Showing papers on "Intramolecular force published in 1979"

Symmetry-adapted double-perturbation analysis of intramolecular correlation effects in weak intermolecular interactions

Abstract: A general symmetry-adapted double-perturbation procedure for treating intramolecular or intra-atomic correlation in the theory of intermolecular forces is developed The method was applied to the interaction of two helium atoms The calculations were made employing the Moller-Plesset partition of atomic hamiltonians and using a large basis set of explicitly correlated gaussian wave functions At the van der Waals minimum the total intra-atomic correlation contribution to the interaction energy amounts to -2·9 K and is mainly due to the change of the dispersion energy The total interaction energy is equal to -10·3 K being in agreement with the latest experimental result of Burgmans, Farrar and Lee

Molecular theory of lipid membrane ordera)

Abstract: Starting from the inter‐ and intramolecular interactions of the hydrocarbon chains the orientational long‐range order in lipid membranes is calculated to study the ordered‐fluid phase transition. The positional order is described by the lateral packing density and treated as an external parameter, in addition to temperature. The results for the orientational order parameter, the phase transition temperature, and the latent heat are analyzed to distinguish between the effects of the different chain interactions, the van der Waals interaction, steric hindrance, and flexibility. The dependence of the results on chain length is compared with existing experimental data.

Vibrational predissociation lifetimes of the van der Waals molecule HeI2

Abstract: In this paper we present a quantum mechanical study of the rates of vibrational predissociation of the T‐shaped HeI2(B) van der Waals molecule. The van der Waals bond is characterized in terms of Morse type atom–atom interaction and by a Buckingham type atom–atom potential. The dynamics of vibrational predissociation is shown to be insensitive to the long‐range part of the van der Waals potential and the Morse form is adequate for the description of this process. The close‐coupling equations for nuclear motion are solved by standard numerical methods and the vibrational predissociation rates are related to the widths of the resulting resonances. The superlinear theoretical dependence of the vibrational predissociation rates on the excess vibrational energy of the molecular I2 bond is in good agreement with the experimental data. The relative contribution of intramolecular and of intermolecular terms to this superlinear dependence are elucidated, demonstrating the effects of the anharmonicity of the molecu...

ENERGY RELAXATION MECHANISM IN Ni(II), Pd(II), Pt(II) and Zn(II) PORPHYRINS

Abstract: —Picosecond absorption spectroscopy was used to determine the intramolecular energy relaxation processes occurring in Ni(II). Pd(II), Pt(II), and Zn(II) protoporphyrin IX dimethyl ester. Picosecond data on the rate of ground state repopulation and the kinetics of a transient intermediate made it possible to determine the lifetimes of the excited singlet state of Ni, Pd, and Zn porphyrins as 10±2ps, 19±3ps, and 2.6±0.5 ps, respectively, and<8 ps for Pt porphyrin. On the basis of these data. the nonfluorescent and nonphosphorescent property of Ni porphyrin can be interpreted in terms of internal conversion to a lower lying singlet d-d level which is not the case for the strongly phosphorescent Pd and Pt porphyrins.

Nuclear-magnetic-resonance study on Met-enkephalin and Met-enkephalinamide. Molecular association and conformation.

Abstract: The 270-MHz 1H and 68-MHz 13C nuclear magnetic resonance spectra of Met-enkephalin (Tyr-Gly-Gly-Phe-Met) and Met-enkephalinamide are analyzed in a variety of solvents. For the dipolar form of Met-enkephalin in (C2H3)2SO solution, significant concentration dependences are found of C-alpha proton chemical shifts, indicating an aromatic ring-current effect in molecular aggregates. An anomalous temperature dependence is observed of the amide proton chemical shift of the Met-5 residue. Furthermore, the chemical shifts of C-alpha protons of the dipolar form are found to depend appreciably on temperature. From the analyses of the temperature dependences together with concentration dependences of C-alpha proton resonances, the dipolar form of Met-enkephalin is found to be in an equilibrium of folded and extended conformations at low concentration in (C2H3)2SO solution. Solvent-composition dependences of the amide and C-alpha proton resonances and carbonyl and alpha-carbon resonances of the dipolar form in 2H2O/(C2H3)2SO solution are consistent with the conformation equilibrium and the association equilibrium. The folded conformation of the dipolar form in (C2H3)2SO solution is stabilized by the intramolecular attraction between the positively charged N-terminal group and negatively charged C-terminal group. The presence of the folded conformation is confirmed by the measurements of Gd(III)-induced relaxation enhancements of C-alpha protons. Nuclear Overhauser effects on the dipolar form are not consistent with the predominant formation of the beta-turn structure with the intramolecular hydrogen bond (Gly-2) C=O . H-N(Met-5). For the dipolar form of Met-enkephalin in 2H2O solution and for the cationic form of Met-enkephalinamide in (C2H3)2SO solution and in 2H2O solution there is no evidence for the formation of folded conformations.

Application of Distance Geometry to Protein Tertiary Structure Calculations

Abstract: Synopsis A general approach to the problem of molecular conformation is advanced. We describe a formalism that permits experimental and theoretical information to be incorporated into a set of upper and lower bounds on intramolecular distances. Structures (conformations) meeting these bounds can be readily generated and compared with each other. To illustrate the use of the method, we have employed a simple “firehose” model for protein folding to predict the long-range hydrophobic interactions in a small protein: pancreatic trypsin inhibitor. Models of this type lead to the proper hairpin turns and a reasonable set of long-range contacts for this protein. Application of the distance geometry method then yields backbone conformations with errors of 4-8 A compared to the native structure. We discuss both the merits and shortcomings of the firehose model and the relation between distance geometry and energy minimization techniques.

Structure and stability of radical cations from cyclic and open-chain dithia compounds in aqueous solutions. [Pulsed irradiation]

Abstract: The formation of intra- and some intermolecular radical cation complexes has been observed during the oxidation of cyclic and open-chain organic dithia compounds (the two S atoms not adjacent) by hydroxyl radicals in aqueous solutions. These species are characterized by a new sulfur-sulfur bond established by interaction of the unpaired p electron from the oxidized sulfur atom with the free p-electron pair of a second sulfur atom. Two of these electrons form a sigma bond while the third is promoted to an antibonding sigma/sup */ level. At low solute concentration (typically around 10/sup -4/M) intramolecular complex formation predominates. The radical cation complexes show characteristic broad optical absorptions which are attributable to a sigma ..-->.. sigma/sup */ transition. The position of the absorption maximum (reflecting the sigma/sigma/sup */ energy difference) and the extinction coefficients are related to the p-orbital overlap between the two interacting sulfur atoms. The extent of overlap depends on the internuclear distance, r(S therefore S), and particularly on the angular relationship between the interacting orbitals. Other structural parameters influencing lambda/sub max/ are the flexibility of the species formed and possible interactions between the sigma(S,S) bond with sigma(C,C) bonds. lambda/sub max/ is found to range from 400 to 650more » nm, these particular values referring to the intramolecular radical cation complexes from1,5-dithiacyclooctane and 1,4-dithiacyclohexane, respectively. epsilon values are found up to 7 x 10/sup 3/ M/sup -1/ cm/sup -1/. The lifetimes of the radical cation complexes depend on the stability of the new sulfur-sulfur bond and may well extend into the millisecond range in aqueous solutions.« less

Charge transfer in peptides . Pulse radiolysis investigation of one-electron reactions in dipeptides of tryptophan and tyrosine

Abstract: SummaryOne-electron oxidation of TyrOH-TrpH or TrpH-TyrOH in aqueous solutions by N·3 radicals occurs predominantly at the tryptophyl residue. The corresponding indolyl radicals (absorbing at 510 nm) are subsequently transformed into phenoxyl radicals (absorbing at 390/405 nm): The first-order radical transformation rates are independent of the (initial) concentration of N·3 or peptide and unaffected by urea (as a modifier of hydrogen bond structures). Intermolecular conversion of indolyl into phenoxyl radicals, e.g. by reaction of GlyH-Trp· with TyrOH-GlyH, is very slow and inefficient. It is concluded that reactions (5) and (7) occur by intramolecular charge transfer across the peptide bond.

On the role of Fermi resonance in the spectrum of water in its condensed phases

Abstract: In this paper we present a theoretical analysis of the intramolecular stretching spectrum of D2O in H2O and H2O in D2O for both ice I and amorphous solid water H2O(as). The model includes the dependence of the harmonic stretching (krr) and stretch–stretch interaction (krr′) force constants on hydrogen bonding as well as Fermi resonance between the stretching fundamentals and the bending overtone. Provided that the width of the bending mode is broadened by intermolecular coupling, then the model reproduces the anomalously large widths and splittings in the stretching spectrum recently reported by Devlin et al. for D2O in H2O ice I and H2O(as). The value of the Fermi resonance interaction force constant k122 required to fit these spectra is accounted for by diagonal harmonic terms in the valence force field representation of the intramolecular potential.

Direct measurement of the rate of intramolecular electron transfer in a diruthenium mixed-valence complex

Abstract: The communication reports the rate of intramolecular electron transfer in the mixed-valence ion (NH/sub 3/)/sub 5/Ru/sup II/pzRu/sup III/(edta)/sup +/ (pz = pyrazine, edta = ethylenediamine-tetraacetate) which does not exhibit intervalence transfer absorption. The results suggest a revised model for the electron-transfer process in systems in which the activation barriers arise largely from solvent reorganization. 43 references, 1 table.

Intramolecular dynamics of an isolated ultracold large molecule

Abstract: Intramolecular, nonreactive, radiationless processes in electronically-vibrationally excited states of collision­free "isolated" large molecules involve interstate elec­tronic relaxation (ER)1 and intrastate vibrational energy redistribution (IVR).2 Interrogation of the excited-state population following photoselective excitation of isolated, large molecules at room temperature

Thermische Cyclisierung von α‐Alkinonen zu 2‐Cyclopentenonen

Abstract: Thermal Cyclization of α-Alkynones to 2-Cyclopentenones Gas phase thermolysis at 600–740° of substituted 1-pentyn-3-ones (α-alkynones), which are easily prepared by acylation of trimethylsilyl acetylenes, leads to substituted 2-cyclopentenones. The intramolecular formation of a new C, C-bond between an acetylenic and a non-activated carbon atom is accompanied by a [1,2]-migration of one of the substituents at the triple bond. This novel ‘-alkynone cyclization’ reaction may be explained by postulating an alkylidene carbene intermediate which inserts into a C,H-bond five carbon atoms away at the non-acetylenic part of the ketone. Several examples demonstrate that the α-alkynone cyclization offers a simple tool for the preparation of certain monocyclic, bicyclic and spiro compounds containing a 2-cyclopentenone moiety.

Collision induced intramolecular vibrational energy transfer in 1B3u pyrazine

Abstract: Collision induced intramolecular vibrational energy exchange in 1B3u(n,π*) pyrazine was investigated by analyzing the pressure dependence of the emission from 10 prepared single vibronic levels. Energy transfer rates are, in general, lower than those previously observed in electronic excited states. Though the magnitude of the energy defect for a process strongly influences the transfer rate, other propensity rules are important. Among the single quantum changes possible, those involving mode 16b predominate. The vibronic coupling to the upper 1B2u (π,π*) state increases the observed rates dramatically.

Infrared studies of water in complexes

Abstract: The intramolecular coupling of the OH stretching vibrational frequencies are studied in binary mixtures (H2O + bases) and ternary mixtures (H2O + bases + CCl4) For water molecules partially H bonded with bases (1:1 complexes) this coupling decreases rapidly with the acceptor strength, the C2ν symmetry changing to Cs In 2:1 complexes the C2ν symmetry is re-established, ν3 and ν1 undergo similar frequency shifts referred to unperturbed H2O The force constants and vibrational modes of water in 1:1 complexes are calculated approximately This paper shows that the frequency region between the unperturbed ν3 and the unperturbed (ν3+ν1)/2 is useful in determining the content of non H bonded OH groups

.alpha.-Deuterium isotope effects and transition-state structure in an intramolecular model system for methyl-transfer enzymes

Abstract: The ..cap alpha..-deuterium isotope effects for the general base catalyzed ring-closure reaction of 2-(2-cis-hydroxycyclopentyl)ethylmethyl-4-nitrophenylsulfonium tetrafluoroborate (1) are k/sub 2H/K/sub 2D/ = 1.7 +- 0.02 (water catalysis), 1.104 +- 0.014 (carbonate buffer catalysis), and 0.998 +- 0.001 (hydroxide ion catalysis) in aqueous solution (..mu.. = 1.0) at 40.00 +- 0.05/sup 0/C. These are all considerably more normal (i.e., larger in the direction k/sub H/ > k/sub D/) than the effect for the catechol O-methyltransferase reaction (k/sub 3H//k/sub 3D/ = 0.83 +- 0.05) for which the reactions of 1 are a model. This indicates the model-reaction transition states to be looser (i.e., to possess a longer oxygen-to-sulfur distance) than the enzymic transition state and suggests that the enzyme may achieve all or part of its catalysis of methyl transfer from processes that are associated with compression of the transition state.

Self-Consistent Calculation of Molecular Chemisorption on Metals

Abstract: As a first step towards an understanding of the reactivity of metals and their ability to break chemical bonds we have performed self-consistent calculations on a model with H2 standing upright on a jellium surface. Data for the adsorbate-induced electron structure and potential-energy curves with substrate parameters characteristic for Al, Mg and Na are presented. We extract concepts and trends that should be useful for other molecule-metal systems as well, e.g., (i) the downshift of the antibonding affinity level, in a symmetry-affected correlation with the effective electron potential of the clean surface, (ii) the filling of the corresponding molecular resonance, localized primarily on the outer H atom, (iii) the thereby weakened intramolecular bond, and (iv) the natural separation of the molecular chemisorption energy into intra- and extramolecular terms, whose relative size determines the molecular adsorption to be dissociative, activated etc. The model calculations provide illustrations of different classes of molecular chemisorption systems and support the Lennard-Jones description of molecular adsorption, provided that the molecular potential-energy curve is due to chemical rather than van der Waals' forces.