We have determined the ground-state energies of para-H2 clusters at zero temperature using the diffusion Monte Carlo method. The liquid or solid character of each cluster is investigated by restricting the phase through the use of proper importance sampling. Our results show inhomogeneous crystallization of clusters, with alternating behavior between liquid and solid phases up to N = 55. From there on, all clusters are solid. The ground-state energies in the range N = 13−75 are established, and the stable phase of each cluster is determined. In spite of the small differences observed between the energy of liquid and solid clusters, the corresponding density profiles are significantly different, a feature that can help to solve ambiguities in the determination of the specific phase of H2 clusters.
The intramolecular interactions responsible for the planarity observed in poly(3,4-ethylenedioxythiophene)
and small 3,4-ethylenedioxythiophene-containing oligomers have been investigated using quantum mechanical
methods. Specifically, the relative influence of electron-donating effects, π-conjugation, and geometric
restrictions induced by the cyclic substituent and attractive S · · ·O intramolecular noncovalent interactions,
which were proposed to be the most relevant factor for such planarity on the self-rigidification observed in
these compounds, have been examined considering a wide number of model compounds. Results evidenced
that noncovalent S · · ·O interactions, which were postulated on the basis that the nonbonded distances between
sulfur and oxygen atoms belonging to neighboring repeating units are significantly shorter than the sum of
the van der Waals radii of sulfur and oxygen, are slightly repulsive destabilizing the planar anti conformation.
In contrast, the latter arrangement is favored by the π-conjugation produced by both geometric restrictions
imposed by the cyclic substituent and the electron-donating effects provided by the oxygen atoms attached
to positions three and four of each tiophene ring. Therefore, these factors produce gain in aromaticity and
favorable electrostatic interactions when the planarity is reached, compensating the Pauli repulsions between
the shared electron pairs of the sulfur and oxygen atoms.