Showing papers on "Intramolecular force published in 1975"

The effect of intramolecular quantum modes on free energy relationships for electron transfer reactions

Abstract: A general quantum mechanical description of exothermic electron transfer reactions is formulated by treating such reactions as the nonradiative decay of a ’’supermolecule’’ consisting of the electron donor, the electron acceptor, and the polar solvent. In particular, the role of the high‐frequency intramolecular degrees of feedom on the free energy relationship for series of closely related reactions was investigated for various model systems involving displacement of potential energy surfaces, frequency shift, and anharmonicity effects. The free energy plots are generally found to pass through a maximum and to be asymmetric with a slower decrease in the transition probability with increasing energy of reaction. For high‐frequency intramolecular modes this provides a rationalization of the experimental observation of ’’activationless’’ regions. Isotope effects are discussed as also are the oscillatory free energy relationships, predicted for low temperatures and high frequencies, and which are analogous t...

Refinement of the structure of beta-chitin.

Abstract: The structure of β-chitin has been refined by rigid-body least-squares methods, based on the intensity data for highly crystalline specimens from the pogonophore Oligobrachia ivanovi. The structure consists of an array of poly-N-acetyl-D-glucosamine chains all having the same sense, which are linked together in sheets by NH … OC hydrogen bonding of the amide groups. In addition to the O-3′H … O-5 intramolecular hydrogen bond, analogous to that in cellulose, the CH2OH side chain forms an intrasheet hydrogen bond to the carbonyl oxygen on the next chain. This structure shows considerably better agreement between observed and calculated intensities than that possessing an intersheet hydrogen bond, as had been proposed previously. The structure is consistent with the swelling properties of β-chitin and can also be seen to be analogous to that of native cellulose.

Study of a central force model for liquid water by molecular dynamics

Abstract: The simulation technique of molecular dynamics has been used to investigate a central‐force model for liquid water at 22 °C and 1 g/cm3. In this model, the same atomic pair potentials are used both for intramolecular and intermolecular interactions. Atomic pair correlation functions and velocity autocorrelation functions have been computed. The results demonstrate that stable, nonlinear, vibrating molecules are present, and that they form a random hydrogen‐bond network of the type usually attributed to water. These exploratory calculations suggest that small modifications in the central potentials VHH, VOH, and VOO would produce a more faithful representation of real water.

The Thermal Cyclisation of Unsaturated Carbonyl Compounds

Abstract: When heated, in a sealed tube or in the vapour phase (250° −380°), unsaturated carbonyl compounds of suitable geometry (e-ethylenic ketones among others) undergo, in their enol form, an intramolecular ene reaction. In this reaction the enol hydrogen is shifted to the terminal carbon atom of the unsaturated center and cyclisation occurs by σ bonding between the other carbon atom of that center and the carbon atom α with respect to the carbonyl group. Such cyclisation is general, and very simple and its interest in synthesis in alicyclic chemistry is clear.

Reactions of Vinyl Azides

Abstract: This review is designed to demonstrate the versatility of vinyl azides in organic reactions. The reactive azide function is susceptible to thermolysis, photolysis, cycloadditions, and attack by nucleophiles and electrophiles. The neighboring double bond accentuates the reactivity of the azide function and provides additional intramolecular pathways for reaction. Last but not least, the presence of an appreciable electron density at the β-vinyl carbon makes this class of compounds comparable with enamines in their reactions with electrophiles and 1,3-dipoles.

Rate theory of irreversible linear random polymerisation. Part 2.—Application to intramolecular reaction in A—A + B—B type polymerisations

Abstract: The theory described in the preceding paper is extended to include intramolecular reactions in A—A + B—B type polymerisations. The extension is described in a general way and a limiting form is applied to previously published data giving total ring fractions during the formation of linear polyurethanes. The interpretations afforded by the present approach and by previous analyses of the data are discussed and compared.

Intramolecular Oxypalladation. Cyclization Reaction of 2-Allylphenols with Palladium Salts

Abstract: The stoichiometric reaction of sodium salts of 2-allylphenols 1 with dichlorobis(benzonitrile)palladium(II) gives generally 2-substituted benzofurans 2 in moderate yields. The formation of cyclized products is explained by intramolecular oxypalladation followed by elimination of a palladium hydride species. The cyclization can be made catalytic by using palladium(II) acetate, cupric acetate, and oxygen.

13C Spin‐Lattice Relaxation Times and the Mobility of Organic Molecules in Solution

Abstract: While the chemical shifts and coupling constants of 13C NMR belong to the most powerful tools available to the organic chemist for the solution of structural problems, increasing interest is being shown in 13C spin-lattice relaxation times T1 as structural parameters. Together with the nuclear Overhauser effects arising by proton decoupling of 13C NMR spectra, the T1 values of 13C nuclei in a molecule permit conclusions to be drawn with regard to relaxation mechanisms. They reflect the inter- and intramolecular mobility of a molecule, and thus complement the results of temperature-dependent NMR spectroscopy. The T1 differences within a molecule show, for instance, whether the molecular motion is anisotropic in solution, whether the internal motion of groups is subject to steric hindrance, the extent to which strong intermolecular or interionic interactions affect the flexibility of the molecule, and which parts of the molecule are rigid and which are flexible. Finally, differences between the T1 values measured for the 13C nuclei of a molecule frequently provide a reliable aid in the assignment of 13C NMR spectra, particularly in cases of signal crowding and multiplet overlapping.

Fluorescence from the second excited singlet of aromatic thioketones in solution

Abstract: The spectral characteristics and the quantum yield of the fluorescence from the second excited singlet state S2 of the aromatic thioketone molecules xanthione (XS) and thioxanthione (TXS) have been determined in solution at room temperature and 77 K. In 3-methylpentane, the measured quantum yields are φf (295 K) = 5.1 × 10−3 and φf(77 K) = 1.0 × 10−2 for XS, and φf (295 K) = 1.5 × 10−3 and φf (77 K) = 2.5 × 10−3 for TXS. Using the Strickler-Berg expression for the radiative lifetime, the decay rate of S2 is derived. It is concluded that internal conversion S2 ⇝ S1 is the dominating deactivation channel of S2 with k77 Knr(S2 ⇝ S1) = 1.0 × 1010 s−1 for XS and k77 Knr (S2→S1) = 2.2 × 1010 s−1 for TXS. Between 295 and 77 K, φf increases by a factor of about 2 following an Arrhenius type expression. This temperature dependence of φf is considered to be intramolecular in nature and is attributed to a temperature sensitive rate constant knr(S2⇝S1) with an activation energy of 190 ± 20 cm−1 and a frequency factor k′nr = 3 × 1010 s−1 for the XS molecule in 3-methylpentane.

Intramolecular excimer formation in 1,3‐bis(N‐carbazolyl)propane: Solvent viscosity effects

Abstract: Rate constants for intramolecular excimer formation and dissociation in 1,3‐bis(N‐carbozolyl)propane have been measured in a number of different solvents using time resolved fluorescence decay techniques. It is found that within homologous series of solvents both rate constants decrease progressively with increasing viscosity and obey a relationship originating with the concept of free volume requirements for intramolecular conformational change. Among a group of chemically and structurally different solvents both rate constants are observed to vary randomly with the macroscopic solvent viscosity indicating that other solvent properties are also important. The results are discussed in terms of a qualitative model in which the microstructure of the solvent shell surround the interacting chromophores, as determined by the degree of solvation, ultimately determines the ease with which this intramolecular photophysical process can occur.

Regiospecific Intramolecular Aldol Condensation Induced by Conjugate Addition of lithium dimethylcuprate to ζ‐Oxo‐α, β‐enones preliminary communication [1]

Abstract: ζ-Oxo-α, β-enones react with lithium dimethylcuprate to give cyclic aldols such as hydroxydecalone 1 and hydroxyspiro[4.5]decanone 2 in a regio- and stereospecific manner. This new reaction, a combination of conjugate methyl addition to α, β-enones with directed intramolecular aldolisation, is suggested to proceed via either enoate anions, such as 8 and 11, or radical anions, such as 9 10 and 12 13, as intermediates.

A NMR investigation of proline and its derivatives. II—Conformational implications of the 1H NMR spectrum of L-proline at different pH

Abstract: The 250 MHz 1H NMR spectrum of L-proline is comprehensively analysed by computer simulation for different pH values. A fast endo–exo interconversion has already been proposed from chemical shift and coupling constant data. A critical comparison of the vicinal coupling constants leads to the conclusion that in basic solution the equilibrium is shifted towards a more endo conformation. The position of the carboxylic plane in the ring space is given by the relative intramolecular chemical shift of the geminal protons and by the typical patterns of the Hα multiplet. These results are applied to the analysis of the spectra of ProNH2 and AcProNH2.

Calculation of structural properties and vibrational frequencies of α‐ and γ‐N2 crystals

Abstract: The effects of the zero‐point energy, residual forces and torques on the molecules, and molar density, on the calculated lattice frequencies and structural properties of solid α‐ and γ‐N2, are investigated. An intramolecular potential and a parametrized 12‐6 atom–atom intermolecular potential are used to calculate lattice modes, intramolecular modes, sublimation energy, equilibrium unit cell parameters, Gruneizen coefficients, and P–V data. The second virial coefficient is also reasonably reproduced. The α‐to‐γ phase transition is not revealed by the employed intermolecular potential.

The Proof of a Triplet Ground State in the N,N′-Di-t-butyl-m-phenylenebinitroxide Biradical

Abstract: The magnetic susceptibility of the N,N′-di-t-butyl-m-phenylenebinitroxide biradical has been measured in the temperature range from 60 to 300 K. The susceptibility follows the Curie-Weiss law, with a paramagnetic Curie constant of 0.90±0.04 K·-emu/mol and a Weiss constant of −19±2 K. This susceptibility suggests that the ground state of the biradical is a triplet and that an intramolecular pure triplet approximation, 2J>>kT, is reliable in the temperature region below 300 K. The negative Weiss constant is due to the negative exchange interaction, J′<0, between the triplets.