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排序方式: 共有205条查询结果,搜索用时 15 毫秒
91.
Paterson MJ Robb MA Blancafort L DeBellis AD 《The journal of physical chemistry. A》2005,109(33):7527-7537
We present a detailed CASSCF study of the mechanism of excited-state intramolecular proton transfer (ESIPT) in the o-hydroxyphenyl triazine class of photostabilizers. The valence-bond analysis of the ground state and the two pipi* excited states permits a simple chemical interpretation of the mechanistic information. Our results show that the barrier to enol-keto tautomerism on the ground-state adiabatic surface is high. Following photoexcitation to the charge-transfer state, the ESIPT is predicted to take place without a barrier. Radiationless decay to the ground state is associated with an extended seam of conical intersection, with a sloped topology lying parallel to the ESIPT path, which can be accessed at any point along the reaction path. Our results show that the triazine class of photostabilizers has the photochemical and photophysical qualities associated with exceptional photostability. 相似文献
92.
Discrete basis many body perturbation calculations have been performed on Ne and CH4 in a basis which mimics Boys' oscillator orbitals. The L-shell correlation energy obtained for Ne was –0.347 which is to be compared with the best numerical basis many body perturbation result of –0.336 ± 0.01 obtained by Lee, Dutta and Das and the best variational result of –0.322 obtained by Sasaki and Yoshimine. An analysis of 3- and 4-body corrections (pair-pair repulsions) and the effects of many body EPV renormalization for both localized and canonical SCF orbitals is presented. 相似文献
93.
Tomasello G Ogliaro F Bearpark MJ Robb MA Garavelli M 《The journal of physical chemistry. A》2008,112(41):10096-10107
The photochemical ring opening of 1,2-dihydronaphthalene (DHN) was investigated using two complementary computational approaches. CASPT2//CASSCF minimum energy paths were characterized for reaction channels on the three lowest-energy singlet excited states, describing initial evolution of the spectroscopic bright (ionic) state and its subsequent decay to dark (covalent) states of benzene-like and hexatriene-like character. Although the benzene-like state is unreactive and can radiate, the hexatriene-like state has indirect access to a low-energy conical intersection seam, at which radiationless decay to the ground state and subsequent product formation can take place. An MMVB molecular dynamics simulation was carried out on the reactive hexatriene-like excited state, suggesting that intramolecular vibrational energy redistribution (IVR) controls the radiationless decay and the photoproduct distribution (which is qualitatively reproduced). 相似文献
94.
Jason J Reddick Cynthia Kinsland Robb Nicewonger Todd Christian Diana M Downs Malcolm E Winkler Tadhg P Begley 《Tetrahedron》1998,54(52):9054-15991
The overexpression, purification and characterization of 4-amino-5-hydroxymethyl-2-methylpyrimidine kinase (HMP kinase) and 4-amino-5-hydroxymethyl-2-methylpyrimidine monophosphate kinase (HMP-P kinase) are described. Surprisingly HMP-P kinase also shows HMP kinase activity. These enzymes are useful reagents for the preparation of intermediates on the thiamin biosynthetic pathway. 相似文献
95.
We report highly selective photocatalytic functionalisations of alkyl groups in aryl alkyl ethers with a range of electron-poor alkenes using an acridinium catalyst with a phosphate base and irradiation with visible light (456 nm or 390 nm). Experiments indicate that the reaction operates via direct single-electron oxidation of the arene substrate ArOCHRR′ to its radical cation by the excited state organic photocatalyst; this is followed by deprotonation of the ArOC–H in the radical cation to yield the radical ArOC˙RR′. This radical then attacks the electrophile to form an intermediate alkyl radical that is reduced to complete the photocatalytic cycle. The oxidation step is selective for activated arenes (ArOR) over their non-activated counterparts and the subsequent deprotonation of the methoxy group affords the α-aryloxyalkyl radical that leads to a wide range of functionalised products in good to excellent yield.We report highly selective photocatalytic functionalisations of alkyl groups in aryl alkyl ethers with a range of electron-poor alkenes using an acridinium catalyst with a phosphate base and irradiation with visible light (456 nm or 390 nm). 相似文献
96.
Merchán M Serrano-Andrés L Robb MA Blancafort L 《Journal of the American Chemical Society》2005,127(6):1820-1825
We address the possibility of populating the lowest triplet state of cytosine by an "intrinsic"mechanism, namely, intersystem crossing (ISC) along the ultrafast internal conversion pathway of the electronically excited singlet species. For this purpose, we present a discussion of the ISC process and triplet-state reactivity based on theoretical analysis of the spin-orbit strength and the potential energy surfaces for the relevant singlet and triplet states of cytosine. High-level ab initio computations show that ISC is possible in wide regions of the singlet manifold along the reaction coordinate that controls the ultrafast internal conversion to the ground state. Thus, the ISC mechanism documented here provides a possibility to access the triplet state, which has a key role in the photochemistry of the nucleic acid bases. 相似文献
97.
98.
Groenhof G Schäfer LV Boggio-Pasqua M Goette M Grubmüller H Robb MA 《Journal of the American Chemical Society》2007,129(21):6812-6819
Multiconfigurational ab initio calculations and QM/MM molecular dynamics simulations of a photoexcited cytosine-guanine base pair in both gas phase and embedded in the DNA provide detailed structural and dynamical insights into the ultrafast radiationless deactivation mechanism. Photon absorption promotes transfer of a proton from the guanine to the cytosine. This proton transfer is followed by an efficient radiationless decay of the excited state via an extended conical intersection seam. The optimization of the conical intersection revealed that it has an unusual topology, in that there is only one degeneracy-lifting coordinate. This is the central mechanistic feature for the decay both in vacuo and in the DNA. Radiationless decay occurs along an extended hyperline nearly parallel to the proton-transfer coordinate, indicating the proton transfer itself is not directly responsible for the deactivation. The seam is displaced from the minimum energy proton-transfer path along a skeletal deformation of the bases. Decay can thus occur anywhere along the single proton-transfer coordinate, accounting for the remarkably short excited-state lifetime of the Watson-Crick base pair. In vacuo, decay occurs after a complete proton transfer, whereas in DNA, decay can also occur much earlier. The origin of this effect lies in the temporal electrostatic stabilization of dipole in the charge-transfer state in DNA. 相似文献
99.
100.
Vocal fold contact behavior was examined in separate groups of boys and girls through application of an electroglottograph(EGG). In general, a contact quotient (EGG duty cycle) showed minimal differences within and between boys and girls during sustained production of the vowels /i/, /u/, and /a/. The findings are discussed with respect to the laryngeal behavior of prepubescent children as well as the clinical utility and applicability of the EGG for examining phonatory behavior among young children. 相似文献