Pressure-Induced Polymorphism of Caprolactam: A Neutron Diffraction Study

Caprolactam, a precursor to nylon-6 has been investigated as part of our studies into the polymerization of materials at high pressure. Single-crystal X-ray and neutron powder diffraction data have been used to explore the high-pressure phase behavior of caprolactam; two new high pressure solid forms were observed. The transition between each of the forms requires a substantial rearrangement of the molecules and we observe that the kinetic barrier to the conversion can aid retention of phases beyond their region of stability. Form II of caprolactam shows a small pressure region of stability between 0.5 GPa and 0.9 GPa with Form III being stable from 0.9 GPa to 5.4 GPa. The two high-pressure forms have a catemeric hydrogen-bonding pattern compared with the dimer interaction observed in ambient pressure Form I. The interaction between the chains has a marked effect on the directions of maximal compressibility in the structure. Neither of the high-pressure forms can be recovered to ambient pressure and there is no evidence of any polymerization occurring.

A synthetic co-crystal prepared by cooperative single-crystal-to-single-crystal solid-state Diels–Alder reaction

Solid-state Diels–Alder reaction in a 2 : 1 donor to acceptor charge-transfer complex leads to a synthetic co-crystal composed of product and unreacted donor. Analysis of close contacts and DFT energy calculations indicate that the reaction occurs cooperatively where the arrangement of molecules shown in (ii) is favoured.