Ionization of sodium dihydrogen phosphate in aqueous solutions of hydrochloric acid of various concentrations has been studied using Raman spectroscopy. The known acidities of phosphoric acid were found to lie at pH 12, 6.5 and 2.1. When the concentration of hydrochloric acid is increased up to 12 mol dm–3, the Raman band at 1176 cm–1(PO like) disappeared and the band at 993 cm–1[P(OH) like] shifted. This can be attributed to the new molecular species P(OH)+4: the perprotonated form of phosphoric acid.
Absorption of ultrasound in L-cysteine has been measured at 37 °C for the pH range 6.8–8.0 for 0.2 and 0.5 mol dm–3 solutions. A new pulse-transmission technique has been used which produces continuous absorption spectra for the range 2–50 MHz, enabling accurate determination of relaxation frequency (f0) within this range. The variation of f0 with concentration and pH is explained in terms of a mechanism involving both intra- and inter-molecular proton transfer. For the first time, values have been determined for the volume change of the intramolecular proton-transfer reaction and for a number of the individual ionization rate constants for cysteine.
The gas-phase ionization energies of 11 ferrocene derivatives have been measured by He(I) photoelectron spectroscopy and compared with the electrode oxidation potentials in aprotic solvents. The first four ionization energies and the oxidation potentials of these compounds change linearly with the Hammett substitution constants. Linear correlations with slopes ca. 0.7 have been obtained between the first adiabatic ionization energies and the oxidation potentials in several aprotic solvents. This result indicates that the solvent effect on Fc/Fc+ couples changes proportionally with the gas-phase ionization energies.
The coordination structures of the Cu ions in the mixed solvent system of HCONH2–HCOONH4 at room temperature and 100 °C have been investigated using the EXAFS (extended X-ray absorption fine structure) method for the Cu K-edge, together with visible absorption spectrum analyses and ion-exchange experiments. The Cu ions, which are divalent, CuII, at room temperature, are reduced to univalence, CuI, by heating with HCOO– ions, ca. 80% of CuII ions are transformed into CuI, ions at 100 °C. In the mixed solvent system, it was found that CuII ions form neutral [Cu(HCOO)2(HCONH2)4] octahedra, distorted by the Jahn–Teller effect, with four short Cu—O bond lengths of ca. 1.97 Å and two long Cu—O bond lengths of ca. 2.82 Å at room temperature, and CuI ions form cationic [Cu(HCONH2)3]+ plane triangles with three Cu—O bond lengths of ca. 1.90 Å at 100 °C. The transition of the coordination structures occurs reversibly in the mixed solvent system as a function of the temperature.
This paper reviews two important problems in the field of defect physics and chemistry, for which solutions are needed. The first refers to amorphous silicon and relates to the properties of 'dangling bonds'. The second is the anodic oxidation of aluminium and the mechanism of atomic transport through the oxide layer.
The second hyperpolarisability, γ, of (HF)n, where n= 6, 8, and the interaction hyperpolarisability, Δγ, of (HF)2 are computed by employing ab initio and semiempirical techniques. The results demonstrate the cooperative character of the intermolecular interactions between the H-bonded hydrogen fluoride molecules and their considerable effect on the second hyperpolarisability. The results also show that remarkable variations in Δγ and γ may be induced by a change in the shape of the HF cluster.
In order to detect the Marcus inverted region in electron-transfer reactions in solution it is more appropriate to follow not the forward but the back reaction. This can be done by means of chemically induced dynamic nuclear polarization, measuring the polarization of re-formed educts. The 13C-CIDNP enhancement factors V(13C) of re-formed arene diazonium salts in the photochemical electron transfer from singlet-excited rubrene to a series of para-susbstituted benene diazonium tetrafluoroborates in chloroform have been determined. On plotting V(13C) against the standard free reaction enthalpies of the back-electron-transfer, bell-shaped behaviour of the Marcus type is obtained. Taking the reorganizational energy (λ= 0.81eV) from the maximum of this curve the Franck–Condon factors FFC= exp (–ΔG‡/RT) were calculated according to the Marcus quadratic equation, which, on plotting against V(13C), give a linear correlation. The Franck–Condon factors (0, 2,…,1) show that the back-electron-transfer proceeds with nearly the theoretical maximum value k0.
Excited-state metal–molecule reactions [Hg(3P1)+ H2, Ca(1P1)+ HCl] have been studied within the complexes formed by the reactants. Under these conditions specific aspects of orbital orientation within the molecular frame are exemplified.
An explanation is given for the red shift of the fluorescence band of ethyl 4-(N,N-diethylamino)benzoate (DEAEB) at ambient temperature (band FX), relative to the normal fluorescence band (FN) of ethyl 4-(N,N-dimethylamino)benzoate (DMAEB) in the same alkane as solvent, which is not based on the previously suggested twisted intramolecular charge-transfer nature of the fluorescent state, but on the proximity of low-lying nπ* and ππ* electronic states, having an energy separation depending on the electron donor strength of the substituted amino group. The magnitudes of the electric dipole moment of the fluorescent state of DMAEB (11.1 D) and DEAEB (12.2 D), in cyclohexane at ambient temperature, do not differ enough to allow a characterization of this fluorescent state of DEAEB as a state resulting from intramolecular electron transfer. The presence of small amounts of polar molecules in the alkane solution of DEAEB and DMAEB leads to the formation of fluorescing solute–solvent exciplexes (band FA). The time dependence of the fluorescence of DEAEB and DMAEB in a viscous polar solvent like butan-1-ol reveals the presence of three types of species, leading to fluorescence of type FN, FX and FA. The species emitting fluorescence of type FN are converted into species yielding fluorescence band FX and the latter react with the solvent to give fluorescing solute–solvent exciplexes.
Several reactions are reported in which photolytically prepared ‘hot’ H atoms react with N2O. Under bulk (i.e. single-collision arrested-relaxation) conditions with H-atom kinetic energies of ca. 2.5 eV, intense OH (A2Σ→ X2Π) chemiluminescence is observed. The relative A 2Σ vibrational populations are (ν= 0)/(ν= 1)/(ν= 2)= 5/3/2. However, with N2O–HBr complexes, no such emission is observed, and OH (X2Π) is easily detected using LIF. In both cases, NH(X 3Σ) is observed using LIF, and the slightly colder rotational distribution obtained with complexes is attributed to simultaneous H–Br and H–N2O entrance channel repulsions. With N2O–HBr complexes, the [NH (X 3Σ)]/[OH(X2Π)] ratio is ca. 0.5, despite the 95 kcal mol–1 difference favouring OH + N2 over NH + NO. There is no evidence of products derived from O(1D) reactions via N2O photolysis. At lower H–atom kinetic energies (e.g. ca. 1.8 eV), there is no chemiluminescence, although OH (A 2Σ) is still energetically accessible.
Molecular-mechanical calculations have been performed on beta-cyclo-dextrin (β-CD) and its inclusion complexes using the MM2 force field. The guest molecules involved were nitrophenol (4-NP, 3-NP, 2-NP), nitroaniline (4-NA, 3-NA, 2-NA) and dihydroxybenzene (1,4-DH, 1,3-DH, 1,2-DH). A symmetric structure of β-CD was optimized as the first step of calculation. The preferred way of a guest molecule approach to the cavity in β-CD is discussed, based on the energy variation in this process. A good correlation was found between the order of variation of van der Waals energy and that of Kd of these inclusion complexes, as obtained by electrochemistry.
Electron-impact excitation of methane, silane, tetrafluoromethane and tetrafluorosilane has been explored in a trapped-electron spectrometer. Extensive vibrational excitation of CF4 was observed with electrons of very low incident energies. There was some evidence of multiple vibrational excitation in SiH4 also but the results for this molecule were not totally unambiguous. Nearthreshold vibrational excitation of SiF4 was masked by effects attributed to a large cross-section for elastic scattering. Some optically forbidden electronic states have been located and assignments suggested. Short-lived negative-ion states (resonances) affect electronic excitation of the lowest excited Rydberg state in SiH4, CF4 and SiF4 as well as negative-ion production.
An arbitrarily organized interface with non-uniform curvature is considered. The force factors (surface tension and bending moments) depend on interface geometry. The general definitions of spontaneous geometrical characteristics and elastic moduli of interface are presented. The spontaneous geometrical characteristics correspond to zero values of interface force factors. The elastic moduli are expressed in terms of derivatives of force factors. The expression for energy of interface deformation including the second-order terms is derived.
The reactions of methyl radicals, generated by the photolysis of acetone, with cyclohexane and a number of methyl-substituted cyclohexanes have been studied. The Arrhenius parameters corresponding to overall hydrogen abstraction have been obtained from experiments in the temperature range 100–200 °C based on the value of the rate constant for recombination of methyl radicals. The overall rate constants for hydrogen abstraction by methyl radicals could be adequately expressed as the sum of contributions from each tertiary, secondary and primary hydrogen atom for all of the following nine compounds which were studied: cyclohexane, methylcy clohexane, 1,2-dimethylcyclohexane, 1,3-dimethylcyclohexane, 1,4-dimethylcyclohexane, 1,1-dimethylcyclohexane, 1,3,5-trimethylcyclohexane, 1,2,4-trimethylcyclohexane and 1,1,3-trimethylcyclohexane. The Arrhenius parameters for the abstraction of secondary hydrogen atoms are log (A/cm3 mol–1s–1)= 10.83, E/kJ mol–1= 37.4, and those for the abstraction of tertiary hydrogen atoms are log (A/cm3 mol–1 s–1)= 11.78, E/kJ mol–1= 35.7.
Calculations on the energies of alkali-metal cations bound to aluminium substitutionals in α-quartz are presented. These are compared with experimental dielectric relaxation and electrical conductivity measurements.
The representation of weak interactions of open-shell atoms as effective anisotropic potentials is discussed. Examples, mainly from recent molecular-beam studies of collisions of magnetically orientated atoms in P states, prove that the spherically averaged component of the interactions follows systematics established for closed-shell van der Waals forces. Data are being collected to unravel similar systematic trends for the anisotropic component of the interactions.
The angular distribution of reactive (and also of non-reactive) scattering has been shown to provide useful insights into the steric requirements of chemical reactions. In a simple optical model, the differential scattering cross-section, d2σR/d2w, and the differential orientation-dependent cross-section, dσR/d cos γ, are both derived from a common opacity function. The angle-dependent line-of-centres model has been used to compute both cross-sections. For the K + CH3l reaction where the barrier, according to Bernstein, is a linear function of the cosine of the approach angle, the two differential cross-sections are closely related. Within the angle-dependent line-of-centres model one can also re-examine the optical mode analysis of the non-reactive scattering. This suggests a reinterpretation of the variable ‘the potential at the distance of closest approach’ as ‘the energy along the line of centres’. With this interpretation, the systematics in the opacity functions, determined from the observed non-reactive scattering, can be simply accounted for. In particular, for several reactions [e.g. K + CH3Br, K +(CH3)3 CBr] the opacity analysis provides evidence for a [‘cone of nonreaction’] of a primarily geometrical nature reflecting presumably the steric hindrance of the organic group.
'Hot' hydrogen atoms produced by photolysis of gaseous HBr or HI react with CD2Cl2 by D abstraction, H*+ CD2Cl2→ HD + CDCl2(1) and by Cl abstraction, H*+ CD2Cl2→ HCl + CD2Cl. (2) The integral probability of reaction (1) has been measured for several defined initial translational energies of the hydrogen atom, and the phenomenological threshold energy is 41 ± 4 kJ mol–1. Integral probabilities for reaction (2) have also been determined, and the threshold for Cl abstraction is 43 ± 10 kJ mol–1. For initial translational energies in the range 66–120 kJ mol–1, the ratio of the integral probabilities of Cl abstraction and of D abstraction varies between 2.1 ± 0.8 and 2.5 ± 0.5.
The photographic process involves the interaction of photoelectrons and holes with ionic defects present near the surface and in the interior of silver halide microcrystals. The physical changes which take place in pure silver halide crystals upon exposure to light have been examined and discussed. In practical photographic systems, microcrystals of mixed halides, such as Ag(Cl, Br) and Ag(Br, I), are often employed. Since these mixed crystals range in size from ca. 0.5–5 µm, they have a high surface-to-volume ratio. Thus a detailed description of defects at silver halide surfaces, as well as in the interior, is important to the understanding of photography. By combining the insights provided from theoretical work with experimental findings, a picture has evolved of the role of ionic defects.
The techniques of translational spectroscopy and glancing-incidence molecular ion beam–surface scattering have been combined to study reactive collisions of molecules with surfaces. A clear correlation between the results and known adsorption and reaction behaviour at thermal energies has been established. The similarity between the collision and molecular vibration times permits the identification and investigation of reaction intermediates which may not be detectable by conventional spectroscopies. The technique, which can in principle also measure the molecular orientation to the surface during scattering, should provide valuable insights into the dynamics of adsorption and reaction, both molecular and dissociative, on surfaces. Explicit results for the interaction of N2 with Ni(110) and (111), O2 with Ag(111) and Ni(110), and CO and CO2 with Ni(110) surfaces are presented.
