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81.
The nucleobase 5-methylcytosine (I) is a minor component of eukaryotic DNA thought to be important in regulation of gene expression. The photochemical reactions of this nucleobase and its 2'-deoxyribonucleoside, 5-methyl-2'-deoxycytidine (II), in water have been studied. These reactions lead, respectively, to 3-amino-2-methylacrylamidine (Ib) and 3-(2-erythro-D-pentopyranos-1-yl)amino-2-methylacrylamidine (IIb) as the main photoproducts. The structure of the photoproducts was established by spectroscopic methods (1H and 13C NMR, UV spectroscopy, electron impact and liquid secondary ion mass spectrometry); in the case of Ib, confirmatory evidence was obtained by chemical methods (photolysis of 5-methyl[2-13C]cytosine, hydrolysis of N-carbomethoxy-3-amino-2-methylacrylamidine and reaction of Ib with 1,1'-carbonyldiimidazole to give I). The quantum yield for formation of Ib was determined to be 1.8 x 10(-3) at pH 7.5 while the quantum yield for formation of IIb has a lower value of 0.2 x 10(-3) at pH 7.5. These quantum yields depend strongly on pH and reach maximum values of 2.0 x 10(-3) at pH 7.0 (Ib) and 0.6 x 10(-3) at pH 5.0 (IIb). The mechanism of formation of Ib (or IIb) is proposed to involve nucleophilic attack of water on the C-2 position of photoexcited I (or II), followed by ring opening and decarboxylation of an intermediate carbamic acid. 相似文献
82.
?ukasz Gurzyński Mariusz Makowski Lech Chmurzyński 《The Journal of chemical thermodynamics》2006,38(12):1584-1591
By using the potentiometric method, acidity constants have been determined in systems of tri- and tetra-substituted pyridine N-oxides. The potentiometric measurements in systems of four 4-chloropyridine N-oxide derivatives containing the chlorine atom at position 4 to the NO2 group and four bromine counterparts were carried out in polar non-aqueous solvents, viz. amphiprotic methanol (MeOH) and aprotic protophilic dimethyl sulfoxide (DMSO). It was found that in all the systems studied the pKa values were readily determinable (as indicated by small standard deviations) in MeOH, whereas in DMSO large standard deviations were obtained making the pKa values either hardly determinable or indeterminable from potentiometric measurements. Furthermore, it was demonstrated that the acidity constants of protonated N-oxides studied in MeOH changed according to the sequence of their acidity constants in water. It was also found that in the polar solvents studied, i.e. in the amphiprotic methanol and the highly basic aprotic dimethyl sulfoxide, the cationic homo-conjugation equlibrium constants could not be determined using potentiometric method. Also, by using ab initio methods at the RHF and MP2 levels and the PCM model, utilizing the Gaussian 6-31++G∗∗ basis set, energies and Gibbs free energies of the protonation reactions of the N-oxides have been determined. The energy parameters have been compared with acidity constants of the protonated N-oxides determined by potentiometric titration in methanol to establish a correlation between these approaches. 相似文献
83.
Roman Luboradzki Janusz Lipkowski Wikor Koźmiński Lech Stefaniak 《Journal of chemical crystallography》1995,25(1):29-35
An X-ray diffraction study is reported for three molecules of salts containing a cation with a positive charged 2,3-diphenyltetrazolium ring. The results confirm aromatic structure of the tetrazole ring and the delocalization of charge on the whole molecules of the cations. The molecular geometry obtained is compared with that for free mesoionic 2,3-diphenyltetrazoles which have already been studied and the ring parameters do not reveal significant differences. But is shows that the geometry of the (NNO)– exocyclic group is changed as a result of alkylation. Compound I C13H12N5Cl, P21/a,a=16.934(4),b=6.575(1),c=26.121(4) Å, =108.15(2)°, II C14H13N6OI, P312,a=10.774(3),c=12.821(4) Å, IV C26H20N8Cl2, C2/c,a=22.316(4),b=6.6903(7),c=20.437(3) Å, =92.51(1)°. 相似文献
84.
New dichloride platinum(II) complexes with 5-methyl-1,2,4-triazolo[1,5-a]pyrimidin-7(4H)-one (HmtpO) have been synthesized and characterized by thermal analysis, infrared and 1H, 13C, 15N, 195Pt NMR spectroscopy. X-ray crystal structures of cis-PtCl2(NH3)(HmtpO) (1) and cis-PtCl2(HmtpO)2 · 4H2O (2b) were determined to R = 0.0332 and R = 0.0802, respectively. In both complexes the Pt(II) ions have a square-planar geometry with two adjacent corners being occupied by two nitrogens of HmtpO molecules for 2b or NH3 and HmtpO molecules for 1, whereas the remaining adjacent corners are occupied by two chloride anions. Spectroscopic data confirm the square planar geometry with N(3) bonded HmtpO, S-bonded dimethylsulfoxide and two trans chloride anions for trans-PtCl2(dmso) · 4H2O (3). 相似文献
85.
Orest Dorosh Ewa Biakowska-Jaworska Zbigniew Kisiel Lech Pszczkowski 《Journal of Molecular Spectroscopy》2007,246(2):228-232
The data available from rotational spectroscopy for chlorobenzene, bromobenzene, and iodobenzene have been extended by new measurements in the mm-wave region and in supersonic expansion in the cm-wave region. All available ground state measurements have been combined in global fits to derive precise rotational, centrifugal, and nuclear quadrupole coupling constants for the molecules. Rotational transitions in first excited states of the lowest frequency normal modes in bromobenzene and iodobenzene have been assigned and fitted. The values of electric dipole moments for 35Cl-, 79Br-, 81Br-, and I-benzene have been determined from Stark effect measurements on selected hyperfine components in the supersonic expansion spectrum, and are compared with values for several other series of monohalogen molecules. 相似文献
86.
Leszek Pazderski Jaromír Toušek Jerzy Sitkowski Kateřina Maliňáková Lech Kozerski Edward Szłyk 《Magnetic resonance in chemistry : MRC》2009,47(3):228-238
1H, 13C and 15N NMR studies of gold(III), palladium(II) and platinum(II) chloride complexes with picolines, [Au(PIC)Cl3], trans‐[Pd(PIC)2Cl2], trans/cis‐[Pt(PIC)2Cl2] and [Pt(PIC)4]Cl2, were performed. After complexation, the 1H and 13C signals were shifted to higher frequency, whereas the 15N ones to lower (by ca 80–110 ppm), with respect to the free ligands. The 15N shielding phenomenon was enhanced in the series [Au(PIC)Cl3] < trans‐[Pd(PIC)2Cl2] < cis‐[Pt(PIC)2Cl2] < trans‐[Pt(PIC)2Cl2]; it increased following the Pd(II) → Pt(II) replacement, but decreased upon the trans → cis‐transition. Experimental 1H, 13C and 15N NMR chemical shifts were compared to those quantum‐chemically calculated by B3LYP/LanL2DZ + 6‐31G**//B3LYP/LanL2DZ + 6‐31G*. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
87.
88.
Lech Kozerski 《Magnetic resonance in chemistry : MRC》1977,9(7):395-400
13C magnetic resonance spectra of several enamino ketones with secondary and tertiary amino groups were obtained for trifluoroacetic acid solutions. In both series O-protonation is predominant and the chemical shifts are related to the electron density changes with respect to the parent base. The spectra of the tertiary compounds are interpreted in terms of slow rotation around the C–1? C–2 and C–3? N bonds discernible at room temperature. O-protonated forms of the secondary enamino ketones undergo further reaction on C–2 yielding pyridinium salts. The mechanism of formation of the quaternary salts is interpreted and the additivity parameters of the 13C n.m.r. chemical shifts in the pyridinium ions is briefly discussed. 相似文献
89.
J.F. Lech D.D. Siemer E. Woodriff 《Spectrochimica Acta Part B: Atomic Spectroscopy》1973,28(11):435-439
A method is described for the determination of mercury in air by furnace atomic absorption. The porous graphite cups used ordinarily fail to adsorb mercury vapors from the air. When these same porous graphite cups are plated on the inner surface with a thin layer of gold the retention of mercury from an air sample passed through the cup is shown to be quantitative. The sensitivity of the analytical system is 3 × 10?10 g which is equivalent to 0.6 for a 500 cm3 air sample. Relative S.D. is 11.2%. The method described should also prove useful for other types of flameless devices. 相似文献
90.
Janusz Lipkowski Lech Gluzinski Kinga Suwinska Giovanni D. Andreetti 《Journal of inclusion phenomena and macrocyclic chemistry》1984,2(1-2):327-332
The crystal structures of layered clathrate inclusion compounds of Ni(CNS)2 (4-methylpyridine)4 with 1-bromonaphthalene (I) and azulene (II) were analyzed by x-ray single-crystal diffractometry [crystal data: I–C46H42N6S2Br2Ni; a=11.88(1), b=11.82(1), c=32.79(3), =102.0(1), Z=4, P21/c; II–C46H44N6S2Ni; a=11.51(2), b=11.64(1), c=32.98(2), =103.4, Z=4, P21/n]. Disordering of guest molecules in I is concluded, based on x-ray study, and positions of the guest (each of the two symmetry independent molecules disordered over two orientations) have been refined. In II disoder of guest-azulene is also observed but only one from the two symmetrically independent guest molecules could be located from the x-ray data. The empirical force field calculations were performed for I and II. The results were used for location of the second azulene molecule and for discussion of the disorder mole (short or long-range). 相似文献