Specific interactions and mechanism of association in highly viscous aldehyde 3-phenylpropanal

In this paper, we investigated the thermal, dynamical, and structural properties, as well as association patterns, in 3-phenyl-1-propanol (3P1Pol) and 3-phenyl-1-propanal (3P1Pal), with special attention paid to the latter compound. Both systems turned out to be good glass formers, differing by 17 K in the glass transition temperature, which indicated a strong change in the self-assembly pattern. This supposition was further confirmed by the analysis of dielectric spectra, where, apart from the α-relaxation, also a unique Debye (D)-mode, being a fingerprint of the self-association, characterized by different dynamical properties (dielectric strength, timescale separation from the α-process), was detected in both samples. Further diffraction and infrared investigations, supported by density functional theory computations and molecular dynamics (MD) simulations, revealed that hydrogen (H)-bonds play a key role in the association in 3P1Pol, while in 3P1Pal, there are at least five different intermolecular interactions involving dipole-dipole forces and π-stacking, as well as extraordinary H-bonds formed between methylene groups neighboring with the formyl unit and carbonyl moiety. Finally, optimized dimeric structures, along with MD simulations, allowed us to propose a molecular mechanism responsible for the D-process in 3P1Pal. The presented data systematize previous understanding of molecular association as well as open new ways of exploring this phenomenon in aldehydes.