Rotational disorder in 2-piperidyl-5-nitro-6 -methylpyridine: structural phase transition and its vibrational characteristics

Abstract

X-ray diffraction (XRD) studies have shown that 2-piperidyl-5-nitro-6-methylpyridine, C₁₁H₁₅N₃O₂, undergoes a structural phase transition at T = 240 K. The room temperature structure is tetragonal, space group I4₁/a, with the unit-cell dimensions a = 13.993(2) and c = 23.585(5) Å. The pyridine ring takes trans conformation with respect to the piperidine unit. While pyridine is well ordered, the piperidine moiety shows apparent disorder resulting from a libration about the linking NC bond. The low-temperature phase is monoclinic, space group I2/a. Contraction of the unit-cell volume by 2.3% at 170 K enables the CH···O linkage between the molecules of the neighbouring stacks. As result, the asymmetric unit becomes bi-molecular. The thermal librations of the piperidine and methyl groups become considerably reduced at 170 K and nearly fully reduced at about 100 K. The IR spectra and polarised Raman spectra agree with the X-ray structure and confirm the disorder effect on the piperidine ring. The assignment of the bands observed was made on the basis of DFT chemical quantum calculations. Copyright © 2008 John Wiley & Sons, Ltd.