Molecular structure of the complex of hexano-6-lactam with calcium bromide

A complex of hexane-6-lactam (ε-caprolactam, CL) with calcium bromide was prepared and characterized. According to TGA analyses its thermal stability is distinctly lower than those of the analogous complex of CL with magnesium bromide. The molecular structure of the complex was studied by IR and NMR spectroscopy. Based on interpretations of NMR and vibrational spectra and well-corresponding quantum mechanical calculations, six molecules of CL are arranged in the complex around the central CaBr2 molecule in almost exactly equivalent positions. The complex molecule CaBr2(CL)6 is quite compact but the coordination bonds are markedly weaker in comparison with the recently studied MgBr2(CL)6 complex. In solution, a concentration dependent dynamic equilibrium between bound and free CL with fast exchange between both forms was observed.