Structure and energetics of imogolite: a quantum mechanical ab initio study with B3LYP hybrid functional

Imogolite (Al-2(OH)(3)SiO3OH) single-walled nanotubes are simulated at the ab initio level by using an all electron Gaussian type basis set and the hybrid B3LYP functional. Full exploitation of the roto-translational symmetry drastically reduces the computational cost. Two kinds of tubes, (n, 0) and (n, n), are considered, resulting from rolling up a hypothetical structure containing a gibbsite-like hexagonal layer linked to a silanolic SiOH unit. In both cases a minimum is observed, corresponding to n = 10 for (n, 0) (the radius, taken as the distance between the tube axis and one of the basal SiO4 oxygen atoms, is 7.26 angstrom) and n = 8 for (n, n) (10.3 angstrom). Hydrogen bonds and orientation of the silanolic group inside the tube play an important role in stabilising the structures. The (10, 0) structure is 10.6 kJ mol(-1) per formula unit more stable than the (8, 8) tube, the difference being due, at least partially, to the formation of hydrogen bonds in the inner wall of the tube (the shortest H center dot center dot center dot O distances are 2.14 and 3.01 angstrom, respectively). Two curves are observed in the (n, 0) case, whose minima, both at n = 10, are separated by 2.0 kJ mol(-1) per formula unit. These two structures are extremely similar, the main difference being the orientation of the OH unit pointing inside the tube.