Activation cross-sections for some (n, 2n), (n,p) and (n, γ)-reactions

The average fission neutron cross-sections of the reactions²³³U(n, 2n)²³²U,⁶⁰Ni(n, p)⁶⁰Co and²⁷Al(n, p)²⁷Mg and the resonance integrals of the (n, γ)-reactions of the nuclides¹⁸¹Ta,¹⁷⁶Lu,¹⁷⁵Lu,⁶⁴Ni,⁵⁹Co and²⁶Mg have been determined by the activation method following the well-known conventions. The results verify some of the existing values and present data for hitherto unknown or poorly known reactions.     

The structure and energetics of (GaAs)n, (GaAs)n(-), and (GaAs)n+ (n=2-15)

Electronic and geometrical structures of neutral, negatively, and positively charged (GaAs)n clusters are computed using density functional theory with generalized gradient approximation. All-electron computations are performed on (GaAs)2-(GaAs)9 while effective core potentials (ECPs) are used for (GaAs)9-(GaAs)15. Calibration calculations on GaAs and (GaAs)9 species support the use of the ECP for the larger clusters. The ground-state geometries of (GaAs)n(-) and/or (GaAs)n+ are different from the corresponding neutral ground-state geometry, except for n=7, 9, 12, 14, and 15, where the neutral and ions have similar structures. Beginning with n=6, all atoms are three coordinate, except for (GaAs)10+ and (GaAs)13+. For the larger species, there is a competition between fullerenes built from hexagons and rhombi and geometrical configurations where Ga-Ga and As-As bonds are formed, which results in the formation of pentagons. As expected, the static polarizability varies in the order of anion>neutral>cation, but the values are rather similar for all three charge states. The thermodynamic stability for the loss of GaAs is reported.     

Chelating properties of n,n,n,n-tetrakis-(phosphonomethyl)-1,2-cyclohexanediamine

The octasodium salt of N,N,N',N'-tetrakis-(phosphonomethyl)-1,2-cyclohoxanediamine, Na₈-CDTMP, can be prepared in 45% yield from chloromethylphosphonic acid and 1,2-cyclohexanediamine. The titration curve resulting from the titration of the acid, CDTMP, with sodium hydroxide at 25° and μ = 0.1 was used to estimate the eight pK values for the free acid. The stability constants of metal chelatles of CDTMP were measured at 25° and μ = 0.1 potentiometrically.     

(2n,n) potential and sticky-sphere fluids

The authors investigate the behavior of a model fluid for which the interaction energy between molecules at a separation r is of the form 4epsilon[(sigma/r)2n-(sigma/r)n], where epsilon and sigma are constants and n is a large integer. The particular properties they study are the pressure p, the mean square force F2, the elastic shear modulus at infinite frequency Ginfinity, the bulk modulus at infinite frequency Kinfinity, and the potential energy per molecule u. They show that if n is sufficiently large it is possible to derive the properties of the system in terms of two parameters, the values of the cavity function and of its derivative at the position r=sigma. As an example they examine in detail the cases with n=144 and n=72 for three different temperatures and they test the theory by comparison with a computer simulation of the system. They use the simulated pressure and the average mean square force to determine the two parameters and use these values to evaluate other properties; it is found that the theory produces results which agree with computer simulation to within approximately 3%. It is also shown that the model, when the parameter n is large, is equivalent to Baxter's sticky-sphere model with the strength of the adhesion determined by the value of n and the temperature. They use Baxter's solution of the Percus-Yevick equations for the sticky-sphere model to determine the cavity function and from that the values of the same properties. In this second approach there are no free parameters to determine from simulation; all properties are completely determined by the theory. The results obtained agree with computer simulation only to within approximately 6%. This suggests that for this model one needs a better approximation to the cavity function than that provided by the Percus-Yevick solution. Nevertheless, the model looks promising for the study of (typically small) colloidal liquids where the range of attraction is short but finite when compared to its diameter, in contrast to Baxter's sticky-sphere limit where the attractive interaction range is taken to be infinitely narrow. The continuous function approach developed here enables important physical properties such as the infinite shear modulus to be computed, which are finite in experimental systems but are undefined in the sticky-sphere model.     

含BF配体的锇羰基化合物Os(BF)(CO)_n(n=4,3)和Os_2(BF)_2(CO)_n(n=7,6,5,4)稳定性的理论研究

采用MPW1PW91和BP86 2种密度泛函方法对中性单核锇羰基化合物Os(BF)(CO)n(n=4,3)及双核锇羰基化合物Os2(BF)2(CO)n(n=7,6,5,4)进行理论计算研究,优化得到22个低能异构体.研究发现,单核配位饱和Os(BF)(CO)4的能量最低的异构体对称性为C2v,其BF基团在三角双锥赤道面上.该异构体失去1个赤道面上的CO可得到Os(BF)(CO)3的能量最低异构体.单核Os(BF)(CO)n(n=4,3)的能量最低异构体的BF基团都位于配体三角双锥及缺顶点结构的赤道面上.配位饱和双核Os2(BF)2(CO)7有4个能量接近的异构体,其中能量最低的异构体结构中含有2个呈蝶形的桥配位BF基团.配位不饱和的Os2(BF)2(CO)6的2个能量接近的异构体结构相似,2个桥配位BF基团与2个Os原子构成平行四边形结构单元.配位不饱和的Os2(BF)2(CO)5和Os2(BF)2(CO)4的能量最低异构体都含有由2个桥配位BF基团与2个Os原子构成的平行四边形结构单元.双核Os2(BF)2(CO)n(n=7,6,5,4)能量最低异构体的BF基团都以桥配位形式和Os原子相连.离解能研究表明,单核配位饱和的Os(BF)(CO)4具有一定的热力学稳定性.双核的Os2(BF)2(CO)n(n=7,6)失去1个CO或者分裂为单核的Os(BF)(CO)4或Os(BF)(CO)3所需能量较高,表明其具有一定的热力学稳定性.     

Structures and energetics of Be(n)Si(n) and Be(2n)Si(n) (n = 1-4) clusters

The structures and energies of Be(n)Si(n) and Be(2n)Si(n) (n = 1-4) clusters have been examined in ab initio theoretical electronic structure calculations. Cluster geometries have been established in B3LYP/6-31G(2df) calculations and accurate relative energies determined by the G3XMP2 method. The two atoms readily bond to each other and to other atoms of their own kind. The result is a great variety of low-energy clusters in a variety of structural types.     

Ab initio calculations of anharmonic vibrational spectroscopy for hydrogen fluoride (HF)n (n = 3, 4) and mixed hydrogen fluoride/water (HF)n(H2O)n (n = 1, 2, 4) clusters

Anharmonic vibrational frequencies and intensities are computed for hydrogen fluoride clusters (HF)n, with n = 3, 4 and mixed clusters of hydrogen fluoride with water (HF)n(H2O)n where n = 1, 2. For the (HF)4(H2O)4 complex, the vibrational spectra are calculated at the harmonic level, and anharmonic effects are estimated. Potential energy surfaces for these systems are obtained at the MP2/TZP level of electronic structure theory. Vibrational states are calculated from the potential surface points using the correlation-corrected vibrational self-consistent field method. The method accounts for the anharmonicities and couplings between all vibrational modes and provides fairly accurate anharmonic vibrational spectra that can be directly compared with experimental results without a need for empirical scaling. For (HF)n, good agreement is found with experimental data. This agreement shows that the M?ller-Plesset (MP2) potential surfaces for these systems are reasonably reliable. The accuracy is best for the stiff intramolecular modes, which indicates the validity of MP2 in describing coupling between intramolecular and intermolecular degrees of freedom. For (HF)n(H2O)n experimental results are unavailable. The computed intramolecular frequencies show a strong dependence on cluster size. Intensity features are predicted for future experiments.     

Structure and stability of (TiO2)n, (SiO2)n, and mixed Ti(m)Si(n-m)O(2n) [n = 2-5, m = 1 to (n - 1)] clusters

Structures, energetics, and vibrational spectra are investigated for small pure (TiO(2))(n), (SiO(2))(n), and mixed Ti(m)Si(n-m)O(2n) [n = 2-5, m = 1 to (n - 1)] oxide clusters by density functional theory (DFT). The BP86/ATZP level of theory is employed to obtain constitutional isomers of the oxide clusters. In accordance with previous studies, our calculations show three-dimensional compact structures are preferred for pure (TiO(2))(n) with oxo-stabilized higher hexavalent states, and linear chain structures are favored for pure (SiO(2))(n) with tetravalent states. However, the herein theoretically first reported mixed Ti(m)Si(n-m)O(2n) oxide clusters prefer either three-dimensional compact or linear chain structures depending upon the stoichiometry of the compound. Vibrational analysis of the important modes of some highly stable structures is provided. Coupled-cluster single and double excitation (with triples) [CCSD(T)] computed energy gaps for the TiO(2) dimers compare well with results from previous study. Excitation energies are computed by use of time-dependent (TD) DFT and equation-of-motion coupled-cluster calculations with singles and doubles (EOM-CCSD) for the most stable isomers.     

Geometry, chemical bonding, and electronic spectra of Si(n) and Si(n)-glycine (n = 3-5) complexes

Structures and spectra are calculated for Si(n) and Si(n)-Gly (n = 3-5) complexes. Relative stability differences of Gly conformers are magnified by interactions with the Si(n) cluster, so that one conformer of Si(n)-Gly is stabilized. Significant charge transfer occurs from the amino group in Gly to a Si atom in the cluster. Interactions with Gly are predicted to shift the excitation energies of Si(n) significantly to the blue to 2.1-2.7 eV, although they are still lower than in a Si cluster passivated by hydrogen.     

Structure and properties of Mn(n), Mn(n)-, and Mn(n)+ clusters (n = 3-10)

Electronic and geometrical structures of Mn(3)-Mn(10) together with their singly negatively and positively charged ions are computed using density functional theory with generalized gradient approximation. The ground-state spin multiplicities in the neutral series are 16, 21, 4, 9, 6, 5, 2, and 5, for Mn(3)-Mn(10), respectively. Thus, there is a transition from a ferromagnetic ground state to a ferrimagnetic ground state at Mn(5). The energy difference between ferrimagnetic and ferromagnetic states in Mn(n) grows rapidly with increasing n and exceeds 2 eV in Mn(10). The corresponding change from ferro- to ferrimagnetic ground state occurs at Mn(6)(-) and Mn(3)(+) in the anionic and cationic series, respectively. Beginning with Mn(6), the ion spin multiplicities differ from that of the neutral by +/-1 (i.e., they obey the empirical "+/-1 rule"). We found that the energy required to remove an Mn atom is nearly independent of the charge state of an Mn(n) cluster and the number of atoms in the cluster, except for Mn(3). The results of our calculations are in reasonable agreement with experiment, except for the experimental data on the magnetic moments per atom, where, in general, we predict smaller values than the experiment.     

Experimental and computational study of the Zn(n)S(n) and Zn(n)S(n)+ clusters

Zinc sulfide clusters produced by direct laser ablation and analyzed in a time-of-flight mass-spectrometer, showed evidence that clusters composed of 3, 6, and 13 monomer units were ultrastable. The geometry and energies of neutral and positively charged Zn(n)S(n) clusters, up to n = 16, were obtained computationally at the B3LYP/6-311+G level of theory with the assistance of an algorithm to generate all possible structures having predefined constraints. Small neutral and positive clusters were found to have planar geometries, neutral three-dimensional clusters have the geometry of closed-cage polyhedra, and cationic three-dimensional clusters have structures with a pair of two-coordinated atoms. Physical properties of the clusters as a function of size are reported. The relative stability of the positive stoichiometric clusters provides a thermodynamic rationale for the experimental results.     

Systematic study of (Na n )2 dimer transient state in Na n +Na n collisions (n=8, 9, 19, and 20)

From distance dependent tight-binding molecular dynamics simulations, we systematically study the Na _n +Na _n collision dynamics around the first two closed shells (n=8 and 20). We investigate the stability of sodium cluster dimers (Na _n )₂, for many events with random relative orientation at finite temperature, various impact parameters and incident energies. We find that (Na₈)₂, (Na₉)₂, (Na₁₉)₂ and (Na₂₀)₂ can exist during about 3000 fs in central collisions while they can exist up to about ten thousands fs in peripheral collisions with larger impact parameters in fusion mechanism at c.o.m energy per atomE _cm/n=0.025 eV. We observe that the lower the incident energy, the longer the lifetime of the cluster dimers in both central and peripheral collisions. There is no apparent difference in the dynamical stability of (Na₈)₂ and (Na₉)₂, (Na₁₉)₂ and (Na₂₀)₂ although (Na₈)₂ and (Na₂₀)₂ are respectively slightly colder than (Na₉)₂ and (Na₁₉)₂ for the same incident energy per atom and the same impact parameter.     

(n, 2n) reactions in iodate and periodate systems

The chemical consequences of (n,2n) reactions on crystalline sodium iodates and sodium periodates, containing¹²⁷I or¹²⁷I+¹²⁹I, were investigated measuring the initial yields and the post irradiation thermal annealing yields at 90°C for three separated fractions: I+I^o, IO ₃ ^– and IO ₄ ^– . The results show different effects for each system and neither isotope effect nor qualitative differences on thermal annealing were observed. The influence of the nuclear reaction type, of the hot-atom's nature and of the structural and chemical environment are discussed.     

Infinite Three-Dimensional Coordination Polymers: Synthesis and Structures of [Cd (4,4'-bpy)2 (H2O)2]n (pic)2n, [Zn (4,4'-bpy)2 (H2O)2 ]n-(pic)2n (H2O)2n, and [Zn (4,4'-bpy)2 (H2O)2]n (4,4'-bpy)n (H2O)n (pic)2n

Recently, a new research realm in crystal engineering of supramolecular architecturesassembled by means of coordinate covalent bonding', hydrogen bonding', or other weakintermolecular interactions= has been rapidly expanding in order to rationally developnew classes of functional materials with cavities or pores. These types of compoundsmay exhibit interesting topological structures and the clathrations of the cavity structuresmay have many potential properties such as catalysis', electrical co…     

Theoretical study on homoleptic mononuclear and binuclear ruthenium carbonyls Ru(CO) n (n= 3–5) and Ru2(CO) n (n = 8, 9)

Science China Chemistry - Homoleptic mononuclear and binuclear ruthenium carbonyls Ru(CO) n (n = 3–5) and Ru2(CO) n (n = 8,9) have been investigated using density functional theory. Sixteen...     

Structure and vibrations of Ga(n)N(n) (n = 3-10) clusters

The structure and harmonic vibrations of Ga(n)N(n) (n = 3-10) clusters have been investigated using the B3LYP (Becke 3-parameter-Lee-Yang-Parr) density functional theory. All structures are found to be cumulenic D(nh) rings (equal bonds, alternating angles), with one intense out of plane mode and three infrared-active degenerate modes, of which the highest one is extremely intense and asymptotically increases to 1029 cm(-1) for n = 10. Comparisons with C2n, B(n)N(n), and Al(n)N(n) clusters, the structure and bonding type for the Ga(n)N(n) (n=3-10) clusters are consistent with those of the C2n (n = 3, 5, 7, ...) clusters, the B(n)N(n) (n = 3-10), and Al(n)N(n) (n = 3-9) clusters.     

Crystal structures, lattice potential energies, and thermochemical properties of crystalline compounds (1-C(n)H(2n+1)NH3)2ZnCl4(s) (n = 8, 10, 12, and 13)

As part of our ongoing project involving the study of (1-C(n)H(2n+1)NH(3))(2)MCl(4)(s) (where M is a divalent metal ion and n = 8-18), we have synthesized the compounds (1-C(n)H(2n+1)NH(3))(2)ZnCl(4)(s) (n = 8, 10, 12, and 13), and the details of the structures are reported herein. All of the compounds were crystallized in the monoclinic form with the space group P2(1)/n for (1-C(8)H(17)NH(3))(2)ZnCl(4)(s), P21/c for (1-C(10)H(21)NH(3))(2)ZnCl(4)(s), P2(1)/c for (1-C(12)H(25)NH(3))(2)ZnCl(4)(s), and P2(1)/m for (1-C(13)H(27)NH(3))(2)ZnCl(4)(s). The lattice potential energies and ionic volumes of the cations and the common anion of the title compounds were obtained from crystallographic data. Molar enthalpies of dissolution of the four compounds at various molalities were measured at 298.15 K in the double-distilled water. According to Pitzer's theory, molar enthalpies of dissolution of the title compounds at infinite dilution were obtained. Finally, using the values of molar enthalpies of dissolution at infinite dilution (Δ(s)H(m)(∞)) and other auxiliary thermodynamic data, the enthalpy change of the dissociation of [ZnCl(4)](2-)(g) for the reaction [ZnCl(4)](2-)(g)→ Zn(2+)(g) + 4Cl(-)(g) was obtained, and then the hydration enthalpies of cations were calculated by designing a thermochemical cycle.     

Darstellung und Charakterisierung der Trisilylphosphane (Ph 3Si) n P(SiMe 3)3−n,n=1,2,3

A simple preparation of the title compounds is achieved by reacting Na₃P/K₃P with stoichiometric mixtures of chlorotriphenylsilane and chlorotrimethylsilane.³¹P- and²⁹Si-NMR-Data as well as Ir and Raman spectra of the compounds are reported.

     

Global optimization of ionic Mg(n)F(2n) (n=1-30) clusters

The global optimization basin-hopping (BH) method has been used to locate the global minima (GM) of Mg(n)F(2n) (n=1-30) clusters using a Born-Mayer-type potential. Some of the GM were particularly difficult to find, requiring more than 1.5 x 10(4) BH steps. We have found that both the binding energy per MgF2 unit and the effective volume of the GM isomers increase almost linearly with n, and that cluster symmetry decreases with cluster size. The data derived from the BH runs reveal a growing density of local minima just above the GM as n increases. Despite this, the attraction basin around each GM is relatively large, since after all their atomic coordinates are randomly displaced by values as high as 2.0 bohrs, the perturbed structures, upon reoptimization, relax back to the GM in more than 50% of the cases (except for n=10 and 11). The relative stabilities derived from energy second differences suggest that n=8,10,13,15, and 20 are probably the magic numbers for these systems. Mass spectrum experiments would be very useful to clarify this issue.