Abstract
We employ temperature- and pressure-dependent dielectric spectroscopy, as well as differential scanning calorimetry, to characterize benzophenone and the singly-substituted ortho-bromobenzophenone derivative in the liquid and glass states, and analyze the results in terms of the molecular conformations reported for these molecules. Despite the significantly higher mass of the brominated derivative, its dynamic and calorimetric glass transition temperatures are only ten degrees higher than those of benzophenone. The kinetic fragility index of the halogenated molecule is lower than that of the parent compound, and is found to decrease with increasing pressure. By a detailed analysis of the dielectric loss spectra, we provide evidence for the existence of a Johari–Goldstein (JG) relaxation in both compounds, thus settling the controversy concerning the possible lack of a JG process in benzophenone and confirming the universality of this dielectric loss feature in molecular glass-formers. Both compounds also display an intramolecular relaxation, whose characteristic timescale appears to be correlated with that of the cooperative structural relaxation associated with the glass transition. The limited molecular flexibility of ortho-bromobenzophenone allows identifying the intramolecular relaxation as the inter-enantiomeric conversion between two isoenergetic conformers of opposite chirality, which only differ in the sign of the angle between the brominated aryl ring and the coplanar phenyl-ketone subunit. The observation by dielectric spectroscopy of a similar relaxation also in liquid benzophenone indicates that the inter-enantiomer conversion between the two isoenergetic helicoidal ground-state conformers of opposite chirality occurs via a transition state characterized by a coplanar phenyl-ketone moiety.
Highlights
We employ temperature- and pressure-dependent dielectric spectroscopy, as well as differential scanning calorimetry, to characterize benzophenone and the singly-substituted orthobromobenzophenone derivative in the liquid and glass states, and analyze the results in terms of the molecular conformations reported for these molecules
While the study of the molecular flexibility of large molecules is necessarily carried out, due to their complexity, on a case-by-case basis, it is important to identify and predict which types of constrained dynamics may take place in smaller subunits that are commonly found in larger organic molecules[5,6]
The second difficulty is that real glassy systems exhibit elementary excitations on very different time scales, which range from the mHz frequency scale of the α relaxation near Tg, through the radio and microwave region where so-called secondary relaxations are observed, up to the THz domain where an excess vibrational density of states (Boson peak) shows u p13–15
Summary
We employ temperature- and pressure-dependent dielectric spectroscopy, as well as differential scanning calorimetry, to characterize benzophenone and the singly-substituted orthobromobenzophenone derivative in the liquid and glass states, and analyze the results in terms of the molecular conformations reported for these molecules. In order to investigate the origin of the secondary dielectric response of BPh and further analyze the effect of constrained internal rotation in molecules with germinal aromatic rings, in the present contribution we analyze the temperature- and pressure-dependent relaxation dynamics of a singly-substituted halogenated derivative of BPh, namely ortho-bromobenzophenone (hereafter, BrBPh), and compare the results with more detailed dielectric spectra of BPh that we have acquired near the glass transition of this compound.
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