识别手性六螺烯分子的计算机模拟

手性拆分分子与手性分子间的短程相互作用会形成非对映异构体的接触对,从而导致的立体选择作用很难传统实验方法进行研究^[1]。蛋白质－核酸识别机理和研究表明,计算机模拟可以很好地研究这个问题^[2]。用高分子分离性分子的计算机模拟尚未见报道。     

Synthesis and X‐Ray Analysis of a New [6]Helicene

A new disubstituted hexahelicene derivative 3 bearing methoxy functions at positions 3 and 14 has been prepared in racemic form through a Heck reaction followed by photocyclodehydrogenation. Suitable crystals of rac‐3 were analyzed by X‐ray crystallography and showed similar geometry to the structure of hexahelicene itself. Deprotection of 3 using boron tribromide led to 3,14‐dihydroxyhexahelicene 4 in quantitative yield. The complexation of transition metal atoms seemed to be quite possible by these two bidentate hexahelicene derivatives.     

Conformational studies on hexahelicenes—II: Synthesis and spectral properties of alkyl hexahelicenes

The synthesis of eight alkyl substituted hexahelicene derivatives by photodehydrocyclization is described.     

An alternative approach to 3-(diphenylphosphino)hexahelicene

3-(Diphenylphosphino)hexahelicene is synthesised in good yield and purity, via a three-step sequence involving a palladium-catalysed Mizoroki-Heck coupling reaction and classical oxidative photocyclisation. Mononuclear ruthenium and palladium complexes of 3-(diphenylphosphino)hexahelicene are prepared and characterised.     

Two strained hexahelicenophanes

The crystal structures of the [6]­helicenes 4,13‐(1,10‐deca­methyl­ene­dioxy)­hexahelicene, C₃₆H₃₄O₂, (I), and 4,13‐(1,8‐octa­methyl­ene­dioxy)­hexahelicene, C₃₄H₃₀O₂, (II), show strong steric interactions between the terminal benzene rings and the poly­methyl­ene­dioxy chains. The shortest ring A and F distances amount to 2.941 (3) and 2.902 (3) Å, respectively. The increased steric energy of the ground state is responsible for a significantly lower racemization barrier of (I) and (II) in comparison to the unsubstituted [6]­helicene.     

Synthesis and characterization of new hexahelicene derivatives

The synthesis and structural characterization of a new hexahelicene derivative 2 is reported. This compound features a new type of structure bearing a bromine atom and a methoxy group. Suitable crystals of the latter indicate that its conformation closely resembles that of the unsubstituted [6]helicene, whose idealized symmetry is C2. This 3-bromo-14-methoxyhexahelicene was subjected to lithiation/phosphinilation and yielded the new 3-methoxy-14-(diphenylphosphino)hexahelicene 3.     

Synthesis,Properties, and Two‐Dimensional Adsorption Characteristics of 5‐Amino[6]hexahelicene

A convergent synthesis of racemic 5‐amino[6]hexahelicene is described. Cross‐coupling reactions are used to assemble a pentacyclic framework, and a metal‐catalyzed ring‐closure comprises the final step. The enantiomers were separated by means of chromatography and the absolute configurations were assigned by comparison of the CD spectra with hexahelicene. The t_1/2 value for racemization at 210 °C was approximately 1 hour. Scanning tunneling microscopy (STM) measurements were carried out on enantiopure and racemic samples of aminohelicene on Au(111) under ultrahigh vacuum (UHV) conditions.     

Synthesis and resolution of a new thiahexahelicene

A new thiahexahelicene bearing a methoxy group, at a selected position on the terminal aromatic ring, was prepared in a good yield through Heck coupling and photocyclization reactions. The X-ray crystal structure of the product indicates that its conformation closely resembles that of the unsubstituted hexahelicene, whose idealized symmetry is C₂. Both enantiomers of the new helically chiral hexacyclic system have been successfully separated using (R)-2-phthalimidopropanoic acid chloride as a chiral resolving agent.     

Synthesis,X-Ray Analysis,and Evaluation of the Optoelectronic Properties of a New Thia[6]helicene

The synthesis and structural characterization of a new hexahelicene derivative is reported. This compound features a new type of structure bearing a bromine atom. Suitable crystals of the latter indicate that its conformation closely resembles that of the unsubstituted [6]helicene, whose idealized symmetry is C2. The optoelectronic properties of this helical hexacyclic system were determined and exhibit a very interesting behavior.     

The crystal and molecular structure of 9-methylphenanthro[4,3-a]dibenzothiophene

The crystal structure of 9-methylphenanthro[4,3-a]dibenzothiophene, C₂₅H₁₆S, M_r = 348.47, has been determined. Monoclinic, P2₁/c, a = 11.364(3), b, = 14.257(3), c = 11.575(2)Å, β = 116.26(2)°, V = 1681.9(7)ų, Z = 4, D_x = 1.38 g/cc, MoKα radiation λ = .71069 Å, F(000) = 728, T = 163K, R = .0458 for 2330 reflections. The structure compares favorably with that of hexahelicene and methylated derivatives. The thiophene moiety increases the helical core radius and decreases the pitch with respect to hexahelicene and its derivatives.     

Extended DLVO interactions between spherical particles and rough surfaces

An "extended DLVO" approach that includes Lifshitz-van der Waals, Lewis acid-base, and electrostatic double layer interactions is used to describe interaction energies between spherical particles and rough surfaces. Favorable, unfavorable, and intermediate deposition conditions are simulated using surface properties representing common aquatic colloids and polymeric membranes. The surface element integration (SEI) technique and Derjaguin's integration method are employed to calculate interaction energy. Numerical simulations using SEI demonstrate that nanometer scale surface roughness features can produce a distribution of interaction energy profiles. Local interaction energies are statistically analyzed to define representative interaction energy profiles-minimum, average, and maximum-for various combinations of simulated particles and surfaces. In all cases, the magnitude of the average interaction energy profile is reduced, but the reduction of energy depends on particle size, asperity size, and density of asperities. In some cases, a surface that is on average unfavorable for deposition (repulsive) may possess locally favorable (attractive) sites solely due to nanoscale surface roughness. A weighted average of the analytical sphere-sphere and sphere-plate expressions of Derjaguin reasonably approximates the average interaction energy profiles predicted by the SEI model, where the weighting factor is based on the fraction of interactions involving asperities.     

Model chemistry calculations of thiophene dimer interactions: origin of pi-stacking

The intermolecular interaction energies of thiophene dimers have been calculated by using an aromatic intermolecular interaction (AIMI) model (a model chemistry for the evaluation of intermolecular interactions between aromatic molecules). The CCSD(T) interaction energy at the basis set limit has been estimated from the MP2 interaction energy near the basis set limit and the CCSD(T) correction term obtained by using a medium-size basis set. The calculated interaction energies of the parallel and perpendicular thiophene dimers are -1.71 and -3.12 kcal/mol, respectively. The substantial attractive interaction in the thiophene dimer, even where the molecules are well separated, shows that the major source of attraction is not short-range interactions such as charge transfer but rather long-range interactions such as electrostatic and dispersion. The inclusion of electron correlation increases the attraction significantly. The dispersion interaction is found to be the major source of attraction in the thiophene dimer. The calculated total interaction energy of the thiophene dimer is highly orientation dependent. Although electrostatic interaction is substantially weaker than dispersion interaction, it is highly orientation dependent, and therefore electrostatic interaction play an important role in the orientation dependence of the total interaction energy. The large attractive interaction in the perpendicular dimer is the cause of the preference for the herringbone structure in the crystals of nonsubstituted oligothiophenes (alpha-terthienyls), and the steric repulsion between the beta-substituents is the cause of the pi-stacked structure in the crystals of some beta-substituted oligothiophenes.     

Rapid Syntheses of Hexa and Heptahelicene

A two-step synthesis of hexahelicene (scheme 1; 53% over-all yield), a three-step synthesis of hexaheliene (scheme 2; 33% over-all yield), and a two-step synthesis of heptahelicene (scheme 3; 14% over-all yield) are described.     

Vibrational Raman optical activity of pi-conjugated helical systems: hexahelicene and heterohelicenes

Helicenes constitute a special class of molecules combining helical conformation with pi-electron delocalization. These confer to helicenes specific chirooptical properties. In this article, we investigate the vibrational signatures thanks to the simulation of vibrational Raman optical activity (VROA) spectra. For that, four representative helicenes: hexahelicene, tetrathia-[7]-helicene, and its pyrrole and furan analogs have been simulated and interpreted using a recently implemented analytical scheme. Helicenes show intense VROA peaks attributed to their pi-conjugated structure and associated with collective vibrational modes. In hexahelicene, the dominant VROA features are due to vibrational modes involving motions of the carbon skeleton and H-wagging, but the intensity finds its source almost exclusively in the former. In the case of the three heterohelicenes, the previous statement is also verified, and on changing the heteroatoms, similar modes presenting comparable atomic contribution patterns have been highlighted, though the vibrational and electronic properties are modified. Some fingerprints could therefore be associated with the helicity of the system. In particular, in forward spectra, most of the VROA bands are positive for left-handed helicenes. Nevertheless, the spectral patterns are quite complex, and no easy rule-of-thumb could distinguish between the different heterohelicenes. Then, considering the fact that most of the contributions originate from the C atoms (group coupling matrices decomposition), it can be concluded that the major role of the heteroatom is restricted to modifying the geometry and the normal modes. At last, the small impact of the gauge-origin on the calculated spectra using a relatively modest basis set (rDPS:3-21G) is demonstrated here in the case of the tetrathia-[7]-helicene molecule presenting a C(2) symmetry. This further demonstrates the adequacy of this basis set for VROA calculations.     

How the stabilization of INK4 tumor suppressor 3D structure evaluated by quantum chemical and molecular mechanics calculations corresponds well with experimental results: interplay of association enthalpy, entropy, and solvation effects

The folding free energy of the INK4c tumor suppressor core, consisting of 10 helices, was determined as the sum of gas-phase interaction enthalpy, gas-phase interaction entropy, and dehydration and hydration free energy. The interaction energy and the hydration free energy were determined using the nonempirical density functional theory (DFT) method, augmented by a dispersion-energy correction term, the semiempirical density-functional tight-binding method covering the dispersion energy, and the density functional theory/conductor-like screening model (DFT/COSMO) procedure, whereas the interaction entropy was calculated with the empirical Cornell et al. force field. Alternatively, all contributions were evaluated consistently using empirical methods. All the values of the interaction energy of helix pairs are stabilizing, and the dominant stabilizing terms stem from the London dispersion energy and, in the case of charged systems, the electrostatic energy. The stabilization energy of the core, determined as the difference of the energy of the core and 10 separate helices, amounts to approximately 450 kcal/mol. Systematically, the difference in the hydration free energy of a helix pair and its separate components is smaller in magnitude than the interaction energy, and it is negative for some pairs while positive for others. The average total free energy of a core formation amounts to -29.6 kcal/mol (yielded by scaled quantum-chemical methods) and +13.9 kcal/mol (resulting from empirical methods). These values are considerably smaller than their single components, which are dominated by the interaction energy. The computationally predicted interval encloses the experimental value of the folding free energy (-2.8 kcal/mol).     

Photodehydrocyclizations of stilbene-like compounds—XI : Synthesis and racemization of the double helicene diphenanthro[4.3-a; 3′.4′-o]picene

The synthesis of a new double helicene, diphenanthro[4.3-a;3′.4′-o]picene, is described. Of the two possible diastereoisomers only one was obtained as a racemate. The compound was partly resolved with TAPA yielding fractions enriched with one or the other enantiomer. They could not be converted into the other diastereomer by heating, but racemized under these conditions at a relatively low temperature and with a rate comparable to that of the racemization of hexahelicene at the same temperature.The NMR spectrum is discussed.     

Genäherte Lösung der Hartree-Fock-Gleichungen wechselwirkender Moleküle im Bereich der geringen Überlappung

A perturbational approach has been developed in order to determine the interaction energy of two molecules in the region of small overlap in an attempt to obtain some information upon reactivity. The total wavefunction of the interacting molecules is set up as a single determinantal wavefunction, whose basisfunctions are constructed as linear combinations of all occupied and unoccupied molecular orbitals. The coefficients are determined by an approximate solution of the Hartree-Fock-Roothaan equations of the total system. The SCF-iteration is performed analytically until all first and second order terms are included in the expression for the interaction energy. Due to the analytical procedure of the SCF-cycle the interaction energy contains no numerically iterated quantities. The interaction between a molecule with an open shell and a molecule with a closed shell is treated in the framework of the unrestricted Hartree-Fock-approximation. The interaction energy in the molecular orbital basis is transformed into an atomic orbital basis by using the LCAO-MO-approximation and introducing approximations for several integrals. Useful expressions for calculations are also derived, which contain only quantities of the isolated molecules, together with interatomic distances and overlap integrals. These formulae for the interaction energy are suitable for detailed interpretation of the different interaction terms and reveal the role of the interacting centres.     

Synthesis,Properties, and Two‐Dimensional Adsorption Characteristics of [6]Hexahelicene‐7‐carboxylic acid

A convergent synthesis of racemic [6]hexahelicene‐7‐carboxylic acid by cross‐coupling of a bicyclic and a tricyclic component is described. A metal‐catalyzed ring‐closure is also a fundamental component of the synthetic approach. Scanning tunneling microscopy (STM) measurements of the racemate self‐assembled on Au(111) at liquid–solid interface revealed the formation of ordered racemic 2D crystals.     

H3势能面的键表方法计算

本文采用键表自洽场方法计算H₃体系的势能面,完成了用键表相互作用和多键表自洽场方法计算H₃体系的程序编写和计算,讨论了各种近似条件下键表方法的特点,得到一些对价键方法应用具有建设性意义的结论。