Hugo Meekes

Emergence of a Single Solid Chiral State from a Nearly Racemic Amino Acid Derivative

The evolution of a single chiral solid state is reported for an amino acid derivative starting from a nearly racemic mixture of solid left- and right-handed crystals. Attrition-enhanced dissolution and recrystallization processes based on solubility considerations of the Gibbs−Thomson rule, coupled with solution-phase racemization, drive this near-equilibrium system inexorably to single chirality in the solid phase.

Report on the sixth blind test of organic crystal structure prediction methods

The results of the sixth blind test of organic crystal structure prediction methods are presented and discussed, highlighting progress for salts, hydrates and bulky flexible molecules, as well as on-going challenges.

Complete Chiral Symmetry Breaking of an Amino Acid Derivative Directed by Circularly Polarized Light

Circularly polarized light (CPL) emitted from star-forming regions is an attractive candidate as a cause of single chirality in nature. It has remained difficult, however, to translate the relatively small chemical effects observed on irradiation of molecular systems with CPL into high enantiomeric excesses. Here we demonstrate that irradiation of a racemic amino acid derivative with CPL leads to a small amount of chiral induction that can be amplified readily to give an enantiopure solid phase. A racemate composed of equal amounts of left- and right-handed crystals in contact with the irradiated solution is converted completely into crystals of single-handedness through abrasive grinding when racemization is effected in the solution. The rotation sense of the CPL fully determines the handedness of the final solid state. These findings illustrate the potential effectiveness of CPL in the control of molecular asymmetry, which is relevant for the origin of the single chirality inherent to many biological molecules.

On the prediction of crystal morphology. I. The Hartman-Perdok theory revisited

The over 40 year old Hartman-Perdok (HP) [Hartman & Perdok (1955). Acta Cryst. 8, 49-52, 521-524, 525-529] theory for predicting crystal morphology is reconsidered. The new approach, which gives a physical foundation to the theory, is based on F faces having a roughening transition temperature higher than 0 K. The aim of this paper is to confront the field of crystal growth and in particular the classical HP theory with modern statistical thermodynamical treatments of models of surfaces of relatively simple crystal structures. It is shown that crystal faces (hkl) containing multiple connected nets with a relatively high energy content may have a very low roughening temperature and an unexpectedly high growth rate. In some cases, crystal faces become rough at 0 K because of multiple connected nets related by symmetry giving rise to symmetry roughening. The use of connected nets in the HP theory offers the possibility of extrapolating the results of statistical thermodynamical models of simple crystal surfaces to more complex crystals as encountered in practice. The role of the step free energy in understanding crystal morphology is emphasized.

Morphology, evolution and other characteristics of gibbsite crystals grown from pure and impure aqueous sodium aluminate solutions

The effect of external conditions on the growth morphology of gibbsite, γ-Al(OH)3, grown under a range of conditions has been studied. The results show that during growth the morphology of gibbsite crystals evolves from thin, rounded hexagons and faceted lozenges into faceted plates and blocks with well-formed basal, prismatic and chamfered faces. This morphology evolution shows only a weak dependence on the growth conditions. Moreover, increasing the driving force or the caustic concentration leads to larger crystals, up to tens to a hundred μm in size. The influence of small amounts of inorganic impurities on the growth of gibbsite crystals turns out to be negligible. Different growth morphologies are obtained from potassium and cesium hydroxide solutions. This indicates that the alkali ions of the solution have a major influence on the morphology of gibbsite. The twinning behaviour is similar for gibbsite crystals grown from sodium, potassium or cesium aluminate solutions.

Complete Chiral Resolution Using Additive-Induced Crystal Size Bifurcation During Grinding

Grinding them down: By using a tailor-made additive, even in the absence of racemization in solution, abrasive grinding can yield an enantiopure solid state. This novel chiral resolution technique is based on an asymmetric bifurcation in the crystal size distribution as a result of stereoselective hampered crystal growth. R = o-tolyl.

Side-Face Structure and Growth Mechanism of Tabular Silver Bromide Crystals

Recently, it has become possible to grow large tabular silver bromide crystals from organic solvents, which makes direct observation of the side faces possible. Solving the side-face structure is a key to understanding the (lateral) growth mechanism of tabular silver bromide crystals. The side-face structure of 42 tabular crystals was determined in detail. The side-face structure of the tabular crystals is built up by flat {111} and {100} cubo-octahedral faces and there are three different kinds of tabular morphology. In all observations, ridge structures were found showing no acute lips. From these determinations, the number of parallel twin planes of the tabular crystals was directly reduced from the side-face structure. The lateral growth of the tabular crystals is described by a substep mechanism. This mechanism explains the increase of growth rate of a {111} side face that is linked with a twin plane to a {100} side face. The proposed substep mechanism is based on a theory of Ming, which is universal and not only valid for the case of silver bromide. The interpretation assumes the presence of only cubo-octahedral faces and the absence of acute lips as observed.

STRUCTURAL PHASE-TRANSITIONS IN C-70

Cubic as well as hexagonal single crystals of C70 have been grown and investigated by electron diffraction, electron microscopy and x-ray diffraction. Several phase transitions have been detected and crystallographic models are proposed. Hexagonal crystals, stable at room temperature with c/a = 1.63 will undergo two transitions, upon cooling. First the c/a ratio will increase to 1.82 owing to c-axis alignment of the molecules; at a lower temperature the molecules will orientationally order, resulting in a monoclinic structure.

Emergence of single-molecular chirality from achiral reactants

The synthesis of enantiopure molecules from achiral precursors without the need for pre-existing chirality is a major challenge associated with the origin of life. We here show that an enantiopure product can be obtained from achiral starting materials in a single organic reaction. An essential characteristic of this reaction is that the chiral product precipitates from the solution, introducing a crystal–solution interface which functions as an asymmetric autocatalytic system that provides sufficient chiral amplification to reach an enantiopure end state. This approach not only provides more insight into the origin of life but also offers a pathway to acquire enantiopure compounds for industrial applications.

On the prediction of crystal morphology. III. Equilibrium and growth behaviour of crystal faces containing multiple connected nets

In this paper, the equilibrium and growth behaviour of faces (hkl) with more than one connected net is studied. It is shown that for these types of orientation different surface phases exist under equilibrium conditions as a function of temperature. Depending on the exact bonding topology at the surface, flat, rough or disordered flat phases are found. Moreover, the growth rate R(hkl) of such faces can differ significantly from the usually calculated relative growth rates based on the attachment energy. Monte Carlo simulations confirm the results from the Hartman-Perdok analyses and offer a tool for the prediction of the crystal habit as a function of supersaturation.