Quantum confinement effects are known to affect the behavior of molecules adsorbed in nanostructured materials. In order to study these effects on the transport of a single molecule through a nanotube, we present a quantum dynamics study on the diffusion of H 2 in a narrow (8,0) carbon nanotube in the low pressure limit. Transmission coefficients for the elementary step of the transport process are calculated using the flux correlation function approach and diffusion rates are obtained using the single hopping model. The different time scales associated with the motion in the confined coordinates and the motion along the nanotube's axis are utilized to develop an efficient and numerically exact approach, in which a diabatic basis describing the fast motion in the confined coordinate is employed. Furthermore, an adiabatic approximation separating the dynamics of confined and unbound coordinates is studied. The results obtained within the adiabatic approximation agree almost perfectly with the numerically exact ones. The approaches allow us to accurately study the system's dynamics on the picosecond time scale and resolve resonance structures present in the transmission coefficients. Resonance enhanced tunneling is found to be the dominant transport mechanism at low energies. Comparison with results obtained using transition state theory shows that tunneling significantly increases the diffusion rate at T < 120 K. Published by AIP Publishing. [http://dx
We present quantum dynamics calculations of the diffusion constant of H 2 and D 2 along a single-walled carbon nanotube at temperatures between 50 and 150 K. We calculate the respective diffusion rates in the low pressure limit by adapting well-known approaches and methods from the chemical dynamics field using two different PES to model the C-H interaction. Our results predict a usual kinetic isotope effect, with H 2 diffusing faster than D 2 in the higher temperature range, but a reverse trend at temperatures below 50-70 K. These findings are consistent with experimental observation in similar systems, and can be explained by the different effective size of both isotopes resulting from their different ZPE.
We report on quantum dynamical simulations of inter-chain exciton transport in a model of regioregular poly(3-hexylthiophene), rr-P3HT, at finite temperature, using the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method for a system of up to 63 electronic states and 180 vibrational modes. A Frenkel Hamiltonian of HJ aggregate type is used, along with a reduced H-aggregate representation; electron-phonon coupling includes local high-frequency modes as well as anharmonic intermolecular modes. The latter are operative in mediating inter-chain transport, by a mechanism of transient localization type. Strikingly, this mechanism is found to be of quantum coherent character and involves non-adiabatic effects. Using periodic boundary conditions, a normal diffusion regime is identified from the exciton mean-squared displacement, apart from early-time transients. Diffusion coefficients are found to be of the order of 3 x 10-3 cm2/s, showing a non-monotonous increase with temperature.
A study on the quantum dynamics of the hydrogen molecule embedded in the hollow cavity of a single-walled carbon nanotube is presented, taking into account for the first time all six degrees of freedom of the confined molecule.A set of initial eigenstates of the trapped H 2 molecule are propagated for 500 fs using the State Average Multiconfigurational Time-dependent Hartree approach. An initial linear momentum is added to the hydrogen molecule in order to mimic high temperature behavior, forming an angle of 0 • and 45 • with respect to the nanotube's axis; an additional propagation is carried out without adding any extra momentum. The wave packet dynamics are analyzed using projections and overlap functions in the appropriate degrees of freedom. The study reveals little correlation between the translation of the confined molecule along the nanotube and the remaining degrees of freedom.
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