Determination of the attenuation of epicadmium neutrons using the method of varying Cd-thickness

A new approach is presented for experimental determination of the cadmium correction factor F_Cd using the method of varyind Cd-thickness. In this method, the shift of the effective Cd cut-off value by varying the Cd-thickness was taken into consideration F_Cd for the following isotopes was determined with 1 mm cadmium covers^110mAg,^114mIn,¹²²Sb,¹⁴⁰La,¹⁵³Sm,^116mIn,¹⁶⁰Tb,¹⁶⁶Ho,^176mLu,¹⁸⁸Re,¹⁹²Ir,¹⁹⁴Ir¹⁹⁸Au,²³³Pa and²³⁹Np.     

Effect of desolvation of the reactants during change in the acidity of the medium on the electrophilic hydroxylation and nitration of alkanes

The effect of the acidity of the medium on the hydroxylation and nitration of alkanes (RH) in 90–98% H₂SO₄ at 25°C is described quantitatively by a model taking account of the thermodynamic activity of the RH, H₂O, and HSO₄^- particles. It was concluded that in the transition states the reagents H₃O₂⁺HSO₄^- and NO₂⁺ HSO₄^- are present as HO⁺ and NO₂⁺ ions without the bases H–O and HSO₄^-, the alkanes are present without hydrophobic shells, and the initial reaction products are ROH₂⁺ and RNO₂H⁺.     

Calculation of the potential energy curves for the low-lying doublet and quartet states of the CN radical

The potential energy curves of the low-lying X²Σ⁺, A²π_i, B²Σ⁺,⁴Σ⁺, and ⁴π states of CN are calculated by the MC SCF (CAS SCF) method. Their vibrational levels and the molecular constants obtained are in good agreement with those determined in our recent experimental analysis of the CN (B²Σ⁺-X²Σ⁺) emission spectrum. several intensity anomalies in the observed spectrum are ascribed to perturbations between the B²Σ⁺ and ⁴π states with the following vibrational quantum numbers: (υ_B, υ_π)=(9,x), (11, x+2), (12, x+3), (14, x+6), (17, x+11), and (18, x+13), where x = 0 is the most probable assignment. Likewise, the perturbations between the B²Σ⁺ and ⁴Σ⁺ states with (υ_B, υ_Σ) = (11, y), (13, y+3) are interpreted as y = 8±1.     

Poor hydration enhances the activation energy of the exchange rate of the base-paired imino protons with water at the core part of the DNA duplex

Here we report the exchange rates (k_{e x}) of imino protons of d[^5′p(T¹G²T³T⁴T⁵G⁶G⁷C⁸)^3′]:d[^{3∼'(A15C14A13A12A11C10C9)p5′}] (duplex I) with water at different pH and temperature to give the life-times (τ_o) of the closed state of the base-pairs. The τ_o of the closed state of the base-pairs is uniform (E_a ≈ 25 ± 5 kcal/mol) in the duplex I, and varies between 0.2 – 4 ms. A plot of the natural log of various exchange rates of the imino protons of the base-pair of the duplex I within the pH range of 6.1 to 8.6 as a function of the inverse of temperature gave the activation energy (E_a) of the exchange process of imino protons with the bound water (hydration). It has been found that although τ_o are in the same range but the E_a of the exchange processes of the open state of imino protons with the bound water are very different, and they are strongly dependent upon the location of the nucleotide residues along the DNA duplex: 22.3±3.3 kcal/mol for the core base-pair T⁴-A¹², 16.2±2.4 kcal/mol for the base-pair T⁵-A¹¹, 10.5±1.6 kcal/mol for the base-pair T³-A¹³. 12.3±1.8 kcal/mol for the base-pair G⁶-C¹⁰ and 2.4±0.4 kcal/mol for the base-pair G²-C¹⁴. The comparison of the activation energies of the exchange process of imino protons and water with that of the water abundance in the first spine of hydration between fully-matched duplex I and the analogous G⁷-A⁹ mismatched duplex II, (d[^5′p(T¹G²T³T⁴T⁵G⁶G⁷C⁸)^3′]: d[^3′(A¹⁵C¹⁴A¹³A¹²A¹¹C¹⁰A⁹)p^5′], determined by a combination of NOESY and ROESY experiments, suggests for the first time that the relative exchange of imino protons of the base-pairs in the DNA duplex is more rapid when there is an abundance of water at the first spine of hydration. This result also showed unambiguously that the core of the DNA is by and large devoid of water and the energy penalty of water entering the core is very high. This is consistent with our earlier work which showed that as the water activity in the minor and major groove of DNA increases, the T_m decreases (ref. 1), suggesting the water poisoning as the principal factor for base-pair mismatch, frame-shift and mutation in our DNA replication machinery.     

Theoretical characterization of the ground state of the alkaline-earth monocarbides: Ordering of the two lower-lying states of the BeC,MgC, and CaC molecules

The two lower-lying electronic states (³Σ⁻ and ⁵Σ⁻) of the BeC, MgC, and CaC molecules were investigated using restricted Hartree-Fock (RHF), generalized valence bond (GVB), and configuration interaction (CI) calculations to establish the relative ordering of those states as a function of the size of the alkaline-earth element. It is shown that as a result of the competition between bonding effects, which predominate for the ³Σ⁻ states, and exchange effects, which stabilize the ⁵Σ⁻ states, the ordering of these states can be reversed as we move from the Be to the Ca atom. For both the BeC and MgC molecules, the ground state was found to be a triplet X³Σ⁻ state, but for the CaC molecule, the high-spin X⁵Σ⁻ becomes more stable. © 1996 John Wiley & Sons, Inc.     

Conditions for the Chemical Precipitation of the Starting Substances from Solutions for the Synthesis of Aluminospinels

The precipitation of hydroxides from the Mg^{2 +}-Al^{3 +}-An^--H₂O system (An = NO₃ ^-, Cl^-, SO₄ ^{2 -}) at the initial spinel molar ratio Mg^{2 +} : Al^{3 +} = 1 : 2 was studied. The optimal conditions were found for obtaining the precipitates exhibiting the best properties in the manufacturing sense; their subsequent heat treatment yields a single phase of aluminomagnesium spinel.     

Determination of the acid dissociation constants of p-benzohydroquinone by the INDO method

The stepwise acid dissociation constants for p-benzohydroquinone (QH₂) in aqueous media have been explicitly calculated for the first time, with the INDO parametrized SCF –MO method. We have optimized the geometries of QH₂, QH^?, and Q^2? and of the QH₂ · 6H₂O, QH^? · (H₃O⁺) · 5H₂O, and Q^2? · (H₃O⁺)₂ · 4H₂O systems that model the solvated species. The presence of the associated water molecules (and hydronium ions) account for the stabilization due to hydrogen bonding as well as for a part of the effect of interaction of these molecules with the respective reaction fields in an aqueous medium. To simulate the first solvation shell in a more complete manner, four more water molecules have been considered to be placed above and below the quinonoid ring and the optimized geometries of the resulting hydrated species, QH₂ · 10H₂O, QH^? · (H₃O⁺) · 9H₂O, and QH^? · (H₃O⁺) · 8H₂O, have been determined. The standard free-energy changes calculated for the dissociation of QH₂ into QH^? and H⁺ is 0.0251 Hartree (65.9 kJ mol^?1) and that of QH^? into Q^2? and H⁺ is 0.0285 Hartree (74.8 kJ mol^?1). Experimentally observed dissociation constants for these two steps correspond to free-energy changes of 0.0214 Hartree (56.2 kJ mol^?1) and 0.0248 Hartree (65.1 kJ mol^?1), respectively. © 1995 John Wiley & Sons, Inc.     

Effect of the Nature of the Cation on the Physicochemical Properties of d-Metal Complexes with α,α-Dipyrrolylmethene

Complexes of Cu^{2 +}, Ni^{2 +}, Co^{2 +}, Zn^{2 +}, and Hg^{2 +} with 4,4'-dibutyl-3,3',5,5'-tetramethyldipyrrolylmethene were synthesized. The stoichiometric compositions of the complexes in solution and in crystal were determined, and their electronic absorption spectra and thermooxidative degradation were studied.     

Exploration of the potential application of plutonium isotopes in source identification of sandstorm in the atmosphere of Beijing

Plutonium (Pu) is an anthropogenic radionuclide which mainly derived from atmospheric nuclear tests in the environment. In this study, the Pu isotopes (²³⁹Pu and ²⁴⁰Pu) in aerosol samples collected during the sandstorm and non-sandstorm period were measured by accelerator mass spectrometry (AMS) and the behavior of Pu was studied. The activity concentrations of ²³⁹Pu and ²⁴⁰Pu in the aerosol samples of Beijing were ranged from 0.62 nBq/m³ to 99.6 nBq/m³ for ²³⁹Pu and 3.51 nBq/m³ to 60.23 nBq/m³ for ²⁴⁰Pu, respectively. ²³⁹Pu and ²⁴⁰Pu concentrations exhibited a remarkable seasonal variation trend, with the higher results showed in spring, and the relatively lower concentrations in winter. The observed higher concentration of ²³⁹Pu and ²⁴⁰Pu detected in sandstorm samples further indicated Pu was closely related to the occurrence of sandstorms. The global fallout characteristics of ²⁴⁰Pu/²³⁹Pu atom ratios (average 0.20, ranging from 0.16 to 0.27) in aerosol samples indicating that global fallout was the major source of Pu in the atmosphere. Using aluminum (Al) as an indicator of soil resuspension, significant positive correlation between ²³⁹Pu and Al (r² = 0.934), ²⁴⁰Pu and Al (r² = 0.525) revealed that soil resuspension was a primary source of atmospheric Pu in Beijing. These results implied that the combination of ²³⁹Pu, ²⁴⁰Pu and Al could be used as the potential tracer of sandstorm.     

Relation of the Gibbs free energy of transfer of ions from water to polar solvents to the properties of the solvents and the ions

A statistical treatment of data for the standard molar Gibbs free energies of transfer of monovalent ions from water to polar solvents has been made in terms of properties of the solvents and the ions. A common multiple regression equation with seven fitting constants, for almost 200 data points, has been found to describe the data in terms of four solvent properties: their electron donor and acceptor abilities, dielectric constant, and cohesive energy density, and three ionic properties: charge, size, and softness. For the ions Na⁺, K⁺, Rb⁺, Cs⁺, Tl⁺, (Ph)₄As⁺, Cl^?, Br^? and N ₃ ^? the predictions of the equation are within acceptable error limits of the data, and encourage its application to solvents beyond the thirteen used for the data base. For other ions, e.g. H⁺, Ag⁺, and the larger anions, further interactions must be taken into account.     

A Raman spectroscopic study of the effect of ion-pairing on the structure of the triiodide and tribromide ions

The changes in the Raman spectra of aqueous solutions of I₂ in HI (1:1 mole ratio) with changing concentration, have been shown to be a result of ion-pairing: H⁺ + I⁻₃ = H⁺I⁻₃.The anomalous high frequency stretching band of the solute at 172 cm⁻¹ for these solutions arises mainly from stretching of the stronger II bond in the H⁺I⁻₃ ion-pair and not from vibrational modes of I₂ or I⁻₅. Ion-pair dissociation constants estimated from the Fuoss equation, combined with the known I⁻₃ ion-pair and not from vibrational modes of I₂ or I⁻₅. Ion-pair dissociation constants estimated from the Fuoss equation, combined with the known I⁻₃ and I⁻₅ formation constants, account for the intensity changes of the stretching bands. The spectra of solutions of Br₂/HBr at 1:1 mole ratio may be interpreted in the same way but other Br species are present. The spectra of the isolated X⁻₃ ions in solution exhibit a shoulder to high frequency of the symmetric stretching band, ν₁. In the spectrum of the I⁻₃ ion, this peak is assigned to one of the two frequencies, resulting from Fermi resonance between ν₁ and 2ν₂ but, in the case of the Br⁻₃ ion, this peak may be due to ν₃, which becomes Raman active as a result of disymmetric solvation. The consequences of such ion-pairing for the nature of I₂ dissolved in polymers are discussed.     

The preparation of the 239Np tracer from 243Am and the purification of the stock solution

The present work describes preparation of ²³⁹Np tracer from ²⁴³Am stock solution and the purification of this solution from ferric cation. The method of the preparation of tracer involves stabilization of Np(IV) by ascorbic acid and ferric nitrate, separation of ²³⁹Np from ²⁴³Am by extraction chromatography and determination of recoveries of ²³⁹Np by means of gamma spectroscopy. We used the commercially available sorbents TEVA^®Resin for the ²³⁹Np preparation and DGA Resin for ²⁴³Am purification. All sorbents were purchased from Eichrom Industries, Inc. The first eluate from the column can be stored for a future preparation of the tracer and fraction with ²³⁹Np will be used to monitor radiochemical yield of ²³⁷Np.     

Accurate values for the nonrelativistic energies of the lowest singlet and triplet S-states of the4He-Isotop

Accurate lower and upper bounds for the nonrelativistic lowest energies¹ E ₀ and³ E ₀ of the singlet and triplet-system of the⁴He-Isotop are calculated with the linearized method of variance minimization. The same was done for¹ E ₁ the energy of the first excitedS-state 2¹ S. The results especially for¹ E ₀ and³ E ₀ in a.u. are −2.9033076997₅ ≤¹ E ₀ ≤ −2.9033076921₈ −2.1749324263₇ ≤³ E ₀ ≤ −2.1749324245₉ i.e. the values are determined with an absolute error smaller than 0.00167 cm⁻¹ for¹ E ₀ and 0.00039 cm⁻¹ for³ E ₀.     

Estimation of a priori errors in ab initio calculations of thermochemical values for the example of the dissociation energies of the ZnO and ZnS molecules

The total energies of ZnO(¹Σ), ZnO(³Π), ZnO^?(²Σ), ZnO⁺(²Σ), ZnS(¹Σ), ZnS(³Π), ZnS^?(²Σ), and ZnS⁺(²Σ) were calculated ab initio by the CCSD(T) method with the use of atomic basis sets including 80, 84, and 93 functions for O, S, and Zn, respectively. Similar calculations were performed for the Zn atom [Zn(¹ S), Zn(³ P), Zn⁺(² S), Zn²⁺(¹ S)] and several oxygen and sulfur states [O(³ P), O^?(² S), O(¹ D), O₂(³Σ), O ₂ ^? (²Π), O₂(¹Δ), S(³ P), S^?(² S), S(¹ D), S₂(³Σ), S ₂ ^? (²Π), and S₂(¹Δ)]. The ideology of engagement groups suggested by us is considered. According to this approach, data treatment can be performed on the assumption that the errors in all the 24 results obey the normal distribution law. As a result, we obtained D _e(ZnO) = 1.70 ± 0.21 and D _e(ZnS) = 1.57 ± 0.25 eV (at a 95% confidence level).     

The scavenging of the precursors of the solvated electron from the positron lifetime spectroscopy and the doppler broadening of annihilation line shape technique

The electron scavenging properties of aqueous solutions of two series of solutes are investigated, using the positron as a probe. For a better interpretation of the data, both lifetime spectroscopy and the Doppler broadening of annihilation line shape technique are used. All solutes inhibit the positronium (Ps) formation, by the scavenging of electrons. The first series consists of the halate ions, that should follow the Hunt linear relation between the rate constant for reaction with the solvated electrons, k_(e^?aq+S), and that for its precursor(s), 1/C₃₇. The Ps inhibition constants, k, are 0.14, 1.44 and 3.45 M^?1 for ClO^?₃, BrO^?₃ and IO^?₃, respectively. This sequence is quantitatively consistent with that of the respective k_(e^?eq+S). The second series includes the SeO⁼₄, Te(OH)₆ and BrO^?₄ species, and the Ps inhibition constants are 5.62, 10.5 and 14.3 M^?1 respectively. These values are much higher than expected from the k_(e^?aq+S) constants, on basis of the Hunt relation, in agreement with previous results from pulse radiolysis experimets.     

Measurement of the neutron capture cross-section of 238U using the neutron activation technique

The ²³⁸U(n, ??)²³⁹U reaction cross-section at average neutron energy of 3.7?±?0.3?MeV from the ⁷Li(p, n)⁷Be reaction has been determined using activation and off-line ??-ray spectrometric technique. The ²³⁸U(n, ??)²³⁹U and ²³⁸U(n, 2n)²³⁷U reaction cross-sections at average neutron energy of 9.85?±?0.38?MeV from the same ⁷Li(p, n)⁷Be reaction have been also determined using the above technique. The experimentally determined ²³⁸U(n, ??)²³⁹U and ²³⁸U(n, 2n)²³⁷U reaction cross-sections were compared with the evaluated data of ENDF/B-VII, JENDL-4.0, JEFF-3.1 and CENDL-3.1. The experimental values were found to be in general agreement with the evaluated value based on ENDF/B-VII, and JENDL-4.0 but not with the JEFF-3.1 and CENDL-3.1. The present data along with literature data in a wide range of neutron energies were interpreted in terms of competition between different reaction channels including fission. The ²³⁸U(n, ??)²³⁹U and ²³⁸U(n, 2n)²³⁷U reaction cross-sections were also calculated theoretically using the TALYS 1.2 computer code and were also found to be in agreement experimental data.     

Studies on the effect of the cation nature on the kinetics of the isotopic exchange reaction between chloride ion and O,O-diphenylphosphorochloridothionate in acetonitrile

Rate constants and activation parameters for the isotopic exchange reactions between (PhO)₂PSCl and M³⁶Cl (M = Me₄N⁺, Et₄N⁺, n-Bu₄N⁺, Et₃HN⁺, EtH₃N⁺, Li⁺) in acetonitrile were measured in order to find the effect of the cation nature onthe kinetics of the reaction. The rate constants measured for a range of concentrations of Et₃HN³⁶Cl, EtH₃N³⁶Cl, and Li³⁶Cl were analyzed using the Acree equation. The equivalent conductance of LiCl in acetonitrile was determined. The nature of the cation has no effect on the mechanism of the reaction. The cation changes only the experimental rate constant proportionally to the dissociation degree of the salt. Smaller values of the rate constant and smaller activation parameters ΔH? and ΔS? for the reaction with Li³⁶Cl indicate the existenceof the intermolecular interaction between lithium ions and O,O-diphenylphosphorochloridothionate.     

The orbital description of the potential energy curves and properties of the lower excited states of the BH molecule

The electronic wavefunctions for the ground (X¹ Σ⁺) and the low-lying excited states (a³Π, A¹Π, ³Σ⁺) of the BH molecule have been calculated as a function of internuclear distance using the ab initio generalized valence bond method (GVB) with optimization of spin coupling (SOGI). The potential curve of the A¹Π state in the zero rotational level is found to have a hump of 0.150 eV at R = 3.8₉a_o (experimentally a hump of unknown size is found at 3.9 ± 0.4 a₀); a smaller hump at larger R (0.02 eV at R = 4.9₂a₀) is also found for the calculated a³Π state. The presence of such humps is found to result from the recoupling of orbitals that must occur as R is decreased from ∞ to R_e and is comparable in origin to the activation barrier in a radical exchange reaction (e.g., H₂ + D ? HD + H). The calculated binding energies of the BH states are 3.272 eV (X¹ Σ⁺), 2.216 eV (a³ Π), and 0.502 eV (A¹ Π). The ³Σ⁺ state is unbound although it does exhibit a small unbound minimum. The dipole moment, quadrupole moment, and electric field gradient are calculated as a funtion of R. The shapes of the potential curves and the properties are interpreted in terms of simple qualitative considerations of the GVB orbitals.     

Molecular structure of 6-methyluracil in the gas phase and characteristics of the 6-methyluracil-water system

Geometric parameters of the 6-methyluracil molecule were determined by gas-phase electron diffraction: interatomic distances (r _a, Å) N¹-C² 1.390(3), C²-N³ 1.384(3), N³-C⁴ 1.407(3), C⁴-C⁵ 1.455(10), C⁵-C⁶ 1.336(20), C¹-C⁶ 1.395(3), C-Me 1.519(5); bond angles (deg) N¹C²N³ 114.1(), C²N³C⁴ 126.3(7), N³C⁴C⁵ 114.3(5), C⁴C⁵C⁶ 121.6(5), C⁵C⁶C¹ 119.7(5), C⁷C⁶C⁵ 11C⁵.4(8), O⁸C²N¹ 123.5(1.5), O⁹C⁴N³ 123.3(10). The heterocycle is planar. One of the C-H bonds of the methyl group and the C⁵=C⁶ bond are coplanar. The nearest surrounding of the heterocycle by water molecules (four and five molecules) was examined by AM1 and B3LYP/6-31G** calculations, and the energies of the hydrogen bonds in the heterocycle-water system were estimated.     

Study of the spectrum of cherenkov light

It is pessible to study experimentally Cherenkov light spectra in a liquid scintillation spectrometer with colour filters. Cherenkov light spectra of⁶⁰Co,¹⁹⁸Au,¹¹⁵Cd,¹⁴³Ce,¹⁴⁰La,³²P,⁸⁶Rb,⁷⁶As,⁴²K and an external standard were studied in a Packard 3375 type liquid scintillation spectrometer, using 11 various Kodak Wratten filters. The absorption maxima of the filters ware in the 410–796 nm interval.