Rationalizing the Lacking of Inversion Symmetry in a Noncentrosymmetric Polar Racemate: An Experimental and Theoretical Study

Abstract

The total charge density of PYRAC, a polar (<i>Pca</i>2₁) organic racemate with <i>Z</i>′ = 2, was derived from high-resolution single-crystal X-ray diffraction data at <i>T</i> = 100(2) K and periodic DFT calculations. The PYRAC asymmetric unit consists of a hydrogen-bonded pair of conformationally different enantiomers, A and B_i, where the subscript “i” indicates a reversed absolute configuration. The lattice stability was compared with that of centrosymmetric possibly competing structures, with the aim of understanding why a noncentrosymmetric lattice framework is obtained from a racemic mixture. The likelihood of specific intermolecular recognition processes among different conformers of PYRAC in the very first stages of nucleation was investigated by DFT simulations in vacuo. Two competing, equivalent interconversion pseudorotatory paths between the most stable A and the least stable B conformers were found. It results that molecules spend most of their time (≈53%) in the A conformation, whereas the B one is far less populated (≈7%). Therefore, centrosymmetric AA_i adducts are formed very frequently in the reaction liquor, whereas the BB_i ones are rare. Nevertheless, AA_i pairs produce crystal forms with cohesive energies and densities significantly less favorable than those estimated for the noncentrosymmetric heterochiral AB_i ones. Therefore, preference for <i>Z</i>′ = 2 in conjunction with noncentrosymmetric point and space groups results from the thermodynamic control of the crystallization process. The capability of forming extended hydrogen bond chains throughout the lattice appears to be a prerequisite to bind together the fundamental AB_i repeating units