Die Abscheidung von kolloidalen Teilchen auf der Oberfl?che einer rotierenden Scheibe

Ohne Zusammenfassung     

On the pairing rules for recognition in the minor groove of DNA by pyrrole-imidazole polyamides

Background: Cell-permeable small molecules that target predetermined DNA sequences with high affinity and specificity have the potential to control gene expression. A binary code has been developed to correlate DNA sequence with side-by-side pairings between N-methylpyrrole (Py) and N-methylimidazole (lm) carboxamides in the DNA minor groove. We set out to determine the relative energetics of pairings of Im/Py, Py/Im, Im/Im, and Py/Py for targeting G·C and A·T base pairs. A key specificity issue, which has not been previously addressed, is whether an Im/Im pair is energetically equivalent to an Im/Py pair for targeting G·C base pairs.Results: Equilibrium association constants were determined at two five-base-pair sites for a series of four six-ring hairpin polyamides, in order to test the relative energetics of the four aromatic amino-acid pairings opposite G·C and A·T base pairs in the central position. We observed that a G·C base pair was effectively targeted with Im/Py but not Py/Im, Py/Py, or Im/Im. The A·T base pair was effectively targeted with Py/Py but not Im/Py, Py/Im, or Im/Im.Conclusions: An Im/Im pairing is energetically disfavored for the recognition of both A·T and G·C. This specificity will create important limitations on undesirable slipped motifs that are available for unlinked dimers in the minor groove. Baseline energetic parameters will thus be created which, using the predictability of the current pairing rules for specific molecular recognition of double-helical DNA, will guide further second-generation polyamide design for DNA recognition.     

Hund's rules

We review the present state of our undertanding of Hund's first and second rules, their domains of validity, and of generalizations in cases where the rules in their original form are invalid. These exceptions occur mainly in atomic configurations with more than one open shell withl ≥ 1, but also in cases with large orbital angular momentaL. We present a derivation of thealternating rule, which is, in some sense, a generalization of Hund's first rule, and present new rules, which generalize Hund's second rule. The importance of the concept ofunnatural parity states for an understanding of these rules is stressed. It is demonstrated that the lowest singlet-triplet average energy for the sameL corresponds to a pair of unnatural parity states (theunnatural parity rule). For sufficiently smalll ₁ andl ₂, the lowest average energy is that of the pair of states with the maximum possibleL (maximum-L rule), though exceptions are found already for moderately large values ofl ₁ andl ₂. Our analysis indicates that the validity of Hund's second rule is to some extent an accidental consequence of the minimisation of an elementary function ofl ₁,l ₂, andL — which does not depend monotonically onL — over states of unnatural parity, and that a more general and more fundamental rule should be formulated in terms of this function. We also discuss the generalisation of Hund's rules to molecules, as well as violations of them, with particular emphasis on the inversion of Hund's first rule by spin-polarization in molecules.     

Extension rules

The statement that the solubility is higher for metastable phases than for stable phases can be proved by a relatively simple imaginary experiment. Hence, it follows that the metastable extensions of the solubility limit curves, surfaces, and hypersurfaces in the phase diagrams of binary, ternary, and multicomponent systems cannot run in the single-phase solvent region. A special case for binary systems is the Hollmann’s rule.     

On the theoretical foundation of Walsh's rules of molecular geometry in terms of the Hellmann–Feynman theorem

A new interpretation of the ordinate in a Walsh diagram for a polyatomic molecule is suggested in terms of the Hellmann–Feynman theorem. This makes use of the fact that in a single-configurational MO wave function the total one-electron density is the sum of individual densities in the occupied orbitals. Walsh-type diagrams have been constructed for three different molecules, water, ammonia and hydrogen peroxide. In H₂O and NH₃ calculation of the force, and thus of the energy, in terms of the valence angle, is made on the assumption that the central (heavy) atom is kept fixed while each of the lighter atoms moves in a plane containing the principal symmetry axis and the relevant bond, in a totally symmetric fashion; for H₂O₂ the two oxygen atoms are kept fixed. The angular correlation diagrams obtained reproduce the general features of those obtained by plotting Hartree–Fock MO energies as functions of the valence angles. The conclusion emerges that the force formulation provides a satisfactory pictorial basis for understanding molecular geometry in terms of the balance between the electron–nucleus attractive forces resulting from the charge densities in the occupied MO'S , and the nuclear repulsive forces. However, in the absence of highly accurate charge distributions such an approach is unsuitable for the quantitative prediction of molecular quantities such as valence angles, force constants or energy barriers.     

Note on the significance of Walsh's rules

The success of Walsh's method is due to the fact that several large errors cancel out approximately. The ordinate of the correlation diagrams corresponds to the eigenvalues of the self-consistant hamiltonian.The authors wish to thank Professor L. D'Or for his interest in this work. Support from the Fonds de la Recherche Scientifique Fondamentale Collective is gratefully acknowledged.     

Connection between zero-pressure mixing rules and infinite-pressure mixing rules

Infinite-pressure mixing rules and zero-pressure mixing rules are related by a very simple equation. The zero-pressure models and the infinite-pressure models are interchangeable through this connection. This paper will show how to convert the Huron–Vidal infinite-pressure mixing rule to a zero-pressure mixing rule, or vice versa. The MHV1 zero-pressure mixing rule will be reformulated to satisfy second virial coefficient constraint. The Twu–Coon and Wong–Sandler infinite-pressure mixing rules will be modified to improve their accuracy for reproducing the incorporated G^E model. A simplification of the Twu–Coon–Bluck [TCB(r)] zero-pressure mixing rule will be presented. The MHV1 mixing rule will be shown to be a special case of TCB(r).     

Simple rules for the understanding of Heusler compounds

Heusler compounds are a remarkable class of intermetallic materials with 1:1:1 (often called Half-Heusler) or 2:1:1 composition comprising more than 1500 members. Today, more than a century after their discovery by Fritz Heusler, they are still a field of active research. New properties and potential fields of applications emerge constantly; the prediction of topological insulators is the most recent example. Surprisingly, the properties of many Heusler compounds can easily be predicted by the valence electron count. Their extremely flexible electronic structure offers a toolbox which allows the realization of demanded but apparently contradictory functionalities within one ternary compound. Devices based on multifunctional properties, i.e. the combination of two or more functions such as superconductivity and topological edge states will revolutionize technological applications. The subgroup of more than 250 semiconductors is of high relevance for the development of novel materials for energy technologies. Their band gaps can readily be tuned from zero to ≈4 eV by changing the chemical composition. Thus, great interest has been attracted in the fields of thermoelectrics and solar cell research. The wide range of their multifunctional properties is also reflected in extraordinary magneto-optical, magnetoelectronic, and magnetocaloric properties. The most prominent example is the combination of magnetism and exceptional transport properties in spintronic devices. To take advantage of the extremely high potential of Heusler compounds simple rules for the understanding of the structure, the electronic structure and the relation to the properties are reviewed.     

An exception to the CIDNP sign rules

It is demonstrated experimentally that the CIDNP sign rules can give erroneous results. Net effects observed in the ¹H NMR spectrum of 1-phenyl[2-¹³C]propan-2-one after photolysis are correctly predicted at 360 MHz but not at 100 MHz by the rules. The converse is true for the ¹H–¹³C multiplet effects. The radical pair theory can account for the CIDNP effects in all cases.     

Resistance distance local rules

In [D.J. Klein, Croat. Chem. Acta. 75(2), 633 (2002)] Klein established a number of sum rules to compute the resistance distance of an arbitrary graph, especially he gave a specific set of local sum rules that determined all resistance distances of a graph (saying the set of local sum rules is complete). Inspired by this result, we give another complete set of local rules, which is simple and also efficient, especially for distance-regular graphs. Finally some applications to chemical graphs (for example the Platonic solids as well as their vertex truncations, which include the graph of Buckminsterfullerene and the graph of boron nitride hetero-fullerenoid B ₁₂ N ₁₂) are made to illustrate our approach.     

On the hypervirial relation in the random phase approximation and the sum rules for ordinary and rotatory intensities in finite bases

It is shown that a one-electron hypervirial relation for transition amplitudes in the random phase approximation (RPA) follows immediately from the double commutator RPA formulation proposed by Rowe. For the transition energy-independent expressions for the ordinary and rotatory intensities it is shown that, in finite basis calculations, the sums of these intensities are independent of whether the single-transition approximation, the Tamm—Dancoff approximation or the RPA method is used. Specific results are quoted for a ab-initio minimal basis calculations on twisted ethylene.     

Defining rules for the shape evolution of gold nanoparticles

The roles of silver ions and halides (chloride, bromide, and iodide) in the seed-mediated synthesis of gold nanostructures have been investigated, and their influence on the growth of 10 classes of nanoparticles that differ in shape has been determined. We systematically studied the effects that each chemical component has on the particle shape, on the rate of particle formation, and on the chemical composition of the particle surface. We demonstrate that halides can be used to (1) adjust the reduction potential of the gold ion species in solution and (2) passivate the gold nanoparticle surface, both of which control the reaction kinetics and thus enable the selective synthesis of a series of different particle shapes. We also show that silver ions can be used as an underpotential deposition agent to access a different set of particle shapes by controlling growth of the resulting gold nanoparticles through surface passivation (more so than kinetic effects). Importantly, we show that the density of silver coverage can be controlled by the amount and type of halide present in solution. This behavior arises from the decreasing stability of the underpotentially deposited silver layer in the presence of larger halides due to the relative strengths of the Ag(+)/Ag(0)-halide and Au(+)/Au(0)-halide interactions, as well as the passivation effects of the halides on the gold particle surface. We summarize this work by proposing a set of design considerations for controlling the growth and final shape of gold nanoparticles prepared by seed-mediated syntheses through the judicious use of halides and silver ions.     

Symmetry rules for predicting the course of chemical reactions

Rules are given for a reaction of any molecularity which show whether it is allowed or forbidden by orbital symmetry properties. The rules are based on a rigorous treatment, but correspond to simple concepts in practice. A rough MO sequence for the reactants is all that is needed.

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Zusammenfassung Für Reaktionen beliebiger Molekularität werden Regeln dafür angegeben, ob die Reaktion aus Symmetriegründen erlaubt oder verboten ist. Diese Regeln werden streng hergeleitet, sind jedoch in der Praxis einfach anwendbar. Benötigt wird lediglich die Reihenfolge der MO der Reaktionen.

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Résumé On donne des règles de sélection pour une réaction de molécularité arbitraire selon les propriétés de symétrie orbitale. Les règles sont basées sur un traitement rigoureux mais correspondent en practique à des concepts simples. On a seulement besoin de connaître l'ordre des orbitales moléculaires des réactants.

     

Selection rules for self-formation in the molecular nanotechnology

Self-formation in molecular nanotechnology is based on interaction between organic and biological molecules. Selection rules of the distinct interactions of the symmetric and topologically similar molecule self-formation are investigated.     

New sum rules for the generalized oscillator strength

Partial sum rules for the generalized oscillator strength are reported. The summation runs here over all final target states having a fixed value of the conserved total angular momentum. Total and partial sum rules have been derived provided the initial target state is aligned. In particular it is shown that an alignment destroys the Bethe sum rule.     

On the origin of the rotational state dependence of the decay of intermediate case molecules. Role of angular momentum selection rules in intersystem crossing

Experimental and theoretical studies of the role of angular momentum in the decay behavior of ¹B_3u pyrazine show that the strong rotational state dependence has its origin in the breakdown of the ΔP = 0 (ΔK = ±1) selection rule for intersystem crossing. Some possible reasons for this are discussed.     

On selection rules of molecules having one,two and three CH3 tops

Symmetry groups are considered which are generated by various associated elements of the molecular symmetry group (PE-group) of the molecule. It is shown that these (associated) symmetry groups are essentially the same symmetry groups used earlier for the classification of the eigen functions and for evaluation of the selection rules for the rotation-torsion and torsional-vibration problem of molecules having one, two, and three CH₃ tops and C_s, C_2v, and C_3v symmetry, respectively.     

An ICSCF investigation of Walsh's rules

The ICSCF method is applied to the calculation of orbital energies as a function of bond angle for several AH₂ molecules. The resulting orbital energy diagrams are quite similar in appearance to the canonical SCF results even though the sum of the ICSCF energies is the SCF energy. The method is also applied to Li₂O, CO₂, HCN and a few AH₃ molecules with similar results. The sum of the ICSCF valence orbital energies generally correlates better with the equilibrium bond angle than does the similar sum of canonical orbital energies.