Acidic microhydrated Portlandite Ca2+(OH−)(H₂O)₂: Temperature driven cyclic to acyclic hydrogen-bonding networks

Here, we have investigated the problem of acidic microhydration of portlandite, focusing on the gas-phase clusters of Ca²⁺(OH⁻)(H₂O)₂, employing ab initio global minimum structure-search method. The competent structures ensued were each established as a local minimum on the free energy hypersurface. We found that while the well-known trigonal Ca²⁺(OH⁻)(H₂O)₂ structure corresponding to a global minimum that is predominantly stabilized by CaO ionic interactions, the contiguous potential isomers constitutes cyclic and acyclic hydrogen-bonded networks with distinct vibrational spectra. In that, the two isomers, a four membered cyclic and an acyclic cis were previously unknown. Interestingly, they are the second-most stable isomers, with one being the most stable at low temperatures and the other at high temperatures, above ∼250 K. The three-dimensional electron density maps examined and the possible interconversion paths by computing reaction-energy profiles through finding transition states suggests that the acidic microhydrated portlandite would have a core that resembles a protonated Ca(OH)₂, which is the [Ca(OH)(H₂O)]⁺ complex, offering novel examples of comprehending proton-transfer mechanisms and cluster stabilities in the complex ionic and hydrogen bonded environments.