Pressure mediated phase transition and dihydrogen bonding formation in trimethylamine borane

Abstract

Trimethylamine borane (TMAB, (CH₃)₃N·BH₃) is widely used as a reliable precursor in various chemical syntheses and has a theoretical hydrogen content of 16.6 wt%, making it a potential candidate for hydrogen storage. Exploring materials under high pressure provides key insights into their structural stability and phase transformations, which are crucial for their design, synthesis, and practical application. In this work, we report the first high-pressure study on TMAB, investigating its structural stability, pressure-induced phase transition, bonding, and electronic properties up to 35 GPa using a comprehensive approach combining vibrational spectroscopy, X-ray diffraction (XRD), evolutionary crystal structure prediction, and density functional theory (DFT). The results indicate that TMAB exhibits an unprecedented high-pressure stability up to nearly 9 GPa, above which it undergoes a phase transition from the R3m phase to a P3₁ phase. The absence of N–H⋯H–B dihydrogen bonds significantly contributes to the robust stability of TMAB, making it suitable for further high-pressure applications and as a stable precursor under extreme conditions. Detailed analysis of TMAB's pressure-dependent crystal lattice parameters, unit-cell volumes and bulk moduli highlights its compressibility, while changes in its electronic band gap suggest improved conductive properties at higher pressures. Our findings provide new insights into the structural, bonding, and electronic properties of TMAB that are essential for its advanced applications.