全文获取类型
收费全文 | 6743篇 |
免费 | 278篇 |
国内免费 | 37篇 |
专业分类
化学 | 4706篇 |
晶体学 | 26篇 |
力学 | 218篇 |
数学 | 1083篇 |
物理学 | 1025篇 |
出版年
2022年 | 40篇 |
2021年 | 69篇 |
2020年 | 85篇 |
2019年 | 105篇 |
2018年 | 93篇 |
2017年 | 84篇 |
2016年 | 161篇 |
2015年 | 155篇 |
2014年 | 178篇 |
2013年 | 286篇 |
2012年 | 358篇 |
2011年 | 399篇 |
2010年 | 245篇 |
2009年 | 244篇 |
2008年 | 366篇 |
2007年 | 350篇 |
2006年 | 382篇 |
2005年 | 339篇 |
2004年 | 300篇 |
2003年 | 270篇 |
2002年 | 308篇 |
2001年 | 121篇 |
2000年 | 119篇 |
1999年 | 70篇 |
1998年 | 64篇 |
1997年 | 78篇 |
1996年 | 83篇 |
1995年 | 64篇 |
1994年 | 73篇 |
1993年 | 65篇 |
1992年 | 62篇 |
1991年 | 61篇 |
1990年 | 56篇 |
1989年 | 55篇 |
1988年 | 47篇 |
1987年 | 51篇 |
1986年 | 48篇 |
1985年 | 99篇 |
1984年 | 97篇 |
1983年 | 63篇 |
1982年 | 98篇 |
1981年 | 66篇 |
1980年 | 87篇 |
1979年 | 66篇 |
1978年 | 84篇 |
1977年 | 70篇 |
1976年 | 65篇 |
1975年 | 50篇 |
1974年 | 53篇 |
1973年 | 45篇 |
排序方式: 共有7058条查询结果,搜索用时 343 毫秒
991.
Begue D Elissalde S Pere E Iratcabal P Pouchan C 《The journal of physical chemistry. A》2006,110(25):7793-7800
An extension of our combined procedure to determine a complete quartic force field and to resolve a vibrational problem thanks to a variational treatment is proposed for quantitative calculations of vibrational spectra in solution. Energies and gradients are obtained through a polarizable continuum model (PCM), the so-called self-consistent isodensity (SCI)-PCM. We present in this paper new experimental results dealing with formaldehyde in solution in cyclohexane, chloroform, THF, acetonitrile, DMSO and water; the obtained vibrational spectra are then compared with CCSD(T)/cc-pVQZ calculations. In addition, density functional theory (DFT) calculations have been carried out with the aim of both anticipating and positioning these approaches for larger sized molecules. 相似文献
992.
An organic mixed valence compound with a spacer length of 25 unsaturated bonds separating two amine redox centres was synthesised and the electron transfer behaviour was investigated in the context of a Mulliken-Hush analysis in order to estimate the longest redox centre separation for which an intervalence charge transfer band can be observed. 相似文献
993.
Thureau P Ancian B Viel S Thévand A 《Chemical communications (Cambridge, England)》2006,(17):1884-1886
The absence of preferential hydration in thymine and its lowest water accessibility with respect to uracil were evidenced by NMR diffusion and HOESY experiments; the hydration differences observed between these pyrimidine bases were attributed to the electronic rather than steric properties of the methyl group. 相似文献
994.
Cravino A Roquet S Leriche P Alévêque O Frère P Roncali J 《Chemical communications (Cambridge, England)》2006,(13):1416-1418
Introduction of dicyanovinyl groups on a triphenylamine-based conjugated system leads to an intramolecular charge transfer which extends the spectral response and raises the open-circuit voltage of the resulting hetero-junction solar cells. 相似文献
995.
Michel F Hamman S Thomas F Philouze C Gautier-Luneau I Pierre JL 《Chemical communications (Cambridge, England)》2006,(39):4122-4124
In copper(ii) complexes of tripodal ligands, the protonation state of the phenol moiety, and its position (axial vs. equatorial), are easily assessed by (19)F NMR. 相似文献
996.
A series of [2.2]paracylophane-bridged bis-triarylamine mixed-valence (MV) radical cations were analyzed by a generalized Mulliken-Hush (GMH) three-level model which takes two transitions into account: the intervalence charge transfer (IV-CT) band which is assigned to an optically induced hole transfer (HT) from one triarylamine unit to the second one and a second band associated with a triarylamine radical cation to bridge (in particular, the [2.2]paracyclophane bridge) hole transfer. From the GMH analysis, we conclude that the [2.2]paracyclophane moiety is not the limiting factor which governs the intramolecular charge transfer. AM1-CISD calculations reveal that both through-bond as well as through-space interactions of the [2.2]paracyclophane bridge play an important role for hole transfer processes. These electronic interactions are of course smaller than direct pi-conjugation, but from the order of magnitude of the couplings of the [2.2]paracyclophane MV species, we assume that this bridge is able to mediate significant through-space and through-bond interactions and that the cyclophane bridge acts more like an unsaturated spacer rather than a saturated one. From the exponential dependence of the electronic coupling V between the two triarylamine localized states on the distance r between the two redox centers, we infer that the hole transfer occurs via a superexchange mechanism. Our analysis reveals that even significantly longer pi-conjugated bridges should still mediate significant electronic interactions because the decay constant beta of a series of pi-conjugated MV species is small. 相似文献
997.
Van-Oanh NT Désesquelles P Bréchignac P 《The journal of physical chemistry. A》2006,110(17):5599-5606
The hydrogen-loss channel, induced by sequential multiphoton absorption, of the vapor-phase fluorene cation was investigated using a pulsed supersonic molecular beam, a time-of-flight mass spectrometer, and pulsed nanosecond lasers. Our new method leads to the determination of the absolute absorption cross section. Its attenuation with the number of absorbed photons has been approximated by means of statistical models. A model-free determination of the evolution of the dissociation rate constant in a relatively large energy range was obtained by solving the set of coupled differential kinetic equations numerically. Particular attention was focused on the data analysis techniques. The free fit of these rate constants is close to the photothermodissociation statistical model, but shows a discrepancy with the Rice and Ramsperger and Kassel model mainly at high energy. The resulting activation energy is in agreement with both that deduced from the ab initio calculations and that from the tight-binding energy potential surface model. 相似文献
998.
Protonated methane, CH(5)(+), is a key reactive intermediate in hydrocarbon chemistry and a borderline case for chemical structure theory, being the simplest example of hypercoordinated carbon. Early quantum mechanical calculations predicted that the properties of this species could not be associated with only one structure, because it presents serious limitations of the Born-Oppenheimer approximation. However, ab initio molecular dynamics and diffusion Monte Carlo calculations showed that the most populated structure could be pictured as a CH(3) tripod linked to a H(2) moiety. Despite this controversy, a model for the chemical bonds involved in this ion still lacks. Here we present a modern valence bond model for the electronic structure of CH(5)(+). The chemical bond scheme derived directly from our calculations pictures this ion as H(3)C...H(2)(+). The fluxionality can be seen as the result of a proton transfer between C-H bonds. A new insight on the vibrational bands at approximately 2400 and approximately 2700 cm(-1) is suggested. Our results show that the chemical bond model can be profitably applied to such intriguing systems. 相似文献
999.
1000.
Summary Given two pointsx, yS
1 randomly chosen independently by a mixing absolutely continuous invariant measure of a piecewise expanding and smooth mapf of the circle, we consider for each >0 the point process obtained by recording the timesn>0 such that |f
n
(x)–f
n
(y)|. With the further assumption that the density of is bounded away from zero, we show that when tends to zero the above point process scaled by –1 converges in law to a marked Poisson point process with constant parameter measure. This parameter measure is given explicity by an average on the rate of expansion off.Partially supported by FAPESP grant number 90/3918-5 相似文献