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The infrared spectra of 33 polycrystalline ammonium salts have been measured at 25C, at pressures up to 45 kbar. The N-H stretching and bending bands of the hydrogen-bonded NH4+ ions of most of the salts shift anomalously to higher and to lower frequencies, respectively, as the pressure is raised. In this sense, the salts behave as if they had very strong hydrogen bonds, instead of quite weak ones. A fairly good correlation exists between the N-H stretching frequencies of salts with N+-H...O bonds and their hydrogen bond strengths as measured by the minimum N+...O distances in their crystals.

The infrared spectra of the title compounds have been measured over the pressure range 0-42 kbar at 25C. The phase change at c. 12 kbar is accompanied by an increase in the NH stretching frequencies and other well-defined spectral changes. Vibrational coupling in phase II of HCO2NH4 between CH in-plane bending in HCO2- and a bending motion in NH4+, and its counterpart for DCO2ND4, account for some unusual frequency changes arising from isotopic substitution in the counter ion. The spectral changes suggest that during the phase change I → II the NH...O hydrogen bonds are weakened and the cations move to positions in close proximity beside the CH bonds.

We have examined the effects of hydrostatic pressures to 8 kbar on the rates of deuteration of formate and acetate ions in pure D2O at temperatures between 160 and 200C. The formate reaction is only slightly accelerated by pressure (the activation volume ΔVo?, at zero pressure, is -2.5 cm3 mol-1). This fact disproves mechanisms that have been proposed previously for the reaction and we propose an alternative, concerted, molecular mechanism that fits the facts. By contrast, the acetate reaction is significantly accelerated by pressure (ΔVo? = -10.5 cm3 mol-1), which is consistent with its occurring by mixed molecular and carbanionic mechanisms.

The infrared spectra of the solid metal formates HCO2Li, HCO2Li,H2O, HCO2Na, DCO2Na, H13CO2Na, HCO2K, DCO2K, HCO2Rb, DCO2Rb, HCO2Cs, DCO2Cs, (HCO2)2Ca, (HCO2)2Sr, (HCO2)2Ba and (HCO2)2Pb have been measured in the pressure range 0-42 kbar at 25C. For the sodium salt two new modifications formed at high pressure have been observed, but the potassium, rubidium and caesium salts show only one phase transition. A phase change, though only at pressures far above 42 kbar, is indicated also for the calcium, the barium and probably the anhydrous lithium salt. The various spectral responses to changes in pressure strongly indicate that in the form prevailing at ordinary pressure and temperature, phase I, the formates of sodium and potassium have the same type of molecular packing in the crystal structure. Phases II of the sodium and potassium salt appear to belong to a new modification (?C?), whereas phase III of the sodium and phase II of the caesium salt belong to the modification ?B? previously observed in disc spectra. In a given phase, the effect of pressure on the formate vibration frequencies is sometimes quite different for the pure crystalline salt in bulk and for the same species when isolated by isotopic dilution. In these cases the main effect of pressure is that on the intermolecular coupling of vibrations. Several frequency changes accompanying phase transitions, by contrast, are essentially unchanged on isotopic dilution and arise mainly from changes in the effective intramolecular force constants.

A simple formula is proposed to describe the pressure dependence of the variable Φ in El?yanov and Gonikberg's linear free energy relationship for ionization reactions in solution at high pressure. The expression, given in equations (10) and (12), provides a good description of the influence of pressure on ionization equilibria in aqueous solutions. It permits El'yanov's general linear relationships between Φ and ionization free energies, enthalpies and entropies, pH and Hammett's p parameter, to be expressed in terms of the pressure in convenient analytical forms. The formula is shown to be consistent with the simple electrostatic theory of ion hydration, allowing for the effect of pressure on the dielectric constant of water. Combined with the theory, it provides a general means of predicting ionization constants over a wide range of pressures and temperatures simply from knowledge of the changes in molar volume, enthalpy and entropy which accompany the reactions at atmospheric pressure.

Measurements have been made of the effects of raising the pressure from 1 bar to 2000 bar on the frequencies and intensities of the low frequency electronic absorption bands of 57 aromatic molecules and ions in aqueous solution at 25C. For comparison, measurements have also been made of frequency shifts produced by changing the solvent from water to ethanol at atmospheric pressure. A remarkable feature of the results is that although compression raises the dielectric constant and refractive index of water, it produces red frequency shifts (shifts to lower frequencies) in the n → π* bands of molecules. These bands are well known to undergo blue shifts when one solvent is changed to another of higher polarity at atmospheric pressure. The apparent anomaly can be explained in terms of McRae's theory of frequency shifts, as arising from a decrease, with increasing pressure, of the energy of interaction between permanent dipoles of the solvent and of the absorbing molecules. The π → π* absorption bands of molecules generally show red shifts with both increasing pressure and increasing solvent polarity: this result is also consistent with McRae's theory. The absorption bands of most of the ions show blue shifts which can be explained by charge delocalization in the excited states. The intensity changes are in general agreement with the predictions of current theories of oscillator strengths in solution. The results provide indirect evidence of the influence of pressure on the ionization constants of excited states of phenols.

Measurements have been made of the influence of hydrostatic pressure on the rates of three kinds of bimolecular substitution reactions of alkyl halides in solution. They are reactions with (a) halide ions in acetone solution (Finkelstein reactions), (b) silver nitrate in aqueous ethanol and (c) mercuric nitrate in aqueous dioxan. The Finkelstein reactions were unsymmetrical with respect to the halogen atoms, for instance I- + PrCl → PrI+Cl-. They were all accelerated by an increase of pressure, in contrast to the sym- metrical exchange I*- + PrI → PrI* + I-, which is known to be retarded by an increase of pressure. This fact supports Gonikberg's view of the role of partial desolvation in the symmetrical reactions. Reactions involving LiCl as the source of halide ions were accelerated much more than those involving KI, simply because an increase of pressure enhances the dissociation of unreactive LiCl ion pairs. Reactions of the types (6) and (c) were also accelerated by an increase of pressure, a fact which argues against a suggestion that the attacking species might be ion pairs.

Measurements have been made of the influence of pressure to 40 kbar on the infrared spectra of the normal and X-deuterated forms of seven solid compounds which have strong X-H...Y hydrogen bonds and whose spectra show very broad and intense X-H stretching bands at low frequencies. For two of the compounds, isonicotinic acid and nicotinic acid, compression causes the smooth transformation of superimposed absorption bands into Fermi resonance ?windows?, or ?Evans holes?. The effect probably arises from increased overlap of the wave functions for in-plane ring-bending vibrations and X-H stretching vibrations.

Infrared measurements have been made of the influence of pressures between 0 and 40 kbar on the stretching and bending frequencies of N-H groups in a range of crystalline compounds which have N-H. . .N, N-H. . .O, N-H. . .S or N-H. . .Cl hydrogen bonds. The frequencies have been found to change in accordance with rules that were derived and tested in Parts I and I1 for other kinds of hydrogen-bond systems.

Infrared measurements have been made of the influence of pressures between 0 and 40 kbar on the asymmetrical stretching frequencies v3 and bending frequencies v2 of the hydrogen-bonded ions FHF- and ClHCl- in the solid salts NaHF2, KHF2, NH4HF2, (CH3)4NHCl2 and (C2H5)4NHCl2 at 25C. The behaviour of the symmetrical stretching frequency v1 for FHF- in KHF2 has been deduced indirectly from the shifts of combination bands. Contrary to the behaviour of compounds with weaker, 'normal', hydrogen bonds, the v3 bands shift to higher frequencies and the v2 bands shift to lower frequencies with increasing pressure. The vl band of KHF2 shifts to higher frequencies. These trends are all in agreement with predictions made in Part I for a simple model of hydrogen bonds. A new high-pressure phase transition has been found to occur in NaHF2 at about 40 kbar.

Measurements have been made of the influence of pressures up to 40 kbar on the hydrogen-bonded OH stretching and bending frequencies of a series of solids whose hydrogen bonds cover the full range of strengths from very weak to very strong. The bending frequencies all increase with increasing pressure, and the stretching frequencies nearly all decrease. The exceptions to this general pattern are shown by compounds with strong, symmetrical, hydrogen bonds, whose stretching frequencies increase with increasing pressure. This reversal of behaviour is consistent with the predictions of a simple model based on Lippincott and Schroeder's empirical potential energy function for hydrogen bonds.

The infrared stretching and bending frequencies of isotopically dilute +NHD3 ions in predominantly +ND4 salts have been measured at pressures up to 5 GPa at 25°C. For most of the 28 salts studied, the stretching bands move to higher frequencies with increasing pressure, these results suggesting the absence of any significant hydrogen bonding between the ammonium ions and the anions in the crystals.

Theoretical calculations have been made, with the aid of the Debye model, of the differences in molar volumes between different isotopic forms of simple crystals. Where comparisons are possible, the agreement with experimentally determined (X-ray) volume differences is fairly good. The theory has been applied to predict the differences between the molar volumes of the 32S and 34S forms of the minerals sphalerite, ZnS, galena, PbS, and pyrite, FeS2, and hence to estimate the possible influence of pressure on the partitioning of sulfur isotopes between pairs of those minerals. This partitioning has been used in the past as an isotopic 'geothermometer', with an assumption that it depends upon temperature but not pressure. It turns out that for the conditions that generally apply in practical isotope geothermometry (pressure ≤ 800 MPa, temperature ≤ 1100 K), the calculated pressure effects are considerably less than the uncertainties in experimental fractionation factors.

Measurements have been made of the infrared spectra of eight solid heterocyclic organosilicon and organogermanium compounds at pressures up to 5 GPa at normal temperature. Evidence is found of reversible conformational changes under pressure, and of phase transitions in some of the compounds. Unexpectedly, it appears that compression does not favour the transannular interaction that exists, in some cases, between the heteroatom (silicon or germanium) and a functional group on the opposite side of the ring.

An applied pressure of 2000 bars (1 bar = 105 Pa ≡ 0.9869 atm) has been found to double the rate of photodimerization of methyl 3-methoxy-2- naphthoate in solution in toluene and in methanol at 20°C. The acceleration corresponds to an average activation volume between 1 and 2000 bars of ΔV‡av ≈ -10 cm3 mol-1, and is much less than the known acceleration, under similar conditions, of analogous thermal reactions.

The model described in Part I1 has been extended to allow for two factors which may explain the abnormally flat titration curves of some amine ion-exchange resins. They are (a) the possible formation of ion pairs within a resin and (b) a possible increase in the ionization constant of the functional groups in a resin as its degree of ionization increases. It is suggested that the change (b) may occur if the di- electric constant of a resin is increased sufficiently by the absorption of ions and water as titration proceeds. Calculations have been made of the influence of both effects on thermal regeneration equilibria.

The compressions of binary mixtures of ten organic liquids with water have been measured at a pressure of 1013 bar (1000 atm) at 30C. The excess compressions for these and six similar systems are compared and used to derive the excess volumes, under pressure, of fourteen of the systems. In all cases the excess volumes are numerically smaller at 1013 bar than at 1 bar and, in this sense, the solutions are more nearly ideal. For the ethanol-water system, the excess volumes at the two pressures are compared with theoretical values derived from the dipole model of Haskell. There is qualitative agreement, although it may be fortuitous.

A theoretical model has been developed to describe the thermal regeneration of ion-exchange resins in the Sirotherm process1-10 for desalting water. It assumes that the thermal effect is primarily due to the large increase that occurs in the ionization of water when the temperature is raised. The hydrogen and hydroxyl ions compete with salt for exchange sites in the resins and for that reason water, itself, acts as a regenerating acid and base. Calculations have been made of the equilibrium conditions, column behaviour, and titration behaviour of mixtures of weak-acid and weak-base resins with aqueous solutions of 1 : 1 and 2 : 2 electrolytes, assuming the systems to obey Donnan's equilibrium relationship. There is satisfactory agreement between the calculated and observed effects of both temperature and pressure.

The second virial coefficients of the mixtures CH4-C(CH3)4, CH4-Si(CH3)4, and CH4-SF6 have been measured for several compositions and at several temperatures. Values of the coefficients B12, derived from the results, fail to conform to Guggenheim and McGlashan's (1951) extension of the principle of corresponding states. Reasons are given for this disagreement.

An examination of the theoretical foundations of Trouton's and Hildebrand's rules gives an explanation of the greater generality of Hildebrand's rule. In a group of liquids obeying the principle of corresponding states those with the larger critical volumes Vc have smaller Hildebrand entropies of evaporation. But in many groups of non-polar liquids an increase in Vc is accompanied by a change in the form of the molecular interaction potential (with consequent departure from the principle of corresponding states) and this change can produce a compensating increase in Hildebrand's entropy.

The standard partial molar volumes of some strong and weak electrolytes have been measured in water and three other solvents. The results have been used to estimate the volume changes which accompany the ionization of weak electrolytes. Solvent effects are discussed.

Consideration of the spherically smoothed mutual potential energy between nearly spherical polyatomic molecules leads to the conclusion that it can often be well represented by a (28,7) type of Lennard-Jones potential. Second and third virial coefficients have been calculated for this potential and also for (∞,6) and (∞,7) potentials. The (28,7) interaction energy gives a good description of the properties of gases of quasi-spherical molecules. For these gases it is markedly superior to the more usual (12,6) potential.

It has been found that the second-order rate constant for the benzoylation of benzene, catalysed by aluminium chloride in benzoyl chloride solution at 29.6, is doubled when the pressure is raised from 1 atm to 1500 atm. The same change of pressure has no measurable effect on the orientation of benzoyl substitution in toluene.

This paper reports measurements of the electrical resistances of some lithium, sodium, and potassium silicate glasses between 1 and 2000 atm and from 15 to 35C. The resistances of all the glasses increased with increasing pressure, and the change was greatest for the potassium glass. The apparent volumes of activation for the conduction process were found to be slightly greater than the volumes of the alkali metal ions that carried the current.

The present paper describes some calculations of the speed of sound u in a theoretical Lennard-Jones and Devonshire liquid. It is found that u decreases as the temperature is raised, but increases if the liquid is compressed. When quantum effects are considered it is found that u decreases as the value of de Boer's quanta1 parameter Λ* is increased. All these effects have been observed in experiments on real liquids.

It is shown that the speed of sound in the simple liquids A, N2, O2, CH4 conforms to the principle of corresponding states as formulated by de Boer in molecular units. The lighter liquids H2 and He show negative deviations which are proportional to their quanta1 parameters Λ*. CCl4 shows a positive deviation which is most likely due to a difference in the form of its intermolecular potential from that of the other liquids. A simple theory is presented for the speed of sound in a hypothetical liquid at absolute zero. The calculated value is consistent with the experimental data at finite temperatures.

The resistivities of B, I, TiS2, TiS3, TiSe2, ZrS2, ZrS3, ZrSe2, ZrSe3, and ZrTe2 have been measured at room temperature under static pressures between 1 and 40,000 atm. In every instance the resistivity decreased with increasing pressure, but the change was never great enough to render the materials metallic. The largest change was shown by iodine, whose resistivity dropped by a factor of 1000 at 40,000 atm. Under explosive shock pressures of about 250,000 atm, sulphur and iodine were found to have very low resistances and were presumably metallic. In similar conditions ZrS3 and anthracene remained semiconductors.

This paper presents a theoretical analysis of the Joule-Thomson cooling of normal hydrogen and deuterium from an initial temperature of 64 K. Below 70 atm the cooling is greater for H2 than for D2, but at higher pressures it is less. It is found that the difference at low pressures is due to the lower heat capacity of H2, associated with the more quantized rotation of its molecules. The difference at high pressures is due to the less negative enthalpy of imperfection of H2, and this in turn can be attributed to the higher zero-point energy for translational motion in the lighter isotope. Both differences can be accounted for quantitatively.

The effect of pressure on the formation of complex ions in solutions of cobaltous chloride and in solutions containing iodine and iodide has been shown to be in qualitative agreement with a theory (Buchanan and Hamann 1953) attributing these effects primarily to changes in the free energy of salvation of the ions. The dissociation constant of tri-iodide ions in water at 22 C has been shown to decrease from 1.6 x 10-3 mol kg-1 at atmospheric pressure to 1.2 x mol kg-1 at 1500 atm. The enthalpy of dissociation of the tri-iodide ion was found to be approximately -5 kcal mol-1 and to be little affected by a pressure of 1500 atm.

Calculations have been made of the conditions that are necessary for the existence of vertical equivalence point inflexions in the titration curves of dibasic, tribasic, and polybasic acids. Account has been taken of the effects of finite acid concentration, of finite concentration of the titrating base, of finite strength of the titrating base, and of activity coefficients. As a rule, each of these effects causes a flattening of the titration curves and the ultimate disappearance of inflexions.

The following compounds have been found to polymerize spontaneously in the solid state at pressures in the range 10-50 kbar, at temperatures between 20 and 200C: acrylamide, p-phenylstyrene, potassium p-styrenesulphonate, itaconic anhydride, maleic anhydride, maleimide, 1,2,3,6-tetrahydrophthalic acid, 1,2,3,6-tetrahydrophthalic anhydride, acenaphthylene, p-benzoquinone, N,N'-p-phenylene-dimaleimide, sulpholene, diphenylacetylene, 8-trioxan. The pressure-temperature requirements for polymerization have been determined in a high-pressure "squeezer" apparatus and in a diamond optical cell which permits infrared spectral analysis of a specimen while it is under compression. Apart from diphenylacetylene and trioxan, the compounds that polymerized were either monosubstituted ethylenes or cyclic 1,2-disubstituted ethylenes. Non-cyclic 1,2-disubstituted ethylenes and tri-substituted and tetra-substituted ethylenes failed to polymerize. There is evidence that shearing stresses played a part in some of the reactions. 1-Allyl-2-thiourea did not polymerize, but transformed from its stable crystal form I to the unstable modification 11.

Solid maleic anhydride undergoes spontaneous polymerization when it is heated to 100-1700 at pressures above 20000 atm. On a pressure- temperature diagram, the regions of monomer stability and of polymerization are separated by a rather well-defined line, which possibly marks the occurrence of a physical polymorphic transition. The structure of the polymer is discussed. Efforts to polymerize some other 1,2-disubstituted ethylenes have been unsuccessful. The following substances were recovered unchanged after being heated to 160-1800 at 30000-45000 atm: maleic acid, fumaric acid, crotonic acid, sorbic acid, coumarin, and trans-stilbene.The melting temperature of maleic anhydride has been measured to 3770 atm and the parameters in the Simon melting equation are reported.