The Influence of Molecular Architecture on the Dynamics of H-Bonded Supramolecular Structures in Phenyl-Propanols

TL;DR

This study investigates how the position of the phenyl ring in phenyl-propanols affects hydrogen-bonded supramolecular structures and their dynamics, revealing that steric hindrance influences the balance between ring and chain formations.

Contribution

It provides a systematic analysis combining BDS, PCS, and calorimetry to disentangle Debye and alpha relaxations in phenyl-propanols, highlighting the effect of phenyl ring position on hydrogen-bonded structures.

Findings

Debye relaxation weakens as phenyl ring moves closer to the hydroxy group.

Hydrogen bonds are not suppressed but shifted towards ring-like structures.

Alpha relaxation shape remains consistent regardless of hydrogen bonding degree.

Abstract

The relaxation behaviour of monohydroxy alcohols (monoalcohols) in broadband dielectric spectroscopy (BDS) is usually dominated by the Debye process. This process is regarded as a signature of the dynamics of transient supramolecular structures formed by H-bonding. In phenyl propanols the steric hindrance of the phenyl ring is assumed to influence chain formation and thereby to decrease or even suppress the intensity of the Debye process. In the present paper we study this effect in a systematic series of structural isomers of phenyl-1-propanol in comparison with 1-propanol. It turns out that by combining BDS, Photon Correlation Spectroscopy (PCS) and calorimetry the dynamics of supramolecular structures can be uncovered. While light scattering spectra show the same spectral shape of the main relaxation for all investigated monoalcohols, the dielectric spectra differ in the Debye contribution. Thus it becomes possible for the first time to unambiguously disentangle both relaxation modes in the dielectric spectra. It turns out that the Debye relaxation gets weaker the closer the position of phenyl ring is to the hydroxy group, in accordance with the analysis of the Kirkwood-Fr\"ohlich correlation factor. Even in 1-phenyl-1-propanol, which has the phenyl group attached at the closest position to the hydroxy group, we can separate a Debye-contribution in the dielectric spectrum. From this we conclude that hydrogen bonds are not generally suppressed by the increased steric hindrance of the phenyl ring, but rather an equilibrium of ring and chain-like structures is shifted towards ring-like shapes on shifting the phenyl ring closer to the hydroxy group. Moreover, the shape of the alpha-relaxation as monitored by PCS and BDS remains unaffected by the degree of hydrogen bonding and is the same among the investigated alcohols.