The effect of bond saturation in five-membered heterocyclic compounds upon their acoustic properties – VT energy migration rate

Abstract

Acoustic investigations were carried out for a series of cyclic and heterocyclic compounds, with particular regard to the dependence of acoustic parameters on chemical structures. Several interesting regularities were found among five-membered cyclic compounds, a few of which have been previously described by Linde et al. [1], but the rest were observed for the first time. In the present paper, the results of investigations in six five-membered heterocyclic compounds are discussed. These compounds are furan, 2,3-dihydrofuran, 2,3-dihydrofuran, tetrahydrofuran, thiophene and tetrahydrothiophene. The acoustic relaxation observed in all the compounds was found to result from Kneser's processes (vibrational relaxation). As can be seen from pressure-dependent acoustic measurements in tetrahydrofuran, both propagation velocity and absorption of ultrasonic waves decrease with increasing pressure. The values of the acoustic absorption coefficient in the low frequency range were several times higher and relaxation times markedly longer than those observed in non-saturated compounds, similarly to the previously considered cyclic and heterocyclic compounds [1–5]. It was found that the changes in acoustic parameters, and hence in the rate of the energy transfer between vibrational and translational degrees of freedom (VT transition), can be accounted for by the effect of the compound's aromaticity. Comparison of the changes in the parameters of furans and thiophenes, with adiabatic compressibility being maintained, shows that acoustic parameters strictly depend on the internal molecular structure. This problem is also dealt with in the present paper. Being unequivocal for the group of six five-membered compounds, the interpretation becomes less obvious in the case of other cyclic compounds, e.g. six-membered substances.