An anomeric effect can favour an axial electronegative substituent at a cyclic acetal carbon. Donation from an endocyclic oxygen lone pair into an antiperiplanar exocyclic C–O antibonding orbital is one important contribution. Steric, electrostatic and solvent effects also influence the observed preference.

An equimolar mixture of two enantiomers.

Steric hindrance is the restriction of access to a reactive site or molecular arrangement by the spatial demands of surrounding groups. It can affect both relative stability and reaction barriers; these are distinct effects.

Two sites are diastereotopic if replacing one or the other with the same test group gives diastereomers. In an achiral solvent they can have different chemical shifts, unlike a simple pair of symmetry-related sites. A rigid chiral metal chelate can make the two hydrogens of a linker methylene diastereotopic.

Topicity classifies the relationship between apparently similar atoms, groups, or molecular faces by a replacement test. Replacing one member and then the other gives identical products for homotopic sites, enantiomers for enantiotopic sites, and diastereomers for diastereotopic sites.

For example, replacing either hydrogen on a suitable prochiral methylene carbon by deuterium can produce opposite enantiomers. Enantiotopic sites are equivalent in an achiral environment but can be distinguished by a chiral reagent or enzyme. Diastereotopic hydrogens can give different chemical shifts even in an achiral solvent. Topicity therefore connects molecular symmetry to spectroscopy and stereoselective reactions.

Epimerization interconverts diastereomers differing at one stereocenter. It can change a product ratio after the initial bond-forming chemical reaction.

Enantioselectivity is preferential formation or reaction of one enantiomer over the other. For a product containing two enantiomers, their imbalance is commonly reported as enantiomeric excess, the absolute difference between their amounts divided by their sum.

A product ratio of $90:10$ corresponds to $80\%$ enantiomeric excess, not $90\%$. A chiral catalyst can make the competing transition states diastereomeric and therefore unequal in free energy. Selectivity depends on the full reaction conditions and must be distinguished from the separate question of overall chemical yield.

A dependence of reactivity or stability on the spatial alignment of donor and acceptor orbitals. Antiperiplanar lone-pair donation into a breaking bond can accelerate a suitable cleavage.

Interconversion of the two spatial orientations of a three-coordinate pyramidal centre through a flatter geometry. Quaternary ammonium nitrogen has no lone pair and cannot follow the ordinary three-coordinate amine pathway.

A chiral catalyst preferentially forms one stereoisomer from an achiral or stereochemically labile substrate. Product stereochemistry depends on competing transition-state free energies, rather than a catalyst simply transferring its own R or S label.

Selective conversion of interconverting substrate stereoisomers to an enriched product. Unlike a simple kinetic resolution, rapid substrate interconversion can in principle allow more than 50% yield of one stereoisomer.

Bond-making or bond-breaking at the same face of the interacting orbital array in a pericyclic reaction. It is a local stereochemical relation, not a statement that all substituents end on one molecular face or a specification of an absolute clockwise rotation.

Chirality associated with the arrangement of substituents relative to a plane, as in an unsymmetrically substituted coordinated cyclopentadienyl ring. Two planar-chiral coordination ligands can give a meso arrangement and a pair of enantiomeric arrangements.

Preferential formation of one stereoisomer over another.

Reversal of the spatial arrangement at a stereogenic center during a reaction.

Preservation of the relative spatial arrangement at a stereogenic center during a reaction.

electron paramagnetic resonance measures resonant transitions involving unpaired electron magnetic moments. A spin label attached to a protein or lipid can report local motion or distances under appropriately designed pulse experiments.

A coherent second-order optical method generating light at the sum of two incident frequencies. Its electric-dipole response is sensitive to broken inversion symmetry and molecular orientation at interfaces.

The wavelength- or frequency-dependent absorption of a specified specimen. Composition, optical path length, concentration, linewidths and illumination determine its transmitted colour; a single band gap does not specify that colour.

An implementation defect that makes a program behave contrary to its intended specification. A faithfully implemented but inappropriate scientific model is a different kind of error.