Preferential formation of one product or reaction at one site. chemoselectivity, regioselectivity and stereoselectivity describe different comparisons.

Interconversion of constitutional forms by proton transfer accompanied by redistribution of pi bonds. Keto–enol interconversion is a familiar example.

Interconversion of isomers without changing the overall molecular formula. Conformational interconversion can be kinetically slow when rotation is hindered.

Addition of Si–H across an unsaturated bond. For an alkene, the net products acquire one C–H and one C–Si bond. Alternative catalytic mechanisms can insert the alkene into either a metal–hydride or a metal–silyl bond; these alternatives should not be conflated.

A rearrangement reaction changes the connectivity of a molecule by moving atoms or groups within its framework. Its product is a constitutional isomer when the overall molecular formula is unchanged, although rearrangements can also occur as steps within a larger reaction.

A carbocation may undergo a hydride or alkyl shift that relocates its positive charge and changes the carbon skeleton. Rearrangements can also be concerted, without a free carbocation intermediate. Identifying which bonds break and form is therefore more informative than treating every rearrangement as the same mechanism.

Formation of a three-membered carbon ring, for example by transferring a carbene-like carbon across an alkene.

A concerted reorganization of bonding through a cyclic orbital interaction, including electrocyclic reactions. The participating orbitals and their electronic occupation determine the relevant symmetry selection rule.

Exchange of ionic partners, often used to replace a metal halide by an anionic ligand while an alkali-metal halide forms. Atom and charge balance distinguish it from an oxidative-addition reaction.

Addition of water to a molecular species. A carbonyl compound can form a geminal diol by hydration, distinct from a solvent hydration shell.

The probability density $E(t)$ of times spent in a flowing system. The surviving fraction of a first-order decaying solute is ${\int }_0^{\infty }E(t)e^{-\lambda t}dt$; it is generally not the exponential function evaluated at the mean residence time.

Cohesion in a metal involving delocalized valence electrons and positive ion cores. A dispersion-only sum between neutral molecular units omits this collective electronic contribution.

Adjacent compatible p atomic orbitals allow electron density to delocalize across more than two atoms. Resonance contributors describe aspects of this shared electronic structure.

Negative hyperconjugation is donation from an occupied lone-pair or related orbital into an antibonding sigma orbital. Nitrogen-to-P–X sigma-star donation contributes to phosphazene bonding. Short P–N bonds therefore do not require a model dominated by phosphorus $3d$ pi orbitals.

The inductive effect is the redistribution of electron density through a molecular framework associated with the electron-withdrawing or electron-releasing influence of substituents. It is usually discussed as polarization transmitted through sigma bonds, distinguished from resonance through conjugated orbitals.

For example, an electron-withdrawing substituent near a carboxylic acid can stabilize its conjugate base and increase acidity. The effect usually weakens with increasing bond distance. Separating an inductive effect from electrostatic field effects is a modeling convention, so a qualitative substituent label is not a universal numerical prediction for every solvent and molecular geometry.

A metal-metal bond joins two metal centers directly through overlap of their atomic orbitals. A single bond contributes one electron to the local count at each metal in conventional electron counting. Bridging ligands can accompany direct bonding, but a short bridged separation alone does not establish its bond order.

Multicentre bonding describes a bonding interaction distributed over more than two atomic centers. A two-center localized electron-pair picture may then fail to represent how the occupied molecular orbitals stabilize the structure.

A standard example is a bridging hydrogen in diborane, where a three-centre two-electron bond extends across a boron-hydrogen-boron unit. The electron pair belongs to the whole three-center interaction, rather than to two independent ordinary bonds. This helps explain electron-deficient molecules without requiring each apparent atom-to-atom connection to contain its own pair of electrons.

Representation of an electronic structure using multiple Lewis contributors with the same nuclear connectivity. The contributors are not separate species interconverting through ordinary chemical reactions.

In a local harmonic approximation, the vibrational wavenumber obeys $\tilde{\nu }=(2\pi c{)}^{-1}\sqrt{k/\mu }$. Comparisons at fixed reduced mass therefore use squared frequency ratios. A local stretching force constant is not identical to a bond-dissociation energy.

Compact valence orbitals favor strong carbon-carbon bonding, but neighboring lone-pair repulsion can weaken nitrogen-nitrogen bonds. Bond strengths depend on orbital overlap, polarity and electronic occupancy together; atom size alone is insufficient.

Bonding with two nodal planes containing the internuclear or coordination axis. metal $d_{xy}$ and $d_{x^2-y^2}$ orbitals can interact with ring combinations of matching delta symmetry; symmetry alone does not imply a strong bond.