Communication: Synperiplanar to antiperiplanar conformation changes as underlying the mechanism of Debye process in supercooled ibuprofen

Abstract

In this Communication, we present experimental studies that put new insight into the puzzling nature of the Debye relaxation found in the supercooled liquid state of racemic ibuprofen. The appearance of D-relaxation in the loss spectra of non-hydrogen bonding methylated derivate of ibuprofen has proven that Debye relaxation is related solely with conformational changes of the carboxyl group, termed in this paper as synperiplanar-antiperiplanar. Our studies indicate that the presence of hydrogen bonding capabilities is not here the necessary condition to observe Debye process, however, their occurrence might strongly influence α- and D-relaxations dynamics. Interestingly, the activation energy of the D-process in ibuprofen methyl ester on approaching T(g) was found to be perfectly consistent with that reported for ibuprofen by Affouard and Correia [J. Phys. Chem. B 114, 11397-11402 (2010)] (~39 kJ/mol). Finally, IR measurements suggest that the equilibrium between conformers concentration depends on time and temperature, which might explain why the appearance of D-relaxation in supercooled ibuprofen depends on thermal history of the sample.

Molecular Dynamics and Physical Stability of Ibuprofen in Binary Mixtures with an Acetylated Derivative of Maltose

“…The molecular origin of the D-process observed in pure IBU has already been widely discussed. 47 − 51 Pure IBU has a big tendency to form H-bonded aggregates such as dimers and trimers (linear and cyclic), and the molecular origin of this process has been related to H-bonds dynamics. Correia and Affuard showed that the D-process in pure IBU is dominated by the cis - to trans -transformation of the carboxyl group (O=C—O—H) in IBU coupled to H-bonds dynamics.…”

Mapping Isobaric Aging onto the Equilibrium Phase Diagram

“…It is for example possible to change the volume without a concomitant change of the microscopic structure. Aging experiments can therefore give new information to the puzzle of what governs the relaxation time and could play a role analogous to that played by high pressure measurements [3,4,9], ultra stable glasses [10], and confinement [11,12], which have led to important experimental advances in glass science during the last couple of decades.…”

Slow rheological mode in glycerol and glycerol–water mixtures

“…The hydrogen-bonded structures can also give rise to an additional peak in the static structure factor at low Q-vectors [25,27], although the reverse is not true, a prepeak in the structure is not systematically associated with a slower than structural relaxation feature in the dynamics [28,29]. The family of liquids displaying a dielectric Debye process [30] includes, besides monoalcohols, secondary amides [31], protic ionic liquids [32], and several pharmaceuticals [33,34].…”