  The donor-acceptor exchange (DAE) is a significant hydrogen bond network
rearrangement (HBNR) mechanism because it can lead to the change of hydrogen
bond direction. In this work, we report a new DAE mechanism found in water
trimer that is realized by sequential flipping (SF) of all molecules rather
than the well-known proton transfer (PT) process. Meanwhile, the SF process has
a much smaller potential barrier (0.262 eV) than the previously predicted
collective rotation process (about 1.7 eV), implying that SF process is a main
flipping process that can lead to DAE. Importantly, high-precision ab initio
calculations show that SF-DAE can make the water ring to show a clear chiral
difference from PT-DAE, which brings the prospect of distinguishing the two
confusing processes based on circular dichroism spectra. The reaction rate
analysis including the quantum tunneling indicates an obvious
temperature-dependent competitive relationship between SF and PT processes,
specifically, the SF process dominates above 65 K, while the PT process
dominates below 65 K. Therefore, in most cases, the contribution for DAE mainly
comes from the flipping process, rather than the PT process as previously
thought. Our work enriches the understanding of the DAE mechanism in water
trimer and provides a piece of the jigsaw that has been sought to the HBNR
mechanism.
