Translational and rotational diffusion of three glass forming alcohols by 1H field cycling NMR relaxometry

Abstract

1H field-cycling NMR relaxometry was applied to investigate the dynamics of the three glass forming alcohols 2-phenylbutan-1-ol (BEP), 2-(trifluoromethyl)phenetyl alcohol (2TFMP), and 4-(trifluoromethyl)phenetyl alcohol (4TFMP), all having a phenyl ring as substituent. 1H longitudinal relaxation rates, R1, were measured at Larmor frequencies (ν) from 0.01 to 35 MHz in the liquid phase of the three alcohols between 213 and 313 K. Data analysis was performed using master curves built on the basis of the frequency temperature superposition principle exploiting the NMR susceptibility representation. Longitudinal relaxation was considered to arise from two dynamic processes, i.e. translational diffusion and molecular rotations. For the first process a force-free hard-sphere model was used, whereas the phenomenological Davidson-Cole function was employed to model the second motional process. The analysis allowed translational and rotational correlation times to be determined over a wide time scale (10−11 - 10−3 s). The ratio between the two correlation times indicated the formation of hydrogen bonded networks for all alcohols, while their trends with temperature suggested that BEP forms stronger hydrogen bonds. The resulting self-diffusion coefficients were in agreement with the values independently determined from the slope of R1vs ν1/2 at low frequencies.