Properties and Exciting Facts About 1945-84-2

Reference of 1945-84-2, Because enzymes can increase reaction rates by enormous factors and tend to be very specific, typically producing only a single product in quantitative yield, they are the focus of active research.you can also check out more blogs about 1945-84-2.

Reference of 1945-84-2, The transformation of simple hydrocarbons into more complex and valuable products via catalytic C–H bond functionalisation has revolutionised modern synthetic chemistry. 1945-84-2, Name is 2-Ethynylpyridine, SMILES is C1=C(N=CC=C1)C#C, belongs to pyridine-derivatives compound. In a article, author is Zhou, Qiaoqiao, introduce new discover of the category.

Selective deoxygenation of biomass volatiles into light oxygenates catalysed by S-doped, nanosized zinc-rich scrap tyre char with in-situ formed multiple acidic sites

In this paper, we report the unique properties and catalytic performance of scrap tyre char, an otherwise low-value waste derivative on the selective deoxygenation of biomass volatiles (derived from flash pyrolysis of lignocellulosic biomass at 500 degrees C) into value-added light oxygenates including furfural and phenol. Due to the inclusion of sulphur (S) and zinc oxide (ZnO) as additives during the prior tyre manufacturing process, the pyrolysis-derived tyre char is rich in both organically bound S and nano-sized zinc sulphide (ZnS) that are highly dispersed within the carbonaceous matrix. Detailed characterisations of the fresh and spent catalysts have been conducted to elaborate the unique mechanisms, upon the use of XPS, TEM-SAED, XAS, NEXAFS, Pyridine-FTIR, and NH3-TPD. As has been confirmed, multiple acidic sites are present in tyre char, including organically bound S serving as a weak Bronsted acid and nano-sized ZnS being a strong Lewis acid. The former acid is active for dehydration, whilst the latter one mainly catalyses the decarboxylation and decarbonylation reactions. Upon the interaction with adsorbed oxygen-bearing species and water molecules, the ZnS-centered active acid site can in-situ transform into a ZnSOx-centered super strong acid site, which is a Bronsted acid that is able to enhance the deoxygenation extent remarkably. More interestingly, the organic S enables an in-situ sulphidation of the less active ZnO on the catalyst surface, thereby enabling a continuous exposure of bio-oil vapour to the highly active ZnS and its sulphate derivative. This in turn enlarges the lifetime of the catalyst and its strong stability upon cyclic tests.

Reference of 1945-84-2, Because enzymes can increase reaction rates by enormous factors and tend to be very specific, typically producing only a single product in quantitative yield, they are the focus of active research.you can also check out more blogs about 1945-84-2.

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

Simple exploration of 503615-03-0

Reference of 503615-03-0, Enzymes are biological catalysts that produce large increases in reaction rates and tend to be specific for certain reactants and products. I hope my blog about 503615-03-0 is helpful to your research.

Reference of 503615-03-0, Enzymes are biological catalysts that produce large increases in reaction rates and tend to be specific for certain reactants and products. 503615-03-0, Name is 3-Morpholino-1-(4-nitrophenyl)-5,6-dihydropyridin-2(1H)-one, SMILES is [O-][N+](=O)C1=CC=C(C=C1)N1CCC=C(N2CCOCC2)C1=O, belongs to pyridine-derivatives compound. In a article, author is Hu, Guangcai, introduce new discover of the category.

Stable and Bright Pyridine Manganese Halides for Efficient White Light-Emitting Diodes

Highly efficient lead halide perovskites with tunable emission performance have become new candidate materials for light-emitting devices and displays; however, the toxicity of lead and instability of halide perovskites greatly limits their application. Herein, rapid and large-scale synthesis of highly emissive organic-inorganic manganese halide perovskites, (C5H6N)(2)MnBr4 and C5H6NMnCl3, are presented by a one-pot solution-based method, of which (C5H6N)(2)MnBr4 displays a high absolute photoluminescence quantum yield (95%) in the solid-state. The developed (C5H6N)(2)MnBr4 perovskite noticeably exhibits high stability. Therefore both as-synthesized green and red emissive manganese-based phosphors with superior optical properties are used to fabricate blue light pumped white light-emitting diodes (WLEDs), displaying excellent quality white light with a high color rendering index value of 91 and a correlated color temperature of 5331 K. This study not only presents the robust large-scale production synthetic approach for organic-inorganic manganese halide perovskites, but also facilitates the development of high-performance phosphors for future lighting and display technologies.

Reference of 503615-03-0, Enzymes are biological catalysts that produce large increases in reaction rates and tend to be specific for certain reactants and products. I hope my blog about 503615-03-0 is helpful to your research.

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

Never Underestimate The Influence Of Pyridin-4-ol

But sometimes, even after several years of basic chemistry education, it is not easy to form a clear picture on how they govern reactivity! 626-64-2, you can contact me at any time and look forward to more communication. Name: Pyridin-4-ol.

Reactions catalyzed within inorganic and organic materials and at electrochemical interfaces commonly occur at high coverage and in condensed media, causing turnover rates to depend strongly on interfacial structure and composition, 626-64-2, Name is Pyridin-4-ol, SMILES is OC1=CC=NC=C1, in an article , author is Bouroumane, Nadia, once mentioned of 626-64-2, Name: Pyridin-4-ol.

New Pyrazole-Based Ligands: Synthesis, Characterization, and Catalytic Activity of Their Copper Complexes

The purpose of this study is to demonstrate the synthesis of pyrazole-based ligands and to evaluate their catalytic properties in the oxidation reaction of catechol to o-quinone. The ligands were prepared via the condensation of (3,5-dimethyl-1H pyrazol-1-yl)methanol A with the appropriate primary amine. Four pyrazole-based ligands were successfully synthesized and characterized. These ligands provide one pyrazole sp(2)-nitrogen, one pyridine sp(2)-nitrogen, and one amine sp(3)-nitrogen, which were capable of coordinating to the metal. For evaluating the catalytic activity, the experiments were tested by varying the type of solvent, metal ion, anion in the metal salt, and ratios of ligands and metal salts. Excellent catalytic activities for the oxidation of catechol to o-quinone were obtained. The copper (II)-based complexes showed better reactions rates than those based on other metals (e.g., nickel, tin, and barium), which was due to the fact that the active catalytic site of the catecholase enzyme has two active sites from the existence of copper (II) ions. The composition ratios of ligands and metal salts as well as the type of anion in the metal salt bring impacts to the formation of complexes. We found also that the type of solvent contributes to the interaction and dilution of reactants in the solvent. This study demonstrated that the present ligands can be used as a model for further developments in catalytic processes relating to catecholase activity.

But sometimes, even after several years of basic chemistry education, it is not easy to form a clear picture on how they govern reactivity! 626-64-2, you can contact me at any time and look forward to more communication. Name: Pyridin-4-ol.

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

Brief introduction of 198904-85-7

Reference of 198904-85-7, Because enzymes can increase reaction rates by enormous factors and tend to be very specific, typically producing only a single product in quantitative yield, they are the focus of active research.you can also check out more blogs about 198904-85-7.

Reference of 198904-85-7, Redox catalysis has been broadly utilized in electrochemical synthesis due to its kinetic advantages over direct electrolysis. The appropriate choice of redox mediator can avoid electrode passivation and overpotential. 198904-85-7, Name is tert-Butyl 2-(4-(pyridin-2-yl)benzyl)hydrazinecarboxylate, SMILES is O=C(NNCC1=CC=C(C2=NC=CC=C2)C=C1)OC(C)(C)C, belongs to pyridine-derivatives compound. In a article, author is Heydari, Fereshteh, introduce new discover of the category.

Synthesis of a novel Pd supported polymeric magnetic nanoparticles with urea-pyridine bridge: application as an efficient catalyst for the C-C and C-N bond formation

Here a novel Pd supported polymeric magnetic nanoparticles with urea-pyridine bridge (denoted as Fe3O4@/Urea-Pyridine/Pd) was synthesized and characterized. The Fe3O4@/Urea-Pyridine/Pd nanocatalyst was synthesized via a four steps process by using Fe3O4 nanoparticles, 3-(triethoxysilyl) propylisocyanate (TESPIC), 2,6 bis(propyl-triethoxysilylureylene) pyridine (BPS) and palladium chloride. The synthesized polymeric Fe3O4@/Urea-Pyridine/Pd nanocatalyst was analyzed through different analytical techniques, including FT-IR, NMR, XRD, VSM, TGA, DTA, ICP, FESEM, EDX, and BET. The described palladium supported polymeric magnetic nanoparticles with urea-pyridine bridge (Pd-MNPs) was used for the C-C and C-N coupling of phenylboronic acid with various amines and aryl halides in DMF as well as Sonogashira and Suzuki reactions in aqueous solution. Also, the Fe3O4@/Urea-Pyridine/Pd nanocatalyst exhibited high structural stability and excellent recyclability.

Reference of 198904-85-7, Because enzymes can increase reaction rates by enormous factors and tend to be very specific, typically producing only a single product in quantitative yield, they are the focus of active research.you can also check out more blogs about 198904-85-7.

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

Top Picks: new discover of C6H5NO

Synthetic Route of 500-22-1, One of the oldest and most widely used commercial enzyme inhibitors is aspirin, which selectively inhibits one of the enzymes involved in the synthesis of molecules that trigger inflammation. you can also check out more blogs about 500-22-1.

Synthetic Route of 500-22-1, The transformation of simple hydrocarbons into more complex and valuable products via catalytic C–H bond functionalisation has revolutionised modern synthetic chemistry. 500-22-1, Name is 3-Pyridinecarboxaldehyde, SMILES is O=CC1=CC=CN=C1, belongs to pyridine-derivatives compound. In a article, author is Reddy Marri, Anil, introduce new discover of the category.

Record power conversion efficiencies for iron(ii)-NHC-sensitized DSSCs from rational molecular engineering and electrolyte optimization

Three Fe(ii) pyridylNHC-carboxylic heteroleptic complexes with (ARM7 and ARM11) or without spacers (ARM13) between the pyridine and the COOH anchoring group have been designed and characterized with the aim to increase the metal to surface charge separation and avoid undesired recombination processes in iron-sensitized DSCCs. The ARM13-sensitized DSSC scored the highest efficiency ever reported for an iron-sensitized solar cell (1.44%) providing that Mg2+ cations and NBu4I were present in the electrolyte, thus substantially boosting the photocurrent. The gain in efficiency derived from the use of MgI2-based electrolytes was rationalized by employing DFT calculations for the isolated dye sensitizers and dye/TiO2 interface models.

Synthetic Route of 500-22-1, One of the oldest and most widely used commercial enzyme inhibitors is aspirin, which selectively inhibits one of the enzymes involved in the synthesis of molecules that trigger inflammation. you can also check out more blogs about 500-22-1.

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

The important role of C5H5ClN2

Sometimes chemists are able to propose two or more mechanisms that are consistent with the available data. If a proposed mechanism predicts the wrong experimental rate law, however, the mechanism must be incorrect.Welcome to check out more blogs about 6298-19-7, in my other articles. Application In Synthesis of 2-Chloropyridin-3-amine.

Chemistry can be defined as the study of matter and the changes it undergoes. You’ll sometimes hear it called the central science because it is the connection between physics and all the other sciences, starting with biology. 6298-19-7, Name is 2-Chloropyridin-3-amine, molecular formula is , belongs to pyridine-derivatives compound. In a document, author is Xu, Longkun, Application In Synthesis of 2-Chloropyridin-3-amine.

Improving the Accuracy of PCM-UAHF and PCM-UAKS Calculations Using Optimized Electrostatic Scaling Factors

The optimal electrostatic scaling factor (ESF) for the calculation of solvation Gibbs free energies with the polarizable continuum model (PCM) was determined via extensive benchmarking against 1719 experimental solvation Gibbs free energies and transfer free energies taken from the MNSol-v2012 database, as well as 45 aqueous pK(a) values covering nine solute types (amines, thiols, carbon acid cations, pyridines, alcohols, anilines, carboxylic acids, carbon acid neutrals, phenols) and 20 pK(a) values in acetonitrile covering four solute types (phenols, carbon acids, carboxylic acids, pyridines). Optimal values for the ESF in a range of solvents are reported. For example, in water, the optimal ESF value is 1.2 and this does not vary with solute charge, radius type, or method. For acetonitrile, we recommend 1.1 for neutrals, 1.0 and 1.1 respectively for cations with UAHF and UAKS radii types, 1.3 for anions for IEFPCM-UAHF, and 1.4 for other anions. At the same time, we show that ESF optimization does not address all errors in PCM, and is thus not a substitute for the appropriate use of explicit solvent molecules, nor for the use of isodesmic methods to enhance systematic error cancellation. For a representative subset of the data we show that the errors in PCM are somewhat larger than in SMD and somewhat smaller than in COSMO-RS, although the latter has not benefited from cavity scaling.

Sometimes chemists are able to propose two or more mechanisms that are consistent with the available data. If a proposed mechanism predicts the wrong experimental rate law, however, the mechanism must be incorrect.Welcome to check out more blogs about 6298-19-7, in my other articles. Application In Synthesis of 2-Chloropyridin-3-amine.

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

What I Wish Everyone Knew About C15H17N3O4

But sometimes, even after several years of basic chemistry education, it is not easy to form a clear picture on how they govern reactivity! 503615-03-0, you can contact me at any time and look forward to more communication. COA of Formula: C15H17N3O4.

Reactions catalyzed within inorganic and organic materials and at electrochemical interfaces commonly occur at high coverage and in condensed media, causing turnover rates to depend strongly on interfacial structure and composition, 503615-03-0, Name is 3-Morpholino-1-(4-nitrophenyl)-5,6-dihydropyridin-2(1H)-one, SMILES is [O-][N+](=O)C1=CC=C(C=C1)N1CCC=C(N2CCOCC2)C1=O, in an article , author is Farmer, Kyle, once mentioned of 503615-03-0, COA of Formula: C15H17N3O4.

mGluR5 Allosteric Modulation Promotes Neurorecovery in a 6-OHDA-Toxicant Model of Parkinson’s Disease

Parkinson’s disease is a neurodegenerative disease characterized by a loss of dopaminergic substantia nigra neurons and depletion of dopamine. To date, current therapeutic approaches focus on managing motor symptoms and trying to slow neurodegeneration, with minimal capacity to promote neurorecovery. mGluR5 plays a key role in neuroplasticity, and altered mGluR5 signaling contributes to synucleinopathy and dyskinesia in patients with Parkinson’s disease. Here, we tested whether the mGluR5-negative allosteric modulator, (2-chloro-4-[2[2,5-dimethyl-1-[4-(trifluoromethoxy) phenyl] imidazol-4-yl] ethynyl] pyridine (CTEP), would be effective in improving motor deficits and promoting neural recovery in a 6-hydroxydopamine (6-OHDA) mouse model. Lesions were induced by 6-ODHA striatal infusion, and 30 days later treatment with CTEP (2 mg/kg) or vehicle commenced for either 1 or 12 weeks. Animals were subjected to behavioral, pathological, and molecular analyses. We also assessed how long the effects of CTEP persisted, and finally, using rapamycin, determined the role of the mTOR pathway. CTEP treatment induced a duration-dependent improvement in apomorphine-induced rotation and performance on rotarod in lesioned mice. Moreover, CTEP promoted a recovery of striatal tyrosine hydroxylase-positive fibers and normalized FosB levels in lesioned mice. The beneficial effects of CTEP were paralleled by an activation of mammalian target of rapamycin (mTOR) pathway and elevated brain-derived neurotrophic factor levels in the striatum of lesioned mice. The mTOR inhibitor, rapamycin (sirolimus), abolished CTEP-induced neurorecovery and rescue of motor deficits. Our findings indicate that mTOR pathway is a useful target to promote recovery and that mGluR5 allosteric regulators may potentially be repurposed to selectively target this pathway to enhance neuroplasticity in patients with Parkinson’s disease.

But sometimes, even after several years of basic chemistry education, it is not easy to form a clear picture on how they govern reactivity! 503615-03-0, you can contact me at any time and look forward to more communication. COA of Formula: C15H17N3O4.

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

Some scientific research about 3-Amino-6-chloropicolinamide

If you are hungry for even more, make sure to check my other article about 175358-01-7, Formula: C6H6ClN3O.

One of the major reasons for studying chemical kinetics is to use measurements of the macroscopic properties of a system, such as the rate of change in the concentration of reactants or products with time. 175358-01-7, Name is 3-Amino-6-chloropicolinamide, formurla is C6H6ClN3O. In a document, author is Deorukhkar, Neel, introducing its new discovery. Formula: C6H6ClN3O.

Tuning spin-crossover transition temperatures in non-symmetrical homoleptic meridional/facial [Fe(didentate)(3)](2+) complexes: what for and who cares about it?

The [FeN6] chromophores found in [Fe(didentate)(3)](2+) complexes, where didentate is a non-symmetrical 2-(6-membered-heterocyclic ring)-benzimidazole ligand (Lk), exist as mixtures of two geometrical mer (C-1-symmetry) and fac (C-3-symmetry) isomers. Specific alkyl-substituted six-membered heterocyclic rings connected to the benzimidazole unit (pyridines in ligands L1-L3, pyrazines in L4-L5 and pyrimidines in L6-L7) control the ligand field strength and the electron delocalization so that [Fe-II(Lk)(3)](2+) display tunable thermally-induced spin transitions in solution. Thermodynamic, spectroscopic (UV-Vis, NMR) and magnetic studies in solution demonstrate that [Fe(L6)(3)](2+) (L6 = 1-methyl-2-(pyrimidin-2-yl)-1H-benzo[d] imidazole) exhibits a close to room temperature spin transition (T-1/2 = 273(3) K) combined with a high stability formation constant (logo beta(Fe,L6)(1,3)) = 21.8(9) in acetonitrile), which makes this complex suitable for the potential modulation of lanthanide-based luminescence in polymetallic helicates. A novel method is proposed for assigning specific thermodynamic spin crossover parameters to fac-[Fe(L6)(3)](2+) and mer-[Fe (L6)(3)](2+) isomers in solution. The observed difference relies mainly on the entropic content Delta S-SCO(mer) – Delta S-SCO(fac) = 11(1) J mol(-1) K-1, which favors the spin transition for the meridional isomer. Intermolecular interactions occurring in the crystalline state largely overcome minor thermodynamic trends operating in diluted solutions and a single configurational isomer is usually observed in the solid state. Among the thirteen solved crystal structures 1-13 containing the [M(Lk)(3)](2+) cations (M = Fe, Ni, Zn, Lk = L6-L7), pure meridional isomers are observed six times, pure facial isomers also six times and a mixture (44% mer and 56% fac) is detected only once. Solid-state magnetic data recorded for the Fe-II complexes show the operation of slightly cooperative spin transitions in 7 (fac-[Fe(L6)(3)](2+)) and 12 (mer-[Fe(L7)(3)](2+)). For the meridional isomer in 6, a two-step spin state transition curve, associated with two phase transitions, is detected.

If you are hungry for even more, make sure to check my other article about 175358-01-7, Formula: C6H6ClN3O.

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

The Absolute Best Science Experiment for C6H7Br2N

Balanced chemical reaction does not necessarily reveal either the individual elementary reactions by which a reaction occurs or its rate law. In my other articles, you can also check out more blogs about 31106-82-8. Application In Synthesis of 2-(Bromomethyl)pyridine hydrobromide.

Enzymes are biological catalysts that produce large increases in reaction rates and tend to be specific for certain reactants and products. 31106-82-8, Name is 2-(Bromomethyl)pyridine hydrobromide, molecular formula is C6H7Br2N, belongs to pyridine-derivatives compound. In a document, author is Peng, Qingpo, introduce the new discover, Application In Synthesis of 2-(Bromomethyl)pyridine hydrobromide.

Synthesis of bio-additive fuels from glycerol acetalization over a heterogeneous Ta/W mixed addenda heteropolyacid catalyst

The heterogeneous heteropolyacid-catalyzed acetalization of glycerol with acetone was investigated under solvent-free conditions. In this work, the calcined Ta/W mixed addenda heteropolyacid catalyst ({H-20}-355) was employed as a solid acid catalyst and showed much higher activity than these soluble Keggin-type catalysts in the acetalization of glycerol with acetone due to its superacidity (H-0 =-12.95). Meanwhile, it showed high stability in catalytic recycles and extended for acetalization of glycerol with the other aldehydes and ketones. Based on the catalyst characterization by X-ray diffraction (XRD), scanning electron microscopy (SEM), P-31 NMR spectra, FT-IR spectra, pyridine-absorbed FT-IR, and Hammett acidity functions (H-0) by UV-vis spectroscopy, it was found that the acidic strength of the Ta/W mixed addenda heteropolyacid was highly dependent on the contents of crystalline water that could be tuned by calcination temperature. Notably, the solid superacid catalyst {H-20}355 was swollen by acetone, exhibiting an interesting pseudo-liquid behavior, which served as a microreactor and facilitated the reaction. Furthermore, after the acetalization reaction of glycerol with furfural (98% yield of acetal) on {H-20}-355 catalyst, the furan ring of the acetal products can be hydrogenated sequentially into dioxolane or dioxane with Pd/C catalyst under room temperature condition without the need of any isolation procedure.

Balanced chemical reaction does not necessarily reveal either the individual elementary reactions by which a reaction occurs or its rate law. In my other articles, you can also check out more blogs about 31106-82-8. Application In Synthesis of 2-(Bromomethyl)pyridine hydrobromide.

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem

Final Thoughts on Chemistry for 1122-54-9

Reference of 1122-54-9, Consequently, the presence of a catalyst will permit a system to reach equilibrium more quickly, but it has no effect on the position of the equilibrium as reflected in the value of its equilibrium constant.I hope my blog about 1122-54-9 is helpful to your research.

Reference of 1122-54-9, The transformation of simple hydrocarbons into more complex and valuable products via catalytic C–H bond functionalisation has revolutionised modern synthetic chemistry. 1122-54-9, Name is 4-Acetylpyridine, SMILES is C1=C(C(=O)C)C=CN=C1, belongs to pyridine-derivatives compound. In a article, author is Han, Ye-Min, introduce new discover of the category.

Two isolated Zn-epsilon-Keggin decorated by pyridine-imidazole with excellent electrocatalytic and third-order NLO properties

Based on {epsilon-(PMo8Mo4O40Zn4)-Mo-V-O-VI} POM units, two novel organic-inorganic hybrids, [TBA](2)[(H3PMo8Mo4O40Zn4)-Mo-V-O-VI](2)[4-pyim](6)center dot 7H(2)O (1) (4-pyim = 4-(imidazol-2-yl)pyridine) and [(H3PMo8Mo4O40Zn4)-Mo-V-O-VI][2-pyim](4)center dot 3H(2)O (2) (2-pyim = 2-(imidazol-2-yl)pyridine), have been triumphantly gained under the hydrothermal conditions via self-assembly method. Compound 1 is made up of two Zn-epsilon-Keggin units jointed by two Zn-O bonds, which modified by three 4-pyim ligands, respectively, to form a dimer, while compound 2 consists of a Zn-epsilon-Keggin unit coordinated with four 2-pyim ligands. The coordination mode of the different ligands causes a change in the number of ligands coordinated on each Zn-epsilon-Keggin unit, resulting in the formation of dimers and monomers. What’s more, the third-order non-linear optical (NLO) properties of the two compounds were detected in detail due to the existence of a large number of conjugated organic ligands. Compound 1 has a superiority over compound 2 in NLO performance and the two-photon absorption (TPA) cross section (sigma) are 7320 GM for 1 and 1573 GM for 2, which is attributed to the fact that 1 own more conjugated organic ligands. The NLO testing results of compound 1 and 2 show that Zn-epsilon-Keggin unit can provide a good NLO response through combining with conjugated organic ligands, indicating that it is a wally choice for NLO materials. Furthermore, the electrochemistry properties of two compounds were also amply explored.

Reference of 1122-54-9, Consequently, the presence of a catalyst will permit a system to reach equilibrium more quickly, but it has no effect on the position of the equilibrium as reflected in the value of its equilibrium constant.I hope my blog about 1122-54-9 is helpful to your research.

Reference:
Pyridine – Wikipedia,
,Pyridine | C5H5N – PubChem