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Strings of gold-organic oligomers of polar units have been formed by on-surface synthesis and investigated with non-contact atomic force microscopy. The mutual alignment of dipoles within the strings is analyzed. While an alternating head-to-tail alignment might be expected from dipolar interactions, a more complicated alignment order is observed. The data suggests that coordination bonding to additional gold adatoms leads to stabilization of parallel pairs of molecules, suppressing a head-to-tail alignment order.

We refer to frequently used determinants suggesting dominant interactions between distant atoms in various dimers. First of all, we show, against the still‐prevailling opinion, that, in general, bond paths have nothing in common with dominant intermolecular interactions and therefore they are useless in such cases. Quite the contrary, reliable information about dominant intermolecular interactions can be obtained by means of electrostatic potential maps, which very convincingly explain mutual orientation of molecules in a dimer. For the first time, numerous examples of interactions that violate both the concept of privileged exchange channels proposed by Pendás and his collaborators as well as inequalities obtained by Tognetti and Joubert for the β parameter related to secondary interactions are presented. The possible cause of this violation is suggested. We also show that the so‐called counterintuitive bond paths result from quite natural behavior of the electron density gradient vector, i. e. searching for those areas of space that are characterized by large values of electron density or the most expanded its distributions.

The path of the Norrish–Yang reaction of methylammonium 4-(2,4,6-triisopropylbenzoyl)benzoate was studied by means of X-ray structure analysis. The following parameters influenced by this photochemical reaction were monitored: (a) intramolecular distances and angles in the reaction center, (b) the size of the free space near the reactive atoms, (c) the mutual orientation of molecular fragments, (d) the cell parameters, and (e) the product content. Product molecules were created in two modes, namely, by the reaction of the 2-isopropyl or 6-isopropyl group, which has not been revealed previously for other 2,4,6-triisopropylbenzophenones. The 2-isopropyl group took part in the photochemical reaction during the whole crystal transformation, whereas the reaction of the 6-isopropyl group started with a delay and ceased before the total crystal conversion. The reason for such behavior was explained by the analysis of changes in the free space near both o-isopropyl groups with the crystal phototransformation.

For interaction energy of mesogenic model rod-like molecules an analytical expression is obtained. Interaction energy of dipole moments interaction is much less than the total interaction energy. T...

Photoactive molecules that reversibly switch upon visible light irradiation are one of the most attractive targets for biological as well as imaging applications. One possible approach to prepare such photoswitches is to extend π-conjugation length of molecules and shift the absorption bands to longer wavelengths. Although several attempts have been demonstrated based on this approach for diarylethene (DAE) photoswitches, photoreactivity of the DAE derivatives is dramatically suppressed when the conjugation length is extended by connecting aromatic dyes at the side positions of aryl groups in the DAE unit. In this study, we successfully prepared a visible-light reactive DAE derivative by introducing an aromatic dye at the reactive carbon atom of the DAE unit, optimizing orbital level of each component, and controlling the mutual orientation of the aromatic dye and the DAE unit. The DAE derivative (3) undergoes a photocyclization reaction upon irradiation with 560 nm light and the closed-isomer converts to the open-ring isomer upon irradiation with 405 nm light. The high photoconversion yields (90%) were achieved for both photocyclization and photocycloreversion reactions. The photoreactivity induced by visible light irradiation and the molecular design strategy were discussed based on theoretical calculations.

The co-crystals of pyrazine with dicarboxylic acids of the form (COOH)–(CH2)n–COOH: oxalic (n = 0), malonic (n = 1) succinic (n = 2), glutaric (n = 3), adipic (n = 4), and pimelic (n = 5) acid, were synthesized and structurally characterized by means of X-ray diffraction. There are two principal structural motifs, depending on the stoichiometry: for 1:1 co-crystals, there are chains of alternate pyrazine and acid molecules, connected by strong O–H···N hydrogen bonds (···pyr···acid···pyr···acid···) while for 2:1 stoichiometry the chains are made of pyrazine molecules and carboxylic acid centrosymmetric dimers (···pyr···acid···acid···pyr···). The first possibility is observed for all acids in the series, while the second one only for these with odd numbers of methylene groups. The geometrical features of the acids do not show the clear indication for a reason of such behavior, the most probable reason can be connected with the possible symmetry of the acid molecule, C i for n even, and with the mutual orientation of terminal carbonyl and hydroxyl groups, defined by improper O=C···C=O torsion angles, which are close to 180° for even and have much smaller values for odd n values.

In the present paper, we introduce a representation of the potential energy surface for the H(2)O...H(2) system based on orthogonal vectors, assuming that the two molecules are rigid. We represent the interaction potential by an expansion in real hyperspherical harmonics depending on the distance between the centers of mass of the two molecules and on four angles, which account for two contributions: an external one depending on the three angle variables which define the mutual orientation of the two molecules and an internal one expressed by the angle which describes the position of the oxygen atom in H(2)O with respect to the H(2)O...H(2) system. The surface was generated in the framework of the supermolecular approach, using the counterpoise-corrected interaction energies at the MP2/aug-cc-pVQZ level. Comparisons with other recent work are presented and features of the representation discussed.

Ab initio molecular dynamics simulations are applied to a study of hydration structure of a chain-like molecule containing vinyl alcohol groups -[CH2-CHOH]n- with n = 4. Partial pair distribution functions and angle distributions are analyzed in order to estimate the features of the hydration shell. It is shown, that hydration process causes essential changes of mutual orientation of hydrophilic groups having on average less than two neighbor water molecules. A tendency of the hydrated poly(vinyl alcohol) chain fragment to backbone torsion is observed.

The emission properties of most fluorescent emitters, such as dye molecules or solid-state color centers, can be well described by the model of an oscillating electric dipole. However, the orientations of their excitation and emission dipoles are, in most cases, not parallel. Although single molecule excitation and emission dipole orientation measurements have been performed in the past, no experimental method has so far looked at the three-dimensional excitation and emission dipole geometry of individual emitters simultaneously. We present the first experimental study, using defocused imaging in conjunction with radially polarized excitation scanning, to measure both the excitation as well as emission dipole orientations of single molecules, which allows us to sample the distribution of their mutual orientation. We find an unexpectedly broad distribution of the angle between both dipoles which we attribute to the interaction between the observed molecules and the substrate they are immobilized on.

A phenomenological scenario is suggested in an attempt to explain controversies in the published data concerning the effect of orientational order on electronic transport in solid C60. The main factor of crucial importance is the dependence of the charge hop probability on the mutual orientation of the molecules. Magnitude and the sign of the conductivity response to the changes in the rotational state of C60 across the orientational phase transition are the results of the interplay between the changes in dynamic and static disorder that accompany the transition and may vary in the samples with different structure perfection.

The paper introduces the concept of using three independent, macroscopic factors affecting mutual orientation of the reactant molecules to accomplish absolute asymmetric synthesis. Unlike with other methodologies of asymmetric synthesis in physical fields, none of the utilized factors is chiral in itself (as, e.g., circularly polarized light would be), but the combination of the three constitutes a macroscopically chiral influence. Examples of applicable directional factors are time-even vector fields (e.g., electric field and the directional effects of surfaces and interfaces) and the time-odd directional transport with encounter control. The directional factors employed may act simultaneously or, if their effect can be preserved, consecutively, thus allowing, e.g., a repeat use of the electric field. The electric field strength needed to achieve a practically useful degree of molecular orientation was estimated to be ca. 3 MV/cm, which is now commonly achieved with organic materials in the area of nonlinear optics. Practical implications are discussed, as well as the implications for the origins of natural homochirality.Key words: macroscopic chirality, absolute asymmetric synthesis, homochirality, physical fields.

In the context of the Silberstein theory, by introducing model atoms, the polarizability of pairs of interacting molecules N2-N2, O2-O2, and N2-O2 was studied in relation to the mutual orientation and the intermolecular distance of the molecules in the pairs. The polarizability tensor for the equilibrium configurations of the (N2)2 and (O2)2 dimers was calculated.

A new phosphaazaallene Mes*PCNAr (Mes*=2,4,6- t Bu 3 C 6 H 2 , Ar= p -ClC 6 H 4 ) ( 1 ) has been synthesized. Complex 1 reacts with Mes*PHSiMe 2 t Bu to form the phosphaazaallene insertion product Mes*PC(PHMes*)NAr(SiMe 2 t Bu) ( 2 ). The treatment of 1 with an excess amount of YCl 3 in THF leads to the isolation of the symmetrical dimer Mes*P(μ-CNAr) 2 PMes* ( 3 ), indicating both of the PC and CN bonds of 1 are reactive sites. X-ray structure analysis of 1 indicates that the distance of the PC bond, 1.642(5) A, is markedly shorter than those of isolated PC bonds (1.66–1.72 A), and the NCP unit slightly deviates from linearity with the PCN angle of 170.8(4)°. The structure determination of 2 revealed the molecule adopts a planar conformation about PC bond and the mutual orientation of two bulky Mes* moieties is almost orthogonal (92.8°). Complex 3 is a symmetrical four-membered ring structure in which the Mes* rings lie nearly perpendicular to the planes of the Ar ring and the P 2 C 2 ring (84.8 and 75.6°, respectively), while the Ar ring is almost parallel to the plane of the P 2 C 2 ring (25.2°).

Our earlier model of re-entrant liquid crystalline phases exhibited by highly polar compounds, in which the mutual orientation of near-neighbour molecules can change from an antiparallel to a parallel configuration, has been extended to include both nematic and smectic interactions. We show that, as the McMillan parameter alpha is decreased, the SmAd-SmA1 line goes over to the SmAd-N1Re line, finally becoming the NdRe-N1Re transition line, the latter ending in a critical point. This sequence is in agreement with the predictions of Prost's Landau model as well as with an experimental result. The phase sequence N-SmAd-NdRe-N1Re-SmA1 is obtained on cooling for a range of alphaA values.

In the framework of studying the nature of the low-temperature phase transition in p-(N,N′-dimethylamino)benzonitrile (DMABN) crystals, the influence of intermolecular interactions on the electronic structure of the molecule was modelled by quantum-chemical calculations using the SCF MO method on the 3-21 G 6-31 G ∗ basis. It was shown that a small variation in the mutual orientation of the molecules at a phase transition leads to transformations in their electronic structure, which can be interpreted as a change in the thermodynamic parameters of the DMABN molecules in the crystal. These data suggest that the changes of electronic structure of molecules in the crystal caused the low-temperature phase transition in the DMABN crystals.

Within a theoretical approach to the problem of antifungal action of Amphotericin B (AmB), a conformational analysis of the neutral and zwitterionic form of this antibiotic in vacuo was performed by the MM2P and AM1 methods. The analysis was carried out with regard to the mutual orientation of the macrolidic and glycosidic fragments of the molecule, which is defined by the φ and ψ steric angles. This orientation defines the overall shape of the molecule and is postulated to be important for the antifungal action of the drug. As a result of the MM2P calculations, φ, ψ steric energy and population maps were prepared. Several conformers were found on these maps but only two of them (one each for the zwitterionic and the neutral forms of the antibiotic) were previously observed experimentally for isolated molecules. Our other calculated conformers were not observed experimentally but we propose that they may also appear in the AmB channel structure. The results of our conformational analysis were compared with experimental NMR data (nuclear Overhauser effects between selected hydrogen atoms) obtained previously [1]. New structural information obtained for AmB in the present work will be useful for building a molecular model of AmB-target interactions as well as for designing new derivatives of AmB.

Weak intermolecular interactions are not only important in modern supramolecular chemistry, they hold the organic world together and are responsible for the very existence of liquids and solids. A characteristic feature of weak interactions is the phenomenon of enthalpy-entropy compensation. Information about weak interactions in crystals is obtained from packing patterns, which provide tests for the quality of atom-atom force fields. On melting a solid, most of the cohesive energy is retained in the liquid state, even though the optimal mutual orientation of interacting parts of molecules is now opposed by thermal disorder. For liquids, reliable structural information is hard to come by, but extensive thermodynamic data are available for certain classes of compounds.

Crystals of the 1-chloroanthracene photodimer, viz. trans-bi(1-chloro-9,10-dihydro-9,10-anthracenediyl), C(28)H(18)Cl(2), were obtained from the solid-state [4+4]-photodimerization of the monomer, C(14)H(9)Cl, followed by recrystallization. The symmetry of the product molecules is defined by the orientation of the reactant molecules in the crystal. The mutual orientation parameters calculated for adjacent monomers explain the reactivity of the compound. The molecules in the crystal of the monomer and the recrystallized photodimer pack differently and the photodimer has crystallographically imposed inversion symmetry.

We use a lattice model with orientation-dependent interactions to study network-forming fluids, such as water and silica. Bonds can form between pairs of molecules that have correct mutual orientation and correct separation; they can be weakened by the presence of other molecules sufficiently close to a bonded pair. These interactions give rise to competition between bonded states of low energy, density, and entropy and nonbonded states of high energy, density, and entropy. By suitable choice of parameters, the mean-field solution of the model yields the two different scenarios (stability limit conjecture and second critical point) that have been proposed to explain the anomalous behavior of supercooled water. The model's generality suggests that similar behavior can occur in other network-forming fluids.

A molecular dynamics statistical mechanical simulation study of solvent‐induced forces (SIFs) acting between two fixed model solutes in a bath of 727 water molecules is reported. Simulations were run up to 500 ps on a multiprocessor system, involving 60 T800‐20 transputers. Results are discussed within the statistical mechanical approach of inherent structures. It is shown that SIFs are associated with the occurrence of nonpermanent, but longer‐lived statistically relevant solvent configurations promoted by solutes. Their size and sign are shown to be critically dependent on the type of solute, solute–solute distance, and mutual orientation. These features give SIFs the specificity character crucially required for functional interactions and for functional conformation and dynamics of biomolecules.

Syntheses of the 1,8-dipyrenylnaphthalenes 1-3 are reported. The stereoisomers 1 and 2 were separated; their structural assignment is based on 1H NMR, on the optical activity of 2, and on X-ray structure analyses of 1 and 2. Kinetic parameters for the isomerisation 2 = 1 were determined by optical rotation measurements. - Emission spectra of 1 - 3 are discussed in comparison to monopyrenyl compounds 4 and 8. For 1 and 3 typical 'excimer-like' fluorescence is observed. The difference between 1 and 2 clearly demonstrates the dependence of excimer interactions between the pyrene units on the mutual orientation of the π-systems involved. - On the basis of X-ray analyses the molecular structures of 1- 3 are discussed with emphasis on π···π-interactions between the pyrene units., Synthesen der 1,8-Dipyrenylnaphthaline 1 - 3 werden mitgeteilt. Die Stereoisomeren 1 und 2 wurden getrennt; ihre strukturelle Zuordnung basiert auf 1H-NMR, auf der optischen Aktivität von 2 und auf den Röntgenstrukturanalysen von 1 und 2. Kinetische Parameter für die Isomerisierung 2 = 1 wurden durch Messung der optischen Rotationen bestimmt. - Emissionsspektren von 1-3 werden im Vergleich zu den Monopyrenyl-Verbindungen 4 und 8 diskutiert. Für 1 und 3 wird eine typische "excimeren-artige" Fluoreszenz beobachtet. Der Unterschied zwischen 1 und 2 demonstriert deutlich die Abhängigkeit der Excimeren-Wechselwirkung zwischen den Pyren-Einheiten von der gegenseitigen Orientierung der beteiligten π-Systeme. - Auf der Grundlage von Röntgen-Strukturanalysen werden die Molekülstrukturen von 1 - 3 im Hinblick auf die π···π Wechselwirkung zwischen den Pyren-Einheiten diskutiert.

The triplet–triplet (T-T) energy transfer is one of the fundamental problems in condensed state physics. From exchange-resonant nature of T-T energy transfer it follows that the efficiency of energy transfer must depend on the separation and mutual orientation of the energy donor and acceptor. To investigate experimentally the influence of the orientational and separation factors on the efficiency of intermolecular energy transfer, we took advantage of the Langmuir–Blodgett (LB) technology which allowed donor-acceptor systems to be formed with predicted separation and mutual orientation of molecules.

In the 1980s and 1990s, x-ray studies of the photochemical reaction course in crystals dealt with the analysis of changes in cell constants or movements of atom groups inside molecules. This review presents the results of crystallographic studies on the monitoring of the behaviour of whole molecules in organic crystals during photochemical reactions. Papers on this subject started to appear only a few years ago. The studies showed quantitatively that reactant and product molecules do not take a fixed position in a crystal during the reaction. The product molecules move smoothly to a position assumed in the pure product crystal and the reactant molecules move from a position occupied in the pure reactant crystal. Moreover, with the reaction progress the adjacent reactant molecules gradually come closer and change their mutual orientation to resemble the product. The analysis of the photoreaction kinetics in crystals is also presented. Copyright © 2004 John Wiley & Sons, Ltd.

The intramolecular energy transfer (IET) in bifluorophoric molecules was researched experimentally and theoretically with the quantum-chemical method. It is shown, that IET realizes through the internal conversion process in bifluorophoric molecules depends on a mutual orientation of a donor and an acceptor and a length of (CH 2 ) n - bridge. The coefficient of IET and the lasing efficiency of the bifluorophores were estimated experimentally.

A new algorithm for quantitative assessment of similarity between two atoms in molecules is presented. Both the atomic similarity index and its derivatives with respect to the three Euler angles that describe the mutual orientation of the atoms under comparison are computed efficiently by taking advantage of the recently developed analytical representations for atomic zero-flux surfaces. The use of such representations makes it possible to substantially enhance the accuracy of the computed similarity indices without increasing the cost of their evaluation. Numerical tests involving oxygen atoms in several carbonyl compounds demonstrate the ability of the new algorithm to discern small changes in atomic similarity that are brought about by second-neighbor effects. Comparisons among hydrogen atoms in the acrolein molecule reveal the usefulness of the similarity index in detection and quantification of the effects of steric interactions on atomic shapes. © 1996 by John Wiley & Sons, Inc.

INTRODUCTIONOne of the major new trends in the design of indicators for optically imaging biochemical and physiological functions of living cells has been the exploitation of fluorescence resonance energy transfer (FRET). FRET is a well-known spectroscopic technique for monitoring changes in the proximity and mutual orientation of pairs of chromophores. It has long been used in biochemistry and cell biology to assess distances and orientations between specific labeling sites within a single macromolecule or between two separate molecules. More recently, macromolecules or molecular pairs have been engineered to change their FRET in response to biochemical and physiological signals such as membrane potential, cyclic AMP (cAMP), protease activity, free Ca2+ and Ca2+-calmodulin (CaM) concentrations, protein-protein heterodimerization, phosphorylation, and reporter-gene expression. Because FRET is general, nondestructive, and easily imaged, it has proven to be one of the most versatile spectroscopic readouts available to the designer of new probes. FRET is particularly amenable to emission ratioing, which is more reliably quantifiable than single-wavelength monitoring and better suited than excitation ratioing to high-speed and laser-excited imaging. This article summarizes the photophysical principles of FRET and the types of indicators used.

Quantum chemical calculations of simple potential energy maps for system composed of toluene and oxygen moieties are presented. Reaction pathways following from energy profiles for interaction between reagents in different mutual orientations are discussed.

The dependence of Forster long-range resonance energy transfer efficiency on the orientational freedom of donor D and acceptor A molecules attached to a macromolecular substrate is examined. The usefulness of polarized emission measurements in determining the mutual orientation as well as the degree of orientational freedom of D and A and thereby deriving maximum and minimum values for the D–A separation from the transfer efficieny is demonstrated.

The relative orientation of benzene and aromatic ring planes in condensed phases of organic compounds is examined. Directional preferences are analyzed in terms of electrostatic and dispersion-repulsion empirical formulations. The atomic charge model is reviewed, particularly the definition of charges and their use in empirical potentials. Data are presented on the distribution of interatomic contact distances and on the orientation of benzene rings in hydrocarbon crystals; interplanar angles between 10° and 30° are not preferred, but not entirely forbidden. The forces are simple to model, but their interplay for complex molecules is not easily understood.

Organic luminophores of the azlactone series that contain a strong electronacceptor substituent, viz., a difluoromethylsulfonyl group, were synthesized. The significant effect of the electronic nature of the substituents on the frequencies of the stretching vibrations of the CO and C=N groups was demonstrated, and the trans configuration of the investigated compounds was confirmed. The introduction of a difluoromethylsulfonyl group in the azlactone molecule promotes the development of intense fluorescence in solutions at room temperature. The character of the substituents and their mutual orientation have a substantial effect on the optical characteristics.

We have investigated the dependence of the efficiency of electron tunneling between planar cyclic molecules on their mutual orientation. An expression has been obtained for the dependence on orientation of the tunneling probability in terms of the “cyclic polyene” model. The magnitude of the orientation effect has been estimated for a change in the orientation of the molecules from the face-to-face to the edge-to-edge position.

1. The nature and mutual arrangement of the ring substitutents are the principal factors affecting the choice of fragmentation pattern in the breakdown of the trialkylcyclopentanes. 2. Quantitative differences in the spectra of structural isomers and stereoisomers can be used for purposes of identification. 3. The possibility that the fragmenting ion retain the mutual orientation of the substituents found in the parent molecule is discussed.

In Langmuir-Blodgett films containing organic dyes, efficient energy transfer over distances exceeding 100 nm is observed. This exceptionally long-range transfer is interpreted as due to special mutual orientation of the dye molecules.

1. In carbon tetrachloride the molecules of 1,1-diphenyl-1,2-dichloro- and 1,1-diphenyl-1, 2-dibromoethanes coexist in the form of a mixture of rotational isomers with two-thirds of the form having the trans arrangement of the C-Hal bonds. 2. In both compounds one of the phenyl rings eclipses the ethane C-C bond, irrespective of the nature of the halogen and the mutual orientation of the C-Hal bonds; the second ring in the trans isomer is orthogonal to it, while in the gauche isomer it is close to eclipsing the geminal C-Hal bond.

Intramolecular triplet-triplet energy transfer between nonconjugated molecular chromophores held together by rigid molecular frame at known separation distance and mutual orientation

(1) By comparison of the data on the photochemical decomposition and the azomethane‐induced decomposition of acetaldehyde vapor, it has been concluded that the chain‐carrying steps in the two cases are different. For the photolysis the following mechanism is suggested: (1)CH3CHO+hν→CH3+CHO, (2)CHO+CH3CHO→CH4+CO+CHO, (3)CHO+CH3→stable products (chain−ending),(4)CHO→stable products (chain−ending). (There are possibly other chain‐ending steps similar to reaction (3)). (2) Experiments have been performed using intermittent light at 200° and 300°. As a result of these experiments, estimates have been made for the individual rate constants, k2, k3, and k4, and it has been found that the activation energies are 2.6, 11.3, and 7.8 kcal., respectively. (3) It has been found that k2 and k3 check closely with the collision number multiplied by the appropriate Arrhenius factor. There is little or no evidence of necessity for mutual orientation of the molecules. The significance of this has ...

A simple model of a solution is constructed, assuming that the interaction energies of the molecules depend on their mutual orientation. The thermodynamic properties of the model are calculated by the combinatory method of the strictly regular solution. Graphs of the excess free energy, entropy, and heat of mixing are obtained numerically for a number of typical cases.

A theoretical expression is derived for the depolarization factor of the Rayleigh light scattered by a system of anisotropic molecules, which have a center of inversion, in the dense gas region. This expression is a generalization of previous theories in that it contains besides the anisotropy arising from the structure of the molecules also anisotropy effects coming from the mutual orientation and from the induced dipole interaction. A qualitative discussion of the results leads to the conclusion that the depolarization factor is finite in the critical region. Numerical calculations have been carried out for CO2; the results are compared with those of the existing theories and with provisional experimental data.

Aggregation of organic molecules can drastically affect their physicochemical properties. For instance, the optical properties of BODIPY dyes are inherently related to the degree of aggregation and the mutual orientation of BODIPY units within these aggregates. Whereas the noncovalent aggregation of various BODIPY dyes has been studied in diverse media, the ill-defined nature of these aggregates has made it difficult to elucidate the structure–property relationships. Here, we studied the encapsulation of three structurally simple BODIPY derivatives within the hydrophobic cavity of a water-soluble, flexible PdII6L4 coordination cage. The cavity size allowed for the selective encapsulation of two dye molecules, irrespective of the substitution pattern on the BODIPY core. Working with a model, a pentamethyl-substituted derivative, we found that the mutual orientation of two BODIPY units in the cage’s cavity was remarkably similar to that in the crystalline state of the free dye, allowing us to isolate and characterize the smallest possible noncovalent H-type BODIPY aggregate, namely, an H-dimer. Interestingly, a CF3-substituted BODIPY, known for forming J-type aggregates, was also encapsulated as an H-dimer. Taking advantage of the dynamic nature of encapsulation, we developed a system in which reversible switching between H- and J-aggregates can be induced for multiple cycles simply by addition and subsequent destruction of the cage. We expect that the ability to rapidly and reversibly manipulate the optical properties of supramolecular inclusion complexes in aqueous media will open up avenues for developing detection systems that operate within biological environments.

A recent theory of the circular dichroism induced in the absorption bands of an achiral molecule (A) in the presence of a chiral molecule (C) is re-examined. The ICD theory for the case of a fixed mutual orientation of (A) and (C) is extended and applied to the case of a mutually random orientation of the two molecules. The treatment spans the related ICD effects of the change in the CD spectrum of a chiral molecule induced by an achiral species, and optical activity changes due to the mutual perturbation of two chiral molecules. The macroscopic ICD effect arising from the differential left and right circular radiation field in a chiral medium is considered, together with order-of-magnitude estimates of the ICD effects.

We calculated the probability of proton transfer within the framework of a two-dimensional model with an arbitrary orientation of the directions of the ruptured and formed bonds. It was shown that the overlap value of the proton wave functions determines the most favorable orientation of the reactants.

Syntheses of the 1,8-dipyrenylnaphthalenes 1-3 are reported. The stereoisomers 1 and 2 were separated; their structural assignment is based on 1H NMR, on the optical activity of 2, and on X-ray structure analyses of 1 and 2. Kinetic parameters for the isomerisation 2 = 1 were determined by optical rotation measurements. - Emission spectra of 1 - 3 are discussed in comparison to monopyrenyl compounds 4 and 8. For 1 and 3 typical 'excimer-like' fluorescence is observed. The difference between 1 and 2 clearly demonstrates the dependence of excimer interactions between the pyrene units on the mutual orientation of the π-systems involved. - On the basis of X-ray analyses the molecular structures of 1- 3 are discussed with emphasis on π···π-interactions between the pyrene units. Synthesen der 1,8-Dipyrenylnaphthaline 1 - 3 werden mitgeteilt. Die Stereoisomeren 1 und 2 wurden getrennt; ihre strukturelle Zuordnung basiert auf 1H-NMR, auf der optischen Aktivitat von 2 und auf den Rontgenstrukturanalysen von 1 und 2. Kinetische Parameter fur die Isomerisierung 2 = 1 wurden durch Messung der optischen Rotationen bestimmt. - Emissionsspektren von 1-3 werden im Vergleich zu den Monopyrenyl-Verbindungen 4 und 8 diskutiert. Fur 1 und 3 wird eine typische "excimeren-artige" Fluoreszenz beobachtet. Der Unterschied zwischen 1 und 2 demonstriert deutlich die Abhangigkeit der Excimeren-Wechselwirkung zwischen den Pyren-Einheiten von der gegenseitigen Orientierung der beteiligten π-Systeme. - Auf der Grundlage von Rontgen-Strukturanalysen werden die Molekulstrukturen von 1 - 3 im Hinblick auf die π···π Wechselwirkung zwischen den Pyren-Einheiten diskutiert.

The method for determining the mutual orientation of molecular interaction tensors in powders is described. The technique is based on 2D nutation exchange NQR and NMR spectroscopy. It is shown that the 2D nutation exchange spectra exhibit characteristic ridges, which reflect the motional mechanism in a model-independent fashion. The angles through which the molecule rotates can be read from elliptical ridges in the 2D spectra. The 2D nutation exchange NMR and NQR powder patterns are calculated for spins I = 1 and I = 3/2 for different symmetry of reorienting molecular groups.

A detailed 1H n.m.r. analysis of a series of trans-dihalogenobis(hydrazone)palladium(II) complexes has been made in order to demonstrate unequivocally the presence of two rotational isomers differing in the mutual orientation of the co-ordinated ligands.