Nucleophile addition to an acyl carbonyl followed by leaving-group elimination, often through a tetrahedral reaction intermediate.

In geminal substitution, two incoming groups replace substituents on the same central atom. In cyclotriphosphazenes, $\mathrm{PC}{\mathrm{l}}_2\to \mathrm{P}{\mathrm{R}}_2$ gives a geminal pair. Primary amino groups can enable a base-catalyzed proton-abstraction/chloride-elimination route that is unavailable after substitution by a secondary amine.

Non-geminal substitution puts incoming groups on different central atoms. On a phosphazene ring, it still permits cis/trans relative configurations and does not by itself specify one stereoisomer.

An atom or group departing with the bonding electron pair in a heterolytic substitution. Its stability, basicity and environment affect reactivity; the substrate geometry also controls access to the substitution pathway.

The Bürgi-Dunitz angle describes the oblique approach of a nucleophile toward a carbonyl carbon, conventionally measured relative to C=O. A typical trajectory is near $107^{\circ }$, with the optimum depending on the system.

A Grignard reaction uses an organomagnesium reagent, conventionally written $\mathrm{RMgX}$, as a carbon-based nucleophile or base. Addition to an aldehyde or ketone, followed by protonation of the alkoxide product, forms an alcohol with a new carbon-carbon bond.

The reagent is not simply a free carbanion: its bonding, aggregation, and coordination depend on its environment. Proton donors can consume it by an acid-base reaction instead of the intended carbonyl addition. This competition makes functional-group compatibility central to understanding its synthetic role.^[ref]

The fraction or percentage of guanine and cytosine residues in a nucleic-acid sequence. Composition contributes to duplex stability but does not establish primer specificity.

The interaction remaining after a chosen nuclear mean field is subtracted from the nuclear Hamiltonian operator. It can split configurations having the same occupied one-particle levels but different coupled angular momentum. Thus the ordering of nuclear orbitals alone does not determine every odd-odd nucleus ground-state spin.

A missing proton relative to a filled nuclear shell. In a closed-core approximation a single hole in a $j$ orbital has the same total angular momentum $j$ and orbital parity as the removed particle. For example, removing a $p_{1/2}$ proton from a spin-zero closed core gives $J^P=1/2^{-}$.

Promoting a nucleon from an occupied orbital to an unoccupied orbital creates a particle-hole configuration. Its possible angular momenta come from coupling the particle and hole, and its parity relative to the reference configuration is the product of their orbital parities. Residual interactions split and mix configurations; a measured nuclear excitation energy is not generally just an uncorrected independent-orbital spacing.

The excitation energy in an ideal $K=0$ nuclear rotational band determines an effective moment of inertia through the displayed equation. Comparing it with $2M{R}^2/5$ tests the rigid-sphere scale. A sequence of approximately constant $E_J/[J(J+1)]$ supports the rotational interpretation; systematic variation indicates that the effective inertia changes along the band. Collective rotation need not have a rigid-body inertia, notably when nuclear pairing is important.

For nonrelativistic elastic scattering of a WIMP of mass $m_{\chi }$ from a stationary nucleus of mass $m_N$, the reduced mass is ${\mu }_{\chi N}=m_{\chi }{m}_N/(m_{\chi }+m_N)$. In the centre-of-momentum description, a scattering angle $\vartheta$ gives momentum transfer squared $q^2=2{\mu }_{\chi N}^2{v}^2(1-\cos \vartheta )$. The target kinetic energy is $E_R=q^2/(2m_N)$, giving the displayed maximum at backscattering. In SI units, using masses in kilograms and velocity in metres per second gives joules. Low recoil energy and a falling recoil spectrum make threshold and response calibration important for dark-matter direct detection.

A water-cooled and water-moderated nuclear reactor whose primary coolant is pressurized to suppress bulk boiling in the core. Heat is transferred to a secondary circuit through steam generators.

A movable neutron-absorbing component used to change reactor reactivity. Insertion can make the fission chain reaction subcritical; residual radioactive-decay heat still requires cooling.

A dimensionless measure of deviation from critical neutron multiplication. Positive reactivity indicates a supercritical neutron balance; zero gives criticality. The population's actual time evolution also depends on delayed-neutron and feedback dynamics.

A reduced time-dependent model for a neutron population amplitude coupled to delayed neutron precursor populations. Spatial and spectral shape changes are neglected or represented through prescribed effective coefficients. Such a model illustrates timescale separation without establishing the spatial behaviour or operating limits of an actual reactor.

The ratio of the rate of formation of fissile nuclei to the rate at which the specified fissile inventory is consumed, under stated reaction and accounting conventions. A conversion ratio below one indicates a replacement shortfall in that inventory. It is distinct from neutron multiplication: a reactor's instantaneous neutron balance and its long-term fuel-inventory balance are different questions.

Energy generated in a period divided by rated power times elapsed time. At fixed efficiency, it scales annual thermal energy and fuel consumption.

A constituent that absorbs neutrons and thereby reduces their availability for a nuclear fission chain reaction. Effectiveness depends on isotope, neutron spectrum and spatial distribution, not just one thermal cross section.

A nuclear reactor on the side of a specified moderation optimum where adding moderator increases neutron multiplication. Removing water then tends to reduce reactor reactivity, under the comparison's other fixed conditions. The optimum and the derivative depend on composition, geometry and operating state.^[ref]