Abstract
Acoustical spectroscopy at frequencies up to 10 GHz gives the possibility of the investigation of liquid substances, where the relaxation process observed is caused by energy transfer between translational and vibrational degrees of freedom. The compounds presented in this article belong to this group of liquids. The acoustic investigations in the group of benzene derivatives, particularly research of the dependencies of acoustic parameters and the structure of organic liquids, demonstrated some interesting regularities in the group of these compounds in gas and liquid states. In this article, the results of research on five cyclic liquids: bromo-, chloro-, fluoro-, iodo-, and nitrobenzene as well as toluene and aniline are discussed and compared to benzene. The acoustic relaxation observed in all these compounds was found to result from Kneser’s processes (vibrational relaxation). Based on investigations reported in this article, as well as by other authors, and taking into account experimental and literature data concerning a great number of compounds, one can draw a conclusion that almost all acoustic relaxation (Kneser-type) processes in liquids can be described using a single relaxation time. It also seems that all vibrational degrees of freedom of the molecule take part in this process. It is known that the appearance of differences in transition probabilities could be caused by additional attraction in interactions of molecules having dipole moments. Halogen derivatives have higher values of dipole moments than benzene. This difference could be responsible for the difference of transition probabilities and changes in the relaxation times. However, benzene derivatives with amino, nitro, and methyl groups and halides show the other type of relaxation.
Highlights
Introduction and TheoryAcoustic spectroscopy at frequencies up to 10 GHz enables investigation of liquid substances with subclassic absorption caused by vibrational relaxation
4 3 ηs 1−1 Cv Cp where f is the frequency of an ultrasonic wave, ρ is the density of the liquid, c0 is the ultrasound speed for frequencies below the range of vibrational relaxation, κT is the coefficient of thermal conductivity, ηs is the coefficient of shear viscosity, and C p and Cv are the molar heat capacities at constant pressure and volume, respectively
For these molecules a significant influence of dipole moments De on thermodynamic properties and kinetics of phenomena is observed—a change of these physical values correlates with changes in the vibrational relaxation time, and with the probability of VT processes
Summary
Acoustic spectroscopy at frequencies up to 10 GHz enables investigation of liquid substances with subclassic absorption caused by vibrational relaxation. Where f is the frequency of an ultrasonic wave, ρ is the density of the liquid, c0 is the ultrasound speed for frequencies below the range of vibrational relaxation, κT is the coefficient of thermal conductivity, ηs is the coefficient of shear viscosity, and C p and Cv are the molar heat capacities at constant pressure and volume, respectively. From acousto-optical investigations of halides in the hypersonic range [9,10,11], a linear dependence of the probability excitation logarithm log(P10) on T −1/3 was found It is one of the most characteristic features of relaxation, related to a delay of the internal energy distribution among vibrational and translational degrees of freedom of the molecules [12,13]. Correlation of the values of C(iacoust) and C(iopt) indicated that in these relaxation processes all vibrational degrees of freedom were involved
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