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Isolated system
An isolated system is a thermodynamic system that exchanges neither matter nor energy with its surroundings; its entropy increases to a maximum at equilibrium, approximated in practice by a thermos flask.
ISOLDE
ISOLDE, the Isotope Separator On-Line Device, is CERN's radioactive ion beam facility. First operated in 1967, it has supplied beams of over 1300 isotopes of more than 70 elements.
Isoleucine
Isoleucine (symbol Ile or I) is an essential, branched-chain amino acid used to build proteins, encoded by the codons AUU, AUC, and AUA and obtained from the diet.
ISOLTRAP
ISOLTRAP is a multi-ion-trap precision mass spectrometer at CERN's ISOLDE radioactive ion beam facility, which in 1987 performed the first Penning-trap mass measurement of a radionuclide.
Isomalt
Isomalt is a sugar alcohol sweetener made from sucrose, about half as sweet as sugar with 2 calories per gram, used in sugar-free candy, gum, and baked goods.
Isomer
Isomers, or molecular isomerism, are chemical species sharing a molecular formula but differing in how atoms connect or arrange in space, such as ethanol and dimethyl ether.
Isomer nomenclature and enumeration conventions
Isomer nomenclature and enumeration conventions are the naming and counting rules for constitutional isomers, which share a molecular formula but differ in how their atoms connect, using locants, prefixes, and graph-based enumeration.
Isomer sets of small homologous series
An isomer set of a homologous series is the complete collection of compounds sharing one molecular formula but differing in atom connectivity; counts grow rapidly with carbon number.
Isomer shift
The isomer shift, also called the chemical shift, is a Mössbauer spectroscopy parameter that measures gamma-transition energy differences between sample and reference, revealing oxidation and spin states.
Isomeric transition
An isomeric transition is the de-excitation of a nuclear isomer, a metastable excited state, by gamma emission or internal conversion, applied when the half-life exceeds about 1 nanosecond.
Isomerization
Isomerization is a chemical reaction in which a molecule is converted into an isomer with the same atoms in a different arrangement, including tautomerization and cis-trans isomerism.
Isomers of butanol
The isomers of butanol are four alcohols with formula C₄H₁₀O: n-butanol, sec-butanol, isobutanol, and tert-butanol, differing in where the hydroxyl group sits and how the carbon chain branches.
Isomers of butylamine
The isomers of butylamine are four primary amines with formula C4H11N: n-butylamine, sec-butylamine, isobutylamine, and tert-butylamine, which differ in chain branching, boiling point, and chirality.
Isometheptene
Isometheptene (CAS 503-01-5, ATC A03AX10) is a sympathomimetic amine causing vasoconstriction, used in migraine combinations such as Midrin (with acetaminophen and dichloralphenazone); FDA removed unapproved isometheptene products from US distribution in 2018.
Isometric log-ratio transformation
The isometric log-ratio (ilr) transformation maps a D-part composition into D − 1 real coordinates that preserve distances, allowing standard statistical methods; it was introduced in 2003.
Isometric projection
Isometric projection, also called isometric view or isometric drawing, is an axonometric method for showing a three-dimensional object in two dimensions, with axes equally foreshortened at 120 degrees.
Isometry
An isometry, also called an isometric transformation, is a map between metric spaces that preserves distances exactly, serving as the notion of sameness in Euclidean and Riemannian geometry and tracing to Klein's 1872 Erlangen program.
Isomorphism
Isomorphism is a correspondence pairing elements of two mathematical objects so every relation and operation is preserved, treating isomorphic objects as the same structure with different labels.
Isomorphism theorems
The isomorphism theorems, also known as Noether's isomorphism theorems, are three or four results in abstract algebra describing how quotients and images of algebraic structures relate.
Isomorphous replacement
Isomorphous replacement is an experimental phasing method in X-ray crystallography that determines protein structures by comparing diffraction intensities from native crystals with crystals containing added heavy atoms, whose intensity differences supply the missing phase information.
Isoniazid
Isoniazid (INH, isonicotinic acid hydrazide) is a first-line antibiotic used to treat and prevent tuberculosis; discovered in 1952, it kills Mycobacterium tuberculosis by blocking mycolic acid synthesis in the bacterial cell wall.
Isonicotinic acid
Isonicotinic acid, also called pyridine-4-carboxylic acid, is one of three pyridine carboxylic acid isomers, best known as the industrial precursor to the anti-tuberculosis drug isoniazid.
Isononyl alcohol
Isononyl alcohol, also called isononanol, is an industrial mixture of branched C9 alcohols produced by the oxo process and used chiefly to make plasticizers such as diisononyl phthalate.
Isooctane
Isooctane, the common name for 2,2,4-trimethylpentane, is a colorless branched alkane with the formula C₈H₁₈ that occurs in petroleum and defines 100 on the octane rating scale.
Isopentane
Isopentane, systematically named 2-methylbutane, is a volatile, highly flammable branched alkane with formula C₅H₁₂, one of three pentane isomers, boiling near 28 °C and used in gasoline blending, foam blowing, and geothermal power generation.
Isopentenyl diphosphate
Isopentenyl diphosphate, also called isopentenyl pyrophosphate, is a five-carbon prenol phosphate that, with its isomer DMAPP, serves as the universal precursor of isoprenoids, from monoterpenes to natural rubber.
Isoperimetric inequality
The isoperimetric inequality states that among all closed curves of a given length, the circle encloses the largest area, in the form L² ≥ 4πA.
Isophorone
Isophorone, also known as isoacetophorone, is a colorless cyclic ketone liquid with a peppermint-like odor, produced from acetone and used as a solvent in inks and paints and as a chemical intermediate.
Isophorone diamine
Isophorone diamine (IPDA) is a colorless, low-viscosity cycloaliphatic diamine sold as a cis/trans isomer mix, used mainly as an epoxy curing agent and to make isophorone diisocyanate for polyurethanes.
Isophorone diisocyanate
Isophorone diisocyanate (IPDI) is a cycloaliphatic diisocyanate made from isophorone, used mainly in light-stable polyurethane coatings, resins, and elastomers that resist yellowing under UV light.