Thermodynamic and Dynamic Transitions and Interaction Aspects in Reorientation Dynamics of Molecular Probe in Organic Compounds: A Series of 1-alkanols with TEMPO

The spectral and dynamic properties of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) in a series of 1-alkanols ranging from methanol to 1-decanol over a temperature range 100–300 K were investigated by electron spin resonance (ESR). The main characteristic ESR temperatures connected with slow to fast motion regime transition; T(50G) ‘s and T(X1)(fast) ‘s are situated above the corresponding glass temperatures, T(g), and for the shorter members, the T(50G) ‘s lie above or close to melting point, T(m), while the longer ones the T(50G) < T(m) relationship indicates that the TEMPO molecules are in the local disordered regions of the crystalline media. The T(50G) ‘s and especially T(X1)(fast) ‘s are compared with the dynamic crossover temperatures, T(X)(VISC) = 8.72M(0.66), as obtained by fitting the viscosity data in the liquid n-alkanols with the empirical power law. In particular, for N(C) > 6, the T(X1)(fast) ‘s lie rather close to the T(X)(VISC) resembling apolar n-alkanes [PCCP 2018,20,11145-11151], while for N(C) < 6, they are situated in the vicinity of T(m). The absence of a coincidence for lower1-alkanols indicates that the T(50G) is significantly influenced by the mutual interaction between the polar TEMPO and the protic polar medium due to the increased polarity and proticity destroyed by the larger-scale melting transition.