全文获取类型
收费全文 | 1436篇 |
免费 | 27篇 |
国内免费 | 10篇 |
专业分类
化学 | 829篇 |
晶体学 | 11篇 |
力学 | 42篇 |
数学 | 233篇 |
物理学 | 358篇 |
出版年
2020年 | 15篇 |
2019年 | 20篇 |
2018年 | 13篇 |
2017年 | 11篇 |
2016年 | 16篇 |
2015年 | 14篇 |
2014年 | 19篇 |
2013年 | 72篇 |
2012年 | 54篇 |
2011年 | 51篇 |
2010年 | 29篇 |
2009年 | 31篇 |
2008年 | 54篇 |
2007年 | 63篇 |
2006年 | 63篇 |
2005年 | 40篇 |
2004年 | 36篇 |
2003年 | 34篇 |
2002年 | 52篇 |
2001年 | 36篇 |
2000年 | 27篇 |
1999年 | 19篇 |
1998年 | 23篇 |
1997年 | 24篇 |
1996年 | 23篇 |
1995年 | 20篇 |
1994年 | 22篇 |
1993年 | 28篇 |
1992年 | 21篇 |
1991年 | 16篇 |
1990年 | 18篇 |
1989年 | 20篇 |
1988年 | 20篇 |
1987年 | 29篇 |
1986年 | 29篇 |
1985年 | 40篇 |
1984年 | 32篇 |
1983年 | 16篇 |
1982年 | 33篇 |
1981年 | 14篇 |
1980年 | 17篇 |
1979年 | 13篇 |
1978年 | 17篇 |
1977年 | 12篇 |
1976年 | 23篇 |
1975年 | 21篇 |
1974年 | 27篇 |
1973年 | 19篇 |
1972年 | 12篇 |
1969年 | 8篇 |
排序方式: 共有1473条查询结果,搜索用时 31 毫秒
61.
Hersh WH Xu P Simpson CK Grob J Bickford B Hamdani MS Wood T Rheingold AL 《The Journal of organic chemistry》2004,69(6):2153-2163
Reaction of the N-toluenesulfonyl derivatives of (S)-alanine, phenylalanine, and valine (4-6) with PhPCl(2) gave in high yield the 4-methyl, benzyl, and isopropyl derivatives (7-9) of 2-phenyl-1-p-toluenesulfonyl-1,3,2-oxazaphospholidin-5-one. The ratios of the (2S,4S)/(2R,4S) diastereomers (cis/trans isomers) were 1:1, 2:1, and 10:1 for the methyl, benzyl, and isopropyl derivatives 7a,b, 8a,b, and 9a,b, respectively. For 7a,b, both isomers could be crystallized, but for the others only the major isomers were isolable. The X-ray crystal structure of 9a shows that the isopropyl and phenyl groups are mutually cis and that the tolyl moiety is oriented s-trans to both the isopropyl and phenyl groups. Reaction of 6 with Cl(2)PCH(2)CH(2)PCl(2) (10) gave a 56:38:7 mixture of the cis/cis, cis/trans, and trans/trans diphosphorus heterocycles 11a-c. The major isomer could be crystallized and isolated free of the other diastereomers. Reaction of 6 with EtPCl(2) gave a 6:1 mixture of cis/trans isomers of the ethyl-substituted heterocycles 12a,b as an inseparable oil but allowed confirmation of the structure of 11a. Slow epimerization at phosphorus may occur by inversion but more likely by ring opening/closure, since 7b, 9a, and 11a give rise upon standing in solution to mixtures containing starting material and 7a, 9b, and 11b, respectively, along with the free amino acid derivatives 4 and 6. The NMR spectra, and in particular the coupling constants between the alpha-hydrogen atom of the amino acid moiety and phosphorus, were used to establish the identities of the cis and trans isomers. Reaction of 9a with (THF)W(CO)(5) gave the phosphorus-ligated adduct (9a)W(CO)(5) (13), and the IR spectrum of this complex shows that 9a is a strongly electron-withdrawing ligand. The geometry of the sulfonamide moiety is discussed in detail, as are the (1)H NMR coupling constants. The data are consistent with the presence of little steric interaction between the cis isopropyl and phosphorus substituent in 9a, 11a, and 12a and orientation of the tolyl moiety s-cis to the isopropyl group in 9b, 12b, and 13. 相似文献
62.
An efficient approach is described for using accurate ab initio calculations to determine the rates of elementary condensation and evaporation processes that lead to nucleation of aqueous aerosols. The feasibility of the method is demonstrated in an application to evaporation rates of water dimer at 230 K. The method, known as ABC-FEP (ab initio/classical free energy perturbation), begins with a calculation of the potential of mean force for the dissociation (evaporation) of small water clusters using a molecular dynamics (MD) simulation with a model potential. The free energy perturbation is used to calculate how changing from the model potential to a potential calculated from ab initio methods would alter the potential of mean force. The difference in free energy is the Boltzmann-weighted average of the difference between the ab initio and classical potential energies, with the average taken over a sample of configurations from the MD simulation. In principle, the method does not require a highly accurate model potential, though more accurate potentials require fewer configurations to achieve a small sampling error in the free energy perturbation step. To test the feasibility of obtaining accurate potentials of mean force from ab initio calculations at a modest number of configurations, the free energy perturbation method has been used to correct the errors when some standard models for bulk water (SPC, TIP4P, and TIP4PFQ) are applied to water dimer. To allow a thorough exploration of sampling issues, a highly accurate fit to results of accurate ab initio calculations, known as SAPT-5s, as been used a proxy for the ab initio calculations. It is shown that accurate values for a point on the potential of mean force can be obtained from any of the water models using ab initio calculations at only 50 configurations. Thus, this method allows accurate simulations of small clusters without the need to develop water models specifically for clusters. 相似文献
63.
A. M. Yusof M. M. Rahman A. K. H. Wood 《Journal of Radioanalytical and Nuclear Chemistry》2007,271(1):191-197
A simple and sensitive method for the determination of some metalloids and heavy metals in water samples is presented. The
method is based on the preconcentration of the attachment of chelating functionalities with metalloids and toxic metals irreversibly
and targeted towards toxic metals adsorbed on modified activated carbon, activated carbon and red soil particles at pH 3.0–9.0±0.2,
followed by quantitative determination using instrumental neutron activation analysis (INAA), on the absorbers. Attachment
results from attraction that may be physical, chemical, electrical, or a combination of all three. The efficient removal of
metalloids and toxic metals, especially arsenic, chromium and mercury is anticipated. The adsorption capacity of the chemically
modified activated carbon materials was evaluated for the above mentioned metalloid and toxic metal ions in the presence of
iron ions and simulated water samples. Red soil particles containing iron was utilized in the control of oxidation-reduction
reaction with metalloids and toxic metals. The preconcentration of the elements of interest on red soil particles, activated
carbon and modified activated carbon at different depths, pH and oxidation states was investigated. The results obtained showed
good agreement with certified values giving relative errors of less than 10%. 相似文献
64.
Zusammenfassung Die Arbeit beschäftigt sich mit der systematischen Untersuchung des Verhaltens der auf Seite 250 aufgezählten, bei toxikologischen Untersuchungen häufiger angetroffenen Alkaloidbasen gegenüber 50 Reagenzien. Die auf dem Objektträger erhaltenen Niederschläge wurden mikroskopisch untersucht und gegebenenfalls durch die Bestimmung einfach und rasch zu erfassender Eigenschaften der Kristalle, wie Profilwinkel, Einfachbrechung, Doppelbrechung, Auslöschungswinkel, Pleochroismus, und Verhalten gegenüber dem Gipsblättchen weiters charakterisiert.Eine Durchsicht der Liste der Reagenzien (S. 251 ff.) zeigt, daß außer altbekannten Alkaloidreagenzien eine größere Zahl von bisher wenig oder gar nicht zur Alkaloiderkennung benutzten Reagenzien studiert wurde. Eine systematische Untersuchung des Verhaltens einer möglichst großen Zahl der häufiger angetroffenen Basen erschien erforderlich, da die Spezifität einer Probe nur auf Grundlage der Kenntnis des Verhaltens des Reagens gegenüber allen anderen in Frage kommenden Stoffen sichergestellt werden kann.Die zur Erkennung der Basen nützlichen Reaktionsbilder sind in Tafeln (S. 323 ff.), nach Reagenzien angeordnet, gesammelt. Der systematische Bericht der Untersuchungsbefunde (S. 256) ist nach den Alkaloidbasen eingeteilt. Eine kurze Zusammenfassung betreffend die für jede Base bestgeeigneten Nachweisverfahren ist jeweils am Ende des der Base gewidmeten Kapitels gegeben.Die Nützlichkeit der studierten Reagenzien für die Erkennung von Alkaloidbasen wird zum Abschluß zusammenfassend besprochen (S. 315). Die Reagenzien Brom-Bromnatrium, Perchlorsäure, Kaliumbromid, Stannochlorid, Titanochlorid, Kaliumjodaurit, Natriumbromaurat, Platinbrom-wasserstoffsäure, Kaliumbrommercurat, Zinnchlorwasserstoffsäure und Palladiumbromwasserstoffsäure geben mit mehreren Basen kristalline Niederschläge, die durch Aussehen und Eigenschaften die Erkennung der Basen ermöglichen. Kaliumxanthat, Wismutchlorid, Rhodiumtrichlorid, Rutheniumtrichlorid und Cadmiumjodid erzeugen mit nur je zwei der untersuchten Alkaloidbasen gut kristallisierte Niederschläge, und die folgenden Reagenzien erscheinen durchaus spezifisch: Kupferron gibt nur mit-Eucain einen kristallinen Niederschlag; primäres Kaliumphthalat und Kaliumantimonyltartrat erzeugen nur mit Strychnin Kristallisationen; Cocain ist das einzige Alkaloid, das ein kristallisiertes Jodargentat gab, und lediglich bei Strychnin konnte ein eindeutig kristalliner Niederschlag mit HSbCl4 erhalten werden.Fulton hat darauf hingewiesen, daß Reagenzien, die mit Brucin, Strychnin und Cocain keine Fällungen geben, nicht als allgemeine Alkaloidreagenzien in Betracht gezogen werden können, da sie wahrscheinlich auch mit anderen Basen keine Niederschläge zu erzeugen imstande sein dürften. Es wird nun darauf hingewiesen, daß Reagenzien, die mit den obgenannten drei Alkaloiden versagen, doch studiert werden sollten, da die Möglichkeit besteht, daß sie mit einem anderen Alkaloid einen spezifischen Nachweis geben.
The work herein reported represents a portion of that presented by Mr.Wood as his dissertation in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Polytechnic Institute of Brooklyn. 相似文献
Résumé (L) Le présent travail étudie systématiquement le comportement (signalé page 250) des alcaloides les plus courants en toxicologie, vis à vis de 50 réactifs.Le précipité obtenu sur porte objet est examiné microscopiquement et éventuellement caractérisé par détermination simple et rapide des propriétés cristallines telles que le profil angulaire, la simple et double réfringence, l'angle d'extinction, le pleochroisme et son comportement en présence d'une lame de gypse.L'examen de la liste (p. 251) révèle qu'un grand nombre de réactifs peu ou pas du tout utilisés jusqu'ici dans l'identification des alcaloides ont été étudiés à côté des réactifs usuels, bien connus. Une étude systématique du comportement du plus grand nombre possible des bases les plus courantes s'imposait, parce que la spécificité d'un essai n'est établie avec certitude que si l'on connait la façon dont se comporte le réactif vis à vis de toutes les autres substances qui interviennent dans l'essai.Le Tableau (p. 323) donne tous les schémas de réaction nécessaires à l'identification des alcaloides, en prenant les réactifs comme base de classification.L'exposé des résultats de la recherche est subdivisé d'après les bases alcaloidiques (p. 256).Chaque chapitre se rapportant à un alcaloide déterminé se termine par un court résumé des procédés d'identification les mieux appropriés.La justification de l'emploi des réactifs étudiés est finalement discutée (p. 315) sous une forme résumée.Les précipités cristallins dont l'aspect et les propriétés permettent l'identification des bases, sont obtenus par les réactifs suivants: le bromo-bromure de sodium, l'acide perchlorique, le bromure de potassium, le chlorure stanneux et titaneux, l'iodoaurite de potassium, le bromoaurate de sodium, l'acide bromoplatinique, le bromomercurate de potassium, les acides chlorostannique et bromopalladique.Les réactifs suivants ne donnent de précipité cristallin qu'avec deux bases alcaloides étudiées: le xanthate de potassium, le chlorure de bismuth, le trichlorure de rhodium, de ruthénium et l'iodure de cadmium. Les réactifs suivants sont apparus comme spécifiques: le cupferron ne donne de précipité cristallin qu'avec la b-Eucaine; le phtalate acide de potassium et le tartrate de potassium et d'antimonyle ne fournissent de précipité cristallin qu'avec la strychnine; la cocaine est le seul des alcaloides formant un iodoargentate et enfin, la strychnine a donné un précipité cristallin avec HSbCl4.Fulton avait avancé que les réactifs ne précipitant pas avec la brucine, la strychnine et la cocaine ne pouvaient pas être considérés comme des réactifs généraux des alcaloides, parce que vraisemblablement ils se comportent de même avec d'autres alcaloides. Le présent travail démontre la nécessité d'étudier des réactifs, même s'ils ne précipitent pas ces trois alcaloides, parce qu'ils peuvent avec d'autres alcaloides, donner lieu à des réactions spécifiques.
The work herein reported represents a portion of that presented by Mr.Wood as his dissertation in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Polytechnic Institute of Brooklyn. 相似文献
65.
G. Z. Gassan-Zadeh M. E. Wood T. G. Alkhazov 《Reaction Kinetics and Catalysis Letters》1987,34(2):289-293
The route of NO reduction to N2 over nickel chromite catalyst is dependent on the nature of the reducing agent. NO reduction by dihydrogen proceeds mainly through the step of N2O formation, whereas NO reduction to N2 by carbon monoxide is mainly direct without a step of N2O formation.
NO N2 NO , , N2O, NO , , NO N2, . . N2O.相似文献
66.
Eric E. Boros James B. Thompson Edgar R. Wood O. Bradley McDonald Timothy D. Spitzer Andrea M. Sefler Bryan R. Reep 《Journal of heterocyclic chemistry》2004,41(3):355-358
5‐Methoxy and 5‐anilinopyrido[2,3‐d]pyrimidin‐7(8H)‐ones 2a‐2f were obtained by a tandem Michael addition‐cyclization reaction of methanol and anilines with pyrimidinylpropynoate 5. Methoxy derivative 2a was obtained in 62% yield by treatment of 5 with methanol and potassium carbonate. Anilino derivatives 2b‐2f were prepared in 31–71% yields by reacting 5 with the corresponding anilines in refluxing methanol. This methodology accomplishes Michael‐addition and pyridopyrimidinone ring formation in one‐pot and affords the desired products in reasonable yield without chromatography. Propynoate 5 did not react with 4‐cyanoaniline under these conditions. Reaction of 5 with 2‐aminopyridine gave the unexpected arylpyrido[2,3‐d]pyrimidinone 8 in 58% yield and reaction of 5 with imidazole afforded Michael‐adduct 9 in 69% yield. Compounds 2a and 5 were submicromolar inhibitors of epidermal growth factor receptor (EGFR) tyrosine kinase. 相似文献
67.
Freezing temperatures of dilute aqueous solutions of ethyl acetate and mixtures with myo-inositol, D-mannitol, formamide, 1,3,5-trioxane, 1,4-dioxane, acetamide, hexamethylenetetramine, and methyl formate have been measured. In addition, freezing temperatures of dilute aqueous solutions of methyl formate and mixtures with the above solutes have been measured. From these data, the pairwise molecular Gibbs energies of interaction between the molecules were calculated. Using the additivity principle, the pairwise functional group Gibbs energies of interaction were calculated for ester group interactions with a variety of other functional groups. 相似文献
68.
Wood TE Dalgleish ND Power ED Thompson A Chen X Okamoto Y 《Journal of the American Chemical Society》2005,127(16):5740-5741
Helical zinc(II) complexes of bis(dipyrromethene)s bearing homochiral amide substituents were synthesized. Analysis of the products by chiral HPLC showed two diastereomeric major products and showed that dipyrromethene double-nuclear helicates are stereochemically stable and do not interconvert. Circular dichroism (CD) studies showed that the complexation reactions had proceeded with modest diastereomeric excesses. Analysis of an analogous symmetric zinc(II) bis(dipyrromethene) lacking homochiral substituents that could act as chromophores discounted induced CD by the chiral auxiliaries. 相似文献
69.
We report improvements of our method for the solid-phase synthesis of mechanism-based mercaptomethyl ketone inhibitors of cysteine proteases (Lee, A.; Huang, L.; Ellman, J. A. J. Am. Chem. Soc. 1999, 121, 9907-9914). Specifically, Fmoc-protected chloromethyl ketones were used, rather than the Alloc-protected counterparts. In addition, we further demonstrated that diverse polar functionality can be incorporated at the R1', R1, and R2 sites, in contrast to our previous efforts, where primarily hydrophobic groups were incorporated at these positions. On the basis of these results, a 2016-membered library of potential mercaptomethyl ketone inhibitors was prepared that incorporated diverse functionality. The library was screened against cathepsin B, which is implicated in cancer, resulting in the identification of single-digit nanomolar inhibitors. Because of the diverse functionality incorporated in this library, it should be a rich source of potent inhibitors against many other cysteine proteases. 相似文献
70.
N. S. Allen J. F. McKellar G. O. Phillips D. G. M. Wood 《Journal of polymer science. Part A, Polymer chemistry》1974,12(11):2647-2650
Phosphorescence from polyolefins was studied from the aspects of excitation and emission wavelength and lifetime. Effects of photochemical oxidation on polypropylene phosphorescence are discussed in contrast to the effects of thermal oxidation. 相似文献