Ammonia Borane Acid–Base Complex: An Experiment Involving 1H and 11B NMR Spectroscopy and the Gas Laws

• Autores: Navamoney Arulsamy
• Localización: Journal of chemical education, ISSN 0021-9584, Vol. 95, Nº 8, 2018, págs. 1381-1385
• Idioma: inglés
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• Resumen

  • An experiment involving a moisture-sensitive synthesis and spectroscopic characterization suitable for the advanced inorganic laboratory course is presented. The acid–base complex ammonia borane (AB) is synthesized from the heterogeneous reaction between sodium borohydride and finely ground ammonium sulfate in tetrahydrofuran solvent. AB is isolated as a fluffy white solid. The 1H and 11B NMR spectra of AB, separately measured in the solvents D2O and CD3CN, provide an excellent opportunity to introduce some of the basic concepts of multinuclear NMR spectroscopy. The triplet and quartet peaks observed for the NH3 and BH3 groups in the 1H spectrum measured in CD3CN can be used to discuss the nature of JNH and JBH coupling. The former coupling is rarely observed because the quadrupolar 14N in nonspherical structures experiences rapid relaxation and large J coupling, leading to broad peaks for −NH and −NH2 protons. Although such coupling due to 11B is more commonly observed, a comparison of the 1H NMR spectra of AB and NaBH4 further illustrates the effect of spherical symmetry on the NMR behavior of quadrupolar nuclei. AB is also valued in the field of energy research as a potential chemical H2 storage material and has assumed new significance as a potential portable energy source. The efficacy of H2 generation from AB is examined, and the purity of student-synthesized samples is determined by applying the gas laws. The experiment can be performed in two 3-hour lab periods.