High Resolution ¹⁵N NMR of the 225 K Phase Transition of Ammonia Borane (NH₃BH₃):  Mixed Order-Disorder and Displacive Behavior

Abstract

We report high resolution 15N NMR probing of the solid-solid phase transition of 15N-labeled ammonia borane (NH3BH3) at 225 K. Both the 15N isotropic chemical shift (delta iso) and the spin-lattice relaxation rate (T1-1) exhibited strong anomalies around 225 K. The analysis of T1-1 using the Bloembergen, Purcell, and Pound model showed that the motional correlation time, tau, increased from about 1 to 100 ps and the corresponding Arrhenius activation energy increased from 6 to 14.5 kJ/mol on going through the transition toward lower temperatures. The temperature dependence of delta iso was interpreted by an extension of the Bayer model. The time scale of the underlying motion was found to be in a reasonable agreement with the T1-1 data. These results imply that the NH3 rotor motion plays a pivotal role in the transition mechanism and that the transition is of both order-disorder and displacive type.

The tetragonal-to-orthorhombic phase transformation in ammonia borane and in its deuterium substituted compounds

“…Such a transition is typical of second-order phase transformations. Our observations of modulus pre-softening are complementary and coherent with previous NMR results in NH 3 BH 3 [8], and make the above hypothesis of B-N axis dynamics more solid.…”

“…It has been suggested [8] that the transition is triggered by the slowing down of the NH 3 motion, but at present the mechanism driving the phase transformation is not well understood.…”

New Insights into Factors Influencing BN Bonding in X:BH3−nFnand X:BH3−nClnfor X=N2, HCN, LiCN, H2CNH, NF3, NH3andn=0–3: The Importance of Deformation

“…Interest in this bond is reflected in a series of important papers addressing its inelastic neutron scattering [3] and its 225 K phase transition, studied using high-resolution solidstate 15 N NMR spectroscopy. [4] In addition, the structures of methylamine borane molecules (Me n H 3Àn N:BH 3 , n = 1-3) have been investigated using X-ray diffraction, gas-phase electron diffraction, and quantum chemical calculations. [5] Plumley and Evanseck have discussed the covalent and ionic nature of the dative bond in ammonia borane based on high-level QCISD(T) calculations, [6] and Anane et al have carried out a comparison of the B À N with the B À P bond [7] by comparing H 3 N:BX 3 with H 3 P:BX 3 (X = H, F, Cl) at the G2/MP2 level.…”

Raman spectroscopy study of ammonia borane at high pressure

“…22 At ambient conditions ammonia borane crystallizes in a tetragonal space group I4mm with a unit cell containing two molecules. 23,24 NMR measurements 25,26 revealed that the NH 3 and BH 3 groups undergo motion about the B-N bond. This motion freezes out at low temperature and at about T t = 225 K a first order rotational order-disorder phase transition with a change in structure from body-centered tetragonal ͑bct͒ to orthorhombic ͑space group Pmn2 1 ͒ occurs.…”