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101.
Boggio-Pasqua M Bearpark MJ Ogliaro F Robb MA 《Journal of the American Chemical Society》2006,128(32):10533-10540
A mechanism for the photochemical conversion of 2-vinyl-1,3-terphenyl to 8,9a-dihydrophenanthrene (Lewis, F. D.; Zuo, X.; Gevorgyan, V.; Rubin, M. J. Am. Chem. Soc. 2002, 124, 13664-13665) is presented in this study, based on ab initio restricted active space self-consistent field calculations and a molecular mechanics-valence bond dynamics simulation of a model system: the syn isomer of 2-vinylbiphenyl. An extended crossing seam between the ground and first excited electronic states was found to be largely responsible for the efficient photocyclization of the photochemically active syn isomer. This mechanism is nonadiabatic in nature, with an excited-state reaction pathway approaching the crossing region during the initial stage of cyclization. Dynamics simulation shows that this seam is easily accessible by vibrational motion in the branching space, once a small barrier is passed on the S1 excited-state potential energy surface. Ultrafast radiationless decay to the ground state then follows, and the cyclization is completed on this surface. A second possible mechanism was identified, which involves complete adiabatic cyclization on the S1 surface, with decay to the ground state (at a different conical intersection) only taking place once the product is formed. Thus, there is a competition between these two mechanisms-nonadiabatic and adiabatic-governed by the dynamics of the system. A large quantum yield is predicted for the photocyclization of the syn isomer of 2-vinylbiphenyl and 2-vinyl-1,3-terphenyl, in agreement with experimental observations. 相似文献
102.
Martial Say Ekaterina Paliakov Maged Henary Lucjan Strekowski 《Journal of heterocyclic chemistry》2006,43(6):1613-1620
Facile synthetic approaches to 2‐phenylquinolin‐4‐amines containing an aminoalkyl group at N4 of the quinolin‐4‐amine and amino or aminoalkyl groups at the phenyl moiety are presented. 相似文献
103.
We report the generation of sub-80 fs pulses with energy in the 100 nJ range at 1050 nm and a repetition rate up to 164 kHz based on a nonlinear fiber amplification system combined with an active control of the spectral phase. This control is performed by an acousto-optic programmable dispersive filter operated at a multiple of its acoustic repetition rate. This result opens up its possible use in highly nonlinear fiber chirped-pulse amplification setups. 相似文献
104.
Rickerby J Vallet M Bernardinelli G Viton F Kündig EP 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(12):3354-3368
The complex [Ru(Cp)(R,R-BIPHOP-F)(acetone)][SbF(6)], (R,R)-1 a, was used as catalyst for asymmetric Diels-Alder reactions between dienes (cyclopentadiene, methylcyclopentadiene, isoprene, 2,3-dimethylbutadiene) and alpha,beta-unsaturated ketones (methyl vinyl ketone (MVK), ethyl vinyl ketone, divinyl ketone, alpha-bromovinyl methyl ketone and alpha-chlorovinyl methyl ketone). The cycloaddition products were obtained in yields of 50-90 % and with enantioselectivities up to 96 % ee. Ethyl vinyl ketone, divinyl ketone and the halogenated vinyl ketones worked best and their reactions with acyclic dienes consistently provided products with >90 % ee. alpha-Chlorovinyl methyl ketone performed better than alpha-bromovinyl methyl ketone. The reaction also provided a [4.3.1]bicyclic ring system in 95 % ee through an intramolecular cycloaddition reaction. Crystal structure determinations of [Ru(Cp)((S,S)-BIPHOP-F)(mvk)][SbF(6)], (S,S)-1 b, and [Ru(Cp)((R,R)-Me(4)BIPHOP-F)(acrolein)][SbF(6)], (R,R)-2 b, provided the basis for a rationalization of the asymmetric induction. 相似文献
105.
Rudolf Brenneisen Pascale Meyer Haithem Chtioui Martial Saugy Matthias Kamber 《Analytical and bioanalytical chemistry》2010,396(7):2493-2502
Since 2004, cannabis has been prohibited by the World Anti-Doping Agency for all sports competitions. In the years since then,
about half of all positive doping cases in Switzerland have been related to cannabis consumption. In doping urine analysis,
the target analyte is 11-nor-9-carboxy-Δ9-tetrahydrocannabinol (THC-COOH), the cutoff being 15 ng/mL. However, the wide urinary detection window of the long-term metabolite
of Δ9-tetrahydrocannabinol (THC) does not allow a conclusion to be drawn regarding the time of consumption or the impact on the
physical performance. The purpose of the present study on light cannabis smokers was to evaluate target analytes with shorter
urinary excretion times. Twelve male volunteers smoked a cannabis cigarette standardized to 70 mg THC per cigarette. Plasma
and urine were collected up to 8 h and 11 days, respectively. Total THC, 11-hydroxy-Δ9-tetrahydrocannabinol (THC-OH), and THC-COOH were determined after hydrolysis followed by solid-phase extraction and gas chromatography/mass
spectrometry. The limits of quantitation were 0.1–1.0 ng/mL. Eight puffs delivered a mean THC dose of 45 mg. Plasma levels
of total THC, THC-OH, and THC-COOH were measured in the ranges 0.2–59.1, 0.1–3.9, and 0.4–16.4 ng/mL, respectively. Peak concentrations
were observed at 5, 5–20, and 20–180 min. Urine levels were measured in the ranges 0.1–1.3, 0.1–14.4, and 0.5–38.2 ng/mL,
peaking at 2, 2, and 6–24 h, respectively. The times of the last detectable levels were 2–8, 6–96, and 48–120 h. Besides high
to very high THC-COOH levels (245 ± 1,111 ng/mL), THC (3 ± 8 ng/mL) and THC-OH (51 ± 246 ng/mL) were found in 65 and 98% of
cannabis-positive athletes’ urine samples, respectively. In conclusion, in addition to THC-COOH, the pharmacologically active
THC and THC-OH should be used as target analytes for doping urine analysis. In the case of light cannabis use, this may allow
the estimation of more recent consumption, probably influencing performance during competitions. However, it is not possible
to discriminate the intention of cannabis use, i.e., for recreational or doping purposes. Additionally, pharmacokinetic data
of female volunteers are needed to interpret cannabis-positive doping cases of female athletes. 相似文献
106.
Raya J Bianco A Furrer J Briand JP Piotto M Elbayed K 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2002,157(1):43-51
Rotational resonance and radiofrequency-driven dipolar recoupling (RFDR) experiments have been used to recover the weak proton dipolar interaction present in peptides bound to swollen resins spun at the magic angle. The intensity of the correlation peaks obtained using these sequences is shown to be significantly stronger than the one obtained using the classical NOESY experiment. In addition, it is found that during the relatively long mixing times required to transfer magnetization in such soft materials, the RFDR sequence also achieves magnetization transfer via the scalar J-coupling. 相似文献
107.
We give the theory of the free induction decay of two-level saturation resonances in the time-resolved laser saturation spectroscopy.
The saturating and probe fields may be either co- or counter-propagating. The change-signal line-shape is analysed in detail
in two cases of practical importance, namely when the saturating field is either strong or weak. The time evolution of the
change-signal exhibits many interesting features which include Ramsey-type fringes, oscillatory decay, narrow resonances,
line-narrowing etc. It is shown that by analysing the decay of the change-signal in time one can distinguish (i) various laser
interaction processes such as population effects, coherence effects, three-photon Raman-type process, dynamic Stark splitting,
power-broadening etc., and (ii) various relaxation mechanisms such as phase-changing collisions, etc.
Work supported in part by D.R.E.T. (Paris). 相似文献
108.
109.
Elbayed K Dillmann B Raya J Piotto M Engelke F 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2005,174(1):2-26
High-resolution magic angle spinning (HRMAS) has become an extremely versatile tool to study heterogeneous systems. HRMAS relies on magic angle spinning of the sample and on pulse sequences originally developed for liquid state NMR. In most cases the outcome of the experiment is conform to what is expected from high-resolution liquid state NMR spectroscopy. However in some instances, experiments run under MAS can produce some very puzzling results. After reviewing the basic hardware which is at the heart of HRMAS spectroscopy, we show that the origin of this behavior lies in the natural time-dependence of some physical quantities imparted by the rotation. We focus in particular on the effects of B1 inhomogeneities on the nutation, the (90 degrees)+x-t-(90 degrees )-x and the MLEV16 experiments. Different models of radiofrequency distribution of B1 fields in a solenoidal coil are derived from simple geometrical considerations. These models are shown by NMR spin dynamics calculations to reproduce the experimental NMR results. They are also consistent with electromagnetic simulations of the B1 field distribution inside a solenoidal coil. 相似文献
110.
By applying Doppler-free heterodyne spectroscopy techniques to the hydrogen Balmer-α line (λ = 656 nm), one observes a dramatic improvement in S/N ratios for large detection frequencies, δ. Short-noise limited signals are obtained for δ > 2 MHz. Signal phase shifts yield the first “in situ” measurement of the collisional quenching of the 2s metastable state in gas discharges. 相似文献