Structure and dynamics of aqueous norspermidine solutions: an in situ ultrasonic relaxation spectroscopic study

Abstract

An in situ ultrasonic relaxation spectroscopic study is presented in an effort to determine the structural changes and the dynamics involved when norspermidine (NSpd) is dissolved in water. Our aim is to elucidate the mechanism responsible for the observed relaxation mechanism in acoustic spectra and estimate the corresponding thermodynamic parameters and the associated volume change. The experimental spectra of aqueous NSpd solutions revealed a single Debye-type relaxation mechanism attributed to proton-transfer reaction. The concentration and temperature dependence of the acoustic parameters supports this assignment. The activation enthalpy and entropy were estimated equal to ΔH * = 1.79 ± 0.20 kcal mol −1 and ΔS * = −18.31 ± 0.73 cal mol −1 K −1 , respectively. The concentration and temperature dependence of the sound velocity and absorption in the solutions exhibit characteristic features that are related to alterations in the network rigidity due to variations in hydrogen-bonding interactions at molecular level. The volume change associated to proton-transfer reaction for NSpd has been estimated and compared with the volume change observed for an analogous guanidine, the 1,1,3,3 tetramethyl guanidine. The obtained results are discussed in the framework of an existing theoretical structural model highlighting the strong molecular association in these liquid mixtures leading to complementary information on the structure and dynamics of guanidine amines. A comprehensive model of the whole relaxation processes is presented and discussed in detail.

The Identification of Structural Changes in the Lithium Hexamethyldisilazide–Toluene System via Ultrasonic Relaxation Spectroscopy and Theoretical Calculations

“…For the i -th ( i = 1, 2) structural mechanisms described by Equations (1) and (2), the isentropic standard volume change can be defined as follows [ 17 ]: where is the mass density of the solution, T is the absolute temperature, u is the speed of sound in the solution and R is the gas constant. Γ i is the concentration parameter, while A i and f r,i correspond to the relaxation amplitude and frequency for each mechanism, respectively.…”

“…Subsequently, an increase in the distance between toluene molecules and LiHMDS species is expected, which is verified experimentally in Figure 8. For the i-th (i = 1, 2) structural mechanisms by Equations ( 1) and ( 2), the isentropic standard volume change ∆V s can be defined as follows [17]:…”

Insight into the Structural and Dynamical Processes of Peptides by Means of Vibrational and Ultrasonic Relaxation Spectroscopies, Molecular Docking, and Density Functional Theory Calculations

“…The second and the third relaxation processes are classified into structural processes, since they involve intermolecular rearrangements that are frequently associated with large volume changes between different equilibrium states. In general, structural processes are linked with shear (structural) viscosity and have been observed to prevail in strongly associated liquids consisting of polar molecules interacting through hydrogen bonding [38,39]. 4.0×10 -13 6.0×10 -13 8.0×10 -13 Relaxation frequencies [Hz] Starting from the lower-frequency mechanism, it seems that the relaxation frequency fr1 appears almost constant with solution concentration, while the corresponding amplitude increases monotonically.…”

Insight on the Structural and Dynamical Processes of Peptides by Means of Vibrational and Ultrasonic Relaxation Spectroscopies, Molecular Docking and DFT Calculations

“…The second and the third relaxation processes are classified into structural processes, since they involve intermolecular rearrangements that are frequently associated with large volume changes between different equilibrium states. In general, structural processes are linked with shear (structural) viscosity and have been observed to prevail in strongly associated liquids consisting of polar molecules interacting through hydrogen bonding [38,39]. The standard volume change for the conformational change, the association-dissociation reaction, and the protonation and deprotonation reactions can be directly evaluated from the experimental maximum excess absorption per wavelength 𝜇 𝑚𝑎𝑥 , the solution density ρ, the sound velocity u, the absolute temperature T, the gas constant R and the concentration term Γ from the following equation [40,41]:…”