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991.
Transesterification of β-keto esters with a variety of alcohols to the corresponding β-keto esters has been carried out efficiently over aluminosilicates simply by refluxing in toluene under liquid-phase conditions. Aliphatic β-keto esters exhibit the higher reactivity than the aromatic and/or cyclic β-keto esters, whereas other esters such as -keto esters, ,β-unsaturated esters, normal esters, and -halo esters fail to undergo the transesterification. The reaction proceeds smoothly with primary alcohols than the tertiary-, cyclic-, and allylic alcohols. The large-pore zeolites such as Y, mordenite, and β show higher activity than the medium-pore ZSM-5 and the partial removal of framework aluminum from the large-pore zeolites by dealumination increases their activity. The aluminum containing mesoporous MCM-41 shows poor activity. Among the different solvents used toluene leads to the highest activity and the optimum catalyst concentration found was ca. 20 wt.% of the substrate. The activity increases with increasing reaction temperature. The reaction pathway consists of formation of acyl ketene intermediate by the interaction of β-keto esters with Brønsted acid sites of the catalyst, followed by nucleophilic attack of the alcohol at the electrophilic center and successive elimination of the proton to give the product. 相似文献
992.
N. Rosales-Conrado M. E. Len-Gonzlez L. V. Prez-Arribas L. M. Polo-Díez 《Analytica chimica acta》2002,470(2):147-154
A simple method for the simultaneous determination of chlorophenoxy acid herbicides and their esters in soil is presented. Compounds studied are: 2,4-dichlorophenoxyacetic acid (2,4-D), 4-(2,4-dichlorophenoxy)butanoic acid (2,4-DB), 2,4-dichlorophenoxyacetic-1-butyl ester (2,4-D-1-butyl ester), and 2,4-dichlorophenoxyacetic-1-methyl ester (2,4-D-1-methyl ester).
The chromatographic analysis was carried out by HPLC, after ultrasonic extraction, on a C18 packed capillary column with temperature gradient, large injection volumes and UV detection at 232 nm. Samples were spiked with amounts between 2.5 and 6.0 μg g−1 of each herbicide; recoveries obtained were between 72 and 97% (n=3 for each spiked level) and detection limits were between 0.3 and 0.5 μg g−1.
Application of this procedure to the analysis of herbicides in ester and acid forms showed the effectiveness of the methodology proposed. 相似文献
993.
Summary Acid-catalyzed esterification of carboxyl groups with acetyl chloride in isopropanol is investigated. Yields are equivalent
to those obtained with reagent prepared conventionally by bubbling gaseous hydrogen chloride into isopropanol. The preparation
of the former reagent is less time consuming and no special equipment is needed. The procedure is applicable for other alcohols
as well. 相似文献
994.
2,4-Dimethoxybenzyl ester of per-O-acetylated N-acetylneuraminic acid thioglycoside was synthesized and attempts at the oxidative (DDQ-induced) addition of O-nucleophiles (water and galactose derivatives with unprotected hydroxy group at C(6)) to the benzylic carbon atom of this ester were made. 相似文献
995.
Ointment-like poly(ortho esters) were synthesized for the first time from the reaction of 3,9-bis(methylene)-2,4,8,10-tetraoxaspiro[5. 5] undecane with poly(ethylene glycol)-400, N,N-bis(2-hydroxyethyl)-n-hexadecylamine and N,N-bis(2-hydroxyethyl) palmitamide, respectively. The obtained polymers were characterized by 1H NMR spectra, 13C NMR spectra, elemental analyses, light scattering, and measurements of intrinsic viscosity. The influence of catalyst on the intrinsic viscosity of polymers was investigated. The 9-[(1,3-dihydroxy-2-propoxy) methyl] guanine controlled release profiles of hydrophobic ointment-like polymers such as polymer PII in vitro were also discussed. 相似文献
996.
Tran Van Chien Nguyen The Anh Nguyen Tuan Thanh Tran Thi Phuong Thao Tran Van Loc Tran Van Sung 《Natural product research》2019,33(2):212-218
Two new prenylisoflavones, 3′,4′,5-trihydroxy-8-prenyl-dihydrofuran[2″,3″:7,6]isoflavone (1) and 4′,5-dihydroxy-8-prenyl-dihydrofuran[2″,3″:7,6]isoflavone (2), along with five known prenylisoflavones (3–7), benzylalcohol-4-O-β-d-glucoside (8) and two cinnamic acid esters (9, 10) were isolated from the leaves of Maclura cochinchinensis (Cudrania cochinchinensis). Their structures were elucidated by analysis of NMR (1H-, 13C-NMR, HSQC, HMBC), MS spectra and comparison with the published data. Compounds 4–10 were the first time isolated from this species. Prenylisoflavones 1–4 and 6–7 were evaluated for their in vitro cytotoxic activity on KB and HepG2 cancer cell lines. Compound 4 showed cytotoxic activity against both cancer cell lines with IC50 values of 26.99 and 19.95 μM, respectively. The other compounds were considered as inactive. 相似文献
997.
A 3-monosubstituted dithiirane 1-oxide, 3-(9-triptycyl)dithiirane 1-oxide, was prepared for the first time, by the reaction of (9-triptycyl)diazomethane and S8O. The dithiirane 1-oxide was obtained as cis- and trans-isomers, and the structure of the trans-isomer was verified by X-ray crystallography. The cis-isomer isomerized gradually to the trans-isomer in solution. The divalent sulfur atom of the cis- and trans-dithiirane 1-oxides were removed on treatment with triphenylphosphine to give the corresponding Z- and E-sulfines, respectively. The reaction of the trans-dithiirane 1-oxide with (Ph3P)2Pt(C2H4) provided the (sulfenato-thiolato)PtII complex, and that with Lawesson's reagent yielded the 1,3,4,2-trithiaphospholane and 1,2,4,5,3-tetrathiaphosphorinane derivatives. 相似文献
998.
Titanosilicon mesoporous materials with a high content of titanium (up to Ti/Si 0.36) and high adsorptive characteristics have been obtain from the titanosilicon ester molecular precursors (BuO)2Ti[OSi(O
t
Bu)3]2 and BuOTi[OSi(O
t
Bu)3]3 by template synthesis in the presence of micelle forming surfactants. A characteristic of these mesoporous matrices is the considerable size of the walls of the mesopores (2-4 nm) which may results from the use of the bulky titanosilicon building blocks in the template synthesis. 相似文献
999.
Santos Fustero Sonia Flores Diego Jiménez Jorge Bueno 《Journal of fluorine chemistry》2007,128(10):1248-1254
A new synthesis of fluorinated allylamines through the reaction of 2-(trimethylsilyl)ethyl sulfones and sulfoxides (as vinyl anion equivalents) with imines and imino esters has been described. The process includes a TBAF-mediated fragmentation of 2-(trimethylsilyl)ethyl sulfones to afford the desired allylic amines. When the reaction is performed with the corresponding sulfoxides, the fragmentation takes place under the addition conditions, affording the final products in a single step. 相似文献
1000.
Diphenyl phosphate is a hydrolysis product and possible metabolite of the flame retardant and plasticiser additive triphenyl phosphate. A molecularly imprinted polymer solid-phase extraction (MISPE) method for extracting diphenyl phosphate from aqueous solutions has been developed and compared with SPE using a commercially available mixed-mode anion exchanger. The imprinted polymer was prepared using 2-vinylpyridine (2-Vpy) as the functional monomer, ethylene glycol dimethacrylate (EGDMA) as the cross-linker, and a structural analogue of the analyte as the template molecule. The imprinted polymer was evaluated for use as a SPE sorbent, in tests with both aqueous standards and spiked urine samples, by comparing recovery and breakthrough data obtained using the imprinted form of the polymer and a non-imprinted form (NIP). Extraction from aqueous solutions resulted in more than 80% recovery. Adsorption by the molecularly imprinted polymer (MIP) was non-selective , but selectivity was achieved by selective desorption in the wash steps. Diphenyl phosphate could also be selectively extracted from urine samples, although the urine matrix reduced the capacity of the MISPE cartridges. Recoveries from urine extraction were higher than 70%. It was important to control pH during sample loading. The MISPE method was found to yield a less complex LC–ESI–MS chromatogram of the urine extracts compared with the mixed-mode anion-exchanger method. An LC–ESI–MS method using a Hypercarb LC column with a graphitised carbon stationary phase was also evaluated for organophosphate diesters. LC–ESI–MS using negative-ion detection in selected ion monitoring (SIM) mode was shown to be linear for diphenyl phosphate in the range 0.08–20 ng L–1. 相似文献