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Absorbed photon flux is the rate at which a specified sample absorbs photons. Number-based flux counts photons per second; amount-based flux counts moles of photons per second. The absorbed rate differs from the incident rate and supplies the denominator of a rate-based photochemical quantum yield. For monochromatic light passing through a homogeneous absorbing sample,where $q_0$ is incident photon flux and $A(\lambda )$ is the decadic absorbance at the excitation wavelength. Reflection and scattering must be absent or accounted for before transmission loss is interpreted entirely as absorption.^[ref]

For example, $q_0=2\mu \mathrm{mol}{\mathrm{s}}^{-1}$ and $A=0.30$ giveA product-formation rate of $0.20\mu \mathrm{mol}{\mathrm{s}}^{-1}$ would then give photochemical quantum yield approximately $0.20/0.998=0.200$, whereas division by incident flux gives $0.10$. The two calculations answer different questions.

The distinction also matters during an experiment: if absorbance changes as a reactant is consumed, one initial flux value cannot represent the whole exposure. Absorbed photon amount is the time integral of the absorbed rate. If the numerator is a change of concentration per second, divide the total absorbed amount rate by the illuminated sample volume as well. A photon rate, a rate per unit area and a rate per unit volume have different dimensions; the word flux alone should not conceal that distinction.^[ref]

An activity coefficient expresses departure from a chosen ideal-mixture activity convention. In a concentration-based convention,The thermodynamic activity $a_i$ and coefficient ${\gamma }_i$ are dimensionless; $c^{\circ }$ is the standard concentration. The reference convention fixes which ideal or limiting state has ${\gamma }_i=1$. A mole-fraction convention uses a different ideal activity and must not be mixed silently with the concentration convention.^[ref]

Substituting activities into an equilibrium constant separates concentration ratios from interaction corrections. For $AB\rightleftharpoons A+B$, the correction contains ${\gamma }_A{\gamma }_B/{\gamma }_{AB}$. Changing composition can therefore change an observed concentration quotient even when the thermodynamic constant at fixed temperature and reference state is unchanged. An activity coefficient can be greater or less than one; it is not a fraction of molecules assigned to an active population.

Beta-elimination, also called 1,2-elimination, removes groups from adjacent atoms while forming a new multiple bond between those atoms. The IUPAC definition allows the two central atoms to be elements other than carbon, although carbon–carbon examples are common. It describes the overall connectivity change, rather than specifying one unique reaction mechanism.^[ref]

The net dehydration of ethanol illustrates the topology:One carbon loses a hydrogen and its neighbor loses the oxygen-containing group; the carbon pair becomes the double bond of ethene. Both sides contain two carbons, six hydrogens and one oxygen. The balanced equation describes the net conversion, without asserting that unactivated ethanol changes spontaneously or that all bond changes occur in one step.

The positional label beta must be separated from the mechanistic label in E2 elimination. Both an E1 elimination and an E2 elimination can produce a beta-elimination transformation. Their differing intermediates, rate laws and geometric requirements are further information beyond the atom-connectivity pattern.

In chain-growth polymerization, each growth step joins a monomer to a reactive site on a polymer chain and regenerates that reactive site. This is the precise mechanistic restriction in IUPAC's definition: growth occurs exclusively through these monomer–chain-site reactions. The word chain refers to the succession of site-regenerating chemical reactions. It does not merely mean that the product looks like a long chain. Chain-growth initiation starts this succession; chain-growth propagation continues it; chain-growth termination and chain transfer in chain polymerization can change its fate.^[ref]

The contrast with step-growth polymerization concerns the permitted joining partners. There, suitable reactive groups can join two existing chains, for example a two-unit chain and a three-unit chain. In chain-growth propagation, a free monomer adds to a polymerization-active site. Release of a small byproduct is a separate classification question: IUPAC explicitly allows condensative chain polymerization.^[ref] The useful consequence is that a few growing chains can incorporate many monomer molecules while much of the feed remains unreacted. Their degree of polymerization depends on how those units are shared among chains, as finite chain-growth bookkeeping makes explicit.

Chain-growth termination removes the ability of a reactive chain carrier to continue the succession of growth steps.^[ref] Two illustrative outcomes have different molecule counts:The first joins the two chains into one inactive polymer molecule. The second leaves two inactive polymer molecules, as can occur when one chain end transfers hydrogen to the other instead of their ends joining. End-group chemistry is suppressed in these unit-count symbols; they are not complete molecular formulas. Losing two polymerization-active sites does not mean losing two chains or any incorporated monomeric units. A temporarily inactive, reactivatable site must not be counted as irreversible chain-growth termination.

In chain transfer in chain polymerization, a growing chain can lose its polymerization-active site while another molecule acquires reactivity. In a common atom-transfer picture, the old chain becomes inactive and the new carrier can initiate further chain-growth propagation.^[ref] The number of reactive carriers can survive even though the identity of the growing chain changes. If the new carrier successfully starts a new chain, $C$ increases; a fixed active-carrier count therefore does not imply a fixed polymer molecule count. Whether transfer is irreversible, reversible, or involves an already existing chain matters for the final bookkeeping.

A chemical structure specifies how chemical atoms are connected and, where relevant, how they are arranged in space. A molecular formula counts atoms; a structural formula supplies connectivity information. Molecular geometry and stereochemistry add spatial distinctions that cannot generally be recovered from the atom inventory alone.^[ref]

For a concrete comparison, ethanol and dimethyl ether both have molecular formula ${\mathrm{C}}_2{\mathrm{H}}_6\mathrm{O}$. Their condensed structural formulas arerespectively. In ethanol, the oxygen is bonded to one carbon and one hydrogen; in dimethyl ether, it is bonded to two carbon atoms. The same atom counts therefore describe different connectivities.

A drawing is a representation of a chemical structure, not necessarily a frozen picture of every atom's position. Rotating around a bond can change a molecule's spatial arrangement while preserving its connectivity. Conversely, two drawings that look different on the page can represent the same chemical structure. Establishing identity requires interpreting bonds and spatial conventions, not comparing the appearance of the diagrams alone.

Chromatographic resolution measures the separation of two peaks relative to their widths. With chromatographic retention times $t_{R2}>t_{R1}$ and baseline widths $w_{b1},w_{b2}$, its conventional definition isThe numerator and denominator have the same units, so $R_s$ is dimensionless. A large time difference does not by itself imply good separation if both peaks are correspondingly broad.^[ref]

For an original example, peaks at $4.0$ and $4.6$ minutes, each $0.40$ minutes wide at the base, have $R_s=2(0.6)/(0.8)=1.5$. If their widths increase to $0.60$ minutes while their peak times stay fixed, $R_s=1$. Measuring only the peak centers would miss the deterioration.

Simply rescaling the time axis cannot change chromatographic resolution. If all times and widths in the first example double, then $R_s=2(1.2)/(1.6)=1.5$ again. This is a statement about a rescaled trace, not a claim that changing the flow of a real column leaves its separation physics unchanged.

The peak-width convention must be stated. Half-height widths are smaller than baseline widths and cannot be substituted into the same formula without the corresponding conversion. Peak asymmetry, overlapping components and unequal signal strengths can also make a single reported $R_s$ an incomplete account of whether a particular quantitative measurement is reliable.

The total chromatographic retention time $t_R$ is the elapsed time from sample injection to the maximum of a specified component's chromatographic peak. It includes the time spent moving through the system as well as the additional delay associated with retention. It is a property of the analyte under specified conditions, not a universal identifying number attached to the molecule.^[ref]

Let $t_M$ denote the hold-up time measured with a suitably unretained marker. The adjusted retention time is $t_R^{\prime }=t_R-t_M$. For example, a peak at $6$ minutes with $t_M=1.5$ minutes has $t_R^{\prime }=4.5$ minutes. Its total passage time and its delay relative to an unretained marker answer different questions. At constant measured volumetric flow $F$, the corresponding total retention volume is $V_R=F{t}_R$; $F=0.8$ milliliters per minute and $t_R=6$ minutes give $V_R=4.8$ milliliters.^[ref]

A sample peak matching a reference peak's chromatographic retention time under the same conditions supports an identity assignment, but another compound may coelute. Changing solvent composition or column conditions can change $t_R$ without changing molecular identity. A detector's peak area and chromatographic resolution supply different information from the time at which the maximum occurs.

The definition uses the peak maximum, rather than the first detectable signal or the mean arrival time of every molecule. For a broad or asymmetric peak, these alternatives need not coincide.

Concentration quantifies how much of a named constituent is present relative to a specified sample measure. In chemical terminology, volumetric concentrations use the final mixture volume: mass concentration is mass per volume and molar concentration is amount of substance per volume. Mass fraction and amount fraction instead compare a constituent with the corresponding total quantity in the mixture.^[ref]

The distinction matters in transport models. If an isolated sample expands without exchanging constituents, its volumetric concentration decreases even though its composition fractions stay fixed. A scalar called concentration in an advection model may represent a fraction instead, so its definition must be stated.

Conservation applies to the transported constituent amount. Reactions, diffusion and exchange then determine how the chosen concentration changes relative to its carrier and denominator.

A condensed structural formula writes connected groups of chemical atoms compactly, omitting some explicit bond strokes. It preserves interpretable connectivity information, unlike a molecular formula that merely totals the atoms. It is one way to represent a structural formula, whose purpose is to describe how atoms are connected.^[ref]

For an original counting example,can be unrolled as $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$. The two terminal groups supply two carbons and six hydrogens; four interior groups supply four carbons and eight hydrogens. Thus the molecular formula is ${\mathrm{C}}_6{\mathrm{H}}_{14}$, since $2+4=6$ and $6+8=14$. The parentheses abbreviate a connected repeated group, rather than introducing a separately floating molecule.

The compression must be interpreted with the chosen convention. A formula such as $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ specifies different connectivity from $\mathrm{C}{\mathrm{H}}_3\mathrm{OC}{\mathrm{H}}_3$ despite their common atom counts. Neither expression by itself gives every bond angle or a unique three-dimensional conformation; those questions involve molecular geometry and stereochemistry.

Copolymerization is polymerization that forms a copolymer: a polymer containing monomeric units derived from more than one monomer species. The different units occur within polymer molecules. Merely mixing two separately prepared polymers produces a polymer blend, which does not by itself establish copolymerization.^[ref]

Reacting-mixture composition and incorporated composition are different quantities. In an original material-balance example, start with $100\mathrm{mmol}$ each of monomers $A$ and $B$. Suppose $60\mathrm{mmol}$ of $A$ and $20\mathrm{mmol}$ of $B$ have entered the growing chains, with no other consumption or material loss. The cumulative mole fraction of $A$ units in the product iswhile the remaining monomer mixture contains $40\mathrm{mmol}$ of $A$ and $80\mathrm{mmol}$ of $B$, givingThe initial mixture was equimolar, but neither current value is $1/2$. Conversion bookkeeping for binary copolymers keeps these inventories distinct. The instantaneous copolymer composition equation predicts incorporation at a particular mixture composition under the terminal model of copolymerization; it does not automatically give the cumulative composition over a changing feed.

Composition also does not determine sequence. The schematic eight-unit segments $\mathrm{AAAAAABB}$ and $\mathrm{AAABAAAB}$ both contain six $A$ units and two $B$ units but have different arrangements. Neither is equivalent to placing an all-$A$ chain and an all-$B$ chain together in a polymer blend. Sequence architecture, overall mole fraction and the growth mechanism are separate specifications.

Experimentally, measuring the initial feed alone cannot identify monomer reactivity ratios. The IUPAC recommendation combines conversion and incorporated-composition observations at several starting compositions to constrain the kinetic model.^[ref] The historical 1944 paper by Frank R. Mayo and Frederick M. Lewis explicitly joined a theoretical comparison of monomer behavior with the copolymerization of styrene and methyl methacrylate.^[ref]

A dissociation constant is an equilibrium constant for a reaction in which a combined species separates into components. For $AB\rightleftharpoons A+B$, a concentration convention writesIt has units of concentration. The dimensionless thermodynamic form instead uses thermodynamic activities, $K_d^{\circ }=a_A{a}_B/a_{AB}$. For ideal concentration activities, $K_d^{\circ }=K_d^{(c)}/c^{\circ }$, with specified standard concentration $c^{\circ }$. The choice of quantity and reference is therefore part of quoting the constant.^[ref]

In the simple dilute $1:1$ model with free $c_A$ held fixed, the fraction of B present as AB is $c_A/(K_d^{(c)}+c_A)$. A smaller $K_d^{(c)}$ therefore means more association at the same free concentration. The inverse association constant describes the reversed reaction only when conventions, solvent conditions and temperature match. A dissociation rate is a kinetic quantity and is not specified by this equilibrium ratio alone.

An electronic state specifies a quantum-mechanically allowed state of the electrons in a chemical atom, molecule or molecular system. It includes the electronic wavefunction and its symmetry and spin properties, rather than merely a drawing of which atomic orbitals or molecular orbitals are occupied. IUPAC defines the term in terms of arrangements allowed by quantum mechanics.^[ref]

For a molecule, the Born-Oppenheimer approximation makes the distinction concrete. Hold nuclear coordinates $R$ fixed and solve the electronic Schrödinger equationHere $r$ denotes electronic coordinates. Each electronic state supplies an energy function of nuclear geometry; including nuclear repulsion gives the corresponding potential energy surface. Nuclear molecular vibration and molecular rotation can then occur on that surface. Different vibrational levels can therefore belong to the same electronic state.

An electron configuration is useful bookkeeping, but it need not determine a unique electronic state. For example, two electrons in different spatial orbitals can couple their spins to a singlet or a triplet, subject to the required exchange symmetry. Their spin multiplicities differ even though the orbital occupancy inventory is the same. A simple orbital-promotion picture leaves out that coupling information.

For an illustrative electronic energy gap $\Delta E=2.00\mathrm{eV}$, a matching photon has wavelength $\lambda =hc/\Delta E\approx 620\mathrm{nm}$. Matching the energy is necessary for a resonant optical transition, but its strength also depends on the relevant transition dipole moment and selection rules. A gap alone does not guarantee a strong absorption line.

This distinction matters in spectroscopy: a measured band can contain vibrational and rotational structure within an electronic transition. Conversely, an infrared vibrational band can change nuclear motion while leaving the electronic state unchanged. Identifying which degrees of freedom changed is part of interpreting the spectrum.

Two epimers are diastereomers with opposite configurations at exactly one of the two or more tetrahedral stereocenters in their molecules. Their connectivity is the same, and configurations at the other centers agree. The definition is more specific than simply calling two compounds stereoisomers.^[ref]

D-glucose and D-mannose illustrate the relationship: their configurations differ at carbon 2 while the other corresponding stereocenters agree. This local change can alter recognition by enzymes without changing the elemental composition or the carbon-chain connectivity.

For a hypothetical molecule with corresponding stereocenters 2 and 3, the configurations $(2R,3R)$ and $(2S,3R)$ differ at one center and can form an epimeric pair. Changing both to $(2S,3S)$ is a different comparison: it does not satisfy the one-center condition. These labels assume the same molecular connectivity and consistently assigned centers.

A molecule with only one stereocenter does not produce an epimer pair merely by reversing that center: its two configurations are enantiomers, whereas the epimer definition requires a diastereomeric relationship. Epimerization names a process that interconverts an epimeric pair; it does not by itself specify the reaction mechanism, rate or equilibrium ratio.

A glycosaminoglycan is a long, unbranched polysaccharide built from repeating two-sugar units containing an amino sugar derivative. The partner is commonly a uronic acid; keratan sulfate instead contains galactose. Many glycosaminoglycans carry sulfate groups and occur as carbohydrate chains of proteoglycans. A proteoglycan is the larger protein–carbohydrate assembly, while a glycosaminoglycan is the sugar chain.^{[ref] [ref]}

Hyaluronic acid is a useful comparison: it alternates glucuronic acid with N-acetylglucosamine, is nonsulfated, and is not covalently attached to a protein core. Thus neither “sulfated” nor “covalently protein-bound” defines every member of the family. At physiological pH its carboxylate groups give the chain negative charge, helping explain its interaction with water and surrounding ions. ^[ref]

The distinction matters in experiments on the endothelial glycocalyx. Inhibition or removal of a particular sugar-chain component can change the environment through which blood cells move without removing the entire endothelial cell membrane. Heparinase treatment in microvessels altered capillary hematocrit, supporting a role for the surface carbohydrate layer in blood-cell distribution; it does not establish that every glycosaminoglycan produces the same effect.^[ref]

High-performance liquid chromatography is liquid chromatography typically using a finely divided stationary packing and a pump supplying the pressure needed to drive the chromatographic mobile phase through it. A sample mixture is injected, its components interact differently with the chromatographic stationary phase, and a detector records their elution. IUPAC recognizes both high-performance and high-pressure terminology for this form of liquid chromatography.^[ref]

The pressure supplies flow through the column; differential interactions produce the separation. Raising pressure alone does not specify which component will elute first. The solvent composition, stationary material and analyte chemistry all matter. The useful result is a set of peaks separated sufficiently for the intended identification or measurement.

An original example illustrates how to assess a recorded separation. Suppose two peaks have chromatographic retention times $t_{R1}=5.0$ minutes and $t_{R2}=6.0$ minutes, with baseline widths $w_1=w_2=0.40$ minutes. Using widths at the base, their chromatographic resolution isIf the widths become $0.80$ minutes each while the peak times stay fixed, $R_s$ falls to $1.25$. Peak timing alone therefore does not establish separation quality. Widths measured at half-height require the corresponding formula rather than substitution into this baseline-width definition.^[ref]

Matching a chromatographic retention time to a standard under the same conditions is useful evidence, but different substances can coelute. Quantifying a peak also requires a calibrated detector response; its area is not automatically an amount independent of molecular identity and instrument settings.

An isosbestic point is a wavelength, wavenumber or frequency at which a sample's total absorbance remains unchanged during a specified chemical or physical transformation. It is a property of the observed spectra and transformation, rather than proof of a particular reaction mechanism.^[ref]

For a simple conversion between two species $A$ and $B$, assume additive Beer-Lambert law response, fixed optical path length $\ell$ and constant total concentration $C=c_A+c_B$. ThenAt a wavelength ${\lambda }_{\ast }$ where ${\epsilon }_A({\lambda }_{\ast })={\epsilon }_B({\lambda }_{\ast })={\epsilon }_{\ast }$,independent of the conversion fraction. For example, stipulated values $\ell =1\mathrm{cm}$, $C=10^{-4}\mathrm{mol}{\mathrm{L}}^{-1}$ and ${\epsilon }_{\ast }=5000\mathrm{L}\mathrm{mo}{\mathrm{l}}^{-1}\mathrm{c}{\mathrm{m}}^{-1}$ give constant absorbance $0.50$.

An isosbestic point does not establish that exactly two species exist. Three species could all have the same absorption coefficient at ${\lambda }_{\ast }$; every mixture with the same total concentration would then give the same absorbance there. More generally, constant stoichiometric combinations of coefficients can produce an invariant signal. Conversely, dilution, changing path length, background absorption or changing spectra can remove a crossing without demonstrating a new reaction intermediate.

In an experimental test, record multiple full spectra while controlling total concentration, path length and background. Inspect the stability of the common intersection relative to measurement uncertainty, rather than selecting one pair of curves that happens to cross. The invariant wavelength alone cannot determine composition in the two-species model: its response has deliberately lost sensitivity to the conversion fraction. Measurements away from that wavelength carry the separating information, a distinction also useful in co-oximetry.

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]

Entry in IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry, issued by International Union of Pure and Applied Chemistry. No individual entry author is credited; editors are not treated as entry authors. The fifth edition and online version 5.0.0 are dated 2025.

Canonical entry: goldbook.iupac.org/terms/view/B00626

Inspected official indexed mirror: mail.goldbook.iupac.org/terms/view/B00626

Cited passages: IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book), Beer–Lambert law: Official indexed mirror mail.goldbook.iupac.org/terms/view/B00626: complete opening definition, both equations, decadic coefficient name, conventional and SI units, spectral-bandwidth qualification, fifth-edition/version citation; Entry names its underlying 1996 glossary, printed p. 2230; underlying recommendation not independently inspected.