C–H···S Hydrogen Bonds Governed Colossal Thermal Expansion: Two Concomitant Crystalline Forms of Ethionamide and 2‑Thiobarbituric Acid

Two crystalline forms (Forms I and II) of ethionamide (ETH) and 2-thiobarbituric acid (TBA), obtained concomitantly, in different stoichiometries [1:1 and 2(1:1)] are reported herein. Both the forms crystallize in the same space group, P1̅, but having different unit cell dimensions (Form I, 300 K, a = 8.2816(12), b = 9.2167(12), c = 10.5164(14) Å; α = 73.554(5), β = 76.635(4) and γ = 68.894(4)°; Form II, 300 K, a = 9.4558(16), b = 10.868(2), c = 15.720(3) Å; α = 82.171(6), β = 77.391(6) and γ = 69.637(6)°). Furthermore, Form II shows unusually high atomic thermal displacements for S atoms in the ETH, at room temperature leading to a disordered structure and failing to refine the position during structure determination, while Form I is perfectly ordered even at room temperature. Considering the unusual high magnitude of displacement parameters in Form II, thermal expansion (TE) studies have been carried out on both forms using variable-temperature single-crystal X-ray diffraction methods. The process reveals unusual colossal positive thermal expansion, along one of the principal axes, X3, in both the forms (Form I, 130 × 10–6 K–1; Form II, 205 × 10–6 K–1), while Form II exhibits negative thermal expansion, along X1 (−85 × 10–6 K–1) also. All these types of TE are well correlated with the packing of the molecules in the form of a cradle, governed by C–H···S hydrogen bonds, adding a new dimension for the assessment of the thermal response of different types of weak hydrogen bonds.