Force dependence of the infrared spectra of polypropylene calculated with density functional theory

The force dependence of the infrared spectrum of isotactic 3,5-dimethylheptane, which serves as a model molecule for polypropylene, was investigated with quantum chemical calculations using the External Force is Explicitly Included (EFEI) method. Normal modes were analyzed with respect to the force dependence of their frequencies and intensities. In general, the frequencies exhibit a monotonic increase or decrease with force, depending on the way the external force is modifying the bonds involved in the normal mode vibration. The behavior of the intensities is in many cases non-monotonic. This is attributed to the force-dependent coupling and mixing of modes, which leads to irregular behavior of the intensities. To compare the calculations with experimental results the modes were broadened with a Lorentzian, and superimposed to simulate the infrared spectrum. The experimentally observed force dependence is in qualitative agreement with the calculated values. Due to the occasionally counteracting behavior of normal modes, peaks originating from several normal modes may move, split, coalesce, emerge or disappear under the influence of an external force.