Lewis acids and bases usually form strong adducts, but in the case of the BH3-H2 adduct this behavior is thwarted by zero-point energy (ZPE). The BH3-H2 coordinated complex (BH5 for short), which is isoelectronic with CH5+, is a highly fluxional molecule that due to its large ZPE can sample multiple equivalent minima by large amplitude internal rotation and hydrogen scrambling motions. In 1989 highly correlated ab initio calculations with extended basis sets by Stanton et al. predicted for the first time that BH5 should be stable at low temperatures. Only five years later Tague and Andrews used argon matrix isolation infrared spectroscopy at temperatures between 10-25 K to spectroscopically characterize BH5. We have recently investigated the infrared spectroscopy of BH5 trapped in a solid parahydrogen (pH2) matrix to revisit the spectroscopy of BH5 in another matrix environment. Solvation effects are likely very important for this type of highly fluxional, large ZPE molecule and therefore it is important to measure the infrared spectrum of BH5 in solid pH2 to compare with the previous Ar matrix studies. We are even more excited to investigate the dynamics of this species trapped in a pH2 matrix. The potential exists that BH5 will tunnel through the matrix via repeated reactions of the type, BH5 + H2 → H2 + BH5. We have started these studies and will report the most recent findings at the meeting. We hope these new experimental results stimulate further theoretical work on BH5 that will be necessary to fully interpret these new pH2 matrix isolation studies.

Creating BH5 in a solid parahydrogen matrix: Challenging infrared spectroscopy and reaction dynamics

ACS Spring 2022

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