锇杂苯的电子结构及芳香性

使用密度泛函理论及片段轨道相互作用分析方法,研究了典型的锇杂苯的电子结构和芳香性.结果表明,锇碳六员环具有较好的环平面性及键的离域性,占据的锇dxz轨道与碳环的3π空轨道之间的反馈π键相互作用,使得环上离域π电子数满足Hückel的4n+2规则,计算的环外质子化学位移、同键反应芳香性稳定化能、绝对硬度和磁化率增量数据均表明锇杂苯具有芳香性.     

The correlation between aromaticity and stability in planar N2X2 (X = O,S, Se,and Te) Species

Five isomers of N₂O₂ and a series of planar alternate four-membered ring N₂X₂ (X?=?O, S, Se, and Te) species have been examined with both the B3LYP and the CCSD methods. The 6-311?+?G* basis set is used for O, S, Se and the SDD pseudo potential basis set is used for the heavier atom Te. The aromaticity, the stability, and the relationship between them, are discussed in terms of the nucleus independent chemical shifts (NICS), the transition state (TS) barriers. Little correlation is observed between aromaticity and stability of the present species.     

对全金属La2-4团簇芳香性的从头算理论研究简

我们将芳香性扩展到全金属阴离子团簇La2-4.运用从头算方法(B3LYP/LANL2DZ,B3PW91/LANL2DZ and MP2/LANL2DZ)进行了结构优化.计算结果显示,阴离子团簇有两个同分异构体,一个是C2v结构,另一个是正方形D4h结构.进一步对能量的分析得出,D4h结构比C2v结构更稳定.对最稳定的D4h结构计算了核独立位移(NICS),结果显示正方形的La2-4环呈现强芳香性.详细的分子轨道(Mos)分析揭示正方形的La2-4环拥有四个独立的离域化成键系统,每一个系统拥有两个电子,分别满足4n+2芳香性电子计算规则,因此呈现四重芳香性.     

Embedding a Planar Hypercoordinate Carbon Atom into a [4n+2] π-System

Through delicate tuning of the electronic structure, we report herein a rational design of seventeen new putative global minimum energy structures containing a planar tetra- or pentacoordinate carbon atom embedded in an aromatic hydrocarbon. These structures are the result of replacing three consecutive hydrogen atoms of an aromatic hydrocarbon by less electronegative groups, forming a multicenter σ-bond with the planar hypercoordinate carbon atom and participating in the π-electron delocalization. This strategy that maximizes both mechanical and electronic effects through aromatic architectures can be extended to several molecular combinations to achieve new and diverse compounds containing planar hypercoordinate carbon centers.     

Towards highly powerful neutral organic superacids—a DFT study of some polycyano derivatives of planar hydrocarbons

Density functional theory (DFT) calculations at B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d) level have been carried out on indene, cyclopentaphenanthrene and 1H-phenalene and their heptacyano and nonacyano derivatives, respectively, in order to examine their acidities in the gas-phase and DMSO. It is found that polycyano derivatives represent powerful organic superacids, the most acidic being nonacyano-1H-phenalene. The origin of the highly pronounced acidity is identified as a strong anionic resonance in the resulting conjugate base. Comparison of the calculated ΔH_acid value for 1H-phenalene with the experimental NIST value shows that the latter is too large by 8-11 kcal mol⁻¹. A possible reason for this error is briefly discussed.     

Tricarbonylchromium complexes of [5]- and [6]metacyclophane: an experimental and theoretical study

Tricarbonylchromium complexes of [5]- and [6]metacyclophane were prepared and the interaction between the Cr(CO)₃ tripod and the cyclophane fragment was evaluated by both an experimental and a theoretical study. The tricarbonylchromium complex of [5]metacyclophane could only be obtained in solution and was characterized by its ¹H NMR spectrum. The tricarbonylchromium complex of [6]metacyclophane was isolated and an X-ray crystal structure was obtained, which reveals that no significant geometric changes occur upon coordination of the severely distorted aromatic ring. Computations on the tricarbonylchromium complexes of m-xylene, [5]- and [6]metacyclophane furthermore demonstrate that the corresponding complexation energy is remarkably unaffected by the degree of distortion of the aromatic ring. Theoretical analyses of the above model systems as well as complexes of planar and artificially deformed benzene with Cr(CO)₃ show that this is primarily the result of two counteracting effects: (i) a stabilization due to an increased back-donation from the metal center to the benzene and (ii) a destabilization due to the increasing strain in the aromatic ring.     

Stabilization of ortho-quinone methides by a bis(sulfonium ylide) derived from 2,5-dihydroxy-[1,4]benzoquinone

The zwitterionic intermediates (2a) in the oxidation of ortho-alkylphenols (1) and bis(sulfonium ylide) 3 form reasonably stable 2:1-complexes (4), in which the ortho-quinone methide (oQM) moieties are not present in quinoid form with the exocyclic in-plane methylene group, but as zwitterionic, aromatic conformer having an out-of-plane exocyclic methylene group. The complex 7 derived from the α-tocopherol model compound PMC (5) was comprehensively characterized. As exemplarily demonstrated, the adducts can be advantageously employed in organic synthesis as ‘stabilized oQMs’.     

Isolation of an Annulated 1,4-Distibabenzene Diradicaloid

The first 1,4-distibabenzene-1,4-diide compound [(ADC)Sb]₂ ( 5 ) based on an anionic dicarbene (ADC) (ADC=PhC{N(Dipp)C}₂, Dipp=2,6-iPr₂C₆H₃) is reported as a bordeaux-red solid. Compound 5 , featuring a central six-membered C₄Sb₂ ring with formally Sb^I atoms may be regarded as a base-stabilized cyclic bis-stibinidene in which each of the Sb atoms bears two lone-pairs of electrons. 5 undergoes 2 e-oxidation with Ph₃C[B(C₆F₅)₄] to afford [(ADC)Sb]₂[B(C₆F₅)₄]₂ ( 6 ) as a brick-red solid. Each of the Sb atoms of 6 has an unpaired electron and a lone-pair. The broken-symmetry open-shell singlet diradical solution for ( 6 )²⁺ is calculated to be 2.13 kcal mol⁻¹ more stable than the closed-shell singlet. The diradical character of ( 6 )²⁺ according to SS-CASSCF (state-specific complete active space self-consistent field) and UHF (unrestricted Hartree-Fock) methods amounts to 36 % and 39 %, respectively. Treatments of 6 with (PhE)₂ yield [(ADC)Sb(EPh)]₂[B(C₆F₅)₄]₂ ( 7 -E) (E=S or Se). Reaction of 5 with (cod)Mo(CO)₄ affords [(ADC)Sb]₂Mo(CO)₄ ( 8 ).     

A Triply [5]Helicene-Bridged (1,3,5)Cyclophane

A rigid propeller-shaped conjugated triple macrocycle consisting of two nearly perfectly stacked benzene rings and three linking [5]helicene moieties has been synthesized using a glyoxylic Perkin approach. Analysis of the electron delocalization in this atypical aromatic molecule revealed global aromaticity and a 78 π-electron circuit along the edge of its triple loop, to the detriment of the two 6 π-electron circuits in the two stacked benzene rings.