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General Understanding
Molecules inside of crystals often associate through bonding interactions such chlorine and pi stacking.  These interactions lay the ground work for how each molecules orientates itself inside the crystal.  One such interaction is that of hydrogen bonding.  Hydrogen bonding, the weakest of all bonds, is the joining of molecules through dipole interactions between a partially electronegative atom and a partially electropositive hydrogen atom.   Hydrogen bonding patterns are clearly seen in the well defined structural motif of carboxylic acid dimers.  Carboxylic acid dimers seem to be a dominant factor in the overall architecture of crystal packing pattern.  Synthetic processes can produce molecules that use this predictable motif to engineer unique properties into crystals.  One unique property is that of second harmonic generation (SHG or Non-Linear Optics NLO) which shortens (by half) the wavelength of electromagnetic radiation thereby increasing its energy.  This work sought to create NLO materials through the creation of dipole inducing molecules.
 
CRYSTAL ENGINEERING:  Predictable Non-Centrosymmetric Molecular Architectures towards the Synthesis of Non Linear Optical (NLO) Materials.
 

Crystal engineering centers on the design and fabrication of predictable molecular motifs in crystals. This approach often strives to construct materials with specific bulk properties using knowledge of only the starting synthons structure. Since the formation of molecular crystals is governed by a complex interplay of strong and weak interactions, any structural feature that forms predictable patterns may be used to advantage.  The inversion symmetry operator is readily accepted as a dominant packing motif in organic crystals. In fact, inspection of the CSD reveals 67% of all organic structures, chiral and achiral molecules, form centrosymmetric patterns. This bias was exploited by the creation of molecules that form quasiracemic crystals. Unlike true racemates, quasi-racemates are composed of pairs of chemically unique R-(A) and S-(B) molecules, where A and B are sterically similar molecules.
Crystal engineers often attempt to program certain properties inside molecular structure by breaking certain bias found in many crystal structures. Recently, crystal engineers have focused on breaking centrosymmetry for the creation non-linear optical properties. Non linear optical properties have the ability to shorten the wavelength of synchronous radiation, thereby increasing its energy. Quasiracemates by design has the ability to break the rigid definition of centrosymmetry found in many crystal systems, therefore they contain non-linear optical properties.
 Fredga’s studies in the 1960’s and 1970’s created phase diagrams of quasiracemic mixtures. From several dozen examples of quasiracemates, 2 systems containing 2-(2-bromophenoxy) propionic acid, 2-(2-chlorophenoxy) propionic acid and 2-(2-methylphenoxy) propionic acid were chosen. Quasiracemates of 2-(2-bromophenoxy) propionic acid / 2-(2-chlorophenoxy) propionic acid and 2-(2-bromophenoxy) propionic acid / 2-(2-methylphenoxy) propionic acid and their respective racemates, were synthesized and crystal structures determined. Comparative studies found a difference of 9 angstroms between the Br and Cl molecules with isomorphic cells found between racemates of 2-Bromo / 2-Methyl and quasi-racemates of 2-Bromo / 2-Chloro and 2-Bromo / 2-Methyl. Packing coefficients, the amount of space used in the unit cell by molecules, were similar in all systems. By understanding these systems we are not only able to break biases that are dominant in many crystals, but we are also able to program certain properties inside crystal systems.