Local spin density investigation of the chromium / polyimide interface

Bonding of chromium to the polyimide, PMDA-ODA, surface is still the subject of debate. In an attempt to clarify this problem, we have performed density functional theory calculations on different model molecules, phthalimide, PMDA, and PAP (p-amino phenol), which represent the most functionalities of the polyimide (PMDA-ODA). If we consider only the low spin case, we find that chromium bonds preferentially to the phenyl ring. However, when we release the spin constraint and optimize the structure, we find that the absolute stable configuration is that of chromium in a quintet state, at a carbonyl group. The energy difference is 0.30 eV. The complete infrared spectrum has been calculated for phthalimide and compared with experimental spectra. The agreement is excellent. A vibrational analysis for the Cr/phthalimide system, in both configurations (Cr on C=O and Cr on phenyl), in their stable spin states, is presented. We find that the former yields better agreement with experiment than the latter.