首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到10条相似文献,搜索用时 156 毫秒
1.
2.
To test the feasibility of local spin theory of Davidson and Clark for ferrodoxin clusters, the models [Fe2S2(SR)4]2− (R=—H, —CH3) are chosen for evaluation. This purpose is realized by calculating the local spin expectation values 〈S A·S B〉, 〈S A 2 〉, and m A and discussing the connection between these expected values and the Heisenberg spin model (HSM) and the Noodleman broken-symmetry approach. In practical calculation, the spin-unrestricted Hartree-Fock (UHF) and spin-polarized density functional theory (DFT) are used and the calculational qualities of these two methods are also discussed. In addition, the theoretical magnetic coupling constants J AB of these models are calculated by various computational schemes for comparison with both theoretical and experimental results previously reported. Supported by the Doctorial Initial Foundations of Hainan Normal University (Grant No. 13140252)  相似文献   

3.
4.
The aerial oxidation of PdII to PdIV has emerged as an integral component of sustainable catalytic C−H functionalization processes. However, a proper understanding of the factors that control the viability of this oxidative process remains elusive. An investigation of the intricate mechanism of the transmetalation reaction of the aerial oxidative transformation of [(Me3tacn)PdIIMe2] (Me3tacn=N,N′,N′′-trimethyl-1,4,7-triazacyclononane) to [(Me3tacn)PdIVMe3]+ has been conducted by using DFT, along with multireference methods, such as second-order n-electron valence-state perturbation theory (NEVPT2) with complete active space self-consistent field theory (CASSCF). The present endeavor predicts that the thermodynamics and kinetics of the oxygen activation step are primarily dictated by the polarity of the solvents, which determine the amount of charge transfer to the oxygen molecule from the PdII center. Additionally, it is observed that the presence of a protic solvent has a significant effect on the spin–orbit coupling term at the minimum energy crossing point of the triplet and singlet surfaces. Moreover, it is shown that the intermetal ligand-transfer phenomenon is an important instance of an oxygen-assisted SN2 reaction.  相似文献   

5.
The regioselectivity of ring-forming radical reactions is investigated within the framework of the so-called spin-polarized conceptual density functional theory. Two different types of cyclizations were studied. First, a series of model reactions of alkyl- and acyl-substituted radicals were investigated. Next, attention was focused on the radical cascade cyclizations of N-alkenyl-2-aziridinylmethyl radicals (a three-step mechanism). In both of these reactions, the approaching radical (carbon or nitrogen centered) adds to a carbon-carbon double bond within the same molecule to form a radical ring compound. In this process, the number of electrons is changing from a local point of view (a charge transfer occurs from one part of the molecule to another one) at constant global spin number Ns (both the reactant and the product ring compound are in the doublet state). It is shown that the experimentally observed regioselectivities for these ring-closure steps can be predicted using the spin-polarized Fukui functions for radical attack, f0NN(r).  相似文献   

6.
7.
The silicon atom may increase its coordination number to values greater than four, to form pentacoordinated compounds. It has been observed experimentally that, in general, pentacoordinated compounds show greater reactivity than tetracoordinated compounds. In this work, density functional theory is used to calculate the global softness and the condensed softness of the silicon atom for SiH n F4−n and SiH n F 5−n 1− . The values obtained show that the global and condensed softness are greater in the pentacoordinated compounds than in the tetracoordinated compounds, a result that explains the enhanced reactivity. If the results are analysed through a local version of the hard and soft acids and bases principle, it is possible to suggest that in nucleophilic substitution reactions, soft nucleophiles preferably react with SiH n F 5−n 1− , and hard nucleophiles with SiH n F4−n .  相似文献   

8.
Coupling of a local triplet carbene with a local triplet nitrene through an acetylene linkage gives a new brand of high spin quintet minima ( \textX-\textC ··-\textC o \textC-\textN ·· ·· {\text{X}}{-}\mathop {\text{C}}\limits^{ \cdot \cdot }{-}{\text{C}} \equiv {\text{C}}{-}\mathop {\text{N}}\limits_{ \cdot \cdot }^{ \cdot \cdot } , where X = H, F, Cl, Br), which are rather experimentally unreachable. Placing the same linkage between the local open-shell singlet carbene (σ1π1) and the local triplet nitrene (π1π1) gives triplet minima which are 54–56 kcal/mol more stable than their corresponding quintets. The carbenic angles in both quintets and triplets follow electropositivity of X (H > Br > Cl > F), with every divalent angle in quintet being smaller than the corresponding one in the triplet. Finally no reactive intermediate is observed through connecting singlet states of carbene and nitrene subunits which gives a neutral linear molecule with X–C≡C–C≡N formula, and show about 70 kcal/mol more stability than the corresponding triplet states. Our results are compared at B3LYP, HF, MP2, MP4(SDTQ), CCSD(T), and QCISD(T) levels using 6-311++G** basis set.  相似文献   

9.
10.
Quantum chemical methods involving studies of transition states of the reaction showed that the main products of N-alkylation of prototropic 2,3-dihydroimidazo[2,1-b]quinazolin-1(10)H-5-one (1) in the gas phase and under neutral conditions in solution occurring via the SN2 mechanism should be N(10)-alkyl-substituted derivatives formed from the 1H-tautomer. Minor N(1)-substituted derivatives in solution can be produced from both tautomers. For the alkylation of the free N-anion of compound 1, position 1 is attacked first. Validity of conclusions concerning the overall regioselectivity of the reaction was confirmed experimentally. In the absence of solvent, the alkylation proceeds abnormally with a sharp increase in the content of the 1-substituted isomers up to inversion of the regioselectivity of the reaction, which is explained by the participation in the process of the H-bonded dimer of the substrate (1a)2, which undergoes alkylation via the cryptoanionic mechanism. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 876–887, May, 2006.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号