OpenStem

@openstem · Joined Jul 2026

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Flashcards12 cards

Define chemiluminescence, and contrast it with photoluminescence.1 / 12

Chemiluminescence is the emission of light produced when a chemical reaction releases enough energy in a single step to directly populate an electronically excited state of one of its products — no photon is ever absorbed beforehand. Photoluminescence (fluorescence/phosphorescence) instead starts by absorbing a photon to create the excited state. Both end the same way: an excited-state molecule relaxing to its ground state by emitting a photon. The difference is entirely in how the excited state is FIRST created.

Chemistry

Chemistry · L5 · Chemiluminescence

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Flashcards10 cards

Why does substituting deuterium (²H, D) for ordinary hydrogen (¹H) at a specific position barely change a molecule's chemistry, yet still matter for its kinetics?1 / 10

Isotopes of the same element have identical numbers of protons and electrons, so the electronic structure, bond lengths, and overall bonding potential-energy surface of a C–D bond are essentially the same as a C–H bond — the chemistry looks the same. What DOES change is the nuclear mass, and that alters how the bond vibrates, which can in turn change how easily it breaks.

Chemistry

Chemistry · L5 · The Kinetic Isotope Effect

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Flashcards10 cards

What is the design philosophy behind 'click chemistry', as originally proposed by K. B. Sharpless and coworkers?1 / 10

Click chemistry describes a small set of reactions chosen for being modular, giving very high yields, generating only inoffensive byproducts that can be removed without chromatography, being stereospecific, and running under simple conditions — often in water, at room temperature, tolerant of many functional groups. The goal is a dependable, 'plug-and-play' bond-forming toolkit rather than one reaction optimised for a single target.

Chemistry

Chemistry · L5 · Click Chemistry and Bioorthogonal Labeling

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Flashcards8 cards

What is atropisomerism?1 / 8

Atropisomerism is a form of stereoisomerism arising from restricted rotation about a single bond — most commonly the C–C bond connecting two aromatic rings in a biaryl — where sufficiently bulky substituents near that bond raise the rotational barrier high enough that the two resulting non-superimposable conformers are configurationally stable and isolable at room temperature, rather than rapidly interconverting.

Chemistry

Chemistry · L5 · Atropisomerism & Axial Chirality

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Flashcards7 cards

What is the 'rocking-chair' mechanism of a lithium-ion battery?1 / 7

In a rocking-chair (intercalation-type) lithium-ion battery, Li⁺ ions shuttle back and forth between two host intercalation materials — a graphite anode and a transition-metal oxide cathode — inserting into and extracting from the layered or channeled crystal structure of each electrode without either host structure being consumed or fundamentally transformed. Charging drives Li⁺ out of the cathode host and into the graphite anode host; discharging reverses the flow — the electrode HOSTS never change identity, only their lithium occupancy, which is why the mechanism is called 'rocking-chair' rather than a conversion or plating reaction.

Chemistry

Chemistry · L5 · Battery Chemistry & Materials

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Flashcards9 cards

What is the FeMo-cofactor, and what reaction does it catalyze?1 / 9

The FeMo-cofactor is nitrogenase's active site: a [7Fe-9S-Mo-C-homocitrate] cluster with an interstitial carbide at its center. It catalyzes the reduction of atmospheric N2 to two molecules of NH3, a reaction that requires breaking the N≡N triple bond (bond energy ~941 kJ/mol) and consumes 16 ATP and 8 electrons per N2 fixed, releasing one H2 as an obligate byproduct.

Chemistry

Chemistry · L5 · Bioinorganic Metalloenzymes

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Flashcards8 cards

What is a carbene, and what are its two possible spin states?1 / 8

A carbene is a neutral carbon species with only six valence electrons and two non-bonding electrons on that carbon, making it strongly electrophilic and highly reactive. Those two non-bonding electrons can occupy the same orbital with paired spins (singlet carbene, one filled sp²-like orbital and one empty p orbital) or occupy two different orbitals with parallel spins (triplet carbene, effectively a 1,1-diradical) — the two spin states have distinct geometries, reactivities, and selectivities.

Chemistry

Chemistry · L5 · Carbene Chemistry

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Flashcards8 cards

What is activity-based protein profiling (ABPP), and how does it differ fundamentally from measuring protein abundance (e.g. by Western blot or mass spectrometry proteomics)?1 / 8

ABPP uses a small-molecule probe that covalently reacts with the catalytically active form of a target enzyme class, so the labeling signal reports specifically on enzymatic ACTIVITY (whether the enzyme's catalytic machinery is functional and accessible) rather than total protein ABUNDANCE. This distinction matters because an enzyme can be abundantly present yet catalytically silent — inhibited, mutated, or held in an inactive conformation/zymogen state — and abundance-only methods cannot detect that difference, while an ABP specifically fails to label an inactive enzyme copy even if it is present in equal or greater amount.

Chemistry

Chemistry · L5 · Chemical Biology & Activity-Based Probes

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Flashcards8 cards

What two computational problems does a molecular docking program solve together, and why is speed a necessary compromise?1 / 8

Docking solves a SEARCH problem (finding the ligand's binding pose — position, orientation, and conformation — within the protein's binding site) and a SCORING problem (estimating the binding affinity of each candidate pose) simultaneously, repeated across a very large virtual library (often millions of compounds). Because a rigorous physics-based free-energy calculation for even one pose can take hours of computation, docking scoring functions must be simplified and fast enough to evaluate thousands of poses per compound across a huge library in a practical timeframe — trading rigor for throughput.

Chemistry

Chemistry · L5 · Computational Docking & Drug Design

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Flashcards8 cards

What is a polarization curve, and what does its overall downward-sloping shape represent?1 / 8

A polarization curve plots a fuel cell's operating cell voltage against the current density it delivers; it always slopes downward from the theoretical open-circuit (equilibrium, zero-current) voltage because drawing any real current necessarily incurs voltage losses (overpotentials) at the electrodes and across the cell — the curve is, in effect, a direct visualization of how much of the cell's theoretical thermodynamic voltage is actually lost to real-world kinetic and transport limitations at a given operating current.

Chemistry

Chemistry · L5 · Electrocatalysis & Fuel Cells

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Flashcards9 cards

What is the fundamental difference between batch and continuous-flow synthesis?1 / 9

In batch synthesis, all reagents are combined in a single vessel and the reaction proceeds over time in that same volume. In continuous-flow synthesis, reagent streams are pumped continuously through a reactor (often narrow tubing or a microstructured chip), and a given parcel of fluid reacts only during the fixed time it takes to traverse the reactor — the residence time — set purely by flow rate and reactor volume, decoupling reaction time from vessel dwell and enabling steady-state operation.

Chemistry

Chemistry · L5 · Flow Chemistry & Continuous Processing

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Flashcards8 cards

What is the core principle that makes isotopic labeling a mechanistic and tracing tool?1 / 8

Isotopes of the same element have (to an excellent first approximation) identical electronic structure and chemical reactivity, differing only in nuclear mass and, for radioisotopes, radioactive decay — so substituting one isotope for another (²H for ¹H, ¹³C for ¹²C, ¹⁸O for ¹⁶O, or a radioactive isotope like ¹⁴C or ³H) lets a chemist follow the FATE of a specific atom through a reaction or biological pathway using a distinguishable physical signal (mass, radioactivity, or NMR/IR shift) without meaningfully perturbing the underlying chemistry itself.

Chemistry

Chemistry · L5 · Isotope Labeling & Tracer Studies

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Flashcards8 cards

What is the lanthanide contraction, and what causes it?1 / 8

The lanthanide contraction is the steady, cumulative decrease in ionic radius across the lanthanide series (La³⁺ to Lu³⁺) as the 4f subshell fills. Each added 4f electron shields the increasing nuclear charge poorly (4f orbitals have a diffuse, multi-lobed shape with significant electron density near the nucleus and poor mutual shielding), so the effective nuclear charge felt by the outer electrons rises steadily across the series, pulling the ion's outer shell inward and shrinking its radius.

Chemistry

Chemistry · L5 · Lanthanide & Actinide Chemistry

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Flashcards9 cards

What is mechanochemistry?1 / 9

Mechanochemistry is the study and use of mechanical force — grinding, milling, shearing, or ultrasound — to induce chemical reactions, either by driving reactants together without bulk solvent (mechanochemical synthesis) or by directly breaking or forming covalent bonds through applied stress (mechanochemical bond activation), as distinct from thermal or photochemical activation.

Chemistry

Chemistry · L5 · Mechanochemistry

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Flashcards9 cards

What is a metal-organic framework (MOF)?1 / 9

A MOF is a crystalline, porous solid built from metal ions or metal-oxo clusters (nodes) connected by multitopic organic ligands (linkers) into an extended, often highly ordered three-dimensional network, leaving a permanent internal pore system accessible to guest molecules once the pores are evacuated of synthesis solvent.

Chemistry

Chemistry · L5 · Metal-Organic Frameworks

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Flashcards9 cards

What is photoredox catalysis, in mechanistic terms?1 / 9

Photoredox catalysis uses a photocatalyst — typically a polypyridyl Ru(II) or Ir(III) complex, or an organic dye — that, upon absorbing visible light, reaches a long-lived triplet metal-to-ligand charge-transfer (³MLCT) excited state that is simultaneously a much stronger oxidant AND a much stronger reductant than the catalyst's ground state. That excited state then engages in single-electron transfer (SET) with a substrate, generating a radical ion intermediate that drives the net organic transformation, before the catalyst is regenerated to close the cycle.

Chemistry

Chemistry · L5 · Photoredox Catalysis

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Flashcards8 cards

Write the fundamental thermodynamic relationship connecting a protein-ligand binding constant to enthalpy and entropy.1 / 8

The binding free energy relates to the association constant Ka by ΔG = -RT ln Ka = ΔH - TΔS, where ΔH is the binding enthalpy (heat released or absorbed on complex formation) and ΔS is the binding entropy (change in disorder of the ligand, protein, and surrounding solvent). A more negative ΔG (tighter binding, larger Ka) can arise from a more favorable (more negative) ΔH, a more favorable (more positive) ΔS, or some combination — the same overall affinity can be reached through very different enthalpy/entropy balances.

Chemistry

Chemistry · L5 · Protein-Ligand Binding Thermodynamics

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Flashcards9 cards

What is a radical clock, and how is it used to measure the rate of an extremely fast radical process?1 / 9

A radical clock is a substrate containing a very fast, well-calibrated unimolecular radical rearrangement (classically the ring-opening of a cyclopropylcarbinyl radical, with a known rate constant on the order of 10⁸ s⁻¹) competing against a bimolecular reaction with an external trapping reagent whose concentration is known. By measuring the ratio of rearranged to unrearranged product as a function of trap concentration, the absolute rate constant of the bimolecular reaction can be calculated relative to the clock's known unimolecular rate — letting chemists measure radical reaction rates far too fast to time directly by any spectroscopic method.

Chemistry

Chemistry · L5 · Advanced Radical Chemistry

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Flashcards8 cards

What is the fundamental limitation of ensemble spectroscopy that single-molecule spectroscopy overcomes?1 / 8

Ensemble spectroscopy measures a signal averaged simultaneously over an enormous number (typically ~10²³) of molecules, so any heterogeneity in behavior between individual molecules, or any dynamics that are not perfectly synchronized across the population, is washed out into a single smooth average signal. Single-molecule spectroscopy isolates and observes ONE molecule (or a very small number) at a time, revealing the underlying distribution of behaviors and the exact time-trajectory of individual events that the ensemble average conceals.

Chemistry

Chemistry · L5 · Single-Molecule Spectroscopy

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Flashcards9 cards

What is spectroelectrochemistry (SEC)?1 / 9

SEC couples an electrochemical perturbation (a controlled potential or current) with a simultaneous spectroscopic measurement (UV-vis, IR, Raman, EPR, or NMR) of the same sample volume, so that the electronic or structural identity of a species can be correlated directly with its formal potential rather than inferred indirectly from a voltammetric peak shape alone.

Chemistry

Chemistry · L5 · Spectroelectrochemistry

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What is retrosynthetic analysis, and who formalized it?1 / 9

Retrosynthetic analysis, formalized by E. J. Corey, works backward from a target molecule by applying disconnections — the mental reverse of a known forward reaction — to generate simpler synthetic precursors, repeating until the precursors are commercially available or trivially accessible starting materials. It reframes synthesis planning as a search problem over a tree of possible disconnections rather than a forward guess-and-check process.

Chemistry

Chemistry · L5 · Total Synthesis Strategy

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Flashcards8 cards

What is femtochemistry, and why is the femtosecond timescale significant for chemical bonds?1 / 8

Femtochemistry is the study of chemical reaction dynamics on the femtosecond timescale (1 fs = 10⁻¹⁵ s), pioneered by Ahmed Zewail (1999 Nobel Prize in Chemistry). This timescale matters because it is comparable to the period of a single molecular vibration (roughly 10–100 fs for typical bond stretches), meaning femtosecond pulses are the only tool fast enough to directly capture the actual motion of atoms as a bond breaks or forms, rather than only the reactant and product states before and after.

Chemistry

Chemistry · L5 · Ultrafast Femtochemistry

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Flashcards8 cards

What is a zeolite, structurally, and where does its catalytic acidity come from?1 / 8

A zeolite is a crystalline, microporous aluminosilicate built from corner-sharing SiO4 and AlO4 tetrahedra arranged into a well-defined, molecular-scale pore and channel network. Because Al³⁺ substitutes isomorphically for Si⁴⁺ in the tetrahedral framework, each substitution leaves the framework with one extra negative charge that must be balanced by a cation; when that charge-balancing cation is a proton (H⁺), it sits on a framework oxygen bridging the Al and Si tetrahedra, creating a strong, well-defined Brønsted acid site (a bridging Si-OH-Al hydroxyl) directly within the pore structure.

Chemistry

Chemistry · L5 · Zeolite Catalysis

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Quiz5 questions

What structural feature does axial chirality arise from, as opposed to point chirality?

AA single tetrahedral stereocenter atom with four different substituentsBA chiral axis, where the spatial arrangement around a restricted-rotation bond is non-superimposable on its mirror image, with no single stereocenter atomCA planar ring system with no rotation at allDA double bond geometry described by E/Z descriptors

Chemistry

Chemistry · L5 · Atropisomerism & Axial Chirality

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Quiz5 questions

What defines the 'rocking-chair' mechanism of a lithium-ion battery?

ALi+ ions shuttle between two intercalation host structures, inserting/extracting without either host structure being consumed or fundamentally transformedBLithium metal is deposited and stripped directly at both electrodesCThe electrodes physically rock back and forth during operationDThe battery consumes its electrode materials irreversibly like a primary battery

Chemistry

Chemistry · L5 · Battery Chemistry & Materials

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Quiz3 questions

Which state of matter keeps its own shape?

ALiquidBGasCSolidDSteam

Chemistry

Chemistry · L1 · States of Matter

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Quiz2 questions

Which of these dissolves in water?

APebblesBSandCSaltDPlastic beads

Chemistry

Chemistry · L1 · Mixing and Dissolving

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Quiz6 questions

Which of these is a chemical change?

AMelting an ice cubeBTearing paperCToasting breadDFolding a towel

Chemistry

Chemistry · L1 · Everyday Chemical Changes

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Quiz5 questions

Which of these is true of most metals?

AThey are dullBThey are shinyCThey are stretchyDThey are see-through

Chemistry

Chemistry · L1 · Metals and Non-metals

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Quiz6 questions

A big ship floats, even though it is very heavy. Why?

AIt is not really heavy at allBIts shape traps air, so it is not packed together tightlyCShips cannot sinkDIt is made of wood

Chemistry

Chemistry · L1 · Floating and Sinking

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