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Picosecond relaxations in model substances for proteins: A millimeter‐wave investigation on crystalline alkyl amides

The dielectric absorption at millimeter‐wave (mm‐wave) frequencies (50–150 GHz) of N‐methylacetamide (NMA), N,N‐dimethylacetamide (DiNMA), and N,N‐dimethylacrylamide (DiNMAcry) is measured. Measurements are performed using the oversized‐cavity technique in the temperature range from liquid helium to room temperature. Additionally, a mm‐wave interferometeric measurement at room temperature is made. NMA and DiNMAcry exhibit monotonic increases of the absorption coefficient with temperature as well as with frequency. For DiNMA a monotonic increase of the absorption coefficient with frequency is also found, while the absorption coefficient as a function of temperature shows a pronounced maximum at approximately 30 K. At this maximum the absorption coefficient of DiNMA exceeds those of NMA and DiNMAcry by about two orders of magnitude. The dielectric behavior of the three substances can be described by relaxation processes in asymmetric double‐well potentials. For the low‐temperature relaxation in DiNMA the double well could be established by two possible positions of the molecule in the crystal that are separated by a rotational movement. Hydrogen bonds and long side chains may hinder these relaxational movements in NMA and DiNMAcry, respectively, and thereby account for their comparatively lower absorption. The results are compared with similar results recently obtained on proteins and synthetic biopolymers.