RESUMO
A consecutive rotation of crystallographic orientation has been observed in lateral crystallization of NH4Cl on a glass substrate, which induces a periodic distribution of faceted and roughened regions on the surface of a crystallite aggregate. Experimental observation indicates that this phenomenon derives from the asymmetric surface energies at the growth front, which deform the nascent nucleus and tilt the crystallographic orientation in the nucleation-mediated layered growth. We suggest that this effect is significant for a class of lateral growth where nucleation plays a dominate role.
RESUMO
Discrepancies between the crystal structures of short n-alkanes as obtained from experiment and as obtained from molecular mechanics tended to worsen at longer chain lengths. The same holds for the relative stabilities of the two experimentally observed polymorphs. In this paper it is argued that the discrepancies are caused by thermal effects, and that the triclinic polymorph is the most stable polymorph for all chain lengths at 0 K. A phase transition is predicted but has yet to be found experimentally. Current force fields cannot reproduce the experimental observations without explicit introduction of temperature by means of molecular dynamics.
RESUMO
It is known that the experimental triclinic crystal structures of even n-alkanes are not well reproduced upon energy minimization with current force fields. The inclusion of electrostatics does not solve this, and, moreover, some charge schemes show unphysical features such as positively charged carbon atoms or charge alternation. The effect of the electrostatics on the energies of the crystal structures of the even n-alkanes, and thereby on their polymorphism, has never been established. A new charge scheme is introduced that yields physically sensible charges without constraints. It will also be shown, however, that electrostatics are relevant neither for the structures of the crystals, nor for their energies.
