Showing papers on "Intramolecular force published in 1976"

Mechanism and Theory in Organic Chemistry

Abstract: 1. The Covalent Bond. 2. Some Fundamentals of Physical Organic Chemistry. 3. Acids and Bases. 4. Aliphatic Nucleophilic Substitution. 5. Intramolecular Cationic Rearrangements. 6. Carbanions, Carbenes, and Electrophilic Aliphatic Substitution. 7. Addition and Elimination Reactions. 8. Reactions of Carbonyl Compounds. 9. Radical Reactions. 10. The Theory of Pericyclic Reactions. 11. Applications of the Pericyclic Selection Rules. 12. Photochemistry. Answer Book.

Determination of the structure of cellulose II.

Abstract: The structure of regenerated cellulose is shown by x-ray diffraction to be comprised of an array of antiparallel chain molecules. The determination was based on the intensity data from rayon fibers and utilized rigid-body least-squares refinement techniques. The unit cell is monoclinic with space group P2(1) and dimensions a = 8.01 A, b = 9.04 A, c = 10.36 A (fiber axis), and gamma = 117.1 degrees. Models containing chains with the same sense (parallel) or alternating sense (antiparallel) were refined against the intensity data. The only acceptable model contains antiparallel chains. The -CH2OH groups of the corner chain are oriented near to the gt position while those of the center chain are near to the tg position. Both chains possess an O3-H-O5' intramolecular hydrogen bond, and the center chain also has an O2'-H-O6 intramolecular bond. Intermolecular hydrogen bonding occurs along the 020 planes (o6-h-o2 bonds for the corner chains and O6-H-O3 bonds for the center chains) and also along the 110 planes with a hydrogen bond between the O2-H of the corner chain and the O2' of the center chain. This center-corner chain hydrogen bonding is a major difference between the native and regenerated structures and may account for the stability of the latter form.

Relative Reactivities of Hydroxyl Groups in Carbohydrates

Abstract: Publisher Summary This chapter discusses the important reactions of the hydroxyl group. Knowledge of the relative reactivities of hydroxyl groups in carbohydrates is fundamental to a thorough understanding of carbohydrate chemistry. The chapter explains esterification, which forms the largest part of the literature since 1953 on selective reactions of carbohydrates. The common methods of esterification utilize an acid chloride or an acid anhydride as the reagent, but other acid derivatives often show different selectivities. An investigation pertinent to the subject of favored reactivity concerned the reactions of certain acyl halides with ethanol, in acetone or chloroform solution. The mixed-order nature of some of these reactions suggested that every rate-determining transition-state contained the substrate, the nucleophile (ethanol), and an acceptor— X—for hydrogen bonding, suitable acceptors being an acetone molecule, a chloride ion, or another ethanol molecule. The possibility that intramolecular hydrogen-bonding does not persist in pyridine and that the advantage of a bonded hydroxyl group would be lost, invalidates the explanation of favored reactivity. But if the hydrogen-bonding to pyridine is hindered in the transition state, intramolecular hydrogen-bonding could conceivably assume importance.

Statistical mechanics of chemical equilibria and intramolecular structures of nonrigid molecules in condensed phases

Abstract: An exact classical statistical mechanical theory is developed which describes how intermolecular forces alter the average intramolecular structures of nonrigid molecules and how these forces affect the equilibrium constant of chemically reacting species. The theory lends itself to computationally convenient approximations. Three illustrative applications of the theory are given. First, the equilibrium constant for the reaction N2O2?2NO2 is studied. Second, the shift in the chemical bond lengths of N2 and O2 from their gas phase values to those in the liquid are calculated from the theory. Third, the average conformational structure of n‐butane in various dense fluid solvents is predicted. The basic methods used in the derivation of the theory are the techniques of physical cluster series (as opposed to the usual Mayer ’’mathematical’’ cluster expansions), and topological reductions. The formalism gives rise to a renormalization of chemical bonding Boltzmann factors so that the theoretical expressions are not swamped by essentially infinite and unnormalized cluster functions. Along with the results on chemical equilibria and intramolecular structures, the interaction site cluster series for intermolecular correlation functions is rederived in a more general manner than the derivation originally presented by Ladanyi and Chandler.

Nonlinear resonance and stochasticity in intramolecular energy exchange

Abstract: The dynamics of intramolecular vibrational energy transfer is studied, with particular reference to its effect on the unimolecular dissociation rates of isolated molecules. The molecule is represented as a set of classical, coupled, nonlinear (anharmonic) oscillators which transfer energy through resonant interactions. We suggest that isolated nonlinear resonances (which predominate at low energies) lead to trapping of the vibrational energy of the system, hence to slow vibrational relaxation, while resonance overlap at higher energies leads to rapid energy exchange and the random lifetime distribution assumed by RRKM theory. Results are presented for some simple model systems, and the approach is compared with other recent theoretical descriptions of vibrational relaxation in isolated molecules.

Nmr studies of the molecular conformations in the linear oligopeptides H-(L-Ala)n-L-Pro-OH.

Abstract: The molecular conformations of the linear oligopeptides H-(L-Ala)n-L-Pro-OH, with n = 1,2 and 3, have been investigated. 13C nmr observation of the equilibrium between the cis and trans forms of the Ala-Pro peptide bond indicated the occurrence of nonrandom conformations in solutions of these flexible peptides. The formation of the nonrandom species containing the cis form of the Ala-Pro bond was found to depend on the deprotonation of the carboxylic acid group of proline, the solvent, and the ionic strength in aqueous solution. The influence of intramolecular hydrogen bonding on the relative conformational energies of the species containing the cis and trans Ala-Pro peptide bond was studied by comparison of the peptides H-(Ala)n-Pro-OH with analogous molecules where hydrogen bond formation was excluded by the covalent structure. In earlier work a hydrogen bond between the protonated terminal carboxylic acid group and the carbonyl oxygen of the penultimate amino acid residue had been suggested to stabilize conformations including trans proline. For the systems described here this hypothesis can be ruled out, since the cis:trans ratio is identical for molecules with methyl ester protected and free protonated terminal carboxylic acid groups of proline. Direct evidence for hydrogen bond formation between the deprotonated terminal carboxylic acid group and the amide proton of the penultimate amino acid residue in the molecular species containing cis proline was obtained from 1H nmr studies. However, the cis:trans ratio of the Ala-Pro bond was not affected by N-methylation of the penultimate amino acid residue, which prevents formation of this hydrogen bond. Overall the experimental observations lead to the conclusion that the relative energies of the peptide conformations including cis or trans proline are mainly determined by intramolecular electrostatic interactions, whereas in the molecules considered, intramolecular hydrogen bonding is a consequence of specific peptide backbone conformations rather than a cause for the occurrence of energetically favored species. Independent support for this conclusion was obtained from model consideration which indicated that electrostatic interactions between the terminal carboxylic acid group and the carbonyl oxygen of the penultimate amino acid residue could indeed account for the observed relative conformational energies of the species containing cis and trans proline, respectively.

Pyrolysis-molecular weight chromatography: A new on-line system for analysis of polymers. II. Thermal decomposition of polyolefins: Polyethylene, polypropylene, polyisobutylene

Abstract: The thermal decomposition of low-density polyethylene, isotactic polypropylene, and polyisobutylene has been studied in helium at a heating rate of 20°C/min using an experimental system which consists of a programmable pyrolyzer, a thermal conductivity cell, and a mass chromatograph. For low-density polyethylene, the formation of a homologous series of volatile products corresponding to alkanes and alkanes is interpreted in terms of an intramolecular radical transfer process in the primary macroradicals to the 5th, 9th, 13th, and 17th carbon atoms of the chain. For isotactic polypropylene, the formation of a homologous series of volatile products corresponding to monomer, dimers, trimers, and higher oligomers is explained also in terms of intramolecular radical transfer processes. Transfers to the 5th, 9th, and 13th carbon atoms in the secondary macroradicals (indexing from the secondary carbon atom at the chain end) and transfers to the 6th, 10th, and 12th carbon atoms in the primary macroradicals are shown to account for the major products of pyrolysis. For polyisobutylene, in addition to the depolymerization process which accounts for the extensive formation of monomer, intramolecular radical transfer processes in the primary and tertiary macroradicals (the processes proceeding predominantly in the primary macroradicals) are shown to account for the formation of the dimers, trimers, and higher oligomers that occur in the volatile products of decomposition.

Conformation of the cyclic tetrapeptide dihydrochlamydocin. Iabu-L-Phe-D-Pro-LX, and experimental values for 3 1 intramolecular hydrogen bonds by X-ray diffraction†

Abstract: Chlamydocin, Iabu-L-Phe-D-Pro-LX, is a naturally occurring cyclic tetrapeptide that exhibits high cytostatic activity. The conformation of the peptide ring, as well as the stereo configuration in the vicinity of the epoxide ring, have been established by a single-crystal X-ray study of dihydrochlamydocin: C28H40N4O6·H2O. It crystallizes in the monoclinic space group P21 with a = 12.616(6) A, b = 12.355(6) A, c = 9.442(5) A, and β = 99.5(1)°. The structure was solved by the symbolic addition procedure for phase determination followed by the tangent formula method of phase refinement. This structure represents the first cyclic tetrapeptide in which all four peptide units have been found in the trans conformation; however, each peptide unit is significantly nonplanar with ω angles deviating by 14–24° from the ideal value of 180°. This molecule contains two intramolecular 3 1 hydrogen bonds and experimentally determined parameters for these seven-membered turns are presented.

Electron spin resonance studies. Part L. Reactions of alkoxyl radicals generated from alkyl hydroperoxides and titanium(III) ion in aqueous solution

Abstract: A variety of alkoxyl radicals has been generated in aqueous solution by the one-electron reduction of alkyl hydroperoxides by titanium(III) ion, and their behaviour has been studied by e.s.r. spectroscopy in conjunction with a rapid-flow system. Although the alkoxyl radicals themselves are not directly detectable by e.s.r., both the adducts which they form with the spin-trap CH2:NO2– and the carbon-centred radicals into which they are transformed can be monitored; in this way, a novel 1,2-shift [e.g. PrO·→·CH(OH)Et], as well as intramolecular 1,5-hydrogen transfer, intramolecular addition to an olefinic bond, and fragmentation, have been demonstrated. Estimates have been obtained for the rate constants for some of these processes (e.g. that for the 1,2-shift mentioned is ca. 107 s–1). and the behaviour of alkoxyl radicals in aqueous and non-aqueous media has been compared.

Convergence properties of the intermolecular force series (1/R-expansion)

Abstract: It is proven that the total energy of interacting molecular systems A and B at large intermolecular distancesR can be expanded in a semi-convergent series in powers of 1/R. It is further proven that “exchange forces” vanish faster than any power of 1/R.

On the Solvent-Induced Changes of Electronic Structures of Intramolecular Exciplexes

Abstract: Effects cf solvent polarity upon the electron donor-acceptor interactions in the excited state of p-(CH3)2N C6H4(CH2)n-(9-anthryl) (n=0, 1, 2, 3) and p-(CH3)2NC6H4-(CH2)n-(1-pyrenyl) (n=1, 2, 3) have been investigated by means of fluorescence spectral measurements and theoretical calculations based on some simplified models. The solvent-induced electronic structural changes during the excited state lifetime have been demonstrated for some of the above intramolecular exciplexes.

Cooperative disordering of single-stranded polynucleotides through copper crosslinking.

Abstract: The thermal transitions of single-stranded polynucleotides are noncooperative. In contrast, Cu(II) cooperatively disorders the single-stranded helical structures of poly(A) and poly(C), as demonstrated by ORD and UV spectral changes as a function of the Cu2+ activity, and by a dramatic chain-length dependence of the spectral changes. Equilibrium dialysis binding studies indicate that the cooperative disordering is paralleled by a somewhat less cooperative binding process. The difference between the thermal- and Cu(II)-induced transition is explained by the following mechanism. (1) Cu(II) initially binds in a noncooperative fashion to phosphate. (2) The Cu(II) so bound forms a second bond to a nonadjacent base site on the same polymer strand or another strand. These intramolecular and intermolecular crosslinks to the bases are responsible for the disordering. (3) The initial crosslinks formed provide nuclei for the cooperative formation of additional crosslinks, producing cooperative spectral changes paralleled by cooperative binding. A comparison of the spectral and binding transitions indicates that there is appreciable noncooperative binding of copper to phosphate, which produces no spectral changes in the presence of added electrolyte. This comparison also indicates that each copper crosslink disorders several bases. The formation of intermolecular crosslinks is demonstrated by a polymer concentration dependence of the disordering. The formation of intramolecular crosslinks can be deduced from the fact that the “cooperative unit” required to explain the differences between the hexamer, which does not readily form intramolecular crosslinks, and the polymer is considerably larger than the cooperative unit determined from the polymer results. The poly(A) disordering transition is less symmetrical than that of poly(C), particularly at low polymer concentrations. These results, together with other phenomena, are explained by a greater flexibility of poly(A), which favors the formation of small intramolecular loops.

1,2,4‐Triazole: Vibrational spectra, normal coordinate calculations, and hydrogen bonding

Abstract: The influence of hydrogen bonding on the vibrational spectra of 1,2,4‐triazole has been studied using infrared and Raman spectroscopy and normal coordinate calculations. The infrared vapor phase fundamentals have been assigned. The polarized infrared and Raman spectra of triazole single crystals (Pbca,Z=8) have been investigated in the 4000 to 10 cm−1 region. An assignment of the 18 intramolecular fundamentals is given with help of −d1 and −d3 isotopic derivatives and normal coordinate calculations. Correlation field splitting is discussed. Thirty‐two of the 45 lattice modes have been assigned to their symmetry species and in terms of rotational, translational, and hydrogen bond vibrations. They are compared to those calculated on the ground of the rigid body and flexible molecule models assuming appropriate intermolecular atom–atom potentials. The variation of the NH stretching and bending force constants upon hydrogen bond formation is correlated with the hydrogen bond force constant (0.35 mdyn/A), N–N ...