Titus A. Beu

Molecular dynamics simulations of ion transport through carbon nanotubes. I. Influence of geometry, ion specificity, and many-body interactions

Extensive molecular dynamics simulations of the flow of aqueous NaCl and NaI solutions through carbon nanotubes are presented, evidencing the dependence of diverse transport features on the solute specificity, the nanotube geometry, and the various atomic models employed, including polarizability. The simulated properties are in agreement with published results, indicating that ion translocation sets in only for nanotubes with chiralities higher than (7,7), and extend the explanation of the mechanisms governing ion transport to larger chiralities. The interpretation of the various dynamic quantities is developed in close connection with the structural details of the solution and the energy barriers the solute components have to overcome. Also, the role and relevance of water and ion polarizabilities are discussed in detail.

Structure of ammonia clusters from n=3 to 18

Optimized structures and bonding energies have been calculated for ammonia clusters from n=3 to n=18 using a pairwise additive model potential. The trimer and tetramer are stable cyclic configurations. From the pentamer onward the structures are three dimensional with an increasing tendency to amorphous behavior. The exceptions are the heptamer with a Cs axis, the hexadecamer with a central atom, and the very stable and completely symmetric dodecamer with the D6h point group. Here each ammonia molecule is bound by two covalent and two hydrogen bonds. In general, the coordination number increases from 2.0 for the rings over 4.0 for n=12 to 4.2 for n=18. The structures agree where available with previously obtained results for a more elaborate potential.

Vibrational spectra of ammonia clusters from n=3 to 18

We have calculated the vibrational spectra of the umbrella (ν2), the symmetric (ν1), and the asymmetric (ν3) N–H stretch mode for ammonia clusters from n=3–18. The results are based on recent structure calculations and a molecular perturbation approach that includes the anharmonicities. Clusters with high symmetry exhibit few lines only and show the expected blue shifts for the umbrella and red shifts for the two N–H stretch modes. The calculated frequencies of the umbrella mode agree very well with experimental results for n=2–5 as far as the general shape is concerned, but overestimate the absolute shifts.

The vapor-liquid interface potential of (multi)polar fluids and its influence on ion solvation

The interface between the vapor and liquid phase of quadrupolar-dipolar fluids is the seat of an electric interfacial potential whose influence on ion solvation and distribution is not yet fully understood. To obtain further microscopic insight into water specificity we first present extensive classical molecular dynamics simulations of a series of model liquids with variable molecular quadrupole moments that interpolates between SPC/E water and a purely dipolar liquid. We then pinpoint the essential role played by the competing multipolar contributions to the vapor-liquid and the solute-liquid interface potentials in determining an important ion-specific direct electrostatic contribution to the ionic solvation free energy for SPC/E water-dominated by the quadrupolar and dipolar parts-beyond the dominant polarization one. Our results show that the influence of the vapor-liquid interfacial potential on ion solvation is strongly reduced due to the strong partial cancellation brought about by the competing solute-liquid interface potential.

Molecular dynamics simulations of ion transport through carbon nanotubes. III. Influence of the nanotube radius, solute concentration, and applied electric fields on the transport properties

The present investigations continue previous research on transport in aqueous ionic solutions through carbon nanotubes. Specifically, the effects of the nanotube radius, solute concentration, and applied external electric fields on the transport properties are investigated in terms of mobilities, currents, and pairing times of the solute ions. The simulated transport features are corroborated with general theoretical results of nanofluidics (such as the linear log-log regime of the nanochannel conductance as function of the solute concentration and the current-voltage curve of the channel). Discontinuities in the partial ionic currents are explained on the basis of a recent theoretical model of quantized ionic conductance in nanopores, developed by Zwolak et al. Correlations between the structural and dynamic properties are established, linking causally the highly structured spatial density profiles, the ion pairing phenomenon and the ionic currents.

Infrared spectroscopy of large ammonia clusters as a function of size

We have measured the vibrational spectra of large ammonia (NH3)n clusters by photofragment spectroscopy in the spectral range from 3150 to 3450 cm(-1) for the average sizes n = 29, 80, 212, 447, and 989 and by depletion spectroscopy for n=8. The spectra are dominated by peaks around 3385 cm(-1) which are attributed to the asymmetric nu3 NH-stretch mode. Two further peaks between 3200 and 3260 cm(-1) have about equal intensity for n = 8 and 29, but only about 0.40 of the intensity of the nu3 peak for the larger sizes. The spectra for the smallest and largest size agree with those obtained by Fourier transform infrared spectroscopy in slit jet expansion and collision cells, respectively. By accompanying calculation we demonstrate that the energetic order of the spectral features originating from the bending overtone 2nu4 and the symmetric NH-stretch nu1 in the range from 3150 to 3450 cm(-1) is changed between n = 10 and 100, while the asymmetric NH-stretch nu3 only exhibits a moderate redshift. The reason is the coupling of the ground state modes to the overtones.

Molecular dynamics simulations of ion transport through carbon nanotubes. II. Structural effects of the nanotube radius, solute concentration, and applied electric fields

The reported work extends previously published research on transport in aqueous ionic solutions through carbon nanotubes. Specifically, the effects of the nanotube radius, solute concentration, and applied external electric fields on the solution structuring are investigated in terms of spatial density distributions, pair distribution functions, and electrostatic potential profiles. Several simulated structural features are consistent with general theoretical results of nanofluidics and can be interpreted fairly well with respect to these (such as the Donnan-type voltages established at the channel apertures depending on the logarithm of the maximum ion concentration). The simulated properties are based on averages over the largest data collection times reported in the literature (0.8 μs), providing accurate estimates of the measured quantities.

A new intermolecular potential for hydrazine clusters: Structures and spectra

The structures of small hydrazine clusters from the dimer to the hexamer have been calculated using a standard site-site intermolecular potential and a newly developed systematic approach which is essentially based on monomer properties. Aside from the repulsive and the attractive dispersion and induction interaction special care is taken for the determination of the electrostatic interaction which is represented by a distributed multipole expansion and a penetration correction. Based on these potentials the vibrational spectra of the N-N stretching and the asymmetric NH2 wagging mode are calculated using degenerate perturbation theory. While the small shifts of the N-N stretching mode are fairly well reproduced by both potential models, large differences are predicted for the asymmetric NH2 wagging mode. Here, redshifts of –30 cm−1 are calculated for the standard and blueshifts of 100 cm−1 are obtained for the systematic potential in agreement with experiment. The analysis shows that the reason for this ...

VIBRATIONAL PREDISSOCIATION SPECTRA OF SIZE SELECTED HYDRAZINE CLUSTERS : EXPERIMENT AND CALCULATIONS

Vibrational predissociation spectra of hydrazine (N2H4)n clusters have been measured from the dimer to the tetramer using a linetunable, isotopically substituted CO2-laser in order to fill the frequency gap between 990 and 1010 cm−1. The clusters are size selected in a scattering experiment with helium atoms. The large blue shifts of the asymmetric NH2 wag mode at 937 cm−1 are completely interpreted by calculations based on a recently determined systematic model potential. The gross shifts of 60 cm−1 for the dimer, 80 cm−1 for the trimer, and 110 cm−1 for the larger clusters are explained by the different structures: Cyclic arrangements with two hydrogen bonds per molecule for the dimer, rings with one hydrogen bond per molecule for the trimer, and three-dimensional structures for the larger ones. The peaks in the spectra are caused by characteristic vibrations to which more than one isomer contributes.

GPU-accelerated computation of electron transfer

Electron transfer is a fundamental process that can be studied with the help of computer simulation. The underlying quantum mechanical description renders the problem a computationally intensive application. In this study, we probe the graphics processing unit (GPU) for suitability to this type of problem. Time‐critical components are identified via profiling of an existing implementation and several different variants are tested involving the GPU at increasing levels of abstraction. A publicly available library supporting basic linear algebra operations on the GPU turns out to accelerate the computation approximately 50‐fold with minor dependence on actual problem size. The performance gain does not compromise numerical accuracy and is of significant value for practical purposes.