Proton-transfer in 1,1,3,3 tetramethyl guanidine by means of ultrasonic relaxation and Raman spectroscopies and molecular orbital calculations.

In this work, we report on the structure and dynamics of the 1,1,3,3 tetramethyl guanidine (TMG) aqueous solutions in a wide concentration and temperature range by combining vibrational and ultrasonic spectroscopies. The experimental Raman spectra have been compared with the corresponding spectra obtained by ab initio quantum mechanical and density functional theory electronic structure calculations. This comparison indicated that only a single mechanism occurs when dissolving TMG in water and this is the proton transfer reaction, while the formation of byproducts during hydrolysis of TMG is dubious. This observation is further supported by the concentration dependence of the Raman spectra. The analysis of the ultrasonic relaxation data also revealed that the system exhibits a single relaxation process associated with this proton transfer reaction. It has been also observed that both relaxation amplitude and frequency exhibit a clear monotonous increase with increasing amine concentration in the solutions supporting the concept of the proton transfer reaction. The corresponding activation enthalpy was estimated directly from the temperature dependence of the acoustic data and found equal to ΔH* = 5.56 ± 0.34 kcal/mol, which seems to be reasonable for hydrogen-bond formation. Furthermore, the concentration dependence of the acoustic parameters and kinematic viscosity data has been used as a probe for the molecular association in these solutions. The results have been discussed in relation to the ability or inability of water molecules to form stable clathrates after the addition of amine molecules in the solutions.