Polymorphism and dynamics of glass forming alcohols studied by NMR spectroscopy and relaxometry

Neohexanol (2,2-dimethyl-1-butanol) and some of its isomers, differing for the positions of the OH and CH3 substituents, constitute a very interesting family of alcohols showing a rich polymorphism at low temperatures with several solid-solid phase transitions. Indeed, due to their globular shape and to the possibility to associate through hydrogen bonds, neohexanols give rise to both plastic crystalline phases and glasses. In particular, some of them show orientationally disordered crystal (ODIC) phases due to the easiness of molecular rotational motions in the solid state.1 Fast field cycling (FFC) NMR relaxometry is a fundamental technique for obtaining information on dynamic properties of viscous liquids, allowing internal, overall and collective molecular motions to be investigated by means of measurement and analysis of 1H longitudinal relaxation times over a wide Larmor frequency range (10 kHz - 40 MHz).2 On the other hand, solid-state NMR spectroscopy is a very powerful technique for the study of dynamics in both amorphous and crystalline solid phases, providing, through a combination of different experiments and observable nuclei, complementary information with respect to FFC NMR. In this work 1H FFC NMR relaxometry and solid-state NMR spectroscopy experiments have been applied to neohexanol and three of its isomers (3,3-dimethyl-2-butanol, 2,3-dimethyl-2-butanol and 3,3-dimethyl-1-butanol) in the temperature range from -60 to 30 °C. In particular, 1H spin lattice relaxation times (T1) have been measured at Larmor frequencies from 10 kHz to 35 MHz with FFC techniques and static 1H, 2H, and 13C solid state NMR spectra have been recorded. The spectroscopic and relaxation data acquired for the different isomers have been compared and analyzed to obtain information on the dynamic processes occurring in the different solid phases as well as to establish relationships between chemical structure and dynamic properties. References: [1] E. Juszy?ska, M. Massalska-Arod?, I. Natkaniec, J. Krawczyk Physica B, 2008, 403,109-114 (2008); E. Juszy?ska-Ga??zka, P. M. Zieli?ski, M. Massalska-Arod?, J. Krawczyk Acta Phys. Pol. A 2013, 124, 917-925 (2013). [2] D. Kruk, A. Herrmann, E. A. Rössler Prog. Nucl. Mag. Reson. Spectrosc. 2012, 63, 33-64.