*2.1. Materials and Physical Measurements*

DL-Mandelic acid and the pyridinecarboxamide isomers were purchased from Sigma-Aldrich (Sigma-Aldrich. Inc., Tres Cantos, Madrid, Spain). Commercially available solvents were used as received without further purification. Melting points were determined on a Büchi melting point apparatus (Büchi Labortechnik AG-Flawil, Switzerland) and are uncorrected. Elemental analyses for carbon, hydrogen and nitrogen were performed with a Fisons-Carlo Erba 1108 microanalyser (CARLO ERBA Reagents SAS, Chaussée du Vexin, France). 1H NMR and 13C NMR spectra in DMSO-d6 were run on a Varian Mercury 300 instrument (Varian Medical Systems, Inc., Palo Alto, CA, USA), using TMS as the internal reference. IR spectra were recorded as KBr pellets (4000–400 cm–1) on a Bruker IFS-66v spectrophotometer (Bruker Corporation, Billerica, MA, USA). TGA experiments were carried out on a Shimadzu Thermobalance TGA-DTG-50H Instrument (TA Instruments, New Castle, DE, EE. UU.) from room temperature to 700 ◦C in a flow of air (100 mL min–1) and series of FTIR spectra (20–30 per sample) of evolved gasses were recorded using a coupled FT–IR Nicolet Magma 550 spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA). Differential scanning calorimetry (DSC) was conducted on a DSC Q100 apparatus of TA Instruments. Accurately weighed samples (1–2 mg) were placed in hermetically sealed aluminum crucibles (40 mL) and scanned from 0 to 350 ◦C at a heating rate of 10 ◦C min–1 under a dry nitrogen atmosphere. Powder X-ray diffraction (XRPD) patterns were collected on a Philips PW1710 (Philips Engineering Solutions, Aachen, Germany) with a Cu-Kα radiation (1.5406 Å). The tube voltage and amperage were set at 40 kV and 30 mA, respectively. Each sample was scanned between 2 and 50◦ 2θ with a step size of 0.02◦. The instrument was previously calibrated using a silicon standard.
