Spontaneous proton transfers induced by beryllium bonds
Through the use of B3LYP/6-311+G(d,p) density functional theory (DFT) calculations, we have shown that when a molecule participates as a proton donor in a complex, it yields much stronger hydrogen bonds (HBs) if it participates simultaneously in beryllium bonds. This is indeed the case of the complexes formed by oxyacids such as acetic, benzoic and phosphinic acids with BeCl2, which yield much stronger HBs with different bases than the isolated oxyacids due to a significant acidity enhancement, triggered by the charge transfer from the oxyacid to BeCl2. More importantly, depending on the intrinsic basicity of the base acting as proton acceptor, a spontaneous proton transfer from the oxyacid to the base may occur, leading to the formation of an ion pair in the gas phase. This is indeed the case in complexes involving ammonia. For slightly weaker bases, such as trimethylphosphine, two local minima are stable: one in which the proton remains attached to the oxyacid and one in which this proton has been transferred to the trimethylphosphine, the latter being always the most stable. When a compound is able to act simultaneously as a proton donor and as a proton acceptor, its participation in a beryllium bond necessarily leads to an enhancement and a dampening of both properties, respectively. Hence, the HB in which they participate as proton donors becomes stronger, whereas the HB in which they act as proton acceptors becomes weaker. The dimers of 1H-tetrazole and the dimers of isatin nicely illustrate this finding.