Utilizing the Combined Power of Theory and Experiment to Understand Molecular Structure – Solid-State and Gas-Phase Investigation of Morpholine Borane

Abstract

The molecular structure of morpholine borane complex has been studied in the solid state and gas phase using single-crystal X-ray diffraction, gas electron diffraction, and computational methods. Despite both the solid-state and gas-phase structures adopting the same conformation, a definite decrease in the B–N bond length of the solid-state structure was observed. Other structural variations in the different phases are presented and discussed. To explore the hydrogen storage potential of morpholine borane, the potential energy surface for the uncatalyzed and BH3-catalyzed pathways, as well as the thermochemistry for the hydrogen release reaction, were investigated using accurate quantum chemical methods. It was observed that both the catalyzed and uncatalyzed dehydrogenation pathways are favourable, with a barrier lower than the B–N bond dissociation energy, thus indicating a strong propensity for the complex to release a hydrogen molecule rather than dissociate along the B–N bond axis. A minimal energy requirement for the dehydrogenation reaction has been shown. The reaction is close to thermoneutral as demonstrated by the calculated dehydrogenation reaction energies, thus implying that this complex could demonstrate potential for future on-board hydrogen generation.