Vibrational spectroscopic and computational studies on diisopropylammonium bromide

Area of Science:

• Solid-state chemistry
• Crystallography
• Spectroscopy

Background:

• Diisopropylammonium bromide (DIPAB) exists in orthorhombic and monoclinic crystal structures at room temperature.
• Phase transitions occur upon heating: orthorhombic to monoclinic-II (ferroelectric) above 421K, and monoclinic-II to monoclinic-I (paraelectric) above 426K.

Purpose of the Study:

• To synthesize DIPAB in the orthorhombic structure and study its phase transitions.
• To investigate the vibrational properties (Raman and IR spectra) of DIPAB in its orthorhombic and monoclinic-II phases.
• To analyze phonon irreducible representations and compare experimental spectra with theoretical calculations.

Main Methods:

• Synthesis of DIPAB in the orthorhombic crystal structure.
• Temperature-dependent Raman and Infrared (IR) spectroscopy.
• Factor group analysis for phonon irreducible representations.
• First-principles calculations using Gaussian 09 and CASTEP.

Main Results:

• Identical intramolecular vibrational spectra (200-3500 cm⁻¹) observed for both orthorhombic and monoclinic-II phases, suggesting weak intermolecular interactions.
• Distinct low-wavenumber Raman spectra (10-150 cm⁻¹) between the phases, sensitive to molecular environment and crystal structure.
• First-principles calculations supported mode assignments, with Gaussian 09 agreeing on intramolecular modes and CASTEP on external modes.

Conclusions:

• The vibrational spectra confirm weak intermolecular forces in DIPAB crystals.
• Phase transitions significantly alter the low-frequency lattice dynamics, reflecting changes in molecular arrangement.
• Computational methods are valuable for understanding vibrational modes in molecular crystals.