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James T.C. Liu, who was an international authority on the politics and history of China, particularly on the Sung dynasty, died in Lawrenceville, NJ, on Sep 30, 1993, aged 74. He served as a teacher in Chinese studies in the University of Pittsburgh, Stanford University and Princeton University and inspired the establishment of the Inter-University Program for Chinese Language Studies in Taipei, Taiwan. Liu performed the role of a cultural ambassador and worked to enhance the cultural exchanges between China and the US. He established his reputation as an eminent authority on Sung history with the publication of his book 'Reform in Sung China: Wang An-shih and His New Policies.'

Anefficient method for the preparation of 5,6-dihydropyrazolo[1,5-c]quinazolines via gold(I)-catalyzed chemoselectivebicyclization of N-propargylic sulfonylhydrazoneshas been developed. This process relies on the chemoselective cyclizationof the hydrazone nitrogen instead of the usually favored aniline nitrogenonto the alkyne. The synthetic utility of the current strategy isdemonstrated through the synthesis of a potential Eg5/Kinesin spindleprotein inhibitor. [ABSTRACT FROM AUTHOR]

A highly efficient Pt-catalyzed [3,3]sigmatropic rearrangement/cyclizationcascade of N-propargylhydrazones is reported.The process provides expedient access to a variety of highly functionalizedpyrazoles. The substrate has good substituted group compatibility,and the bioactive 3-CF3pyrazoles could be synthesizedeasily with this method. [ABSTRACT FROM AUTHOR]

An efficient Cs2CO3-promoted synthesis of α-amino ketones using hydrazines, aldehydes, and α-haloketones as starting materials through a cascade condensation/nucleophilic substitution/NN bond cleavage route is developed. The carbonyl group plays a key role in this novel NN bond cleavage process. [ABSTRACT FROM AUTHOR]

A new method for the stereoselective synthesis of highly substituted (1 E,3 E)-2-sulfonyl-1,3-dienes from N-propargylic sulfonohydrazone derivatives has been developed via copper(I)-catalyzed [3,3] rearrangement and highly regioselective migration of the sulfonyl group. [ABSTRACT FROM AUTHOR]

The development of atom- and step-economical methods for converting hazardous CS2 into harmless chemicals is a challenging endeavor. Herein, we disclose a feasible strategy for the electrochemical dehydrogenative coupling of CS2 and amines with β-ketoesters, which leads to dithiocarbamate intermediates in the presence of bases. In addition, inexpensive starting materials, broad substrate scope, and compatibility with natural product moieties make this efficient and sustainable reaction practical. [ABSTRACT FROM AUTHOR]

Static chamber-GC technique was employed to study the effects of different treatment winter cover crops, including no-tillage and directly sowing ryegrass (T1), no-tillage and directly sowing Chinese milk vetch (T2 ), tillage and transplanting rape (T3 ), no-tillage and directly sowing rape (T4), and fallowing (CK), on the CH4 and N2O emission from double cropping rice paddy field. During the growth period of test winter cover crops, the CH4 and N2O emission in treatments T1 -T4 was significantly higher than that in CK (P<0.01). Treatments T1 and T3 not only had the largest CH4q emission (0.60 and 0.88 g·m-2 ), but also had the largest N2O emission (0.20 and 0.23 g·m-2, respectively). After the winter cover crops returned to field, the CH4 emission from early and late rice fields in treatments T1, T2, T3, and T4 was larger than that in CK. In early rice field, treatments T1 and T2 had the largest CH4 emission (21.70 and 20.75 g·m-2); while in late rice field, treatments T3 and T4 had the largest one (58.90 and 54.51 g·m-2, respectively). Treatments T1 -T4 also had larger N2O emission from early and late rice fields than the CK did. The N2O emission from early rice field in treatments T1, T2, T3, and T4 was increased by 53.7%, 12.2%, 46.3%, and 29.3%, and that from late rice field in corresponding treatments was increased by 28.6%, 3.8%, 34.3%, and 27.6%, respectively, compared with CK. [ABSTRACT FROM AUTHOR]

Dithiocarbamate is a key structural sequence in pharmaceuticals and agrochemicals, and its synthesis is crucial in organic chemistry. Although significant progress has been made in related synthesis research, developing a practical and universal synthesis method remains fascinating. Herein, we report a new visible‐light‐induced decarboxylation coupling reaction between N‐hydroxyphthalimide esters and tetraalkylthiuram disulfides, which uses Ir(ppy)3 as a photocatalyst to promote the generation of corresponding decarboxylation thioacylation product‐dithiocarbamates in high yields. This redox‐neutral protocol uses inexpensive and readily available starting material under mild reaction conditions, exhibiting broad substrate scope and wide functional group compatibility. This method can be further used for post modification of complex natural products and bioactive drugs. [ABSTRACT FROM AUTHOR]

A convenient and practical strategy was developed for the synthesis of carbamo(dithioperoxo)thioates through the simple dehydrogenation coupling of thiols and amines and the insertion of CS2 under mild reaction conditions and in the absence of oxidizing reagents. This environmentally friendly electrosynthesis method has a broad substrate scope and can produce a series of bioactive compounds. This strategy is further employed in an electrochemical continuous flow reactor for the preparation of some antifilarial activity compounds, implying its potential application. [ABSTRACT FROM AUTHOR]

Homogeneous electrocatalysts can indirect oxidate the high overpotential substrates through single-electron transfer on the electrode surface, enabling efficient operation of organic electrosynthesis catalytic cycles. However, the problems of this chemistry still exist such as high dosage, difficult recovery, and low catalytic efficiency. Single-atom catalysts (SACs) exhibit high atom utilization and excellent catalytic activity, hold great promise in addressing the limitations of homogeneous catalysts. In view of this, we have employed Fe-SA@NC as an advanced redox mediator to try to change this situation. Fe-SA@NC was synthesized using an encapsulation-pyrolysis method, and it demonstrated remarkable performance as a redox mediator in a range of reported organic electrosynthesis reactions, and enabling the construction of various C-C/C-X bonds. Moreover, Fe-SA@NC demonstrated a great potential in exploring new synthetic method for organic electrosynthesis. We employed it to develop a new electro-oxidative ring-opening transformation of cyclopropyl amides. In this new reaction system, Fe-SA@NC showed good tolerance to drug molecules with complex structures, as well as enabling flow electrochemical syntheses and gram-scale transformations. This work highlights the great potential of SACs in organic electrosynthesis, thereby opening a new avenue in synthetic chemistry., (© 2024 Wiley-VCH GmbH.)

An electrochemical facilitated three-component trifluoromethylation/spirocyclization reaction of N -(arylsulfonyl)acrylamides, CF 3 SO 2 Na, and H 2 O has been developed. Without the requirement of chemical oxidants, a number of unexplored trifluoromethylated 4-azaspiro[4.5]decanes were obtained in satisfactory yields under mild conditions. This work provides a new synthetic strategy for fluorine-containing spirocyclic compounds and shows a new perspective for the reactivity study of N -(arylsulfonyl)acrylamides.

Industrial waste gas is one of the major sources of atmospheric CO 2 , yet the direct conversion of the low concentrations of CO 2 in waste gases into high value-added chemicals have been a great challenge. Herein, a copper-based N-heterocyclic carbene porous polymer catalyst (Cu@NHC-1) for the direct conversion of low concentration CO 2 into oxazolidinones was successfully fabricated via a facile copolymerization process followed by the complexation with Cu(OAc) 2 . A continuous flow device was designed to deliver a continuous and stable carbon source for the reaction. Due to the triple synergistic effect of its porous structure, nitrogen activation sites and catalytic Cu center, Cu@NHC-1 shows highly efficient and selective adsorption, activation, and conversion of the low concentration CO 2 (30 vol%). Its practical application potential is demonstrated by the ability to successfully convert the CO 2 in lime kiln waste gas into oxazolidinones in satisfactory yields under mild conditions., (© 2023 Wiley-VCH GmbH.)

Azoles and organoselenium compounds are pharmacologically important scaffolds in medicinal chemistry and natural products. We developed an efficient regioselective electrochemical aminoselenation reaction of 1,3-dienes, azoles, and diselenide derivatives to access selenium-containing allylazoles skeletons. This protocol is more economical and environmentally friendly and features a broad substrate scope; pyrazole, triazole, and tetrazolium were all tolerated under the standard conditions, which could be applied to the expedient synthesis of bioactive molecules and in the pharmaceutical industry.

During the chlor-alkali process, in operation since the nineteenth century, electrolysis of sodium chloride solutions generates chlorine and sodium hydroxide that are both important for chemical manufacturing 1-4 . As the process is very energy intensive, with 4% of globally produced electricity (about 150 TWh) going to the chlor-alkali industry 5-8 , even modest efficiency improvements can deliver substantial cost and energy savings. A particular focus in this regard is the demanding chlorine evolution reaction, for which the state-of-the-art electrocatalyst is still the dimensionally stable anode developed decades ago 9-11 . New catalysts for the chlorine evolution reaction have been reported 12,13 , but they still mainly consist of noble metal 14-18 . Here we show that an organocatalyst with an amide functional group enables the chlorine evolution reaction; and that in the presence of CO 2 , it achieves a current density of 10 kA m -2 and a selectivity of 99.6% at an overpotential of only 89 mV and thus rivals the dimensionally stable anode. We find that reversible binding of CO 2 to the amide nitrogen facilitates formation of a radical species that plays a critical role in Cl 2 generation, and that might also prove useful in the context of Cl - batteries and organic synthesis 19-21 . Although organocatalysts are typically not considered promising for demanding electrochemical applications, this work demonstrates their broader potential and the opportunities they offer for developing industrially relevant new processes and exploring new electrochemical mechanisms., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

The electrocatalytic ring-opening dihydroalkoxylation of N -aryl maleimides with alcohols under metal- and oxidant-free conditions is described. This electrochemical process consists of anodic single-electron transfer oxidation, cathodic radical reduction, rearrangement-ring cleavage and nucleophilic addition cascade, which employs tetrabutylammonium bromide not only as a redox catalyst but also as an efficient supporting electrolyte, and offers a practical and environmentally friendly route to ring-opening difunctionalization products.

CO 2 is an important C1 resource. We report a method for the synthesis of pharmacologically active 2-oxazolidinones by reacting CO 2 with allylic amines. As opposed to previous addition reactions, the unsaturated double bonds are preserved. Thus, the product is more plastic and easier to use for subsequent structural modification. Furthermore, this method can also be applied to the synthesis of six-membered heterocycles (1,3-oxazinan-2-ones) and the participation of a low concentration of CO 2 , indicating it has certain practicability.

Improving the stability of sensitive catalytic systems is an emerging research topic in the catalysis field. However, the current design of heterogeneous catalysts mainly improves their catalytic performance. This paper presents a single-atom catalyst (SAC) strategy to improve the cobalt-catalysed fluorination of acyl chlorides. A stable Co-F intermediate can be formed through the oxidative fluorination of Co 1 -N 4 @NC SAC, which can replace the unstable high-valent cobalt catalytic system and avoid the use of phosphine ligands. In the SAC system, KF can be employed as a fluorinating reagent to replace the AgF, which can be applied to various substrates and scale-up conversion with high turnover numbers (TON=1.58×10 6 ). This work also shows that inorganic SACs have tremendous potential for organofluorine chemistry, and it provides a good reference for follow-up studies on the structure-activity relationship between catalyst design and chemical reaction mechanisms., (© 2022 Wiley-VCH GmbH.)

An efficient one-pot reaction for the synthesis of oxoaporphine alkaloids has been developed. Twenty-three compounds of oxoaporphine alkaloids were prepared and assessed for their antitumor activities. Most compounds inhibited the growth of T-24 tumor cells in vitro. Particularly, 4B displayed the most potent activity with an IC 50 value of 0.5 μM, which was 19-fold more potent than the parent compound 4. The substitution at C3-position of oxoaporphine core by -NO 2 significantly enhanced the anticancer activity. Mechanism studies indicated that 4 and 4B induced cell cycle arrest at G2/M phase; in contrast, 4V induced cell cycle arrest at the S phase. Increase of mitochondrial ROS/Ca 2+ and decrease of MMP, accompanied by activation of caspase-3/9, were observed in T-24 cells after exposure to compounds 4, 4B and 4V, suggesting that the mitochondrial pathway was involved in the induced apoptosis. Moreover, compound 4B effectively inhibited tumor growth in a mouse xenograft model bearing T-24., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Masson SAS. All rights reserved.)

Ethnopharmacological Relevance: Huangkui capsule (HKC), extracted from Abelmoschus manihot (L.) medic (AM), as a patent proprietary Chinese medicine on the market for approximately 20 years, has been clinically used to treat chronic glomerulonephritis. Renal fibrosis has been implicated in the onset and development of diabetic nephropathy (DN). However, the potential application of HKC for preventing DN has not been evaluated., Aim of the Study: This study was designed to investigate the efficacy and underlying mechanisms of HKC combined with metformin (MET), the first-line medication for treating type 2 diabetes, in the treatment of renal interstitial fibrosis., Materials and Methods: A rat model of diabetes-associated renal fibrosis was established by intraperitoneal injection of streptozotocin (STZ, 65 mg/kg) combined with a high-fat and high-glucose diet. The rats were randomly divided into five groups: normal control, DN, HKC (1.0 g/kg/day), MET (100 mg/kg/d), and HKC plus MET (1.0 g/kg/day + 100 mg/kg/d). Following drug administration for 8 weeks, we collected blood, urine, and kidney tissue for analysis. Biochemical markers and metabolic parameters were detected using commercial kits. Histopathological staining was performed to monitor morphological changes in the rat kidney. High-glucose-induced human kidney HK-2 cells were used to evaluate the renal protective effects of HKC combined with MET (100 μg/mL+10 mmol/L). MTT assay and acridine orange/ethidium bromide were used to examine cell proliferation inhibition rates and apoptosis. Immunofluorescence assay and Western blot analysis were performed to detect renal fibrosis-related proteins including Klotho, TGF-β1, and phosphorylated (p)-p38., Results: Combination therapy (HKC plus MET) significantly improved the weight, reduced blood glucose (BG), blood urea nitrogen (BUN), total cholesterol (T-CHO), triglycerides (TG), low-density lipoprotein (LDL) and increased the level of high-density lipoprotein (HDL) of DN rats. Combination therapy also significantly reduced urine serum creatinine (SCR) and urine protein (UP) levels as well as reduced the degrees of renal tubule damage and glomerulopathy in DN rats. Combination therapy ameliorated renal fibrosis, as evidenced by reduced levels of alpha-smooth muscle actin and fibronectin and increased expression of E-cadherin in the kidneys. Moreover, HKC plus MET alleviated the degree of DN in part via the Klotho/TGF-β1/p38MAPK signaling pathway. In vitro experiments showed that combination therapy significantly inhibited cell proliferation and apoptosis and regulated fibrosis-related proteins in high-glucose (HG)-induced HK-2 cells. Further studies revealed that combination therapy suppressed cell proliferation and fibrosis by inhibiting the Klotho-dependent TGF-β1/p38MAPK pathway., Conclusions: HKC plus MET in combination suppressed abnormal renal cell proliferation and fibrosis by inhibiting the Klotho-dependent TGF-β1/p38MAPK pathway. Collectively, HKC combined with MET effectively improved DN by inhibiting renal fibrosis-associated proteins and blocking the Klotho/TGF-β1/p38MAPK signaling pathway. These findings improve the understanding of the pathogenesis of diabetes-associated complications and support that HKC plus MET combination therapy is a promising strategy for preventing DN., (Copyright © 2021 Elsevier B.V. All rights reserved.)

A metal-free and efficient visible-light-induced spirocyclization of indolyl-ynones with diselenides at room temperature under air atmosphere to prepare 3-selenospiroindolenines in moderate to good yields has been developed. The resulting products were tested for in vitro anticancer activity by MTT assay, and compounds 3 c and 3 e showed potent cancer cell-growth inhibition activities., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Heterogeneous, metal, single-site catalysts often exhibit higher catalytic performance than other catalysts because of their maximized atom efficiency of 100 %. Reported herein is a precoordination/solvothermal polymerization strategy to fabricate a stable mononuclear Pd-metalized porous organic polymer catalyst (Pd@POP). Pd@POP was easy to use in regioselective organic reactions because the internal structure of this Pd@POP can be easily modified. The catalyst was used to solve the intractable regioselectivity problems of Heck reactions. Pd@POP-9 can efficiently activate the ends of olefins, thereby leading to high selectivity for substitution at the external position. To understand the reason underlying the high selectivity and activity of the catalyst, the systemic characterization of Pd@POP-9 and density-functional theory calculations were conducted. This Heck reaction is the first to be catalyzed by a recyclable mononuclear metal catalyst with unprecedented catalytic activity and regioselectivity., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A Co(acac) 2 /POL-Xantphos@10PPh 3 -catalyzed hydrosilylation of unsymmetrical internal alkynes with Ph 2 SiH 2 has been developed for the synthesis of highly selective syn-α-vinylsilane products. Furthermore, terminal alkynes were also used and gave the products with excellent regioselectivity and a wide functional group tolerance. Because this porous organic polymer combines the selectivity and activity merits of Xantphos with the stability advantage derived from the high concentration of PPh 3 , the Co(acac) 2 /POL-Xantphos@10PPh 3 can be recycled multiple times without loss of activity and selectivity. This heterogeneous catalyst is expected to find promising applications in industrial synthesis., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Breast cancer is a major cause of cancer-related death in women. Antitumor T cell responses play critical therapeutic roles, including direct cytotoxicity mediated by CD8 + T cells and immunomodulatory roles mediated by CD4 + T cells. The IL-9-expressing Th9 cells are recently found to present antitumor immunity in melanoma and lung adenocarcinoma. In this study, we found that IL-9 expression in the serum and in circulating CD4 + T cells were significantly upregulated in breast cancer patients compared to healthy controls. The IL-9-expressing Th9 cells were enriched in the CCR4 - CCR6 - CXCR3 - subset. Upon TCR stimulation, this subset also presented potent IL-10 and IL-21 expression in addition to IL-9 expression. CCR4 - CCR6 - CXCR3 - CD4 + T cells also assisted in the killing of autologous tumor cells by CD8 + T cells, but did not initiate cytotoxicity by themselves. This enhancement in CD8 + T cell-mediated cytotoxicity was dependent on IL-9 as well as on IL-21. Interestingly, the tumor-infiltrating Th9 cells presented comparable IL-9, reduced IL-10, and elevated IL-21 expression compared with their counterparts in the peripheral blood. Together, these results demonstrated that IL-9-expressing Th9 cells were upregulated in breast cancer patients and potentially possessed antitumor roles by enhancing CD8 + T cell-mediated cytotoxicity., (Copyright © 2017 Elsevier B.V. All rights reserved.)

This research is to analyze the resourceful chemical composition in different tissues (root, stem, leaf and flower) of Abelmoschus manihot and evaluate their utilizing value. The flavonoids, soluble polysaccharides, cellulose, nucleosides and amino acids in the different tissues of A. manihot were determined by HPLC coupled with UV-Vis spectrophotpmetry, and UPLC-TQ/MS. The flowers are rich in the resourceful chemical compositions of flavonoids which mainly consist of hyperoside, isoquercitrin, cotton-8-O-glucuronide, myricetin, quercetin-3'-O-glucoside, rutin and quercetin. The total content of these flavonoids is 25.450 mg•g-1 in the flowers, while they are trace in the other tissues.Different tissues of A. manihot are rich in soluble polysaccharides and celluloses and the stems have the highest content(19.76%) of soluble polysaccharides, while the roots have the highest content (29.88%) of cellulose. Total of 21 amino acids and 9 nucleosides were detected in this plant, and the flowers have the highest content of amino acids(4.737 mg•g⁻¹), while the leaves have the highest content of nucleosides (1.474 mg•g⁻¹). A. manihot is rich in the resourceful chemical compositions, and its constituents and contents are various in different tissues of this plant.The results provided a scientific basis for the utilization and industrial development of A. manihot plants., Competing Interests: The authors of this article and the planning committee members and staff have no relevant financial relationships with commercial interests to disclose., (Copyright© by the Chinese Pharmaceutical Association.)

A regioselective efficient synthetic approach to N-imino-γ-carbolinium ylides via AgOTf-catalyzed iminoannulation has been developed. This transformation proceeds via a silver(i) triflate-catalyzed consecutive Friedel-Crafts reaction/N-C bond formation sequence between the readily available indole derivatives and propargylic alcohols.

Objective: To establish HPLC fingerprint of Ophiopogonis Radix of Sichuan and simultaneously determine two homoisoflavonoids (methylophiopogonanones A and B)., Methods: Full-quantified HPLC fingerprint was used to establish the HPLC fingerprint and determine the active ingredients of the daodi medicinal material Ophiopogonis Radix of Sichuan in Shengmai injection. Chromatographic condition was as follows: The analytical column was Waters symmetry shield RP 18 (4.6 mm x 250 mm, 5 microm) with a pre-column of symmetry shield RP 18. The mobile phase was acetonitrile-water (containing 0.1% phosphoric acid) with gradient elution. The flow rate was 1.0 mL/min, the detection wavelength was set at 280 nm and the column temperature was maintained at 30 degrees C., Results: The HPLC fingerprint of Ophiopogonis Radix of Sichuan was established with good separation and repeatability. 24 common peaks were defined in the HPLC fingerprint. The similarity among batches was more than 0.98. Compared with standard reference substances, No. 14 peak was methylophiopogonanone A and No. 15 peak was methylophiopogonanone B. Similarity determine system was applied to evaluate them., Conclusion: This analytical method is highly sensitive with strong specificity, which can be used efficiently in the quality control of Ophiopogonis Radix of Sichuan in Shengmai injection.

Objective: To Set up a method for determining the contents of the five flavonoids simultaneously in the HuangKui capsule and analyze their specific chromatograms., Methods: HPLC method was used. The analytical column was Thermo scientific Hypersil GOLD (250 mm x 4.6 mm, 5 microm). The mobile phase was composed of acetonitrile (A) and 0.2% orthophosphoric acid (B) with gradient elution. The flow rate was 0.8 mL/min and detection wavelength was 360 nm. The column temperature was maintained at 30 degrees C., Results: Contents of the five flavonoids (Rutin, Hyperoside, Isoquercitrin, Myricetin, Quercitrin) had good resolution with the correlation coefficients exceed 0.9999 and the average percent recovery lied in 98.46% to 100.33%. The chromatograms of the HuangKui capsule shared 15 common peaks in which 5 of them were recognized by the reference standard. Chromatograms of 10 lots of HuangKui capsule were analyzed with the similarities over 0.95., Conclusion: The proposed method of contents determination and chromatogram analysis has strong characteristic and specificity. This method is fast, easy and reliable, and can be applied for quality control of the preparation.

Static chamber-GC technique was employed to study the effects of different treatment winter cover crops, including no-tillage and directly sowing ryegrass (T1), no-tillage and directly sowing Chinese milk vetch (T2), tillage and transplanting rape (T3), no-tillage and directly sowing rape (T4), and fallowing (CK), on the CH4 and N2O emission from double cropping rice paddy field. During the growth period of test winter cover crops, the CH4 and N2O emission in treatments T1-T4 was significantly higher than that in CK (P < 0.01). Treatments T1 and T3 not only had the largest CH4 emission (0.60 and 0.88 g x m(-2)), but also had the largest N2O emission (0.20 and 0.23 g x m(-2), respectively). After the winter cover crops returned to field, the CH4 emission from early and late rice fields in treatments T1, T2, T3, and T4 was larger than that in CK. In early rice field, treatments T1 and T2 had the largest CH4 emission (21.70 and 20.75 g x m(-2)); while in late rice field, treatments T3 and T4 had the largest one (58.90 and 54.51 g x m(-2) respectively). Treatments T1-T4 also had larger N2O emission from early and late rice fields than the CK did. The N2O emission from early rice field in treatments T1, T2, T3, and T4 was increased by 53.7%, 12.2%, 46.3%, and 29.3%, and that from late rice field in corresponding treatments was increased by 28.6%, 3.8%, 34.3%, and 27.6%, respectively, compared with CK.

In the mol-ecule of the title compound, C(14)H(9)ClF(3)NO(2), the aromatic rings are oriented at a dihedral angle of 66.49 (3)°. Intra-molecular C-H⋯F and C-H⋯O inter-actions result in the formation of one planar five- and one non-planar six-membered ring. In the crystal structure, inter-molecular N-H⋯O hydrogen bonds link the mol-ecules into chains.