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Nucleophilic acyl substitution
Nucleophilic acyl substitution is the reaction in which a nucleophile replaces the leaving group on a carbonyl carbon, converting one carboxylic acid derivative into another.
Nucleophilic aromatic substitution
Nucleophilic aromatic substitution (SNAr) is a class of organic reactions in which a nucleophile replaces a leaving group on an aromatic ring, typically requiring electron-withdrawing groups.
Nucleophilic aromatic substitution of hydrogen
Nucleophilic aromatic substitution of hydrogen, abbreviated SNArH, is a reaction in which a nucleophile directly replaces a hydrogen atom on an aromatic ring, usually requiring an oxidant.
Nucleophilic substitution
Nucleophilic substitution is an organic reaction in which a nucleophile replaces a leaving group on a substrate, proceeding mainly by the SN1 and SN2 mechanisms.
Nucleophilic substitution of alcohols
Nucleophilic substitution of alcohols is the family of reactions replacing an alcohol's hydroxyl group with a nucleophile, proceeding by SN1 or SN2 pathways after acid activation.
Nucleophilicity
Nucleophilicity is the kinetic ability of a chemical species, the nucleophile, to donate electron pairs to an electrophilic partner, measured by reaction rate and distinct from basicity.
Nucleoside phosphoramidites
Nucleoside phosphoramidites are protected nucleoside monomers with DMT, base-acyl, and 2-cyanoethyl phosphoramidite groups, introduced in 1981 and used as the standard building blocks for automated DNA and RNA oligonucleotide synthesis.
Nucleoside triphosphate
A nucleoside triphosphate (NTP) is a molecule of a nitrogenous base, a sugar, and three phosphate groups that serves as the substrate for RNA and DNA synthesis.
Nucleosynthesis
Nucleosynthesis, also called nucleogenesis, is the set of nuclear processes that create atomic nuclei, from big bang hydrogen and helium to elements forged in stars.
Nucleus-independent chemical shift
Nucleus-independent chemical shift (NICS) is a computed magnetic criterion of aromaticity introduced by Paul von Ragué Schleyer and co-workers in 1996; negative values indicate aromaticity and positive values antiaromaticity.
Nuclide
A nuclide is a species of atom distinguished by its mass number, atomic number, and nuclear energy state, identifying specific nuclei like carbon-12 rather than a chemical element generally.
nude mouse
The nude mouse is a hairless laboratory mouse with a Foxn1 mutation that leaves it without a thymus and mature T cells, widely used in cancer research.
Nugget effect
The nugget effect is the nonzero value a variogram approaches as sample separation tends to zero, caused by measurement error and microscale variation; a term coined in mining geostatistics.
Nuh Gedik
Nuh Gedik is an experimental condensed matter physicist who is the Donner Professor of Physics at MIT, known for creating time-resolved techniques that probe charge, spin, and lattice dynamics in quantum materials.
Null geodesics in the Schwarzschild metric
Null geodesics in the Schwarzschild metric are the paths of light rays in the curved spacetime outside a spherical mass, with unstable circular photon orbits at radius 3M.
Null hypothesis
The null hypothesis (H0) is the default proposition, typically stating that an effect is absent, which a statistical test attempts to reject in favor of an alternative.
Null infinity
Null infinity, denoted ℐ, is the boundary surface where outgoing light rays of an asymptotically flat spacetime end, introduced by Roger Penrose in 1963 via a conformal transformation.
Null set
A null set, also called a measure-zero set, is a measurable set of measure zero, like the rationals or the uncountable Cantor set, that integration treats as negligible.
Null test
A null test is a statistical validation procedure in physics and astronomy that checks whether data constructed to contain no signal, such as differences of data splits, look like noise.
Number
A number is a mathematical object used to count, measure, and label, built up in nested systems from the natural numbers to the integers, rationals, reals, and complex numbers.
Number concentration
Number concentration is the number of entities of a constituent in a mixture divided by the volume of that mixture, measured in m−3 and also called number density.
Number density
Number density is an intensive physical quantity giving the number of specified particles, such as atoms or electrons, per unit volume, written n = N/V.
Number field
A number field is a subfield of the complex numbers that is a finite extension of the rational numbers, and the central object of algebraic number theory.
Number needed to treat
The number needed to treat (NNT) is a medical measure of treatment effect: the average number of patients who must receive a treatment for one additional patient to benefit.
Number of the beast
The number of the beast, also called the beast number (Greek: Χξς), is 666, given in Revelation 13:18 as the number of a beastly figure who forces everyone to receive a mark.
Number operator
The number operator is the operator formed from a creation and an annihilation operator whose eigenvalues count the quanta in a mode of a quantum harmonic oscillator or field.
Number theory
Number theory, also called the theory of numbers or higher arithmetic, is the branch of mathematics studying the integers, primes, divisibility, and equations solved in whole numbers.
Number-theoretic transform
The number-theoretic transform (NTT) is a discrete Fourier transform computed modulo a prime instead of over the complex numbers, giving exact convolution for polynomial multiplication and cryptography.
Numbering (computability theory)
A numbering in computability theory is a surjective map from the natural numbers onto a countable collection of objects, such as the partial computable functions or the computably enumerable sets.
Numeral system
A numeral system, also called a scale of notation, is a system for representing numbers using digits and rules for combining them, either additively or by place value.