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81.
82.
Brahmananda Ghosh Vincent J. Catalano David A. Lightner 《Monatshefte für Chemie / Chemical Monthly》2004,135(12):1503-1517
Summary. Crystal structures determined for three bilirubin analogs with gem-dimethyl groups at C(10) are reported, including the first X-ray structure of a bilirubin dimethyl ester. Conformation-determining torsion angles and key hydrogen bond distances and angles were compared to those from molecular dynamics calculations. Like other rubins, the component dipyrrinones of the three compounds were found to adopt the syn conformation, with Z-configuration double bonds at C(4) and C(15) and bis-lactam tautomeric structures of the end rings. No large differences in bond lengths and bond angles at C(10) were found, and the crystal structures of the two 10,10-dimethyl rubin acids showed considerable similarity to that of bilirubin: both pigments adopt a folded, intramolecularly hydrogen bonded ridge-tile conformation stabilized by six hydrogen bonds, with an interplanar angle in ridge-tile of 98° and 86°. In contrast, the dimethyl ester is intermolecularly hydrogen bonded in the crystal. Each molecule of the ester has its two syn-Z-dipyrrinones rotated into a conformation syn to the gem-dimethyl group, whereas in the acids they are anti. 相似文献
83.
Various oxodipyrromethenes with varying β-substituents have been synthesized and their reaction with singlet oxygen studied. The rates of chemical reactivity (kR) and physical quenching (kQ) of singlet oxygen by those substrates approach the diffusion threshold in both chloroform and methanol solvents, with kQ generally larger than kR in chloroform but of comparable magnitude in methanol. The range of (kQ+kR) values is 0.2–4.2 × 109 M?1 s?1. 相似文献
84.
Stefan E. Boiadjiev David A. Lightner 《Monatshefte für Chemie / Chemical Monthly》2002,1(1):1469-1480
Malonic ester derivatives of ethyl and methyl 3,5-dimethyl-4-(1′-iodoneopentyl)-1H-pyrrole-2-carboxylate exhibit restricted rotation about the pyrrole C(4)–C(1′) bond due to the bulky 1′-tert-butyl and malonic ester groups and the ortho effect at C(4) of the sterically crowded 3,5-dimethylpyrrole. The malonates belong to a rare class of atropisomers with restricted rotation about an sp3–sp2 C–C bond, and they undergo diastereomeric separation by TLC and crystallization: the diastereomers are stable in solution at room temperature. A crystal of one of the diastereomers, suitable for X-ray crystallography, gave the relative configuration of the chiral axis and stereogenic center at C(1′). Dynamic NMR studies of the purified diastereomers provide kinetic and thermodynamic parameters associated with the atropisomerism: ΔG ‡ = 132–134 kJ/mol (∼32 kcal/mol) at 383 K in C2D2Cl4 solvent. 相似文献
85.
Diethyl 3,3′-di-tert-butyl-4,4′-dimethyl-2,2′-bipyrrole-5,5′-dicarboxylate was synthesized in four steps from ethyl 3,5-dimethyl-1H-pyrrole-2-carboxylate. The CH2 hydrogens of the ethyl ester groups of the former are diastereotopic in the 1H-NMR, consistent with axial chirality of the bipyrrole and restricted rotation about the 2,2′-bipyrrole bond, due to the
tert-butyl groups. An X-ray structure of the crystalline target compound shows the pyrrole rings are twisted out of coplanarity
by 84.5°. 相似文献
86.
Unsymmetric bile pigment analogs of mesobilirubin-XIIIα and mesobilirubin-IVα were synthesized following oxidative coupling of dipyrrinones: methyl xanthobilirubinate with kryptopyrromethenone; methyl ψ-xanthobilirubinate with kryptopyrromethenone. 相似文献
87.
Stefan E. Boiadjiev David A. Lightner 《Monatshefte für Chemie / Chemical Monthly》2003,134(4):489-499
Summary. Pyrrole α-aldehydes (2-formyl-4,5-dimethyl-1H-pyrrole and 2-formyl-N-methylpyrrole) condense readily at C(3) of indolin-2-ones to give dipyrrinone analogs, such as (3Z)-[(4,5-dimethylpyrrol-2-yl)-methylidenyl]-indolin-2-one and (3E)-[(1-methylpyrrol-2-yl)-methylidenyl]-indolin-2-one. 1H-NMR NOE analyses and X-ray crystallography confirm the syn-(Z) configuration for the former and the syn-(E) configuration for the latter. The former is stabilized by intramolecular hydrogen bonding. Molecular mechanics calculations
of the latter indicate no energy difference between the syn and anti conformations.
Corresponding author. E-mail: lightner@scs.unr.edu
Received August 2, 2002; accepted August 30, 2002 相似文献
88.
Stefan E. Boiadjiev David A. Lightner 《Monatshefte für Chemie / Chemical Monthly》2002,133(11):1469-1480
Summary. Malonic ester derivatives of ethyl and methyl 3,5-dimethyl-4-(1′-iodoneopentyl)-1H-pyrrole-2-carboxylate exhibit restricted rotation about the pyrrole C(4)–C(1′) bond due to the bulky 1′-tert-butyl and malonic ester groups and the ortho effect at C(4) of the sterically crowded 3,5-dimethylpyrrole. The malonates belong to a rare class of atropisomers with restricted
rotation about an sp3–sp2 C–C bond, and they undergo diastereomeric separation by TLC and crystallization: the diastereomers are stable in solution
at room temperature. A crystal of one of the diastereomers, suitable for X-ray crystallography, gave the relative configuration
of the chiral axis and stereogenic center at C(1′). Dynamic NMR studies of the purified diastereomers provide kinetic and
thermodynamic parameters associated with the atropisomerism: ΔG
‡ = 132–134 kJ/mol (∼32 kcal/mol) at 383 K in C2D2Cl4 solvent.
Corresponding author. E-mail: lightner@scs.unr.edu
Received July 1, 2002; accepted July 8, 2002 相似文献
89.
A fluorocarbon-modified poly(N-isopropylacrylamide) has been synthesized by copolymerization of N-isopropyl acrylamide with a small amount of acrylate or methacrylate containing a perfluoroalkyl group. It was found that the hydrophilicity of macromolecular backbone is an important factor to the solution properties of the copolymers and that hydrophobic association between fluorocarbon groups is stronger than that between the corresponding hydrocarbon analogies. The viscosity of some of the copolymer solutions was very sensitive to temperature. It was dilatant at higher fluorocarbon comonomer content ( > 0.20-1.0 mol%) and was Newtonian at very low fluorocarbon comonomer content (0.03-0.2 mol% ) . Evidence for hydrophobic association of the fluorocarbon groups was obtained from the effects of adding Nad and surfactants on the solution viscosity. The LC-ST properties of these copolymers were studied by DSC method and this was also found to be consistent with hydrophobic association between the fluorocarbo 相似文献
90.