全文获取类型
收费全文 | 546篇 |
免费 | 23篇 |
国内免费 | 1篇 |
专业分类
化学 | 470篇 |
晶体学 | 6篇 |
力学 | 5篇 |
数学 | 10篇 |
物理学 | 79篇 |
出版年
2023年 | 1篇 |
2022年 | 2篇 |
2021年 | 3篇 |
2020年 | 5篇 |
2019年 | 8篇 |
2018年 | 3篇 |
2016年 | 17篇 |
2015年 | 10篇 |
2014年 | 11篇 |
2013年 | 34篇 |
2012年 | 34篇 |
2011年 | 33篇 |
2010年 | 20篇 |
2009年 | 15篇 |
2008年 | 22篇 |
2007年 | 34篇 |
2006年 | 41篇 |
2005年 | 43篇 |
2004年 | 40篇 |
2003年 | 23篇 |
2002年 | 28篇 |
2001年 | 13篇 |
2000年 | 11篇 |
1999年 | 9篇 |
1998年 | 3篇 |
1997年 | 5篇 |
1996年 | 11篇 |
1995年 | 8篇 |
1994年 | 1篇 |
1993年 | 9篇 |
1992年 | 1篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1987年 | 2篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1975年 | 2篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1970年 | 2篇 |
1924年 | 1篇 |
排序方式: 共有570条查询结果,搜索用时 15 毫秒
141.
Density functional theory calculations were conducted on the title reactions with explicit inclusion of a variety of water molecules. Concerted reaction paths were examined first in the reaction model, ester(H2O)n --> MeCOOH(H2O)(n-1)EtOH, with n = 1-4. Their Gibbs activation energies are much larger than the experimental value, and the concerted paths are unfavorable. Various stepwise paths were investigated, and the ester(H2O)4 reactant gives a likely stepwise path. The n = 4 based reaction models, n = 4 + 5 and n = 4 + 12, were found to have similar proton-relay shapes with good hydrogen-bond directionality. The distinction of either the concerted or the stepwise path is described by the position of only one proton in the "junction" water molecule. 相似文献
142.
Yuzuru Sanemitsu Akio Manabe Shinichi Kawamura Junichi Satoh Yoo Tanabe 《Journal of heterocyclic chemistry》1990,27(5):1517-1518
2-Alkylamino-4H-1,3,4-benzothiadiazines 3 are prepared in a one-pot synthesis from 1-phenyl-3-thiosemi-carbazides 1 and bromine through an oxidative cyclization. 相似文献
143.
The mechanism of stereoselective addition of Grignard reagents to carbonyl compounds has been investigated using B3LYP density functional theory calculations. The study of the reaction of methylmagnesium chloride and formaldehyde in dimethyl ether revealed a new reaction path involving carbonyl compound coordination to magnesium atoms in a dimeric Grignard reagent. The structure of the transition state for the addition step shows that an interaction between a vicinal-magnesium bonding alkyl group and C=O causes the C-C bond formation. The simplified mechanism shown by this model is in accord with the aggregation nature of Grignard reagents and their high reactivities toward carbonyl compounds. Concerted and four-centered formation of strong O-Mg and C-C bonds was suggested as a polar mechanism. When the alkyl group is bulky, C-C bond formation is blocked and the Mg-O bond formation takes precedence. A diradical is formed with the odd spins localized on the alkyl group and carbonyl moiety. Diradical formation and its recombination were suggested to be a single electron transfer (SET) process. The criteria for the concerted polar and stepwise SET processes were discussed in terms of precursor geometries and relative energies. 相似文献
144.
Yoshihiro Osamura Kichisuke Nishimoto Shinichi Yamabe Tsutomu Minato 《Theoretical chemistry accounts》1979,52(4):257-265
The photodecomposition of C3O2 into C2O and CO is studied with the ab initio MO calculation. It is found that, starting from the second excited state of C3O2 (u), the ground-state C2O (triplet) is yielded through the bent dissociation. The stable structure of the excited triplet state of C3O2 as an intermediate is also demonstrated. 相似文献
145.
Shinichi Kikkawa 《Journal of solid state chemistry》1980,31(2):249-255
The layer compound FeOCl absorbs pyridine molecules into its interlayer regions. The kinetics of this intercalation were investigated in the temperature region from room temperature to 130°C. The reaction products had different compositions, and , in the temperature regions below and above 60°C, respectively. Reaction mechanisms were explained by nucleation and diffusion processes. The nucleation processes followed different rate equations in the respective temperature regions, the first-order rate law at lower temperatures, and the two-dimensional Avrami-Erofeev equation at higher temperatures. The diffusion processes were well described by two-dimensional Jander's diffusion equation. Activation energies of all processes were about 10 kcal/mole. 相似文献
146.
Keiichi Moriya Toshihiro Yamane Toshiya Suzuki Tsuyoshi Masuda Hiroshi Mizusaki Shinichi Yano 《Phosphorus, sulfur, and silicon and the related elements》2013,188(6-7):1427-1432
Cyclotriphosphazene and cyclotetraphosphazene derivatives with mesogenic 4-N-(4-alkoxyphenyl)iminomethyl)phenoxy and 4-(4'-alkoxy)biphenoxy moieties were synthesized. Schiff base moiety has higher mesomorphic stability than that in biphenyl moiety both in cyclotriphosphazene and cyclotetraphosphazene. Cyclotriphosphazenes possess higher thermal stability in the mesomorphic phase than the cyclotetraphosphazenes, suggesting the difference in the molecular structure. 相似文献
147.
Abstract Mesomorphic phase transitions of 4′-n-alkoxy-3′-nitrobiphenyl-4-carboxylic acids (ANBC) with numbers of carbons (n) in the alkoxy group ranging from 11 to 22 have been studied by differential scanning calorimetry (DSC) and polarizing optical microscopy. The D phase, a mesophase of particular interest through its being optically isotropic, was observed for the n = 17, 19, 20, 21, and 22 members of the ANBCs, as well as for the n = 16 and 18 members, as reported previously. The Sc-D phase transition temperature decreased with increasing n, so that the temperature range of the D phase extended over 64° at n = 22. In the n = 15 member, the D phase was certainly observed on first heating, but was not seen on subsequent cooling and second heating processes. 相似文献
148.
Surasak Seesukphronrarak Shinichi Kawasaki Kana Kobori Toshikazu Takata 《Journal of polymer science. Part A, Polymer chemistry》2007,45(14):3073-3082
Several poly(arylene thioether)s ( PTEs ) containing a fluorene moiety were synthesized by the polycondensation of masked dithiols such as 9,9‐bis(4‐(N,N‐dimethyl‐S‐carbamoyl)phenyl)fluorene and various difluoroarenes. All PTEs were obtained in quantitative yields. The PTEs showed good thermal stability: the 10% weight loss temperature was over 480 °C under both nitrogen and air atmosphere by TGA, and glass temperature was within a range of 204–275 °C by DSC. Most PTEs exhibited remarkably high refractive index values in a range of 1.66–1.72 at 589 nm, whereas they had a very low degree of birefringence properties. Furthermore, the PTEs showed high solubility in ordinary organic solvents such as chloroform, N‐methylpyrrolidone, and tetrahydrofuran. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3073–3082, 2007 相似文献
149.
Density functional theory (DFT) calculations were made on the hydrolysis of hydantoin (2,4-imidazolidinedione). In the neutral hydrolysis, reacting systems composed of hydantoin and (H2O)n with n = 1+3, 2+3, 3+3, and 4+3 were adopted. Three water molecules (“+3”) participate in the in-plane hydrogen-bond circuit, and the n–3 = 1, 2, 3 or 4 water cluster works for the out-of-plane nucleophilic attack onto the carbonyl carbon of hydantoin. Transition states (TSs) involving bond interchanges prompted by proton transfers were determined. The reaction path with n = 3+3 containing N-carbamoyl glycine, N-carboxy glycine and three tetrahedral intermediates was found to be most likely. In the acid-catalyzed hydrolysis, a reacting system composed of hydantoin and H3O+(H2O)7 was employed. Ten TSs and nine intermediates were obtained. N-carbamoyl glycine and N-carboxy glycine were confirmed to be detectable stable species. The product consists of glycine, carbonic acid (not CO2), NH4+, and (H2O)5. It has the exothermic energy, whereas the product in the neutral hydrolysis is of the endothermic one for all n values. For both neutral (n = 3+3) and acid-catalyzed hydrolyses, the rate-determining steps were calculated to be for formation of the tetrahedral intermediate, HOOC-CH2-NH-C(OH)2NH2. The pattern of proton transfers along hydrogen bonds was carefully investigated. 相似文献
150.