Jichen Li

A charge-driven molecular water pump

Understanding and controlling the transport of water across nanochannels is of great importance for designing novel molecular devices, machines and sensors and has wide applications, including the desalination of seawater. Nanopumps driven by electric or magnetic fields can transport ions and magnetic quanta, but water is charge-neutral and has no magnetic moment. On the basis of molecular dynamics simulations, we propose a design for a molecular water pump. The design uses a combination of charges positioned adjacent to a nanopore and is inspired by the structure of channels in the cellular membrane that conduct water in and out of the cell (aquaporins). The remarkable pumping ability is attributed to the charge dipole-induced ordering of water confined in the nanochannels, where water can be easily driven by external fields in a concerted fashion. These findings may provide possibilities for developing water transport devices that function without osmotic pressure or a hydrostatic pressure gradient.

THE PARAMETRIZATION OF A THOLE-TYPE ALL-ATOM POLARIZABLE WATER MODEL FROM FIRST PRINCIPLES AND ITS APPLICATION TO THE STUDY OF WATER CLUSTERS (N=2-21) AND THE PHONON SPECTRUM OF ICE IH

We present the parametrization of a new polarizable model for water based on Thole’s method [Chem. Phys. 59, 341 (1981)] for predicting molecular polarizabilities using smeared charges and dipoles. The potential is parametrized using first principles ab initio data for the water dimer. Initial benchmarks of the new model include the investigation of the properties of water clusters (n=2–21) and (hexagonal) ice Ih using molecular dynamics simulations. The potential produces energies and nearest-neighbor (H-bonded) oxygen–oxygen distances that agree well with the ab initio results for the small water clusters. The properties of larger clusters with 9–21 water molecules using predicted structures from Wales et al. [Chem. Phys. Lett. 286, 65 (1998)] were also studied in order to identify trends and convergence of structural and electric properties with cluster size. The simulation of ice Ih produces a lattice energy of −65.19 kJ/mol (expt. −58.9 kJ/mol) with an average dipole moment of 2.849 D. The calculated...

A controllable molecular sieve for Na+ and K+ ions.

The selective rate of specific ion transport across nanoporous material is critical to biological and nanofluidic systems. Molecular sieves for ions can be achieved by steric and electrical effects. However, the radii of Na(+) and K(+) are quite similar; they both carry a positive charge, making them difficult to separate. Biological ionic channels contain precisely arranged arrays of amino acids that can efficiently recognize and guide the passage of K(+) or Na(+) across the cell membrane. However, the design of inorganic channels with novel recognition mechanisms that control the ionic selectivity remains a challenge. We present here a design for a controllable ion-selective nanopore (molecular sieve) based on a single-walled carbon nanotube with specially arranged carbonyl oxygen atoms modified inside the nanopore, which was inspired by the structure of potassium channels in membrane spanning proteins (e.g., KcsA). Our molecular dynamics simulations show that the remarkable selectivity is attributed to the hydration structure of Na(+) or K(+) confined in the nanochannels, which can be precisely tuned by different patterns of the carbonyl oxygen atoms. The results also suggest that a confined environment plays a dominant role in the selectivity process. These studies provide a better understanding of the mechanism of ionic selectivity in the KcsA channel and possible technical applications in nanotechnology and biotechnology, including serving as a laboratory-in-nanotube for special chemical interactions and as a high-efficiency nanodevice for purification or desalination of sea and brackish water.

Neutron spectroscopic investigation of dynamics of water ice

Quantitative studies of the properties of water require detailed investigation of the intramolecular and intermolecular interactions acting between the atoms and the molecules. Experimental information about the strength of the hydrogen bond interaction in water can be obtained directly by measuring its vibrational spectra. This is because a particular vibrational mode (or phonon) is determined by the interatomic force constants, which in turn are the double-differentials of the potential function at its minima. Therefore, measuring dynamical properties is one of the most powerful ways of investigating interatomic potentials in a given material. In this article we report series of inelastic neutron scattering studies for various forms of exotic ices, the aim of the investigation is to provide a better understanding of the nature of hydrogen bonding in water, which has considerable implications to large segment of scientific community for which the properties of water are important.

The proton momentum distribution in water and ice

Deep Inelastic Neutron Scattering (Neutron Compton Scattering), is used to measure the momentum distribution of the protons in water from temperatures slightly below freezing to the supercritical phase. The momentum distribution is determined almost entirely by quantum localization effects, and hence is a sensitive probe of the local environment of the proton. The distribution shows dramatic changes as the hydrogen bond network becomes more disordered. Within a single particle interpretation, the proton moves from an essentially harmonic well in ice to a slightly anharmonic well in room temperature water, to a deeply anharmonic potential in the supercritical phase that is best described by a double well potential with a separation of the wells along the bond axis of about 0.3 Angstrom. Confining the supercritical water in the interstices of a C60 powder enhances this anharmonicity and enhances the localization of the protons. The changes in the distribution are consistent with gas phase formation at the hydrophobic boundaries and inconsistent with the formation of ice there.

Potential lattice dynamical simulations of ice

Abstract Lattice dynamical calculations have been carried out for ice Ih, ice XI, ice II and ice VIII using seven rigid pair-wise potentials and a M-point polarizable potential that is a modification of the potential constructed by Dang and Chang [J. Chem. Phys. 106 (1997) 8149]. It was found that the M-point polarizable potential gives out a wide range of hydrogen bonds and a narrow librational band. MCY makes stretching and bending interactions too weak and it gives O–O bond lengths too long (∼5%) so its lattice densities are obviously lower than other potential lattices or experimental values. Other classic potentials (BF, SPC, Rowlinson, TIP3P, TIP4P and TIPS2) produce reasonable results and slightly short O–O bond lengths. The mean two-body interaction energy of M-point polarizable potential is −0.370 eV/molecule and the mean polarizable energy is −0.180 eV/molecule. Nonadditive energy takes about 32.7% of total ice Ih lattice energy, which is less than Molecular Dynamical result 39% for liquid.

Investigation of the hydrogen bonding in ice Ih by first-principles density function methods

It is a well recognized difficult task to simulate the vibrational dynamics of ices using the density functional theory (DFT), and there has thus been rather limited success in modelling the inelastic neutron scattering (INS) spectra for even the simplest structure of ice, ice Ih, particularly in the translational region below 400 cm(-1). The reason is partly due to the complex nature of hydrogen bonding (H-bond) among water-water molecules which require considerable improvement of the quantum mechanical simulation methods, and partly owing to the randomness of protons in ice structures which often requires simulation of large super-lattices. In this report, we present the first series of successful simulation results for ice Ih using DFT methods. On the basis of the recent advancement in the DFT programs, we have achieved for the first time theoretical outcomes that not only reproduce the rotational frequencies between 500 to 1200 cm(-1) for ice Ih, but also the two optic peaks at ∼240 and 320 cm(-1) in the translational region of the INS spectra [J. C. Li, J. Chem. Phys 105, 6733 (1996)]. Besides, we have also investigated the impact of pairwise configurations of H(2)O molecules on the H-bond and found that different proton arrangements of pairwise H(2)O in the ice Ih crystal lattice could not alter the nature of H-bond as significantly as suggested in an early paper [J. C. Li and D. K. Ross, Nature (London) 365, 327 (1993)], i.e., reproducing the two experimental optic peaks do not need to invoke the two H-bonds as proposed in the previous model which led to considerable debates. The results of this work suggest that the observed optic peaks may be attributed to the coupling between the two bands of H-O stretching modes in H(2)O. The current computational work is expected to shed new light on the nature of the H-bonds in water, and in addition to offer a new approach towards probing the interaction between water and biomaterials for which H-bond is essential.

A quasi-elastic neutron scattering study of the dynamics of supercritical water

Dynamics of supercritical water was studied by quasi-elastic neutron scattering. Two series of measurements, respectively, at constant temperature of 673 K and constant density of 0.578 g/cm 3 were carried out. Experimental data were fitted properly by the jump-diffusion model. The self-diffusion coefficient, typically in the interval 3.71–5.21 � 10 � 8 m 2 /s, depends both on temperature and density. The mean residence time is, however, fairly constant at 0.30 ps in all cases. From the temperature dependence of self-diffusion coefficient which is of Arrhenius type the activation energy Ea ¼ 2:25 kcal/mol is being estimated. 2003 Elsevier Science B.V. All rights reserved.

Quasi-elastic neutron scattering study of hydrated DNA

Abstract Dynamics of water around DNA molecules was studied by incoherent quasi-elastic neutron scattering. The mean residence time is about 10 times higher than for liquid water indicating that mobility of water adsorbed on DNA is reduced very significantly. The mean residence time increases from 10 ps for the sample with 2.5 g water/g DNA to 18 ps for a water content of 0.31 g water/g DNA. The activation energy of diffusion for water in hydrated DNA samples determined by the Arrhenius plot is Ea=6.9 kcal/mol, a value significantly higher than 4.2 kcal/mol estimated for liquid water.

Neutron spectroscopy of high-density amorphous ice.

Abstract Vibrational spectra of high-density amorphous ice (hda-ice) for H 2 O and D 2 O samples were measured by inelastic neutron scattering. The measured spectra of hda-ice are closer to those for high-pressure phase ice-VI, but not for low-density ice-Ih. This result suggests that similar to ice-VI the structure of hda-ice should consist of two interpenetrating hydrogen-bonded networks having no hydrogen bonds between themselves.