Type Of Heteroatom Research Articles

N-derivatives of formaldimines: the reason for the high nitrogen inversion barriers in N-methyl- and N-chloroimines

The energy and electronic parameters of the nitrogen inversion in imines Н 2 С=NХН n (ХН n = СН 3 , NH 2 , OH, F, SiH 3 , PH 2 , SH, Cl) have been calculated with the DFT method (B3LYP 6-311+G(d,p)) in terms of natural bond orbital. It has been established that the interactions of the nitrogen lone pair (LP) with the bond orbitals at the imino carbon atom are practically independent of the X atom and contribute to the decrease of the inversion barriers (Δ Е і ≠ ). While nN®σ * X–H , nN«σ X–H and nN«nX interactions substantially depend on the heteroatom type and promote the increase in the Δ Е і ≠ values with the rise in electronegativity of the X atom. The contribution of the interactions of the nitrogen LP with the Rydberg orbitals of the C=N–X group atoms is small and they cannot be the main reason of the decrease in the Δ Е і ≠ values when X atoms of the second period are replaced by atoms of the third period of the same group. The interactions of the LP of the X atoms and the X–H bond orbitals with the C=N bond orbitals have the main influence on the inversion barriers. The contribution of nX®π * C=N interactions to the Δ Е і ≠ values is dominant. The main reason of the “anomalous” inversion barriers of N-methyl- and N-chloroformaldimines is the destabilization of inversion transition states because of the reduction in the energies of σ X–H ®π * C=N and nX→π * C=N interactions and the rise in the energies of nN«nСl interactions. The contributions of electronegativity of ХН n substituents and energies of intramolecular interactions to the Δ Е і ≠ values have been determined.

Advances and Trends in Chemically Doped Graphene

AbstractChemically doped graphene materials are fascinating because these have different desirable attributes with possible synergy. The inert and gapless nature of graphene can be changed by adding a small number of heteroatoms to substitute carbon in the lattice. The doped material may display superior catalytic activities; durable, fast, and selective sensing; improved magnetic moments; photoresponses; and activity in chemical reactions. In the current review, recent advances are covered in chemically doped graphene. First, the different types of heteroatoms, their bonding configurations, and briefly their properties are discussed. This is followed by the description of various synthesis and analytical methods essential for assessing the characteristics of heterographene with specific focus on the selected graphene materials of different dopants (particularly, single dopants, including N, B, S, P, first three halogens, Ge, and Ga, and codopants, such as N/O), and more importantly, up‐to‐date applications enabled by the intentional doping. Finally, outlook and perspectives section review the existing challenges, future opportunities, and possible ways to improve the graphitic materials. The goal is to update and inspire the readers to establish novel doped graphene with valuable properties and for current and futuristic applications.

Effect of Kerogen Thermal Maturity on Methane Adsorption Capacity: A Molecular Modeling Approach.

The presence of kerogen in source rocks gives rise to a plethora of potential gas storage mechanisms. Proper estimation of the gas reserve requires knowledge of the quantities of free and adsorbed gas in rock pores and kerogen. Traditional methods of reserve estimation such as the volumetric and material balance approaches are insufficient because they do not consider both the free and adsorbed gas compartments present in kerogens. Modified versions of these equations are based on adding terms to account for hydrocarbons stored in kerogen. None of the existing models considered the effect of kerogen maturing on methane gas adsorption. In this work, a molecular modeling was employed to explore how thermal maturity impacts gas adsorption in kerogen. Four different macromolecules of kerogen were included to mimic kerogens of different maturity levels; these were folded to more closely resemble the nanoporous kerogen structures of source rocks. These structures form the basis of the modeling necessary to assess the adsorption capacity as a function of the structure. The number of double bonds plus the number and type of heteroatoms (O, S, and N) were found to influence the final configuration of the kerogen structures, and hence their capacity to host methane molecules. The degree of aromaticity increased with the maturity level within the same kerogen type. The fraction of aromaticity gives rise to the polarity. We present an empirical mathematical relationship that makes possible the estimation of the adsorption capacity of kerogen based on the degree of polarity. Variations in kerogen adsorption capacity have significant implications on the reservoir scale. The general trend obtained from the molecular modeling was found to be consistent with experimental measurements done on actual kerogen samples. Shale samples with different kerogen content and with different maturity showed that shales with immature kerogen have small methane adsorption capacity compared to shales with mature kerogen. In this study, it is shown for the first time that the key factor to control natural gas adsorption is the kerogen maturity not the kerogen content.

NMR study of dyadic and triadic splitting in copoly(arylene)phthalides based on diphenyl oxide and diphenyl sulfide.

All 13 C NMR signals of the poly(arylene) polymers, O-1, S-7, OS-4, OOS-3, OOOS-2, SSO-5, and SSSO-6 (where O is a diphenyleneoxiphthalide unit and S is a diphenylenethiophthalide unit) in dyads and triads were assigned unequivocally with two-dimensional NMR techniques (ge-2D [1 H-1 H] COSY, ge-2D [1 H-13 C] HSQC, and ge-2D [1 H-13 C] HMBC), and for each atom, the increments of the shifts are determined. For structurally similar carbon atoms of the phthalide cycle and heteroaromatic fragments of the skeletal chain, additive signal splitting schemes in phthalide centered dyads and in diphenylene oxide and in diphenylene sulfide centered triads are considered, based on taking into account the contributions to their shielding of adjacent and distant substituents. It was shown that the nature of the splitting of the signals of each of the 20 carbon atoms in 3,3-bisphenylphthalide fragments is determined by the type of carbon atom (tertiary or quaternary, even or odd), the type of heteroatoms in adjacent heteroaromatic fragments, their distance from the identified carbon nucleus, and their polyad symmetry. The results obtained in this article will greatly facilitate our further studies and, in particular, will allow us to study the microstructure of statistical copolymers based on the asymmetric OS monomer at the dyad and triad levels.

Polymer-Derived Heteroatom-Doped Porous Carbon Materials.

Heteroatom-doped porous carbon materials (HPCMs) have found extensive applications in adsorption/separation, organic catalysis, sensing, and energy conversion/storage. The judicious choice of carbon precursors is crucial for the manufacture of HPCMs with specific usages and maximization of their functions. In this regard, polymers as precursors have demonstrated great promise because of their versatile molecular and nanoscale structures, modulatable chemical composition, and rich processing techniques to generate textures that, in combination with proper solid-state chemistry, can be maintained throughout carbonization. This Review comprehensively surveys the progress in polymer-derived functional HPCMs in terms of how to produce and control their porosities, heteroatom doping effects, and morphologies and their related use. First, we summarize and discuss synthetic approaches, including hard and soft templating methods as well as direct synthesis strategies employing polymers to control the pores and/or heteroatoms in HPCMs. Second, we summarize the heteroatom doping effects on the thermal stability, electronic and optical properties, and surface chemistry of HPCMs. Specifically, the heteroatom doping effect, which involves both single-type heteroatom doping and codoping of two or more types of heteroatoms into the carbon network, is discussed. Considering the significance of the morphologies of HPCMs in their application spectrum, potential choices of suitable polymeric precursors and strategies to precisely regulate the morphologies of HPCMs are presented. Finally, we provide our perspective on how to predefine the structures of HPCMs by using polymers to realize their potential applications in the current fields of energy generation/conversion and environmental remediation. We believe that these analyses and deductions are valuable for a systematic understanding of polymer-derived carbon materials and will serve as a source of inspiration for the design of future HPCMs.

Improving the Catalytic Activity of Carbon-Supported Single Atom Catalysts by Polynary Metal or Heteroatom Doping.

Single atom catalysts (SACs) are widely researched in various chemical transformations due to the high atomic utilization and catalytic activity. Carbon-supported SACs are the largest class because of the many excellent properties of carbon derivatives. The single metal atoms are usually immobilized by doped N atoms and in some cases by C geometrical defects on carbon materials. To explore the catalytic mechanisms and improve the catalytic performance, many efforts have been devoted to modulating the electronic structure of metal single atomic sites. Doping with polynary metals and heteroatoms has been recently proposed to be a simple and effective strategy, derived from the modulating mechanisms of metal alloy structure for metal catalysts and from the donating/withdrawing heteroatom doping for carbon supports, respectively. Polynary metals SACs involve two types of metal with atomical dispersion. The bimetal atom pairs act as dual catalytic sites leading to higher catalytic activity and selectivity. Polynary heteroatoms generally have two types of heteroatoms in which N always couples with another heteroatom, including B, S, P, etc. In this Review, the recent progress of polynary metals and heteroatoms SACs is summarized. Finally, the barriers to tune the activity/selectivity of SACs are discussed and further perspectives presented.

Toward Soluble 5,10-Diheterotruxenes: Synthesis and Reactivity of 5,10-Dioxatruxenes, 5,10-Dithiatruxenes, and 5,10-Diazatruxenes.

The following work presents three general approaches allowing, for the first time, the synthesis of 5,10-diheterotruxene derivatives containing two identical heteroatoms, namely, oxygen OOC, nitrogen NNC, or sulfur SSC. Two of described pathways involve the photocyclization of the corresponding triene 2 as a key step leading to a heptacyclic aromatic system. The third approach is based on the acidic condensation between ninhydrin 14 and benzo[b]heteroole 15. Typical functionalizations of the 5,10-diheterotruxene core have also been presented. In addition, the article discusses the advantages and limitations of the three suggested paths for receiving specific 5,10-diheterotruxene derivatives because the universal method suitable for obtaining molecules with any type of heteroatoms is not known so far.

Heteroatom-Doped Nanographenes with Structural Precision.

ConspectusNanographenes, which are defined as nanoscale (1–100 nm) graphene cutouts, include quasi-one-dimensional graphene nanoribbons (GNRs) and quasi-zero-dimensional graphene quantum dots (GQDs). Polycyclic aromatic hydrocarbons (PAHs) larger than 1 nm can be viewed as GQDs with atomically precise molecular structures and can thus be termed nanographene molecules. As a result of quantum confinement, nanographenes are promising for next-generation semiconductor applications with finite band gaps, a significant advantage compared with gapless two-dimensional graphene. Similar to the atomic doping strategy in inorganic semiconductors, incorporation of heteroatoms into nanographenes is a viable way to tune their optical, electronic, catalytic, and magnetic properties. Such properties are highly dependent not only on the molecular size and edge structure but also on the heteroatom type, doping position, and concentration. Therefore, reliable synthetic methods are required to precisely control these structural features. In this regard, bottom-up organic synthesis provides an indispensable way to achieve structurally well-defined heteroatom-doped nanographenes.Polycyclic heteroaromatic compounds have attracted great attention of organic chemists for decades. Research in this direction has been further promoted by modern interest in supramolecular chemistry and organic electronics. The rise of graphene in the 21st century has endowed large polycyclic heteroaromatic compounds with a new role as model systems for heteroatom-doped graphene. Heteroatom-doped nanographene molecules are in their own right promising materials for photonic, optoelectronic, and spintronic applications because of the extended π conjugation. Despite the significant advances in polycyclic heteroaromatic compounds, heteroatom-doped nanographene molecules with sizes of over 1 nm and their relevant GNRs are still scarce.In this Account, we describe the synthesis and properties of large heteroatom-doped nanographenes, mainly summarizing relevant advances in our group in the past decade. We first present several examples of heteroatom doping based on the prototypical nanographene molecule, i.e., hexa-peri-hexabenzocoronene (HBC), including nitrogen-doped HBC analogues by formal replacement of benzene with other heterocycles (e.g., aromatic pyrimidine and pyrrole and antiaromatic pyrazine) and sulfur-doped nanographene molecules via thiophene annulation. We then introduce heteroatom-doped zigzag edges and a variety of zigzag-edged nanographene molecules incorporating nitrogen, boron, and oxygen atoms. We finally summarize heteroatom-doped GNRs based on the success in the molecular cases. We hope that this Account will further stimulate the synthesis and applications of heteroatom-doped nanographenes with a combined effort from different disciplines.

Role of oxygen, nitrogen and sulfur functionalities on the surface of nanoporous carbons in CO2 adsorption: A critical review

Abstract In todays’ perspective, separation of CO2 from different gas streams is extremely important. Besides the key separation need of CO2 that lies with its isolation from N2 in flue gas, different other separation needs of CO2 includes natural gas separation, biogas upgrading, direct CO2 capture from air and purification other gas streams from the mixtures of CO2. The technical challenges of CO2 separation varies with the type of gas mixtures and CO2 concentration. Within different CO2 separation techniques, adsorptive separation of CO2 has gained attention owing to its low cost, regenerability and zero hazard. Among different types of adsorbents, nanoporous carbons are highly attractive because of their lowest cost, versatile synthesis protocol and efficient separation. In past several years, both computation and experimental results established that few selected heteroatoms, including oxygen, nitrogen and sulfur on the surface of porous carbon influences CO2 adsorption to a great extent compared to pristine carbons. This critical review summed up the synthesis procedure, characterization protocol and surface characteristics of oxygen, nitrogen and sulfur functionalized porous carbons, specific type of functionalities in each type of heteroatom that particularly influences CO2 adsorption and computational results as well as experimental findings that corroborated the positive role of heteroatoms in CO2 separation. At the end, few sets of critical comments and recommendations have been provided on the need of accurate analysis, exact interpretation of the role of heteroatoms in CO2 separation and future direction of research that might benefit the CO2 separation by heteroatom doped carbons in real-world.

Structural Behavior of Isolated Asphaltene Molecules at the Oil–Water Interface

Asphaltenes are the heaviest component of crude oil, causing the formation of a stable oil–water emulsion. Even though asphaltenes are known to behave as an emulsifying agent for emulsion formation, their arrangement at the oil–water interface is poorly understood. We investigated the effect of asphaltene structure (island type vs archipelago type) and heteroatom type (Oxygen-O, Nitrogen-N, and Sulfur-S) on their structural behavior in the oil–water system. Out of six asphaltenes studied here, only three asphaltenes remain at the oil–water interface while others are soluble in the oil phase. Molecular orientation of asphaltene at the interface, position, and angle of asphaltene with the interface has also been determined. We observed that the N-based island type asphaltene is parallel, while the O-based island type asphaltene and N-based archipelago type are perpendicular to the interface. These asphaltene molecules are anchored at the interface by the heteroatom. The S-based asphaltenes (both island and ...

Effect of Heteroatoms on Field-Induced Slow Magnetic Relaxation of Mononuclear FeIII ( S = 5/2) Ions within Polyoxometalates.

In this paper, the synthesis and magnetic properties of mononuclear FeIII-containing polyoxometalates (POMs) with different types of heteroatoms, TBA7H10[(A-α-XW9O34)2Fe] (IIX, X = Ge, Si; TBA = tetra- n-butylammonium), are reported. In these POMs, mononuclear highly distorted six-coordinate octahedral [FeO6]9- units are sandwiched by two trivacant lacunary units [A-α-XW9O34]10- (X = Ge, Si). These POMs exhibit field-induced slow magnetic relaxation based on the single high-spin FeIII magnetic center ( S = 5/2). Combining experiment and ab initio calculations, we investigated the effect of heteroatoms of the lacunary units on the field-induced slow magnetic relaxation of these POMs. By changing the heteroatoms from Si (IISi) to Ge (IIGe), the coordination geometry around the FeIII ion is mildly changed. Concretely, the axial Fe-O bond length in IIGe is shortened compared with that in IISi, and consequently the distortion of the [FeO6]9- unit in IIGe from the ideal octahedral coordination geometry becomes larger than that in IISi. The effective demagnetization barrier of IIGe (11.4 K) is slightly larger than that of IISi (9.2 K). Multireference ab initio calculations predict zero-field splitting parameters in good agreement with experiment. Although the differences in the coordination geometries and magnetic properties of IIGe and IISi are quite small, ab initio calculations indicate subtle changes in the magnetic anisotropy which are in line with the observed magnetic relaxation properties.

Preparation and Study of Multi‐Heteroatom Carbon Nanotube as Excellent Electrocatalyst for Oxygen Reduction Reaction Using Polydopamine Derivative

F and other heteroatom codoped multiwalled carbon nanotubes as nonmetal electrocatalyst were developed through pyrolysis of polydopamine derivative under high temperature. The influence of the amount and type of heteroatom on the catalytic activity was investigated. Especially, N/S/F‐codoped carbon nanotubes exhibit the most excellent electrocatalytic activity for oxygen reduction reaction and stability. The method afforded an excellent building block to universally design multi‐heteroatom‐doped or F‐doped carbon materials for ORR or other energy‐relevant applications.

N-Substituted 1H-Pyrimidin-4-One Derivatives Possessing Anxiolytic Activity

The logical framework approach was implemented for designing CNS-active N-substituted 1H-pyrimidin-4-ones. Formation energies of possible ligand—receptor complexes with D2-dopamine and GABAA Receptors were determined from molecular docking calculations. The results allowed the probability of psychotropic activity manifesting in the modeled compounds to be estimated. The most promising compounds were prepared by a modified synthetic procedure. The effect of the heteroatom type in N-heterocyclic 1H-pyrimidin-4-ones on the product yield was studied. Compounds I – IV and VI at a dose of 50 mg/kg exhibited anticonflict activity. The mercaptotriazole derivative of pyrimidin-4-one had the greatest anticonflict activity.

Molecular docking studies of fused coumarin derivatives as inhibitors of GlcN-6

ABSTRACTThe biological activity of heterocyclic compounds depends on their structure, the type of hetero atoms in the ring and on the type of substituents present. In this paper, some heterocyclic compounds with coumarin moieties S1–S5 and novobiocin known as coumarin antibiotic were subjected to the molecular docking studies as important tools for drug discovery. Glucosamine-6-phosphate synthase is selected as a suitable target in this study. In silico studies reveal that all synthesized compounds S1–S5 are good inhibitors of GlcN-6 and the docking results are in agreement with in vitro antibacterial evaluation of compounds S1–S5

Comparing the effects of different oxygen-containing functional groups on sulfonamides adsorption by carbon nanotubes: Experiments and theoretical calculation

Combined with batch adsorption experiments of sulfadimethoxine (SDM), sulfamethizole (SMZ), sulfamethazine (SMT), sulfamethoxazole (SMX) by carbon nanotubes (CNTs) with different oxidation degree, this study investigated the effect of different oxygen-containing functional groups (COC, OH, CO, COO−) on sulfonamides (SAs) adsorption by CNTs using density functional theory (DFT) calculation. The experimental results showed that the SAs adsorption decreased with increasing oxidation degree of CNTs, and followed the order of SDM>SMZ>SMT>SMX. The π-π interactions of the benzene and heterocyclic rings in SAs with the benzene-type rings of CNTs were the predominant adsorption mechanisms. DFT calculation showed that the oxygen-containing functional groups inhibited SAs adsorption, and the inhibitory effects declined in the order of COO−>CO>OH>COC. The COO− and CO groups could damage the ring structure of CNTs while OH and COC only damaged the conjugated structure of benzene-type rings of CNTs if they were located on the basal plane of CNTs. This disabled the π-π interactions of the damaged benzene-type ring and weakened the π-π interactions of the adjacent rings with SAs. However, the functional groups could only weaken the π-π interactions between CNTs and SAs if they were located on the edge of CNTs. The extension degree of V-shaped SAs and the type of heteroatoms in heterocyclic groups of SAs led to their different adsorption by CNTs. The results highlight the type of oxygen-containing functional groups is great importance in their effects on pollutants adsorption by CNTs.

Aromaticity and conformational flexibility of five-membered monoheterocycles: pyrrole-like and thiophene-like structures

Aromaticity and conformational flexibility of the series of five-membered monoheterocycles with group 14–16 heteroatoms, having one or two lone pairs, were studied with ab initio methods using NICS, ASE and I5 indices. For non-planar molecules like phosphole, aromaticity of their planar transition states was also studied, and a special modification of ASE index was proposed to that end. It was found that the presence of two lone pairs is generally preferable for aromaticity of all heterocycles except CPD and silolyl dianions. Heterocycles with group 16 heteroatoms have consistently lower aromaticity compared to other groups. A lot of structures should be classified as moderate aromatic and non-aromatic. Energies of out-of-plane deformation do not correlate with other indices studied, but reveal the same qualitative trends. Generally, aromaticity of five-membered monoheterocycles depends strongly on both heteroatom type and number of lone pairs on it.

Theoretical Study of Electronic Spectra of Lignite Structural Units

Electronic spectra of the lignite structural units and the effects of structural modification were explored by extensive density functional calculations. Based on calibration calculations on six representative compounds by different functionals,the feasibility of both B3 LYP and HSEH1 PBE for this kind of systems was validated. Predicted absorption spectra indicate that the main bands appear in near Ultraviolet-Visible region,while for the lignite structural unit with the extended conjugated components the λmaxabsorption occurs in visible region. Note that there are quite strong absorptions in the visible region of 460—480 nm as the side conjugated chain is linked to the structural unit through C C. The types of hetero-atoms and their positions show remarkable effects on the electronic absorption features,and the frontier orbitals in the structural units with a symmetric hetero-atom are well delocalized,especially for LUMO,resulting in notable red-shifted absorption ofλmax. The predicted spectroscopic properties of the lignite structural units provide a basis to understand their features of light response.

Noble-metal-free electrocatalysts with enhanced ORR performance by task-specific functionalization of carbon using ionic liquid precursor systems.

The synthesis and characterization of functionalized carbon using variable doping profiles are presented. The hybrids were obtained from nitrile-functionalized ionic precursors and a ferric chloride mediator. This way, novel nitrogen doped and nitrogen-sulfur, nitrogen-phosphorus, and nitrogen-boron codoped carbon hybrids with a morphology containing microporous nanometer-sized particles were obtained. As-prepared heteroatom doped carbons exhibited superior electrocatalytic activity toward the oxygen reduction reaction (ORR) in alkaline and acid electrolytes. In particular, both the heteroatom type and iron were found to play crucial roles in improving the catalytic activity of functionalized carbon. It is worth noting that sulfur-nitrogen codoped functionalized materials synthesized in the presence of ferric chloride showed higher activity and stability in comparison to those of the commercial state-of-the-art Pt catalyst in alkaline electrolyte. Moreover, in acid electrolyte, sulfur-nitrogen codoped catalyst rivaled the activity of Pt with a stability outperforming that of Pt. Our X-ray photoelectron spectroscopy (XPS) investigation revealed a distinctive atomic structure in nitrogen-sulfur codoped material in comparison to other codoped catalysts, most likely explaining its superior electrocatalytic activity. This work presents a novel toolbox for designing advanced carbon hybrids with variable heteroatom doping profiles which presents tunable and enhanced ORR performance.

Structure-reactivity relationship in catalytic hydrogenation of heterocyclic compounds over ruthenium black; Part B: Effect of carbon substitution by heteroatom

The effect of the type of heteroatom in the structure on the recyclability of possible candidate compounds for application as LOC (liquid organic carriers) was studied by comparing the rate and selectivity obtained in hydrogenation of carbazole, dibenzothiophene, dibenzofuran and fluorene. The effect of a partial saturation of the compound on its hydrogenation yield and reaction pathway was also considered by studying hydrogenation of 1,2,3,4-tetrahydrocarbazole. Using Ru black catalyst, the rate of hydrogenation was found to decrease in order; dibenzofuran>1,2,3,4-tetrahydrocarbazole>carbazole>>fluorene. No reaction was observed in the hydrogenation of dibenzothiophene under the conditions studied which was attributed to the immediate poisoning of ruthenium metal by sulphur compounds. The rate of hydrogenation of fluorene was around 3 times lower as compared to carbazole and over 8 times lower as compared to that of dibenzofuran under the same reaction conditions. With the exeption of S containing dibenzothiophene, the presence of the heteroatom in the structure was found to be beneficial in terms of increasing the rate of hydrogen loading step. Additionally, a higher reaction rate was obtained in the hydrogenation of the partially saturated 1,2,3,4-tetrahydrocarbazole as compared to the substrate carbazole. The structure and stability of intermediates was found to be significantly influenced by the type and presence of a heteroatom in the structure. A stable octahydro-intermediate was observed only with N-heterocycles, whereas a stable hexahydro-intermediate was produced in the polyaromatic hydrocarbon-fluorene. Additionally, the theoretically obtained lowest total enthalpies using DFT calculations agreed well with the stable intermediates observed experimentally in the hydrogenation of fluorene. Theoretical DFT differences in enthalpies also indicated the products of hydrogenolysis of perhydro-dibenzofuran to be the most favourable products of its hydrogenation, which agreed well with the experimental observations. Overall, taking into account the recyclability of LOC, substitution of carbon with a N heteroatom was demonstrated to be one of the promising approaches to improve the kinetics of the hydrogen loading step.

Intuitive ordering of scaffolds and scaffold similarity searching using scaffold keys.

Scaffold Keys-scaffold descriptors based on simple topological parameters such as number of ring and chain atoms, number and type of heteroatoms, and other simple structural features-are presented. Scaffold Keys enable intuitive ordering of scaffolds from small and simple to large and complex, ordering that is consistent with the way medicinal chemists themselves classify scaffolds. Scaffold Keys may be also used as descriptors for scaffold similarity searches, providing results compatible with expectations of chemists and well-suited for use in scaffold bioisosteric replacement and scaffold hopping. Scaffold Keys also support visualization of large chemical data sets. Scaffold Keys descriptors are easy to understand by chemists as well as easy to implement.