Isotopic Polymorphism in Pyridine

Abstract

The ab initio calculation of the relative stabilities of isomers of gas phase molecules rates as one of the outstanding scientific achievements of the twentieth century. Our understanding of the structure of solid state is, by comparison, much less well advanced. The problem is illustrated by the crystal structure of pyridine. Pyridine (C5H5N or ‘h5’ hereafter) is one of the simplest heteroaromatic compounds but its crystal structure (phase h5-I) is unusually complicated, having four independent molecules in the asymmetric unit (Z’ = 4). [1] Price et al. have surveyed the potential for polymorphism in pyridine using ab initio crystal structure prediction methods, finding over a dozen crystal structures that were energetically competitive with h5-I. [2] In parallel with Price’s theoretical work an intense experimental search was made by one of us (RB group) for new low-temperature polymorphs of pyridine. Though all attempts to crystallize h5 failed to yield anything but the h5-I phase, crystallization of pyridine-d5 (d5) from pentane yielded a new phase, d5-II, at 188 K. Recrystallization from a low-melting solvent such as pentane has been shown in the past to circumvent hightemperature phases because saturation of the solution occurs below the temperature of the phase transition. [3] The new d5-II phase has one molecule in the asymmetric unit (Z’ = 1), but does not correspond to any of the predicted polymorphs of h5. The effect of temperature on the crystal structure of pyridine-d5 was subsequently investigated further using neutron powder diffraction. The sample was ground at 77 K [4] and then rapidly cooled to 2 K. The powder pattern was successfully modelled as d5-I (see Fig. S1a in the Supplementary Information). The sample was then warmed in steps of 2 K, with patterns being acquired at each temperature. When the sample reached 170 K it began to undergo a sluggish phase transition into d5-II. After collecting a clean d5-II neutron powder