NMR crystallography of p-tert-butylcalix[4]arene host-guest complexes using 1H complexation-induced chemical shifts

(1)H magic-angle spinning (MAS) NMR spectra of p-tert-butylcalix[4]arene inclusion compounds with toluene and pyridine show large complexation-induced shifts of the guest proton resonances arising from additional magnetic shielding caused by the aromatic rings of the cavities of the host calixarene lattice. In combination with ab initio calculations, these observations can be employed for NMR crystallography of host-guest complexes, providing important spatial information about the location of the guest molecules in the host cavities.     

Molecular dynamics study of structure H clathrate hydrates of methane and large guest molecules

Methane storage in structure H (sH) clathrate hydrates is attractive due to the relatively higher stability of sH as compared to structure I methane hydrate. The additional stability is gained without losing a significant amount of gas storage density as happens in the case of structure II (sII) methane clathrate. Our previous work has showed that the selection of a specific large molecule guest substance (LMGS) as the sH hydrate former is critical in obtaining the optimum conditions for crystallization kinetics, hydrate stability, and methane content. In this work, molecular dynamics simulations are employed to provide further insight regarding the dependence of methane occupancy on the type of the LMGS and pressure. Moreover, the preference of methane molecules to occupy the small (5(12)) or medium (4(3)5(6)6(3)) cages and the minimum cage occupancy required to maintain sH clathrate mechanical stability are examined. We found that thermodynamically, methane occupancy depends on pressure but not on the nature of the LMGS. The experimentally observed differences in methane occupancy for different LMGS may be attributed to the differences in crystallization kinetics and/or the nonequilibrium conditions during the formation. It is also predicted that full methane occupancies in both small and medium clathrate cages are preferred at higher pressures but these cages are not fully occupied at lower pressures. It was found that both small and medium cages are equally favored for occupancy by methane guests and at the same methane content, the system suffers a free energy penalty if only one type of cage is occupied. The simulations confirm the instability of the hydrate when the small and medium cages are empty. Hydrate decomposition was observed when less than 40% of the small and medium cages are occupied.     

Simulations of structure II H2 and D2 clathrates: potentials incorporating quantum corrections

Molecular dynamics simulations are used to study the stability of structure II H(2) and D(2) clathrates with different large and small guest occupancies at 160 and 250 K and 2.0 kbars. Simulations are performed with the recently proposed anisotropic site-site potentials of Wang for H2 and D2 [J. Quant. Spectrosc. Radiat. Transf. 76, 23 (2003)] which are parameterized to account for quantum corrections of order variant Planck's over 2pi(2) in the second virial coefficient. Occupancies of 0-2 in the small cages and 2-5 in the large cages are considered. Thermodynamic integration is used to determine the most stable guest occupancy at each temperature. Since lattice free energy and configurational energy differences are small for a number of different combinations of cage occupancies, one must expect that in bulk samples various combinations will indeed be observed. Special attention is given to the differences between H(2) and D(2) guests and implications on the hydrogen storage capacity of the clathrates are discussed.     

How much carbon dioxide can be stored in the structure H clathrate hydrates?: a molecular dynamics study

The stability of structure H (sH) carbon dioxide clathrate hydrates at three temperature-pressure conditions are determined by molecular dynamics simulations on a 3x3x3 sH unit cell replica. Simulations are performed at 100 K at ambient pressure, 273 K at 100 bars and also 300 K and 5.0 kbars. The small and medium cages of the sH unit cell are occupied by a single carbon dioxide guest and large cage guest occupancies of 1-5 are considered. Radial distribution functions are given for guests in the large cages and unit cell volumes and configurational energies are studied as a function of large cage CO(2) occupancy. Free energy calculations are carried out to determine the stability of clathrates for large cage occupancies at three temperature/pressure conditions stated above. At the low temperature, large cage occupancy of 5 is the most stable while at the higher temperature, the occupancy of 3 is the most favored. Calculations are also performed to show that the CO(2) sH clathrate is more stable than the methane clathrate analog. Implications on CO(2) sequestration by clathrate formation are discussed.     

A mathematical modeling approach toward magnetic fluid hyperthermia of cancer and unfolding heating mechanism

Journal of Thermal Analysis and Calorimetry - Magnetic nanoparticles (MNPs)-induced hyperthermia is capable of heating the tumor without side effects. In this technique, the tumor temperature is...     

On quantitative structure of small Ree groups

The main aim of this article is to study quantitative structure of small Ree Groups ²G₂(q). Here, we prove that small Ree groups are uniquely determined by their orders and the set of the number of elements of the same order.     

A theoretical study of the solvent effect on the interaction of C<Subscript>20</Subscript> and N<Subscript>2</Subscript>H<Subscript>2</Subscript>

In this work, the interaction of C₂₀ and the N₂H₂ fragment is investigated at the M062X/6-311G(d,p) level of theory in both gas and solution phases. The interaction energies obtained by the standard method are corrected by the basis set superposition error (BSSE) during the geometry optimization for all molecules at the same levels of theory. The results obtained from these calculations reveal that the interaction between C₂₀ and N₂H₂ increases in the presence of more polar solvents. Values of the electrophilic charge transfer show the charge flow from C₂₀ to N₂H₂. The influence of the solvent on the hyperpolarizability indicates that β_tot values decrease on passing from vacuum to the solution phase.     

Analytical solution to transport in Brownian ratchets via the gambler's ruin model

We present an analogy between the classic gambler's ruin problem and the thermally activated dynamics in periodic Brownian ratchets. By considering each periodic unit of the ratchet as a site chain, we calculated the transition probabilities and mean first passage time for transitions between energy minima of adjacent units. We consider the specific case of Brownian ratchets driven by Markov dichotomous noise. The explicit solution for the current is derived for any arbitrary temperature, and is verified numerically by Langevin simulations. The conditions for current reversal in the ratchet are obtained and discussed.     

Stochastic perturbation theory: a low-scaling approach to correlated electronic energies

We discuss a stochastic implementation of M?ller-Plesset (MP) theory based upon the concept of a "graph," a set of connected Slater determinants. We show how contributions from an arbitrary level, MPn, of perturbation theory can be expressed diagrammatically in terms of graphs, and that these may be stochastically sampled to give a good estimate of the energy. We show this to be the case for Ne, Ar, N2, and H2O molecules. N-molecule chains of He atoms and H2 molecules at equilibrium and stretched geometries show an effective scaling of O[N(2.6)] and O[N(5.6)] for MP2 and MP3 theories.