Crystal engineering

Scientists study how tiny parts fit together.

They want to make new solids. They do this by using special bonds. These bonds act like glue.

One kind of bond is a strong one. It helps parts stay in place. Other bonds are weaker. This can help make better medicine. Scientists can even design how a solid works. It is like building with very small blocks. Do you like to build things?

Crystal engineering is a way to design new solids. Scientists want to make materials with special powers. They do this by controlling how tiny parts fit together. They use small forces to guide these parts. These forces are called noncovalent bonds.

One main way is using hydrogen bonds. These are strong bonds that act like glue. Another way uses halogen bonds. Scientists also use coordination bonds to build shapes. They use building blocks called supramolecular synthons. These synthons help order groups in a solid.

This work helps make better medicine. Scientists can make cocrystals. These are crystals made of more than one part. They can change how a drug works in the body.

Sometimes, one chemical can make different crystal shapes. This is called polymorphism. It is important for drug companies. Some shapes pack tightly. Others form quickly. Scientists even use computers to predict these shapes. This helps them design new materials with the right strength or feel.

Crystal engineering is a way to design new solid materials. Scientists want to create solids with special properties. They do this by controlling how tiny molecules fit together. This field connects two types of chemistry. It bridges solid-state chemistry and supramolecular chemistry. By guiding how molecules pack, researchers can control how a material behaves.

This work works through a process called molecular self-assembly. Molecules use small forces to organize themselves into patterns. These forces are called noncovalent bonds. Scientists use three main strategies to guide this. They use hydrogen bonding, which acts like a strong glue. They also use halogen bonding and coordination bonding. They use building blocks called supramolecular synthons to order specific groups.

The history of this field began several decades ago. R. Pepinsky first used the term in 1955. Later, Gerhard Schmidt provided a starting point in 1971. He studied reactions in crystalline cinnamic acids. In 1988, Gautam Desiraju gave a modern definition. He explained that we must understand how molecules interact to design new solids. This understanding helps us create materials with specific physical and chemical properties.

Crystal engineering is very important for making medicines. Scientists create pharmaceutical cocrystals to help people. These crystals use one active ingredient and other safe substances. These extra parts can change how a drug dissolves or moves through the body. This is helpful for making better medicines. Some crystals can even be built in two dimensions. These are very thin layers called monolayers.

Sometimes, one chemical can form different crystal shapes. This is called polymorphism. This happens because of a competition between two factors. One factor is kinetics, which is how fast crystals form. The other is thermodynamics, which is how closely molecules pack. Scientists even use computers for crystal structure prediction. This helps them find the best way to build a material. They can design for strength, elasticity, or even brittleness.

Crystal engineering is an interdisciplinary field that focuses on designing and synthesizing solid-state structures. Scientists aim to create materials with specific, desired properties. They achieve this through the deliberate control of intermolecular interactions. This field acts as a bridge between solid-state chemistry and supramolecular chemistry. By understanding how molecules organize, researchers can dictate how a bulk material behaves.

The core mechanism of this field is molecular self-assembly. This process relies on noncovalent bonding to organize molecules and ions in a solid state. Scientists use three primary engineering strategies to guide this organization. The first is hydrogen bonding, which often serves as a primary director for crystal organization. The second is halogen bonding, which provides additional control in crystal design. The third strategy is coordination bonding.

To manage these complex assemblies, researchers use specific building blocks. One key concept is the supramolecular synthon. A synthon is a pair of molecules that form relatively strong intermolecular interactions during the early phases of crystallization. These pairs act as the basic structural motifs found in a crystal lattice. Another important concept is the secondary building unit. These tools allow scientists to order specific molecular groups within a solid structure.

The history of crystal engineering spans several decades of discovery. The term was first introduced by R. Pepinsky in 1955. However, the field's practical starting point is often credited to Gerhard Schmidt in 1971. Schmidt worked with photodimerization reactions in crystalline cinnamic acids. In 1988, Gautam Desiraju provided a modern, useful definition. He described the field as the understanding of intermolecular interactions to design new solids with specific properties.

One major application of this science is in the pharmaceutical industry. Scientists design multi-component crystals known as pharmaceutical cocrystals. These crystals consist of one active pharmaceutical ingredient (API) and other substances considered safe by the WHO. By forming cocrystals, researchers can modulate the solubility, bioavailability, and permeability of an API. This process is vital for improving how medicines work in the human body.

Crystal engineering also explores the phenomenon of polymorphism. Polymorphism occurs when the same chemical compound exists in more than one crystal form. This happens due to a competition between kinetic and thermodynamic factors. Kinetic factors involve how crystals form, while thermodynamic factors involve how closely molecules pack. There are three main types of polymorphism in organic molecules. Packing polymorphism occurs when molecules pack in different ways. Conformational polymorphism happens in flexible molecules with multiple shapes. The rarest type is synthon polymorphism, which arises from differences in the primary synthon.

Modern scientists also use computational tools like Crystal Structure Prediction (CSP). CSP is a method used to generate energetically feasible crystal structures from a molecular structure. This is a difficult task because experimental structures are often kinetic structures. A major advance occurred in 2007 with a hybrid method. This method uses tailor-made force fields followed by dispersion-corrected density functional theory (DFT). This allows for the precise calculation of lattice energies.

Finally, crystal engineering is used to design specific mechanical and optical properties. Scientists can target properties like plasticity, elasticity, brittleness, and shear strength. They can also design non-linear optical materials with second harmonic generation (SHG) properties. They even use synthons to design supramolecular gels. By manipulating the interaction network, they can control the entire architecture of the material.