together all these ideas. He wrote a famous book in 1939 called The Nature of the Chemical Bond. This book helped many chemists learn about quantum chemistry and how it explains chemical bonds. In 1937, Hans Hellmann also published one of the first books on quantum chemistry.

Over time, many other scientists like Robert S. Mulliken, Max Born, and J. Robert Oppenheimer made important contributions. They helped apply these new quantum ideas to understand how molecules are built, how they react, and how their parts are connected.

Electronic Structure

The electronic structure of an atom or molecule describes how its electrons are arranged. The first step in solving a quantum chemistry problem is usually to figure out this electronic structure. This often means solving the Schrödinger equation for the electrons in the molecule.

It's only possible to solve the Schrödinger equation exactly for a very simple atom like hydrogen. For all other atoms and molecules, scientists have to use clever approximations and computer methods. This area of finding computer solutions is called computational chemistry.

Valence Bond Theory

The method developed by Heitler and London was later expanded by scientists like Slater and Pauling. This became known as the valence-bond (VB) method. It focuses on how pairs of atoms interact and form bonds. It's a bit like how chemists draw bonds with lines between atoms. This method looks at how the electron clouds (called atomic orbitals) of different atoms combine to make individual chemical bonds.

Molecular Orbital Theory

An anti-bonding molecular orbital of Butadiene. This is a picture of where electrons might be in a molecule.

Another way to think about electrons in molecules was developed by Friedrich Hund and Robert S. Mulliken in 1929. In this method, electrons are described by mathematical functions that spread out over the entire molecule, not just between two atoms. This "molecular orbital" (MO) method is a bit less like our everyday idea of bonds. However, it's very good at predicting things like what colors a molecule will absorb, which is useful for spectroscopy.

Density Functional Theory

In 1927, L. H. Thomas and Enrico Fermi tried to describe systems with many electrons using their electron density, instead of complex wave functions. This idea wasn't perfect at first, but it led to what we now call density functional theory (DFT).

Modern DFT methods are very popular in computational chemistry. They are less demanding on computers than some other methods. This means they can be used to study larger molecules, even very big ones like macromolecules. DFT is often quite accurate and is a powerful tool for chemists.

Chemical Dynamics

Beyond just looking at the structure of molecules, quantum chemistry can also study how molecules move. This is called chemical dynamics.

If we solve the Schrödinger equation directly to see how molecules move, it's called quantum dynamics. If we use some shortcuts, it's semiclassical dynamics. When we use simpler physics rules, it's called molecular dynamics. There are also mixed methods that combine quantum and classical ideas.

Adiabatic Chemical Dynamics

In adiabatic dynamics, the interactions between atoms are described by simple "potential energy surfaces." This idea comes from the Born–Oppenheimer approximation by Max Born and Robert Oppenheimer in 1927. It's like having a map that shows the energy of a molecule at different shapes.

Scientists like Rice, Ramsperger, and Kassel used these ideas to estimate how fast chemical reactions happen. Later, Rudolph A. Marcus developed the RRKM theory, which is still used today. These methods help us understand how quickly molecules change from one form to another.

Non-adiabatic Chemical Dynamics

Sometimes, the Born-Oppenheimer approximation isn't enough. In non-adiabatic dynamics, we consider that molecules can switch between different electronic energy states as they move. This is like having several energy maps, and the molecule can jump from one map to another.

Pioneering work in this area was done by Ernst Stueckelberg, Lev Davidovich Landau, and Clarence Zener in the 1930s. Their work helps explain how molecules can change their electronic state during a reaction. For example, some reactions are "spin-forbidden," meaning they involve a change in the electron's spin state, and these are often non-adiabatic.

Quantum chemistry facts for kids

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