Norbert Muller

C13 Splittings in Proton Magnetic Resonance Spectra. I. Hydrocarbons

Indirect spin‐spin couplings between protons and C13 nuclei have been measured for a number of hydrocarbons. The results are interpreted on the basis of a simple semiempirical equation relating the coupling constants with the amounts of s character of the carbon‐atomic orbitals involved in the bonds. No evidence is found for a dependence of the coupling constants on supposed changes in ionic character of the bonds.An equation is also presented relating the coupling constants and the C–H internuclear distances. This provides a new indirect means of evaluating C–H distances, with a reliability apparently at least comparable with that of methods in current use.

C13 Splittings in Proton Magnetic Resonance Spectra. II. Bonding in Substituted Methanes

We have measured indirect spin‐spin couplings between protons and C13 nuclei in a variety of substituted methanes. As found earlier for hydrocarbons, the coupling constants, JC–H, appear to be linearly related both to the percent s character of the carbon atomic orbital participating in the C–H bond and to the bond length. On the basis of the large variations of the JC–H values and of the C–H bond lengths in these compounds we conclude that the state of hybridization of the carbon atom cannot be reliably inferred from the observed, essentially tetrahedral valence angles. This means that the bonds must in many cases be considered as somewhat bent.We investigated the dependence of the C–H bond hybridization in compounds of type CH3X upon the properties of the substituent group. The results can be correlated on the assumption that the effective electronegativity and the size of the atom directly attached to the methyl carbon primarily determine the hybridization.

Temperature Dependence of Chemical Shifts of Protons in Hydrogen Bonds

The observed variations with temperature of the NMR chemical shifts of protons involved in hydrogen bonds have hitherto been attributed to shifts in association equilibria favoring a larger fraction of broken hydrogen bonds with increasing temperature. We now suggest that another mechanism may account for a large part of these variations. The chemical shift for the associated species depends quite strongly on the degree of excitation of the hydrogen‐bond‐stretching vibrational mode. Because this is an unusually low‐frequency motion, several excited states are significantly populated even at temperatures as low as 200°K. When an appropriate average over vibrational levels is used to evaluate the chemical shift, the results show a temperature dependence similar to those found experimentally even though it is assumed in the calculations that no dissociation of the hydrogen‐bonded species occurs.

C13 Splittings in Proton Magnetic Resonance Spectra. III. Formyl Compounds

Electron‐coupled spin‐spin interaction constants JC–H for a series of formyl compounds are presented and interpreted. It is concluded that JC–H is essentially independent of the polarity of the C–H bond and provides the best experimental measure of the hybridization of the carbon atomic orbital used in the bond. Some of the C–H bonds involve carbon orbitals of surprisingly high s character, especially in view of the available data on the bond lengths. It is suggested that the electrostatic interaction between the C–H proton and the π electron density may play an important part in fixing the lengths of these C–H bonds.

Is there a region of highly structured water around a nonpolar solute molecule

Published measurements of water proton chemical shifts for dilute solutions of alcohols and other hydrocarbon derivatives surprisingly seem to imply that hydrophobic groups enhance water structure near 0°C but disrupt it at elevated temperatures. A model is presented which allows these observations to be rationalized and is consistent with experimental values of enthalpies and heat capacities of solution of hydrocarbon gases. It requires the assumption that hydration-shell H-bonds have higher bond-breaking enthalpies and entropies than those in bulk water. These quantities are evaluated from available thermochemical data. Using the corresponding free energies of bond breaking, it is then calculated that the fraction of broken H-bonds is larger in the hydration shell than in the bulk liquid even at temperatures near the freezing point. The Model does not invoke formation of extended ordered regions that could be described as icebergs and that “melt” when the solutions are heated.

Attempt at a unified interpretation of the self-association of 1-1 ionic surfactants in solvents of low dielectric constant

Abstract On the basis of electrostatic energies alone, 1-1 ionic surfactants in nonpolar solvents are expected to form cyclic dimers and compact oligomers. However, approximate model calculations show that when internal entropy contributions are included, strongly polar acyclic dimers and oligomers may become the species of lowest free energy. This implies that two distinct patterns of aggregation are available for these solutes, as seems to be required by available experimental results. The calculations suggest that when the solvent has a relatively high dielectric constant, ϵ, or the sum of the radii of the ionic headgroups, d, is large, association is best represented by stepwise sequential formation of open-chain oligomers with approximately equal equilibrium constants for the binding of additional monomers. When ϵ and d are both small, compact clusters are preferred. Their electrostatic binding energy increases with increasing aggregation number, n, but their growth is eventually limited at n = nmax by the steric demand of the hydrocarbon chains. When nmax ≥ 16 most of the aggregated surfactant should be present in the form of clusters with n ⋟ n max , and then the association process is much more nearly analogous to micellization in aqueous solutions.

Fluorine magnetic resonance and equilibrium dialysis study of the binding of sodium 12,12,12-trifluorododecylsulfate by polyethylene glycol

Abstract Fluorine chemical shifts have been measured for sets of solutions containing fixed amounts of polyethylene glycol and variable concentrations of the anionic surfactant sodium 12,12,12trifluorododecylsulfate. Equilibrium dialysis experiments with similar solutions were used to determine the binding isotherm. Polymer samples with nominal average molecular weights of 7000 and 20 000 exhibit identical binding behavior, but there is little or no binding when the molecular weight is 1500. For the samples of higher molecular weight, binding occurs only when the surfactant concentration exceeds 9.6 m M , or about two-thirds of the critical micelle concentration, 14.6 m M . The number of moles of surfactant bound per gram of polymer reaches a maximum near 7.7, but the data suggest that this is only about 90% of the amount required to saturate the polymer. Micellization then intervenes to block the further increase in the free detergent concentration that would be needed to produce an increase in the amount bound. The chemical shift of the trifluoromethyl groups of the bound anions is independent of the amount bound and is essentially equal to the shift of micellized anions. The results may be rationalized using a model involving the binding of approximately 15 anions in one step to form micelle-like clusters each stabilized by an “effective segment” of the polymer chain with a molecular weight of about 1800. The unitary free energies of binding and micellization, computed with similar simplifying assumptions, are −5.07 and −4.94 kcal/mole of detergent ions, respectively.

Fluorine chemical shift of 6,6,6-tritluoro-1-hexanol in mixtures of water with organic cosolvents

Abstract Fluorine chemical shifts are reported as a function of solvent composition for dilute solutions of 6,6,6-trifluoro-l-hexanol in mixtures of water and organic liquids, mainly at 35°C. The cosolvents are acetone, dioxane, tetrahydrofuran, ethylene glycol, 2-methoxyethane, 1,2-dimethoxyethane, methanol, t-butanol, dimethyl formamide, and dimethyl sulfoxide. The data are interpreted assuming that they reflect primarily variations of the shielding contributions arising from bulk magnetic susceptibility and dispersion interactions. The shapes of the curves of shift versus composition depend greatly on the nature of the cosolvent, and they can be explained only by invoking several types of structural effects, similar to those suggested by other work on water-cosolvent systems. A few observations at 10°C and some data for D 2 O-cosolvent mixtures are consistent with the finding that structural effects are enhanced by reducing the temperature or substituting D 2 O for H 2 O. The results are helpful in evaluating the merits and deficiencies of proposed methods of using 19 F NMR shifts to measure the extent of exposure to water of a fluorine-containing probe molecule bound to a micelle, lipid vesicle, or protein.

Model calculations of changes of thermodynamic variables for the transfer of nonpolar solutes from water to water-d2

A recently introduced modified hydration shell hydrogen bond model for rationalizing the thermodynamic consequences of hydrophobic hydration is adapted for use with heavy water. The required adjustment of parameters employs the assumption that breaking hydrogen bonds in water-d2 involves a greater enthalpy change and a larger entropy increase than bond breaking in ordinary water. It also makes some use of information derived from studies of gas solubilities in the two solvents, although a review of the data leads to serious questions about the reliability of results obtained in this way. The model permits calculations of hydrogen bonding contributions to the changes, ΔGto, ΔHto, ΔSto, and ΔCp,to, for transfer of nonpolar solutes from water to water-d2 and implies that such data should show regular trends. Although some of the numerical results depend strongly on the values chosen for the parameters, the pattern defined by these trends is nearly independent of parameters. Predicted values of ΔCp,to are large and positive for all nonpolar solutes, while ΔSto is expected to be negative near 0°C, becoming progressively less negative on warming and eventually positive. Both of these quantities should be proportional to the molecular surface area of the solute. Analogous predictions regarding ΔGto and ΔHto can also be made, but only if it is permissible to neglect possible contributions to these quantities from van der Waals interactions.

Synthesis of 5,5,5-trifluoro-DL-isoleucine and 5,5,5-trifluoro-DL- alloisoleucine

Abstract Three methods of synthesis are described for 2-amino-3-methyl-5,5,5-trifluoropentanoic acid, a mixture of diastereomers which on separation afford the title compounds. The least cumbersome involves bromination and subsequent amination of 3-methyl-5,5,5-trifluoropentaonic acid, with overall yields near 45% in several trials. The starring materials is readily obtainable from the products of an anodic trifluoromethylation of menthallyl cyanide.