Intermolecular interactions in solid benzene

Area of Science:

• Solid State Physics
• Materials Science
• Computational Chemistry

Background:

• Understanding molecular vibrations and lattice dynamics is crucial for predicting material properties.
• Benzene and its deuterated forms serve as model systems for studying intermolecular interactions and crystal dynamics.

Purpose of the Study:

• To calculate lattice dynamics and molecular vibrations of benzene and deuterated benzene crystals.
• To compare theoretical predictions with experimental inelastic neutron-scattering data.
• To investigate the coupling between molecular modes and the sensitivity to crystal cell size.

Main Methods:

• Utilizing force constants derived from density-functional theory (DFT) calculations.
• Performing inelastic neutron-scattering (INS) experiments for spectral comparison.
• Analyzing the influence of lattice parameter variations on vibrational frequencies.

Main Results:

• A minor adjustment (0.5%) in lattice parameters yielded real lattice-mode frequencies across the Brillouin zone.
• Strong coupling observed between wagging and breathing modes away from the zone center.
• Identified two primary interactions governing benzene crystal dynamics: strong in-layer interactions and weaker inter-layer interactions.
• Demonstrated that alterations in lattice parameters affect inter-layer interactions, consequently modifying lattice dynamics.
• Observed a small auxetic effect where elongation along the b-axis induced expansion in the ac-plane.

Conclusions:

• DFT calculations accurately predict benzene crystal dynamics when accounting for minor lattice parameter adjustments.
• Intermolecular interactions, particularly H-atom to ring interactions, are key drivers of benzene's lattice dynamics.
• Benzene crystals exhibit auxetic behavior, showing coupled responses to dimensional changes.