Experimental evidence of picosecond to femtosecond molecular motion of themacrocycles 12-crown-4 and 15-crown-5 in cyclohexane at 25 degrees C

Values of the dielectric loss coefficient, epsilon ", part of the complex p ermittivity, epsilon* = epsilon'-i epsilon ", from UHF to terahertz (THz) f requencies, are reported for the macrocycles 12-crown-4 (12C4) and 15-crown -5 (15C5), in the solvent cyclohexane at 25 degrees C. The coefficients of the real part of the complex permittivity epsilon' at UHF-microwave frequen cies, and the refractive indices in the visible, at the sodium doublet (lam bda = 589.3 nm), are also reported. The data are described by the sum of th ree Debye relaxation functions in the microwave region, and by three Rocard -Powles functions in the terahertz, far-IR range of frequencies. The molecu lar origin of the three Debye functions, following Vij and Hufnagel (J. Phy s. Chem., 1991, 95, 6142), is attributed to rotation of the macrocycles alo ng the molecular axes. The source of two Rocard-Powles functions is assigne d to molecular librations while one describes a weak solvent absorption.