Under kinetic control, the observed product distribution reflects competing formation rates rather than an equilibrated distribution. Heating may speed access to a lower-Gibbs free energy product, but does not itself prove that the equilibrium favors it.

A kinetic isotope effect is a change in a reaction rate constant caused by isotopic substitution. Solvent isotope effects can constrain proton-transfer pathways, but coupling and kinetic masking prevent interpreting every rate ratio as one isolated bond-breaking event.

The rate constant for formation of an associated complex, with units of concentration inverse times time inverse for a bimolecular elementary step.

The first-order rate constant for dissociation of a specified complex. For a single exponentially dissociating state its mean lifetime is the reciprocal of this constant.

A rapid temperature perturbation that shifts equilibrium, allowing relaxation toward the new state to reveal kinetic processes.

An isotope effect is a change in a physical property, equilibrium, or reaction rate caused by replacing an atom with one of its isotopes. A kinetic isotope effect compares rate constants, often as $k_{\mathrm{H}}/k_{\mathrm{D}}$ for hydrogen and deuterium substitution.

Isotopes have nearly the same electronic potential-energy surface but different masses, so vibrational frequencies and zero-point energies change. If a carbon-hydrogen bond is substantially altered in a rate-controlling step, replacing it by carbon-deuterium can produce a measurable rate difference. Such a measurement constrains a reaction mechanism, but its interpretation can also involve tunneling, equilibrium steps, and competing pathways.^[ref]

Association stabilized by energy transfer to another collision partner. Counting removed reactive intermediates requires their stoichiometric factors, not just the event rate.

A feedback in which heat release raises temperature sufficiently to accelerate further heat release. Its occurrence depends on kinetics, heat capacity, transport and depletion; exothermicity alone does not prove an inevitable explosion.

Approximation that formation and consumption of selected reactive intermediates nearly balance after an initial transient. It does not set their concentrations to zero or imply equilibrium of the stable reagents.

Product selection governed by equilibrium free energies after reversible interconversion, in contrast to kinetic control before equilibration.

Persistence over the observation time because reaction pathways have sufficiently large barriers. It does not require the state to be the thermodynamic minimum.

A set of species interconverted rapidly relative to the production or removal of the set as a whole. Fast interconversion can justify a concentration ratio without establishing that the total family concentration is in steady state.

An approximation setting the time derivative of a transient population or intermediate concentration to zero when production and loss balance on the timescale considered. It differs from thermal equilibrium.

For competing irreversible pseudo-first-order channels with rates $k_j$, the fraction following channel $j$ is $f_j=k_j/{\sum }_l{k}_l$ when those conditions stay fixed during the intermediate's lifetime. A photochemical destruction rate must count only branches that actually remove the chosen species after fast recycling. For excited oxygen, collisional deactivation followed by ozone reformation is different from reaction with water to make hydroxyl radicals.

A time correlation $C_{fs}(t)=⟨\delta (s_0){\dot{s}}_0\Theta (s_t)⟩$ correlating signed initial flux through a dividing surface with product-side occupation at a later time. Dividing by reactant population gives a rate on a plateau after microscopic recrossing and before macroscopic equilibration.

For a one-dimensional channel, $k(T)Q_r=(2\pi \hslash {)}^{-1}{\int }_0^{\infty }{e}^{-\beta E}P(E)dE$. The reactant partition function and any length normalization must be specified.

The number of rings and pi-bond equivalents needed by a conventional closed-shell formula. For a neutral formula containing C, H, N and halogens, it is $(2C+2+N-H-X)/2$; oxygen does not change the count. A carbonyl contributes one and a triple bond two.

Exchange much faster than the resonance frequency difference gives one population-weighted resonance. The averaging does not prove a single static geometry.

Arsenic is the chemical element with atomic number 33 and symbol $\mathrm{As}$. It is a metalloid. Its chemistry includes oxidation states $-3$, $+3$, and $+5$, and it occurs in both inorganic and organic compounds. Chemical form matters: elemental arsenic, arsine, and arsenic oxides differ in bonding and physical behavior.^[ref]

Zirconium is a group-4 metal; Zr(IV) is formally $d^0$. Its cationic alkyl complexes can catalyze polymerization through alkene migratory insertion.