Atomistic Assessment of Melting Point Depression and Enhanced Interfacial Diffusion of Cu in Confinement with AlN.

The continuing trend in heterogeneous integration (i.e., miniaturization and diversification of devices and components) requires a fundamental understanding of the phase stability and diffusivity of nanoconfined metals in functional nanoarchitectures, such as nanomultilayers (NMLs). Nanoconfinement effects, such as interfacial melting and anomalous fast interfacial diffusion, offer promising engineering tools to enhance the reaction kinetics at low temperatures for targeted applications in the fields of joining, solid-state batteries, and low-temperature sintering technologies. In the present study, the phase stability and atomic mobility of confined metals in Cu/AlN NMLs were investigated by molecular dynamics, with the interatomic potential compared to the ab initio calculations of the Cu/AlN interface adhesion energy. Simulations of the structural evolution of Cu/AlN nanomultilayers upon heating in dependence on the Cu nanolayer thickness demonstrate the occurrence of interfacial premelting, a melting point depression, as well as extraordinary fast solid-state diffusion of confined Cu atoms along the defective heterogeneous interfaces. The model predictions rationalize recent experimental observations of premelting and anomalous fast interface diffusion of nanoconfined metals in nanostructured Cu/AlN brazing fillers at strikingly low temperatures.

Exploring water adsorption and reactivity in a series of doped aluminum cluster anions.

We present a systematic density functional study of central- and surface-doped aluminum cluster anions Al12X- (X = Mg, B, Ga, Si, P, Sc-Zn), their interactions and reactivity with water. Adsorption of water molecules on central-doped clusters is governed by the cluster electron affinity. Doping introduces a dramatic change in the cluster electronic structure by virtue of different ordering and occupation of super-atomic shells, which leads to the creation of complementary active sites controlling the reactivity with water. Surface doping creates unequal charge distribution on the cluster surface, resulting in the adsorption and reactivity of surface-doped clusters being dominated by electrostatic effects. These results demonstrate the strong influence of the doping position on the nature of the interaction and reactivity of the cluster, and contribute to a better understanding of doping effects.

NMR absolute shielding scales and nuclear magnetic dipole moments of transition metal nuclei.

This work reports new, accurate nuclear magnetic dipole moments for NMR-active transition metal nuclei where the long-standing systematic error due to obsolete diamagnetic correction has been eliminated by ab initio calculations of absolute NMR shielding constants. The error of the diamagnetic correction reaches ≈ -14 000 ppm for rhenium, which results in magnetic dipole moment corrections of about -3 × 10-2µN for the 185Re and 187Re nuclei. Such extreme values are one to two orders of magnitude larger than the corrections reported in the literature so far. These findings may help to resolve discrepancies in hyperfine splitting experiments involving rhenium ions. To obtain the corrected transition metal nuclear magnetic dipole moments, NMR shielding constants for a series of transition metal complexes defined as NMR standards [Harris et al., Pure Appl. Chem., 2001, 73, 1795] were calculated using the non-relativistic coupled cluster method and four-component relativistic density functional theory. To reproduce the experimental conditions of the NMR standards, the solvent effects were incorporated by explicit and implicit solvent models.

DFT Modeling of Cross-Linked Polyethylene: Role of Gold Atoms and Dispersion Interactions.

Using DFT modeling, we analyze the concerted action of gold atoms and dispersion interactions in cross-linked polyethylene. Our model consists of two oligomer chains (PEn) with 7, 11, 15, 19, or 23 carbon atoms in each oligomer cross-linked with one to three Au atoms through C-Au-C bonds. In structures with a single gold atom the C-Au-C bond is located in the central position of the oligomer. Binding energies (BEs) with respect to two oligomer radical fragments and Au are as high as 362-489 kJ/mol depending on the length of the oligomer chain. When the dispersion contribution in PEn-Au-PEn oligomers is omitted, BE is almost independent of the number of carbon atoms, lying between 293 and 296 kJ/mol. The dispersion energy contributions to BEs in PEn-Au-PEn rise nearly linearly with the number of carbon atoms in the PEn chain. The carbon-carbon distance in the C-Au-C moiety is around 4.1 Å, similar to the bond distance between saturated closed shell chains in the polyethylene crystal. BEs of pure saturated closed shell PEn-PEn oligomers are 51-187 kJ/mol. Both Au atoms and dispersion interactions contribute considerably to the creation of nearly parallel chains of oligomers with reasonably high binding energies.

Density functional theory modeling of C-Au chemical bond formation in gold implanted polyethylene.

We have studied processes of gold ion implantation in polyethylene (PE) by theoretical chemistry methods. Car-Parrinello molecular dynamics (CPMD) simulations of collisions and following chemical kinetics considerations lead to the conclusion that chemical bonds between gold atoms and PE chains are formed. We have identified and characterized by a DFT method various stable structures with C-Au, C-Au-C, C-Au-H and C-AuH2 types of chemical bonds. The binding energies (BE) of C-Au bonds are as high as 227 kJ mol-1 and the bond analysis reveals a covalent bonding character. For the experimental detection of these structures in gold implanted PE, we predicted characteristic infra-red (IR) frequencies. The C-Au stretching vibrational modes lie around 500 cm-1. Other characteristic frequencies lie in a band between 730 cm-1 and 1500 cm-1.

NMR absolute shielding scale and nuclear magnetic dipole moment of (207)Pb.

An absolute shielding scale is proposed for (207)Pb nuclear magnetic resonance (NMR) spectroscopy. It is based on ab initio calculations performed on an isolated tetramethyllead Pb(CH3)4 molecule and the assignment of the experimental resonance frequency from the gas-phase NMR spectra of Pb(CH3)4, extrapolated to zero density of the buffer gas to obtain the result for an isolated molecule. The computed (207)Pb shielding constant is 10 790 ppm for the isolated molecule, leading to a shielding of 10799.7 ppm for liquid Pb(CH3)4 which is the accepted reference standard for (207)Pb NMR spectra. The new experimental and theoretical data are used to determine µ((207)Pb), the nuclear magnetic dipole moment of (207)Pb, by applying the standard relationship between NMR frequencies, shielding constants and nuclear moments of two nuclei in the same external magnetic field. Using the gas-phase (207)Pb and (reference) proton results and the theoretical value of the Pb shielding in Pb(CH3)4, we find µ((207)Pb) = 0.59064 µN. The analysis of new experimental and theoretical data obtained for the Pb(2+) ion in water solutions provides similar values of µ((207)Pb), in the range of 0.59000-0.59131 µN.

Indirect NMR spin-spin coupling constants in diatomic alkali halides.

We report the Nuclear Magnetic Resonance (NMR) spin-spin coupling constants for diatomic alkali halides MX, where M = Li, Na, K, Rb, or Cs and X = F, Cl, Br, or I. The coupling constants are determined by supplementing the non-relativistic coupled-cluster singles-and-doubles (CCSD) values with relativistic corrections evaluated at the four-component density-functional theory (DFT) level. These corrections are calculated as the differences between relativistic and non-relativistic values determined using the PBE0 functional with 50% exact-exchange admixture. The total coupling constants obtained in this approach are in much better agreement with experiment than the standard relativistic DFT values with 25% exact-exchange, and are also noticeably better than the relativistic PBE0 results obtained with 50% exact-exchange. Further improvement is achieved by adding rovibrational corrections, estimated using literature data.

Temperature dependence of the (1)J((11)B(19)F) spin-spin coupling in BF(3) molecule.

The (1)J((11)B(19)F) spin-spin coupling of gaseous BF(3) was observed in (11)B NMR spectra as a function of density in a wide range of temperatures. Following the extrapolation of the measured values to the zero-density limit, the coupling constant free from intermolecular effects (1)J(0)((11)B(19)F) was obtained for each temperature. In contrast to previous investigations, the final results indicate a nonlinear dependence of (1)J(0)((11)B(19)F) on temperature. In the corresponding ab initio calculations of spin-spin coupling constants performed at the coupled cluster singles and doubles (CCSD) level to obtain a reliable result for this coupling constant we had to take into account large vibrational corrections.

NMR shielding constants in BF(3) and magnetic dipole moments of (11)B and (10)B nuclei.

Gas-phase NMR spectra of (11)B, (10)B, and (19)F in BF(3) are reported, and high-level ab initio calculations of the corresponding NMR shielding constants are described. Extrapolation of the measured resonance frequencies to the zero-density limit ensures that the results correspond to the ab initio values for an isolated molecule. Simultaneous measurements of (3)He resonance frequencies and application of the calculated shielding constants allow us to determine improved values of the nuclear magnetic dipole moments of (11)B and (10)B. The magnetic moments of both isotopes are also determined independently by comparing with the (19)F spectral parameters (frequencies and shielding constants). The separately derived nuclear magnetic moments are in good agreement, whereas the literature moments of both (11)B and (10)B are noticeably less accurate.

Ab initio study of interaction-induced NMR shielding constants in mixed rare gas dimers.

The interaction-induced contribution to the NMR shielding constants in homonuclear A2 and heteronuclear AB (A,B=He,Ne,Ar) dimers is obtained ab initio by employing a coupled cluster singles and doubles with perturbative treatment of triples wave function model and extended correlation-consistent basis sets. The second virial coefficients entering the expansion of the property with the density are then computed in a fully quantum mechanical approach, for temperatures ranging from the limit of dissociation of the dimer to well above standard conditions. The results can be used to describe the density and temperature dependence of the shielding constants in binary mixtures of helium, neon, and argon. The predicted effects should be observable for the interaction of 21Ne with other rare gases.