Spectroscopic nuclear quadrupole moments

A ‘year-2001’ set of nuclear quadrupole moments, Q, is presented. Compared to the previous, ‘year-1992’ set, a major revision of the value or a considerable improvement of the accuracy is reported for ⁶ ₃Li, ₇N, ¹⁹ ₉F (197 keV, I = 5/2), ₁₁Na, ₁₃Al, ₂₁Sc, ₂₂Ti, ₂₆Fe (14 keV, I = 3/2 Mössbauer state), ₃₁Ga, ₃₂Ge, ⁷⁷ ₃₄Se (250 keV, I = 5/2 state), ₃₅Br, ₃₆Kr, ₃₇Rb, ₃₉Y, ₄₀Zr, ¹⁰⁰ ₄₅Rh, ₅₀Sn (24 keV, I = Mössbauer state), ₅₃I, ₅₄Xe, ₅₅Cs and ₈₃Bi.     

Fluorine hyperfine coupling constants: an INDO study of the fluorobenzyl radicals

The underlying assumptions of Roothaan's symmetry-restricted SCF method for open-shell systems are considered. A ‘restricted Brillouin's theorem’ is formulated and applied in a discussion of total energy discontinuities arising in restricted SCF calculations of systems which exhibit Jahn-Teller instability, such as the tetrahedral ² T ₂ state of CH₄ ⁺.     

Steric effects on the mechanism of reaction of nucleophilic substitution of β‐substituted alkoxyvinyl trifluoromethyl ketones with four secondary amines

The kinetics of the reaction of β‐substituted β‐alkoxyvinyl trifluoromethyl ketones R¹O‐CR²?CH‐COCF₃ ( 1a – e ) [( 1a ), R¹?C₂H₅, R²?H; ( 1b ), R¹?R²?CH₃; ( 1c ), R¹?C₂H₅, R²?C₆H₅; ( 1d ), R¹?C₂H₅, R²?V^?pNO₂C₆H₄; ( 1e ), R¹?C₂H₅, R²?C(CH₃)₃] with four aliphatic amines ( 2a – d ) [( 2a ), (C₂H₅)₂NH; ( 2b ), (i‐C₃H₇)₂NH; ( 2c ), (CH₂)₅NH; ( 2d ), O(CH₂CH₂)₂NH] was studied in two aprotic solvents, hexane and acetonitrile. The least reactive stereoisomeric form of ( 1a – d ) was the most populated ( E‐s‐Z‐o‐Z ) form, whereas in ( 1e ), the more reactive form ( Z‐s‐Z‐o‐Z ) dominated. The reactions studied proceeded via common transition state formation whose decomposition occurred by ‘uncatalyzed’ and/or ‘catalyzed’ route. Shielding of the reaction centre by bulky β‐substituents lowered abruptly both k′ (‘uncatalyzed’ rate constant) and k″ (‘catalyzed’ rate constant) of this reaction. Bulky amines reduced k″ to a greater extent than k′ as a result of an additional steric retardation to the approach of the bulky amine to its ammonium ion in the transition state. An increase in the electron‐withdrawing ability of the β‐substituent increased ‘uncatalyzed’ k′ due to the acceleration of the initial nucleophile attack (k₁) and ‘uncatalyzed’ decomposition of transition state (k₂) via promoting electrophilic assistance (through transition state 8 ). The amine basicity determined the route of the reaction: the higher amine basicity, the higher k₃/k₂ ratio (a measure of the ‘catalyzed’ route contribution as compared to the ‘uncatalyzed’ process) was. ‘Uncatalyzed’ route predominated for all reactions; however in polar acetonitrile the contribution of the ‘catalyzed’ route was significant for amines with high pK_a and small bulk. Copyright © 2007 John Wiley & Sons, Ltd.     

Piezoelectric properties of non-uniform polymeric structures under static load

ABSTRACT

Piezo-electrical properties of structures containing ‘soft’ and ‘hard’ dielectric layers with charge stored on the interfaces are described in the paper. The piezo-activation process of structures containing layers with gas voids by partial discharges is described. The influence of the mechanical properties of the ‘soft’ layer on the piezoelectric parameter d₃₃ value and its dependence on the static pressure p are also discussed. It was found experimentally, that for the fibrous type of ‘soft’ dielectric layer, the dependence of the piezoelectric parameter d₃₃ (p) can be described by the function d₃₃ ÷ p^?n, where n ≤ 1.     

Defined-sector explicit solvent in the continuum model approach for computational prediction of pK a

Accurate prediction of the acidity dissociation constant (K _a) is a challenge for the theory of proton-transfer reactions, making first-principles prediction of pK _a within 0.5?pK units of experimental values a benchmark of broad interest. In the present contribution, the defined-sector explicit solvent in continuum cluster model, which considers the structure-to-chemical affinity relationship of the carboxyl functional group, is presented. The model demonstrates predictable solvent networks based on established ‘preferred’ conformations found in a training set. Predictability within 1?kcal?mol^–1 accuracy is shown for a full set of carboxylic acid systems with varying functionality.     

Electron paramagnetic resonance and electron nuclear double resonance of the AsO4 4- centre in X-irradiated KH2AsO4

Electron paramagnetic resonance studies of the AsO₄ ^4- centre in X-irradiated crystals of KH₂AsO₄ in the ferroelectric phase at 77 and 4·2 K are reported. The symmetry of the spin-hamiltonian has been found to be orthorhombic, and the g tensor and the A tensor describing the interaction of the unpaired electron with the arsenic nucleus (I = 3/2) have been obtained. Domain splitting has been observed in he spectra recorded in the ab plane of the crystal. By studying these spectra in the presence of an applied electric field, it has been possible to plot the hysteresis curve of ferroelectric KH₂AsO₄. Electron-nuclear double resonance (ENDOR) of protons surrounding the AsO₄ ^4- units has been studied at 4·2 K. Two sets of ENDOR lines have been found arising from the protons in the two equilibrium positions (labelled ‘ close ’ and ‘ far ’) along the hydrogen bonds linking the AsO₄ tetrahedra. The angular variation of the ENDOR lines from both ‘ close ’ and ‘ far ’ protons has been plotted in the crystal symmetry planes. The observed ENDOR frequencies have been fitted to those calculated from the numerical diagonalization of the Hamiltonian. The superhyperfine parameters for the ‘ close ’ and ‘ far ’ protons thus obtained are found to be quite anisotropic. The ENDOR results are shown to explain all details of the partially-resolved proton superhyperfine structure at room temperature as well as at low temperatures. An isotropic contact hyperfine coupling of -2·875 MHz of the unpaired electron to the proton in the ‘ far ’ position of the hydrogen bond has been determined, providing direct evidence for covalency in the hydrogen bonding in KH₂AsO₄ crystals.     

A cellular ligand-field model for l-l spectral intensities

The intensity distributions in the ‘d-d’ spectra of three planar CuCl^2- ₄ chromophores are reproduced quantitatively within the theoretical model described in the preceding paper. Intensity for these species, namely bis(1-methyl-4-oxo-3,3-diphenylhexyldimethylammonium)tetrachlorocopper(II), bis(2-iminol-1-methy-4-imidazolidinium)tetrachlorocopper(II) and bis(N-methylphenylethylammonium)-tetrachlorocopper(II), is deemed to arise dynamically via the bending vibrations. Fair reproduction of experimental absorbances is achieved using the b _2u mode alone, this being known to be the softest vibration in these systems. The roles of spin-orbit coupling and small departures from D _4h symmetry are discussed. Nearly perfect reproduction of experiment is obtained with addition of small contributions from the e_u bend. The intensity distribution in the ‘d-d’ spectrum of the pseudo-tetrahedral chromophore, bis(N-benzylpiper-azinium)tetrachlorocopper(II), is reproduced quantitatively within the ‘static’ model described in part I of this series. Comparisons between the parameter values obtained from the ‘static’ analysis with those from the ‘dynamic’ analysis for the planar chromophores suggest that dynamic modelling with both b _2u and e_u modes is the more physically persuasive. The efficacy of the model in part IV is critically reviewed in this first application.     

Observation of the fast component in the fluorescence of gaseous SO2 excited to the A1A2 state in the presence of a magnetic field

Effects of an external magnetic field (B) on the SO₂ fluorescence have been examined under excitation of the ‘C’ band and the single rotational levels of the ‘B’ and ‘E’ bands of the A(¹A₂) ← X¹A₁ transition. For the ‘C’ band, the total SO₂ fluorescence was studied, while for excitation of the single rotational levels, fluorescence of the single rotational lines of (^rR₅(5) (31711 cm^-1), ^pP₇(7) (31662 cm^-1)) (‘E’ band) and of (^rR₈(8) (31000 cm^-1), ^pP₁₀(10) (30927 cm^-1)) (‘B’ band) was studied. Decay of the SO² fluorescence was studied with nanosecond time resolution in the 1?5–50 mTorr region. In the presence of a magnetic field, decay of the total SO₂ fluorescence and of the fluorescence belonging to the single rotational lines were fitted by the bi-exponential functions. In the case of the total SO₂ fluorescence, this function also includes the time independent term, which, however, is dependent on a magnetic field. The time independent term belongs to the long-living component, which can be approximated by a constant in the time-scale studied. Radiationless processes induced by an external field were directly observed. The magnetic field and pressure dependences of the processes induced by a field were studied under excitation of the SO₂ fluorescence by light of a different wavelength. The data obtained were explained by the direct mechanism.     

Analysis of the heterogeneous dynamics of imidazolium-based [Tf2N−] ionic liquids using molecular simulation

The complex dynamic behaviour of the imidazolium-based ionic liquids [C_nmim⁺][Tf₂N^?], n = 4, 8, 12 is examined at various temperatures and at atmospheric pressure using molecular dynamics simulation. An existing all-atom force field is further optimised in order to attain reasonable agreement with experimental data for transport properties, such as self-diffusivities and viscosities. Dynamical heterogeneity phenomena are quantified through the calculation of the non-Gaussian parameter and the deviation of the self-part of the van Hove correlation function from the expected normal distribution. From this analysis, ions that move faster or slower than expected are detected in the system. These subsets of ‘fast’ and ‘slow’ ions form individual clusters consisting of either mobile or immobile ions. Detailed analysis of the ions’ diffusion reveals preferential motion along the direction of the alkyl tail for the cation and along the vector that connects the two sulphur atoms for the anion. For the longest alkyl tails, the heterogeneity in the dynamics becomes more pronounced and is preserved for several nanoseconds, especially at low temperatures.     

Rotational diffusion of carbon monoxide in various simple liquids

Infra-red spectra of CO dissolved in liquid N₂, O₂, Ar, Xe and CH₄ have been recorded both in the fundamental and the first overtone. From the band profiles, rotational correlation functions, band moments and intermolecular mean-square torques have been calculated. With the help of Gordon's rotational diffusion model [1] we computed the theoretical correlation functions in the ‘M-diffusion’ (including or not a distribution of angular momentum correlation times) and ‘J-diffusion’ limits. The validity of the Gordon and Debye diffusion models in the various dense gases is discussed.     

MCSCF study of the spin-orbit coupling in OH (X 2Π i )

Full-valence MCSCF calculations were performed for the X ²Π _i state of OH in that it was hoped that these would predict the spin-orbit coupling A(r) to within a few tenths of a wavenumber over the entire range of r. The functional form was correct but the actual values did not meet our expectations. We briefly considered higher-order relativistic corrections.

A universal even-tempered basis set was tested relative to an ‘extended’, optimized basis. The universal basis set did remarkably well and highlighted significant r-dependent and, presumably, state dependent problems with the smaller basis.     

Etude ENDOR d'un radical HAsO3 - dans la phase anti-ferroélectrique du cristal d'arséniate d'ammonium NH4H2AsO4

L'identification par ENDOR d'un centre radicalaire HAsO₃ ^- créé par irradiation gamma d'un monocristal de NH₄H₂AsO₄ a été réalisée à 1,5 K. En particulier, les interactions hyperfines correspondant aux protons des trois liaisons hydrogène O-H … O reliant le radical à des tétraèdres AsO₄ ^3- sont déterminées. Un de ces protons est en position ‘proche’ par rapport à un oxygène du radical, en accord avec la formule HAsO₃ ^-, tandis que les deux autres protons sont en position ‘éloignée’. Les parties isotropes des interactions des protons ‘éloignés’ sont trouvées négatives, comme le veut un mécanisme de polarisation de spin à travers une liaison hydrogène partiellement covalente; les protons ‘éloignés’ sont trouvés être en configuration de Slater latérale, ce qui correspond au niveau du défaut à l'ordre antiferroélectrique des protons proposé par Nagamiya.     

Dimple‐assisted dewetting: heterogeneous nucleation in undercooled wetting films

Undercooled wetting films near a first‐order wetting transition exhibit an unusually long lifetime: the thermal nucleation barrier for formation of a critical hole in a film of thickness F diverges according to Γ ∼ exp (ℰ_c/k_BT) where the excess free energy ℰ_c ∼ F^ζ with ζ ≥ 2. Localized perturbations of the liquid‐vapor interface (‘dimples’) are shown to be a useful tool in reducing Γ in a controlled way: they act as heterogeneous nucleation centers for thermal critical nuclei. For ⁴He wetting films on weak‐binding alkali substrates (Cs, Rb) dimples can be generated either by vortices in a superfluid film or by surface electrons. The theory of the heterogeneous nucleation process initiated by the presence of surface dimples (‘dimple‐assisted dewetting’) is developed, accompanied by quantitative predictions for experiment.     

EPR studies of highly pure,pure and doped chromium oxide powders

Abstract

EPR absorption measurements on ‘pure’, highly pure and A1₂O₃ doped Cr₂O₃ powder have been made. The EPR absorption in the ‘pure’ powders obtained below Ntel temperature is shown to be due to background magnetic impurities present in the powders and not due to superparamagnetism as suggested by earlier authors. No EPR absorption could be observed below Nkl temperature in highly pure powders (total background impurity concentration less than 5 ppm). ‘Pure’ powders or highly pure powders mixed with A1₂O₃ powder and annealed at high temperatures showed a symmetrical EPR absorption line at room temperature. The shape and the g value of this line are practically the same as those obtained for Cr³⁺ ions in Cr₂O₃ above Nee1 temperature or in other nonmagnetic crystals. It is concluded from these results that the impurities diffuse into Cr₂O₃ powder, the antiferro-magnetic coupling between some of the Cr³⁺ ions is broken and these Cr³⁺ ions become paramagnetic, even when the bulk of the material is in antiferromagnetic state. The variation of half-width of EPR lines with impurity concentration shows that the dipolar coupling between Cr³⁺ ions decreases with the increase in impurity concentration and when the impurity concentration is high the Néel temperature seems to shift to lower temperatures. A longer heat treatment of the ‘pure’ B powder resulted in the production of shining metal particles in the powder. The EPR of this powder showed excessive increase in the intensity of EPR absorption when the temperature of the powder was raised to a value just above the Néel temperature. A comparison of these reuslts with the work of earlier authors suggests that the shinning metal particles are those of chromium metal and are responsible for this increase in EPR absorption.     

Dielectric properties of a mixture of dipolar hard spheres

A theory for the dielectric constant, ε, of a fluid mixture of dipolar hard spheres is formulated by generalizing the methods developed by Ramshaw and Wertheim for the pure fluid case. The resulting expression for ε depends on the pair distribution functions, g _αβ(r ₁, θ₁, r ₂, θ₂) for a dipolar mixture. Due to the unavailability of exact representations for these dipolar pair distribution functions, the results of the mean spherical approximation are employed in the formalism developed. Numerical results are given for ε as calculated from the pair distribution functions for a spherical volume of macroscopic dimensions. The compositional dependence of the ε obtained in this way for a specific mixture is compared with the corresponding properties of the well established theories of Clausius-Mossotti-Debye and Onsager. In addition, the relative importance of the dipole moment and size of the hard sphere parameters in determining ε for a dipolar mixture (the correlative behaviour of which is described by the mean spherical approximation) is evaluated. It is found that the differences in hard core diameters can be largely ignored, in that ε for an ‘effective’ single component fluid can be given to within 2–5 per cent relative error (at worst) of the mean spherical approximation's result. Such an ‘effective pure fluid’ is described as having the same polarization content as the actual mixture being considered. Thereby, the properties of the effective fluid are determined by the quantity y = 4πβ(m ₁ ² ρ₁ + m ₂ ² ρ₂)/9 where m_i and ρ _i are the dipole moment and number density of component i in the binary mixture, with β = (kT)^-1.     

The structure of 16O; A review of the theory

The nucleus ¹⁶ ₈O₈ is the prototype for a large number of developments in nuclear structure theory. It is a doubly magic N=Z nucleus, light enough that an isotopic spin formalism should be a valid approximation. The Brueckner-Hartree-Fock procedure in a spherical basis should be capable of describing the gross properties of the ground state. The excited states of negative parity exhibit the characteristic low-lying ‘octupole vibrational state’ and there is a much studied ‘giant dipole region’ which should be amenable to the analysis of the ‘random phase approximation’. The first excited state is the ‘mysterious second zero’ par excellence and a great deal of work on describing it via the method of ‘deformed state admixtures’ has been carried out. The first excited state and a number of other excited states appear to support spectra reminiscent of rotational bands and the collective character of these states has been extensively studied in both the Bloch-Horowitz and α-cluster model schemes.     

Physical cluster mechanics: Statistical thermodynamics and nucleation theory for monatomic systems

We extend previous computations of mechanical stability of atomic microclusters to the realm of statistical thermodynamics, obtaining thermodynamic functions for small, solid-like Van der Waals clusters of less than some 100 atoms possessing non-crystalline structures of ‘polytetrahedral’ type. These are shown to be almost invariably at a thermodynamic advantage over alternative lattice structures of the same number of atoms, at least for the Lennard-Jones potential in the harmonic-oscillator/rigid-rotor approximation. The dependence of thermodynamic functions upon cluster size appears to be essentially monotonic in the number of internal degrees of freedom; although there are certain exceptional structures, particularly with icosahedral symmetry, there proves to be little evidence for the occurrence of ‘magic numbers’ for stability at any temperature and within the size-range considered. Particular attention is given to the heat capacity of model systems in relation to their vibrational spectra. The Debye T ³ law appears reasonably well obeyed at low temperatures with no evidence for the existence of either ‘soft modes’ or distinct surface contributions.

The results for the free energy of formation of minimal clusters ΔG _f are then applied to the computation of nucleation rates in terms of the Becker-Döring-Volmer-Zeldovitch quasi-equilibrium theory. Gibbsian behaviour in the form of a maximum in the curve of ΔG _f versus size is observed with a critical nuclear size at realistic temperatures and pressures of the order of that predicted by macroscopic liquid-drop theories. These figures and those derived for nucleation rate and critical supersaturation appear remarkably insensitive to the details of the model used, in particular to the distinction between ‘microcrystalline’ and ‘amorphous’ atomistic models.

The general status of atomistic nucleation theory is critically examined in the light of these and similar results.     

A new theoretical development of the limiting electric conductivity of ions in solution

The physical interpretation of the limiting equivalent conductivity of binary electrolytes in solution, Λ⁰ (=Σ_i?=?+,– λ⁰_i), has been a challenge for almost a century. Here, as a follow-up on a preliminary communication [D. Fraenkel, Mol. Phys. 115, 2944 (2017)], a quantitative theoretical treatment is offered and fully developed for the limiting ionic equivalent conductivity, λ⁰_i, based on assuming that around any singled-out ion i in solution, there is an electrostatic atmosphere that includes oriented solvent dipoles and the counterion as ‘charge density’. This leads to a simple mathematical expression for λ⁰_i as the result of the tandem operation of the dipole viscosity and dipole relaxation forces on the drifting ion. Contrary to literature teaching, λ⁰_i is found proportional to the effective ionic radius, r_i. An ‘electric ion-radius scale’ derived from experimental λ⁰_i’s for aqueous solutions, compares reasonably well with known ion-size scales. A newly defined solvent-specific dipole random-walk correction factor, ξ, is used for nonaqueous solvents as an adjustable parameter, to attain a parallel electric ion-size scale. The new theory provides overall a comprehensive view of the limiting ion mobility in solution, based on the ionic properties, solvent variables and universal constants.     

Sauna,sweat and science II – do we sweat what we drink?

ABSTRACT

Inspired by a previous ‘Sauna, sweat and science’ study [Zech et al. Isot Environ Health Stud. 2015;51(3):439–447] and out of curiosity and enthusiasm for stable isotope and sauna research we aimed at answering the question ‘do we sweat (isotopically) what we drink’? We, therefore, pulse-labelled five test persons in a sauna experiment with beverages that were ²H-enriched at about +25,600?‰. Sweat samples were collected during six sauna rounds and the hydrogen isotope composition δ²H_sweat was determined using an isotope ratio mass spectrometer. Before pulse labelling, δ²H_sweat – reflecting by approximation body water – ranged from –32 to –22?‰. This is ～35?‰ enriched compared to usual mid-European drinking water and can be explained with hydrogen-bearing food as well as with the respiratory loss of ²H-depleted vapour. The absence of a clearly detectable ²H pulse in sweat after pulse labelling and δ²H_sweat results of ≤+250?‰ due to a fast ²H equilibration with body water are moreover a clearly negative answer to our research question also in a short-term consideration. Given that the recovery of the tracer based on an isotope mass balance calculation is clearly below 100?%, we finally answer the question ‘where did the rest of the tracer go?’