Non-equimolar discrete compounds in binary chiral systems of organic substances

Abstract

Since knowledge on the occurrence of non-equimolar discrete compounds in binary systems containing chiral molecules is very limited, this study reviews and systematizes the current state of investigating such systems and summarizes the results on two example systems studied in detail by the authors. In particular, the identification and verification of the non-equimolar discrete compounds compared to other discrete solid phases occurring in the two systems are discussed by presenting the results of related SCXRD, PXRD, TRPXRD, DSC, IR, and HSM studies. The (S)-malic acid–(R)-malic acid system has been found to contain non-equimolar 1 : 3 and 3 : 1 stable (S₃R and SR₃) and metastable (3S1R and 1S3R) discrete compounds, along with the equimolar compounds RSI and RSII (known monoclinic modifications) and the recently discovered RSIII modification. Polymorphic transformations of the discrete phases are debated, and the crystal structure of the stable compound S₃R is identified (S. G. P1). The L-valine–L-isoleucine system has been stated to contain a non-equimolar 2 : 1 discrete compound, V₂I, that could independently be proven by the ternary solubility diagram in water and its crystal structure solved (S. G. C2). The results obtained are discussed in conjunction with the findings reported in the literature. In order to systematize the variety of terms used for the description of discrete phases in binary chiral systems of organic substances, a systematization of equimolar and non-equimolar compounds based on chemical and crystallographic characteristics is proposed.