Quantum chemistry

Quantum chemistry is a specialized branch of physical chemistry. It applies the rules of quantum mechanics to chemical systems. Scientists use it to study molecules, materials, and solutions at the atomic level. The field focuses on calculating how electrons contribute to physical and chemical properties. These calculations help us understand the structure of atoms, molecules, and ions. They also allow us to describe chemical reaction pathways, intermediates, and transition states.

To understand these systems, researchers must determine the electronic structure. This refers to the quantum state of a molecule's electrons. The primary goal is to find solutions to the Schrödinger equation. This equation describes how particles behave at a tiny scale. However, an exact solution only exists for the hydrogen atom. Most systems involve three or more particles moving at once. Because of this complexity, scientists must use computational chemistry. They use systematic approximations to make these math problems solvable by computers. One common method is the Born–Oppenheimer approximation. This assumes the electronic wave function is shaped by the positions of the nuclei.

The history of this field involves many important discoveries. Gilbert N. Lewis provided a critical framework in 1916. He developed the first working model of valence electrons. In 1927, Walter Heitler and Fritz London reached a major milestone. They applied quantum mechanics to the diatomic hydrogen molecule. This explained the phenomenon of the chemical bond. Later, Linus Pauling integrated many ideas into a new framework. He wrote a series of articles during the 1930s. His 1939 text became a standard university textbook. It introduced many chemists to valence bond theory. Other key contributors include Robert S. Mulliken and Maria Goeppert Mayer.

There are two main ways to describe how atoms bond. The first is valence bond theory. This method focuses on the pairwise interactions between atoms. It looks at how atomic orbitals combine to form individual bonds. A covalent bond forms when half-filled orbitals overlap to create an electron pair. The strength of this bond depends on the amount of overlap. Atoms also have an ideal bond distance. This is the stable length where attractive and repulsive forces balance. Orientation matters for the type of bond created. A sigma (σ) bond forms from direct orbital overlap. A pi (π) bond forms from a side-to-side overlap of p-orbitals.

The second approach is molecular orbital theory. Friedrich Hund and Robert S. Mulliken developed this in 1929. This method is less intuitive than valence bond theory. Instead of focusing on local overlaps, it describes the whole molecule as one system. Electrons are described by mathematical functions that are delocalized over the entire molecule. This approach is the basis for the Hartree–Fock method. It is very useful for predicting spectroscopic properties. This is how light interacts with different types of matter.

Another essential tool is density functional theory, or DFT. The Thomas-Fermi model began this work in 1927. Modern DFT uses the Kohn-Sham method to describe many-electron systems. Instead of using wave functions, it uses electronic density. This makes the math much easier for computers to handle. The computational requirements for DFT scale much more slowly than other methods. This allows scientists to study large polyatomic molecules and even macromolecules. It is one of the most popular methods in computational chemistry today.

Quantum chemistry also explores chemical dynamics. This is the study of how molecules move and interact over time. Scientists can use quantum dynamics to solve the Schrödinger equation directly. They can also use semiclassical dynamics for a simpler approach. Some researchers use molecular dynamics to simulate motion using classical physics. Other methods, like path integral molecular dynamics, add quantum corrections to these simulations. These studies help us understand chemical kinetics, which is the study of reaction rates. By mastering these tools, scientists can predict how the building blocks of our world behave.