Showing papers on "Intramolecular force published in 1969"

Energy Migration in Lanthanide Chelates

Abstract: Intramolecular energy transfer in lanthanide chelates can occur through different paths depending on the nature of the chelate. Contrary to prevailing notions, it is shown that energy transfer does not require the participation of the lowest triplet level of the chelate. The experimental results from a study of solutions of more than 600 chelate systems are consistent with a transfer mechanism via the ligand‐excited singlet state, with rate constants ket of about 1011 sec−1 and higher. Transfer via the lowest triplet state may predominate, however, when the rate of energy transfer from the singlet excited level is smaller than the rate of intersystem crossing to the triplet level. Whatever the mechanism, intramolecular energy transfer is generally a very efficient process, regardless of the fluorescence quantum yield of the chelated lanthanide ion. Low quantum yields of sensitized fluorescence are usually the result of radiationless decay processes following intramolecular energy transfer. This quenching ...

Electronic Relaxation in Large Molecules

Abstract: In this paper we consider the problem of the radiative decay of electronically excited states of a large molecule. We have considered both intramolecular vibronic coupling and the interaction with the radiation field. Compound states for a system of decaying indistinguishable levels are constructed using the Fano method. General expressions for the radiative decay rate are derived and applied for the statistical limit of intramolecular vibronic coupling. On a time scale shorter than a typical intramolecular recurrence time the radiative decay is exponential, and the reciprocal lifetime consists of independent contributions of radiative and nonradiative components. The experimental implications of these results for large and medium‐size molecules are discussed.

Molecular forces governing non-electrolyte permeation through cell membranes.

Abstract: The relations between the chemical structure of non-electrolytes and their ability to permeate cell membranes are analysed at the level of molecular forces, using the measurements of reflexion coefficients in gall-bladder epithelial cells tabulated in the preceding paper. Stronger solute: water forces and weaker solute: membrane forces are associated with lower permeating power. The portions of the membrane controlling non-electrolyte permeation behave as nearly pure hydrocarbons with very few hydrogen-bonding sites. Most substituents (hydroxyl, ether, carbonyl, ester, amino, amide, urea, nitrile) are shown to decrease permeation in proportion to the number and strength of intermolecular hydrogen bonds which they form with water, while intramolecular hydrogen bonding accelerates permeation. Carbon-carbon double bonds and triple bonds and aromatic residues decrease permeability due to hydrogen bonds involving $\pi $ electrons. Inductive effects, in which a substituent indirectly modifies permeability by withdrawing or releasing electrons at an adjacent hydrogen bonding site, are most noticeable for halogens, the nitro group, double and triple bonds, and branched alkyl groups. Altered forces between membrane hydrocarbons and the solute retard the permeation (weaker forces) of fluorine compounds and branched compounds, and slightly accelerate the permeation (stronger forces) of other halogen derivatives and compounds with long carbon chains. The main factor in the increase of permeability with increasing hydrocarbon chain length is an entropy effect associated with a change in local water structure; and this effect is partly responsible for the decrease in permeability with chain branching, whose origin is particularly complex.

Etude, par spectroscopie infra-rouge, de la conformation de quelques composés peptidiques modèles

Abstract: Some experimental data are given on the infrared spectra between 3300 and 3500 cm−1 of dilute solutions in carbon tetrachloride of three types of model compounds: CH3−CONH-CH(R1)-CONH(R2), (I); CH3-CON(CH3)-CH(R1)-CONH(R2), (II) and CH3-CONH-CH(R1)-CON(R2)2, (III). In studying the N-H stretching bands, it was found that there are two types of intramolecular hydrogen bonds in these molecules; these result in two different cyclized conformations, C5 and C7, which contain respectively, five and seven atoms in the ring. By using model substances I, II, and III, in which the nitrogen atoms are unequally substituted, it is possible to identify the N-H stretching bands which are to be ascribed to the N-H oscillators included in the two different chelated conformations. It is found also that the stretching frequency of a free N-H oscillator depends upon the substituent on the nitrogen atom. Thus, it is possible to observe, with some of the model compounds I, four different absorption bands located at 3340, 3420, 3440, and 3460 cm−1. The first two are ascribed to the N-H oscillators included in the Hbonds which lock the C7 and C5 conformations; the last two correspond to free N-H which differ with the substituent on the nitrogen atom.

Optically Active Crystal Vibrations of the Alkali‐Metal Nitrates

Abstract: The infrared and Raman spectra for the three alkali‐metal nitrates LiNo3, NaNO3, and KNO3 have been measured in the region from 1600 to 30 wavenumbers, and a normal‐coordinate analysis has been made for these compounds with a special emphasis on the rotational and translational lattice modes in the crystal. All three compounds were studied as D3d6 models and a modified Urey–Bradley force field was used in the calculations. The intramolecular and interaction force constants for all three nitrates are given. The effect of the interaction potential constants on the NO3− ion is discussed as well as the effect of the metal ion–nitrate ionic interaction in the crystal from a comparison of the calculated force constants. Although there are large changes in the rotational and translational lattice frequencies when comparing one alkali‐metal nitrate with another, the lack of force constant dependence is interesting.

Intramolecular Triplet Energy Transfer in Poly(1‐vinylnaphthalene)

Abstract: Poly(1‐vinylnaphthalene) in a rigid glass at 77°K exhibits a delayed fluorescence due to triplet–triplet annihilation following intramolecular triplet energy transfer through the naphthalene chromophores. Delayed fluorescence does not appear in the spectrum of 1‐ethylnaphthalene at equivalent concentrations, about 10−3M. Delayed emission from the polymer but not from 1‐ethylnaphthalene is quenched by piperylene. End groups are more important than chain length in the control of triplet migration in the homopolymer; experiments with 1‐vinylnaphthalene‐methyl methacrylate copolymers, however, indicate that some minimum chain length is required for efficient annihilation.

Raman spectra of hydrogen and deuterium halide crystals.

Abstract: The crystal Raman Spectra of HCl, DCl, HBr, and DBr have been observed for the lattice and intramolecular vibrational regions. The results for the intramolecular region indicate that the crystal structure of the low‐temperature orthorhombic phase is C2v9 − Pn21a with four molecules in a unit cell, containing nonplanar hydrogen‐bonded zigzag chains. However, the nonplanarity of the chain is so small that it is not sensitively reflected to the spectra of lattice vibrational region. As a result, the lattice vibrational bands may be assigned adequately based on a planar structure C2v12 − Bb21m, which is the structure determined by neutron diffraction measurement. Normal coordinate calculations have been made by using several kinds of intermolecular potentials. A central force field which assumed dispersion and repulsion forces and dipole–dipole forces failed to explain the observed lattice frequencies. A potential which assumed hydrogen bonding explicitly gave a reasonable fit of the calculated frequencies wi...

Raman Spectrum and Intermolecular Forces of the Chlorine Crystal

Abstract: The Raman spectrum of the chlorine crystal has been observed at low temperatures for lattice and intramolecular vibrational regions. The observed spectrum supports the x‐ray determined crystal structure Cmca with two molecules in a unit cell, with which the lattice vibrational Raman lines have been assigned. Normal‐coordinate calculations showed that the central force field is inadequate for the chlorine crystal and noncentral forces must be taken into account to reproduce the observed frequencies. The force constants obtained based on a central and noncentral force field indicated the importance of charge‐transfer interaction in the crystal.

Conformational studies of β-D-1,4′-xylan†

Abstract: The nonbonded interaction energy was computed for xylobiose and xylan as a function of the dihedral angles (ϕ,ψ). The energy maps indicate that interactions higher than the second neighbor are negligible. Of the four possibilities, the left-handed helical conformation with (ϕ,ψ) = (63°,25°) is of the lowest energy. The hydrogen bond search and the energy maps reveal that the xylan helix is stabilized mainly by van der Waals forces. The allowed region map shows that the freedom of rotation of the monomer units in cellulose is more restricted than that of the monomer units in xylan, because of the presence of the CH2OH group in the former. The intramolecular hydrogen bond of the O5 O3′ type is stronger in cellulose.