Steric effects are the interaction of molecules dictated by their shape and/or spatial relationships. There are several names for steric effects, usually named for the particular action caused by the steric effect:

Steric hindrance or Steric resistance occurs when molecular groups on a molecule (or molecules individually) that would normally be attracted to one another cannot interact due to their special relationship. For example due to the shape of a molecule atoms that have affinity for one another may not be at a appropriate distance to attract each other, or may have other atoms blocking them.

Steric shielding occurs when a charged molecular group on a molecule is seemingly weakened or covered up spatially be less charged (or oppositely charged) atoms. In some cases for an atom to interact with these sterically shielded atoms it would have to approach from a vicinity where there is less shielding, thus controlling where and what direction a molecular interaction can take place.

Steric attraction occurs when molecules have shapes or geometries that are optimized for interaction with one another. In these cases molecules will react with each other most often in specific special arrangements with one another.

Understanding steric effects is critical to chemistry, biochemistry and pharmacology. In chemistry steric effects are nearly universal and effect the rates and energies of most chemical reactions to varying degrees. In biochemistry steric effects determine the shapes and affinities bio-molecules such a proteins and nucleic acids have, also determines the rates of enzymatic reactions. In pharmacology steric effects determine how and at what rate a drug will interact with its target bio-molecules.

External links

http://www.chem.swin.edu.au/modules/mod4/qsarwebp4.html

http://www.gh.wits.ac.za/craig/steric/

http://jilawww.colorado.edu/pubs/thesis/fair/