isodesmic reaction
An isodesmic reaction is a type of chemical reaction used primarily as a theoretical tool in the field of computational chemistry and thermochemistry. In an isodesmic reaction, the number and type of bonds in the reactants and products are the same, but their arrangement is different. Here's a more detailed explanation:
- Bond Conservation: In isodesmic reactions, the types of bonds broken in the reactants are the same as the types of bonds formed in the products. This means that if a C-H bond is broken in the reactant, a C-H bond is formed in the product, and so on for all bond types in the reaction.
- Purpose and Use: Isodesmic reactions are often used in computational chemistry to calculate the enthalpies of formation or other thermodynamic properties of compounds. Since the bond types are conserved, many systematic errors in computational methods (like basis set superposition error) can be minimized or canceled out, leading to more accurate results.
- Example: An example of an isodesmic reaction is the conversion of butane to two molecules of ethane. In this reaction, the number and type of C-C and C-H bonds remain constant, but their arrangement changes.
- Comparison with Homodesmotic Reactions: Isodesmic reactions are similar to but distinct from homodesmotic reactions. In homodesmotic reactions, not only are the types of bonds conserved, but also their environments (i.e., the bonding context or the hybridization states of the atoms involved) are preserved. Homodesmotic reactions are often used for more accurate thermodynamic calculations.
- Significance in Research: Isodesmic reactions are valuable for theoretical studies and for understanding the stability and reactivity of molecules, especially when experimental data is difficult to obtain or interpret.
In summary, isodesmic reactions are a theoretical construct used in computational chemistry to accurately calculate thermodynamic properties of molecules, taking advantage of bond conservation to reduce errors in calculations.