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61.
A quantum-chemical investigation of the location and form of the solitons in the dications of polyenes was undertaken. It
was shown that two soliton waves (both charge and geometric) in opposite phases are generated in the doubly charged conjugated
systems; they coincide in form with that for the solitons in the single-charge π systems. In the dications of polyenes with a long chain both solitons are repelled from each other. In short chains a minimum
distance is maintained between them, and both centers of the soliton waves can even be outside the molecule. The introduction
of hydroxy or phenyl terminal groups into the dications of the polyenes leads to displacement of the centers of the charge
and geometric waves without substantially distorting their form. The introduction of an amino group or tropylium residue is
accompanied by displacement of the solitons to the end of the conjugation chain so that half of the soliton waves projects
onto the molecule.
Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 44, No. 5, pp. 272–278, September–October, 2008. 相似文献
62.
We examine the singlet stability of symmetry adapted, restricted Hartree-Fock (RHF) solutions and the implied symmetry breaking
for various planar, π-electron systems with conjugated double bonds as described by the semiempirical Pariser-Parr-Pople Hamiltonian.
In particular, we explore the energy and charge- density waves (CDWs) in various real and hypothetical structures that result
by a systematic deformation of the nuclear framework: we start with a highly symmetric cyclic polyene C
N
H
N
having a nondegenerate ground state (N = 2n = 4ν + 2, ν = 1, 2,...), whose sites form a regular N-gon (D
Nh
point group), and proceed to structures with lower symmetry (D
6h
, D
3h
, D
2h
point groups), or with only the planar symmetry of the conjugated π-electron system (C
1h
). The objective of this study is to explore the phenomenon that could be referred to as a breaking of an approximate symmetry or an implied symmetry breaking. 相似文献
63.
Kelsey L. Horvath Dr. Christopher G. Newton Dr. Kimberley A. Roper Dr. Jas S. Ward Prof. Michael S. Sherburn 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(16):4072-4076
The first general synthesis of compounds of the tetravinylethylene (TVE) family is reported. Ramirez-type dibromo-olefination of readily accessible penta-1,4-dien-3-ones generates 3,3-dibromo[3]dendralenes, which undergo twofold Negishi, Suzuki–Miyaura or Mizoroki–Heck reactions with a wide variety of olefinic coupling partners. This route delivers a broad range of unsymmetrically substituted tetravinylethylenes with up to three different alkenyl substituents attached to the central C=C bond. The extensive scope of the approach is demonstrated by the preparation of the first higher order oligo-alkenic through-conjugated/cross-conjugated hybrid compounds. An unsymmetrically substituted TVE is shown to undergo a domino electrocyclization–cycloaddition with high site-selectivity and diastereoselectivity, thereby demonstrating the substantial synthetic potential of substituted TVEs for controlled, rapid structural complexity generation. 相似文献
64.
Dressed Time-Dependent Density Functional Theory (Maitra et al., J Chem Phys 2004, 120, 5932) is applied to selected linear polyenes. Limits of validity of the approximation are briefly discussed. The implementation strategy is described. Results for the 2(1)B(u) and 2(1)A(g) states of selected linear polyenes are presented and compared with accessible experimental and theoretical results. 相似文献
65.
《化学:亚洲杂志》2017,12(5):605-614
The BN‐doped organic analogues are interesting as aliphatic amineboranes for hydrogen storage, precursors for aromatic borazines and adsorbent cage azaboranes. However, BN‐doped aliphatic polyenes remained undeveloped. Herein, we perform theoretical calculations on two mono BN‐doped aliphatic lower polyenes, 1,3‐butadiene and 1,3,5‐hexatriene. A general rule is proposed, i.e., isomers with terminal nitrogen and directly BN‐connected, N−B(R), in particular, are of significant thermodynamic stability as compared with their inverse analogues (where boron is at the terminal position). The N−B(R) type isomers are found to be the most stable ones in both polyenes. Isomers with terminal B and N are of intermediate stability. Highly destabilized isomers are those with one terminal methylene group and one terminal heteroatom in the butadiene series, and two terminal methylene groups in the hexatriene series. Rules established here may lead researchers to synthesize isomers with particular thermodynamic stability. 相似文献
66.
测量了非极性分子β胡萝卜素和极性分子角黄素, 在非极性溶剂CS2和极性溶 剂1,2二氯乙烷中243–293 K的温度范围内的共振拉曼光谱. 结果表明, 溶质和溶剂的极性对拉曼光谱影响很大. 非极性分子β胡萝卜素在非极性溶剂CS2中的拉曼散射截面最大, 线宽最窄, 而极性分子角黄素在极性溶剂1,2二氯乙烷中的拉曼散射截面最小, 线宽最大. 用溶剂效应及线性多烯分子的“相干弱阻尼电子-晶格振动”, “有效共轭长度”模型给予了解释. 相似文献
67.
Tomohiko Ohwada Hirotaka Kagawa Hiroshi Ichikawa 《International journal of quantum chemistry》1998,68(1):65-72
Solving the constrained Hartree–Fock equation and using the method of the energy component analysis, we have determined the fundamental factor which causes the electrons at the negative center of conjugated carbanion to delocalize over the whole system. Constraint on the π-electron flow between the anion center and polyethylenic system produces the electronic state that the lone-pair electrons are localized at the anionic center. By doing so, a prominent increase in the kinetic energy of π electrons was observed. Such an increase was found to quantitatively fit in with that by the model that electrons are packed in a box. Thus, we could clearly show that delocalization of the carbanion in the polyethylenic system is brought forth by the relaxation of the kinetic energy pressure. We also examined conjugated carbodianion systems. Dianion involves repulsive interaction between anions causing less conjugation energy. Namely, conjugation itself is given by the release in the kinetic energy pressure, but, it is restrained by the electrostatic repulsion. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 68: 65–72, 1998 相似文献
68.
The large N expansion of the restricted Hartree–Fock (RHF) exchange energy per atom E(N) of the Pariser–Parr–Pople (PPP) model of cyclic polyenes (annulenes) CNHN is derived in detail. We explicitly derive the coefficients E0 and E1 of the asymptotic expansion: E(N)=E0+E1 ln N/N2+O(N−2), N→∞, in the very simple case of half-filling and no bond alternation. The exchange energy per atom in the infinite chain can be written as Eex=(2/π2)∑∞j=1{[γ∞(2j−1)]/[(2j−1)2]}, where γ∞ is the two-electron repulsion integral in the infinite chain. On the other hand, the second coefficient E1 giving a finite-size correction is found to be 1/2b, where b is the bond length. This value of E1 differs slightly from that of a linear chain with periodic boundary conditions because the distance between sites depends upon the radius of the ring, i.e., upon N. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 397–407, 1998 相似文献
69.
70.