X-ray Diffraction, FT-IR, and ¹³C CP/MAS NMR Structural Studies of Solvated and Desolvated C-Methylcalix[4]resorcinarene

Solid C-methylcalix[4]resorcinarene solvated by acetonitrile and water (CAL-Me) and then modified by slow solvent evaporation (CAL-Me*) was studied using single-crystal and powder X-ray diffraction, FT-IR, and ¹³C CP/MAS NMR. The CAL-Me solvate crystallizes in the monoclinic P2₁/n space group with three CH₃CN and two H₂O molecules in the asymmetric part of the unit cell. The CAL-Me molecules adopt a typical crown conformation with all of the hydroxyl groups of the aryl rings oriented up and all of the methyl groups disposed down (the rccc isomeric form). The crystalline network is formed by resorcinarene, CH₃CN, and H₂O molecules and assembled by intermolecular hydrogen bonds and weak C−H···A or C−H···π interactions. The desolvated CAL-Me* loses its crystalline character and becomes partly amorphous. It is devoid of CH₃CN and deficient in water. However, the resorcinarene molecules still remain in the crown conformation supported by intramolecular hydrogen bonds, while intermolecular hydrogen bonds are considerably disintegrated. The work directs general attention to the problem of stability and polymorphism of resorcinarene solvates. It shows that the joint use of diffractometric and spectroscopic methods is advantageous in the structural studies of complex crystalline macromolecular systems. On the other hand, the solid-state IR and NMR spectroscopic analyses applied in tandem have been found highly beneficial to elucidate the disordered structure of poorly crystalline, desolvated resorcinarene.