Ernest Grunwald

Rates and Mechanisms of Protolysis of Methylammonium Ion in Aqueous Solution Studied by Proton Magnetic Resonance

The application of the nuclear magnetic resonance technique to the study of fast protolysis reactions of methylammonium ion in water is described. It is shown that reaction rates can be measured quantitatively for half‐times in the range, 0.002 to 0.2 sec. Reaction rates were measured as a function of the methylammonium ion concentration and of the hydrogen ion concentration. Detailed information on the reaction mechanism was obtained by correlating the rate constants obtained from the CH3, the NH3+ and the H2O lines of the NMR spectrum. In the pH range employed (between 3 and 5), the exchange rate is very nearly proportional to [CH3NH3+][CH3NH2], the second‐order rate constant being 6×108 sec—1M—1 at 19°. Two separate reactions contribute to this rate constant: One of them, which accounts for about 40% of the total rate, is a direct proton transfer from CH3NH3+ to CH3NH2. The other involves also at least one molecule of water, CH3NH2 receiving a proton from the solvation shell of CH3NH3+ while simultaneo...

Application of Nuclear Magnetic Resonance to the Study of Acid‐Base Equilibria

Acid‐base equilibria in aqueous solutions of methylamine, dimethylamine, and trimethylamine have been studied by nuclear magnetic resonance. It is shown that the chemical shift of the methyl protons is a linear function of the concentration ratio of acid to base plus acid. The NMR spectrum of tetramethylammonium ion is also given.

Thermodynamic properties of nonpolar solutes in water and the structure of hydrophobic hydration shells

Experimental partial molar entropies and heat capacities in water for noble gases, nonpolar diatomic gases, and hydrocarbons were analyzed thermodynamically by delphic dissection to evaluate the contributions from solute-induced perturbations of the water network. These contributions, which are typically large, were then interpreted in terms of the familiar two-state model of liquid water. Changes in the fractions of the two states and of their relative enthalpies, per mole of solute, were thus evaluated. The numbers of water molecules that are nearest neighbors to the solute, and the manner in which the nearest neighbors are tied to the bulk water network, could be elucidated.

Electric dipole moments in polar solvents. I. Audiofrequency measurement with slightly conducting solutions

Audiofrequency methods were used to measure dielectric constants of dilute solutions containing electrolytes up to free-ion concentrations of 10−4M. Using a calibrated transformer bridge, capacitance was measured with an accuracy of 0.03 pF at a conductance of 100 μmho and within 0.3 pF at 800 μmho. Evaluation of the double-layer capacitance from the frequency dependence of the data is discribed. The effect of the free ions on the dielectric constant is found to be relatively large and in reasonable agreement with the prediction of the Debye-Falkenhagen theory. The calculation of the electric dipole moment for polar solutes, including ion pairs, is discussed in terms of Kirkwoods theory. Experimental tests are described for finding out whether possible deviations of Kirk-woods correlation factors from unity may be neglected. These tests involve changing the solvent and the temperature. The tests were satisfied for the ion pairs of tetraisoamylammonium nitrate and for nitrobenzene in chlorobenzene and acetic acid.

A thermodynamic analysis of the first solvation shells of alkali and halide ions in liquid water and in the gas phase

Abstract Let n denote the number of water molecules in the nearest-neighbour shell (NS) of an ion J± in liquid water, and denote J± ·nH2O in the gas phase by J± · NSG (g). The standard free energy of hydration ΔG°hyd (J± ·NSG) can then be deduced by a thermodynamic cycle involving ΔG°n for the formation of J± · NSG (g) and ΔG°hyd for the transfer of J± (g) to water. Values of ΔG°hyd (J± ·NSG) for alkali and halide ions are substantial, ranging from 48 % to 86 % of ΔG0 hyd. The values of ΔG0 hyd(J± ·NSG) can be accounted for largely by the calculated work—electrostatic (ΔWelec) and surface (ΔWsurt)—in the process J± ·NSG (g) → J± (aq). ΔWelec is the major contributor. ΔWsurf depends on whether (i) J± ·NSG (g) can be represented by a cluster consisting of the ion and n separate water molecules, or (ii) there is some molecular complex formation within that cluster. In fact, ΔWsurt < 20 kJ mol−1 for the alkali ions and of the order of 100 kJ mol−1 for the halide ions. A reasonable case can be built that the a...

Participation of Hydroxylic Solvent Molecules

Proton transfer reactions are often depicted as simple bimolecular processes in which a proton is transferred directly from the acid to the base. However, with the development of proton magnetic resonance techniques for measuring the rates of fast reactions, it has become clear that many proton transfer reactions in hydroxylic solvents actually proceed with participation of one or two solvent molecules. The solvent molecule then acts as a bifunctional catalyst, that is, both as a proton acceptor and a proton donor. Thus, termolecular proton transfer from an acid HA to a base B with participation of a water molecule is shown in equation 1.

Vibrational energy redistribution and hot-band spectrum for hexafluorobenzene following infrared multiphoton excitation

Abstract IR-UV double resonance techniques were employed to study the formation of transient hot bands of hexafluorobenzene. It is shown that under various conditions of pressure and IR laser excitation, vibrational energy redistribution relaxation times are of the order of 30–100 ns.

Application of Kirkwood's Theory of the Dielectric Constant to Solutes Hydrogen-Bonded in the Water Network

In a quasi-thermodynamic treatment, the partial molar polarization of a solute in a network liquid is expressed in terms of dipole moments, molalities. Kirkwoods formal correlation factors, and solute-induced changes in the correlation factors. The formal correlation factors are then resolved into explicit terms for solvent-solvent, solvent-solute and solute-solute dipole correlation, which convey specific (though limited) information about the stoichiometry and geometry of the respective hydrogen-bonding. Experimental partial molar polarizations are analyzed for aqueous solutions of p-dioxane, pyrazine, quinoxaline, acetone, pyridine, N,N,N′,N′-tetramethylurea, acetonitrile, and dimethylsulfoxide. The treatment does not yield unique hydrogen-bonded structures but, when combined with other evidence, it greatly limits the possibilities. Water molecules appear to donate hydrogen bonds exhaustively to ether and carbonyl oxygen atoms, and to aza-aromatic nitrogen atoms. Water molecules also appear to donate hydrogen bonds to aza-aromatic π-systems, and to the triple bond in acetonitrile.

Electric dipole moments in polar solvents. II. Tetraisoamylammonium nitrate ion pairs in chlorobenzene. Effect of mutual polarization of the ions

The dielectric constant and conductivity of dilute solutions of tetraisoamylammonium nitrate in chlorobenzene are measured between −34.6° and 99.0°C to give association constants for the formation of ion pairs (KA) and triple ions, and electric dipole moments. The quantityKA as a function of temperature is reproduced by the Denison-Ramsey-Fuoss treatment for unolarized ion pairs [Eq. (2)] with a distance of closest approach of 4.90 Å. The dielectric data are reproduced by Onsagers equation with an inherent (gas-phase) dipole moment of the ion pairs of 14.2±0.3 D. Other methods of calculation lead to consistent dipole moments, confirming that the mutual polarization of the ions is important. The energetics of ionic association is considered on the basis that the ion pair may be treated as a polarizable dipole in a spherical cavity.

Kinetics of ion-pair exchange in acetic acid

The rate constants for the ion-pair metatheses p-toluidinium toluene-p-sulphonate + metal acetate ⇌p-toluidinium acetate + metal toluene-p-sulphonate have been measured using an n.m.r. technique; the rate constants are ca. 109M–1 S–1 and show that the slow step in the exchange involves cation–acetate cleavage.