Theoretical and experimental examination of -caprolactam dimer stability

Abstract

The cyclic dimer of ε-caprolactam (C2) is a very stable compound as indicated by its high melting point (347 °C), low solubility in most solvents, and relatively low reactivity toward hydrolysis. FTIR measurements in solid state confirmed the existence of hydrogen bonds between NH protons and CO groups. Upon heating to 200 °C these bonds are not broken, but are merely weakened. This is shown by shifts of the NH stretching band toward higher wave numbers as well as by shifts of other characteristic bands. Temperature controlled NMR measurements on 0.3 and 30% C2 solutions in formic acid gave temperature coefficients of NH proton signals of −0.006 and −0.009 ppm/K, respectively, strongly indicating intermolecular hydrogen bonding. In the same concentration range signals for NH protons and CO carbons were shifted toward higher fields with higher C2 concentration. Using density functional theory (DFT) calculations it was not possible to successfully perform geometry optimization on any C2 structure with intramolecular hydrogen bonds. The trans form of C2 was found to be 7.0 kcal/mol lower in energy than the cis form. Symmetrical structures of C2 based on published trans X-ray structures were found to have comparable energies. Hydrogen bonds between trans C2 and discrete molecules of formic acid, water and N-methylacetamide, as model for solvents and a neighboring C2 molecule, were found to be comparable in energy. Full geometry optimization of a dimer of trans C2 gave energy stabilization of 8.25 kcal/mol per H-bond. These results show C2-C2 polar interactions to be comparable with polar interactions of C2 with solvents. The stability of C2, however, cannot be explained only by hydrogen bonding. It is likely that nonpolar interactions between C2 molecules complement the stabilization by hydrogen bonding. Optimized dimer of two trans cyclic-ε-caprolactam-dimer molecules (Etot = −1 461.0682582 H).