Showing papers on "Intramolecular force published in 1988"

Intramolecular Long-Distance Electron Transfer in Organic Molecules

Abstract: Intramolecular long-distance electron transfer (EI) has been actively studied in recent years in order to test existing theories in a quantitative way and to provide the necessary constants for predicting ET rates from simple structural parameters. Theoretical predictions of an "inverted region," where increasing the driving force of the reaction will decrease its rate, have begun to be experimentally confirmed. A predicted nonlinear dependence of ET rates on the polarity of the solvent has also been confirmed. This work has implications for the design of efficient photochemical charge-separation devices. Other studies have been directed toward determining the distance dependence of ET reactions. Model studies on different series of compounds give similar distance dependences. When different stereochemical structures are compared, it becomes apparent that geometrical factors must be taken into account. Finally, the mechanism of coupling between donor and acceptor in weakly interacting systems has become of major importance. The theoretical and experimental evidence favors a model in which coupling is provided by the interaction with the orbitals of the intervening molecular fragments, although more experimental evidence is needed.

Intramolecular vibrational excitations accompanying solvent‐controlled electron transfer reactions

Abstract: The theory of dynamic solvent effects on outer‐sphere electron transfer (ET) was extended to incorporate the modification of the high‐frequency quantum modes, which is manifested by the reduction of the electronic coupling by nuclear Franck–Condon factors and by the change of the energy gap Explicit expressions for the ET rates were obtained in terms of a sum over parallel vibronic channels, each involving a distinct intramolecular vibrational excitation of the final state In the solvent‐controlled adiabatic limit, the effects of intramolecular vibrational excitation are exhibited by the modification of the (partial) activation energies, while the frequency factor is dominated by the longitudinal dielectric relaxation rate of the solvent

Determination of long distance intramolecular triplet energy transfer rates. A quantitative comparison with electron transfer

Abstract: Recent efforts in these laboratories have been directed toward understanding the factors governing long distance intramolecular electron transfer (ET). In the models chosen for study the electronic coupling between donor and acceptor is sufficiently weak to assure nonadiabatic reactions. It occurred to them that long distance triplet energy transfer by the Dexter mechanism (TT) of which there are several examples in the literature should exhibit features similar to nonadiabatic electron transfer because both reactions are governed by the same theory of radiationless transitions. They therefore have started a program aimed at finding quantitative similarities and differences in these two processes when studied on directly comparable systems. Also, with one exception, the absolute rates of intramolecular triplet energy transfer have never been measured directly in liquid solution. In this communication they report their first results and conclusions.

Synthetic Applications of Intramolecular Enone-Olefin Photocycloadditions

Abstract: The intramolecular enonmlefin photocycloaddition, the light-induced [2+2] cycloaddition of a ground-state olefin tethered to an excited-state enone to form a cyclobutane, was first reported by Ciamician in 1908 when he observed the formation of carvone camphor on prolonged exposure of carvone to \"Italian sunlight\"' (Figure 1). Buchi's reinvestigation of this process confirmed the isomerization and sparked new interest in the reaction? The rapidity with which complex systems could be constructed by this method was recognized by Cookson3 who irradiated the Diels-Alder adduct of quinone and cyclopentadiene and by Eaton' who utilized a similar intramolecular photocycloaddition in his synthesis of cuhane. This reaction was first applied to the total synthesis of a natural product by Wiesner who prepared 12-epilycopodine utilizing an intramolecular photocycloaddition5 (Figure 2). Corey: Eaton,' and de May0,8.~ among others, subsequently began to investigate the intermolecular enone-olefin photocycloaddition, and these studies culminated in several successful synthetic applications, including the synthesis of caryophyllene'O and bourbonene,'l as well as a proposed mechanistic rationale for the reaction. While the major disadvantage of the intermolecular photocycloaddition is its low regioselectivity in some systems, this problem can be substantially overcome by incorporating the olefin and the enone in the same molecule. Although many of the early examples of the reaction were intramolecular, this variation saw only limited use until the late 1970s when its potential for the rapid construction of systems of

Bonding and structure of water molecules in solid hydrates. Correlation of spectroscopic and structural data

Abstract: Solid hydrate research of the last fifteen years is critically evaluated with regard to bonding and structure of water molecules. This review focusses on new results of structure determination and infrared and Raman studies in terms of hydrogen bonding and other intermolecular bonding interactions, distortion and disorder of water molecules, intermolecular and intramolecular coupling and anharmonicity of water bands, isotopic effects, and phase transitions. The techniques used for structure determination and spectroscopic measurements of solid hydrates are discussed.

Intramolecular vibrations of small water clusters

Abstract: The structures, binding energies, and vibrational frequencies of fully optimized water clusters (H2O)n, n=1– 4, were computed with ab initio molecular orbital theory at the SCF level using different basis sets. The SCF procedure allows satisfactory predictions for these properties compared with experimental results. For quantitative predictions of binding energies and geometrical parameters, a basis set including polarization functions is needed. With respect to intramolecular vibrational frequencies and frequency shifts, however, the split valence basis set 4‐31G leads in all cases to the best rationalization of the available experimental data. Analysis of intramolecular force constants, frequencies, and eigenvectors for n=2 to 4 shows that (i) a transition from highly localized (n=2) to highly delocalized (n=4) vibrational modes takes place; (ii) the delocalized O–H vibrations of cyclic (H2O)n clusters (n≥3) can be described as longitudinal/transverse optical phonons; (iii) internal force constants for ...

Translocation of radical sites by intramolecular 1,5-hydrogen atom transfer

Abstract: Cyclisation radicalaire de bromo-2 hexenes-1 substitues et fonctionnalises, en methylcyclopentanes correspondants

Intermolecular Complexes: The Role of Van Der Waals Systems in Physical Chemistry and in the Biodisciplines

Abstract: I. Introduction. The hierarchy of interactions between elementary particles, atoms and molecules. The origin and phenomenological description of weak interactions. Classification of van der Waals systems. Survey of published monographs and reviews. II. Methods of Calculating Weak Interactions. General consideration of the usefulness of various types of quantum chemical and other methods. Methods of calculating accurate (nonrelativistic) energies. The Hartree-Fock (SCF) method. Empirical methods. The molecular electrostatic potential (MEP) and field (MEF). Statistical thermodynamic treatment. Molecular dynamics and the Monte Carlo Method. III. Experimental Methods Yielding Information on Weak Intermolecular Interactions. Survey of methods. Elastic scattering in molecular beams. Spectroscopic methods. Bulk properties and transport phenomena. IV. Applications in Molecular Physics and Chemistry. The physical properties of individual van der Waals molecules. The reactivity of van der Waals molecules. V. Applications in the Biodisciplines. Nonbonding intramolecular and intermolecular interactions in large molecules. Important processes. Important and common characteristic of bioreactions. Interactions between selected systems. Notes on the mechanism of selected processes. References. Appendix. Index.

Structure and Optical Properties of a Thermochromic Schiff Base. Thermally Induced Intramolecular Proton Transfer in the N,N′-Bis(salicylidene)-p-phenylenediamine Crystals

Abstract: A thermochromic derivative of salicylideneaniline, N,N′-bis(salicylidene)-p-phenylenediamine (BSP), has been prepared and subjected to structural and optical studies in the crystalline state. An X-ray crystallographic analysis has shown that the molecules are planar and are stacked in a parallel fashion to form one-dimensional columns. The interplanar spacing has been found to be quite short, suggesting the existence of an intermolecular charge-transfer interaction. The molecule contains fairly short O–H··· N hydrogen bonds, the strength of which is manifested in an OH stretching absorption band in the infrared region, showing considerable broadening and a low-frequency shift. The BSP crystals are remarkably thermochromic, and visible absorption and emission spectral changes with temperature have been interpreted in terms of an intramolecular proton transfer from the hydroxyl oxygen to imine nitrogen through the O–H··· N hydrogen bond. The emission spectra have also been examined under high pressures.

Quantification of intramolecular ligand equilibria in metal-ion complexes

Abstract: That in a metal-ion chelate one binding site may be more weakly bound than another is often expected, though it is not equally well realized that such differences in metal-ion affinities may give rise to intramolecular equilibria between open and closed, i.e. chelated, forms. Quantification procedures for such equilibria are outlined and applied to describe the situation in complexes of N-substituted iminodiacetate and α-substituted acetate ligands. Corresponding procedures are applicable to the characterization of intramolecular ligand-ligand interactions in mixed ligand complexes. Isomerization equilibria are of general importance; e.g. in metal-ion catalyzed or facilitated technical or biological processes.

Photoinduced intramolecular electron transfer in peptide-bridged molecules

Abstract: Two series of molecules have been prepared and characterized in which a polypyridyl Ru(II) complex is linked to p-dimethoxybenzene (DMB) and p-benzoquinone (Q) moieties by peptide bridges containing the amino acid L-proline (Pro). The photophysical properties of the metal-to-ligand charge transfer (MLCT) excited state of the Ru(II) chromophore have been examined for the complexes with 0, 1, 2, 3, and 4 intervening Pro residues. Steady state and time resolved luminescence experiments on the Pro-bridged DMB system show that the properties of the Ru MLCT excited state are only slightly modified from those of an unsubstituted model complex by the presence of covalently attached DMB peptides. Experiments on the Pro-bridged Ru-Q complexes show that the Q site quenches the yield and lifetime of the Ru MLCT emission. Furthermore, the quenching efficiency is diminished as the number of peptide spacers is increased. The quenching process is ascribed to long-range intramolecular Ru-to-Q electron transfer. This hypothesis is supported by time-resolved luminescence data which suggest that the average electron transfer rate falls sharply with an increase in the peptide bridge length.

A molecular dynamics study of the structure of a model Langmuir monolayer of amphiphile molecules

Abstract: We study the thermodynamics and structure of a model monomolecular film of pentadecanoic acid on water. In this model the water is treated as a uniform polarizable continuum and the pentadecanoic acid molecules as chains of 15 pseudoatom with internal bond constraints, angle bending and torsional intramolecular interactions, and Lennard‐Jones atom–atom intermolecular interactions. The only low pressure phases exhibited by this model at temperatures between 300 and 400 K are the very low density vapor phase and a well ordered condensed phase with a reciprocal density ≤23 A2 per molecule. This is in sharp contrast with experimental studies which show the existence of a stable liquid‐expanded phase with a density of about 35 A2 per molecule at pressures <15 dyn cm−1.

Evidence for dynamical solvent effects on the photoisomerization of 4,4'-dimethoxystilbene

Abstract: The photoisomerization dynamics of 4,4'-dimethoxystilbene have been studied in n-alkanes and n-alkyl alcohols as a function of temperature at ambient pressure. By comparison with unsubstituted stilbene results the influence of the methoxy group, both intramolecular and intermolecular, on the isomerization process has been evaluated. The studies in n-alkane solvents reveal larger barrier heights and a weaker viscosity dependence for dimethoxystilbene compared to those of stilbene. This observation is in agreement with earlier ideas concerning frequency-dependent solvent effects but contrary to free volume models of solute-solvent friction. The studies in n-alkyl alcohol solvents demonstrate the inability of separating intramolecular from intermolecular effects. The potential energy surface associated with the reaction is strongly coupled with solvent dynamics, hence time dependent. These results demonstrate the importance of understanding microscopic friction and dielectric interactions on ultrashort time scales.