Quantum confinement of molecular deuterium clusters in carbon nanotubes: ab initio evidence for hexagonal close packing

Abstract

An ab initio study of quantum confinement of deuterium clusters in carbon nanotubes is presented.First, density functional theory (DFT)-based symmetry-adapted perturbation theory is used to derive parameters for a pairwise potential model describing the adsorbate-nanotube interaction. Next, we analyze the quantum nuclear motion of N D 2 molecules (N o 4) confined in carbon nanotubes using a highly accurate adsorbate-wave-function-based approach, and compare it with the motion of molecular hydrogen. We further apply an embedding approach and study zero-point energy effects on larger hexagonal and heptagonal structures of 7-8 D 2 molecules. Our results show a preference for crystalline hexagonal close packing hcp of D 2 molecules inside carbon nanotubes even at the cost of a reduced volumetric density within the cylindrical confinement.

Spectroscopy of a rotating hydrogen molecule in carbon nanotubes

“…38). Considering a (5,5) CNT as an example of tube with subnanometer diameter, we have applied the SAPT(DFT) method as implemented within the MOLPRO electronic structure package 39 including density fitting (DF). 40 The dangling bonds were saturated with hydrogen atoms.…”

“…40 The dangling bonds were saturated with hydrogen atoms. We followed the same computational setup reported in previous works, 5,8 using the Perdew-Burke-Ernzerhof (PBE) den-sity functional, 41 and a modified version of the augmented polarized correlation-consistent triple-zeta basis 42 (aug-cc-pVTZ) for the nanotube carbon atoms as well as for the adsorbate hydrogen atoms. The DF of Coulomb and exchange integrals employs the auxiliary basis set developed for the aug-cc-pVTZ basis by Weigend, 43 while the aug-cc-pVTZ/MP2Fit basis 44 was used for fits of integrals containing virtual orbitals.…”

“…The parameters of the APPM were extracted by fitting to SAPT(DFT)-based values of dispersionless and dispersion terms. This APPM is specially designed to characterize the interaction of adsorbates with curved carbon-based nanostruc-tures 5,8,23 and metallic solid surfaces. 48 It should be stressed that the influence of the nanotube curvature-induced dipole 49 in the H 2 -CNT interaction is implicitly accounted for in the potential model through the dependence of dispersionless and dispersion components on the angle between the radial vector going from the nanotube center to one carbon atom and the vector pointing from the H 2 center-of-mass to the same C atom.…”

“…2 The general goal in hydrogen storage methods being to pack hydrogen as close as possible, the existing remarkable experiments provide evidences for solid-like packing of hydrogen at supercritical temperatures of industrial importance 3 as well as for highdensity orientationally ordered phases at 75 K. 4 On a theoretical side, a very recent study showed possibility for an hexagonalclose-packing of deuterium molecules in carbon nanotubes. 5 The crucial role of quantum nuclear effects in the motion of light molecular species inside carbon nanotubes has been demonstrated in experimental observations at low temperatures. Thus, a recent work by Ohba 6 revealed a quenched adsorption of helium atoms in carbon nanopores with diameter below 0.7 nm at very low temperatures of 2−5 K. The same work also showed that more molecules of nitrogen than helium atoms adsorb in these narrow nanotubes despite of the larger kinetic diameter of molecular nitrogen.…”

“…This method has very recently been applied to precise characterizations of the nature of bound states of atomic helium, molecular nitrogen, and molecular deuterium clusters inside carbon nanotubes. 5,8 In previous works 5,8 molecules of nitrogen and deuterium were treated as structureless point-like particles using the Full Configuration Interaction nuclear orbital (FCI-NO) approach (see, e.g., Refs. 33-35 and references therein).…”