Nuclear quantum effects on intramolecular hydrogen bonds and backbone structures in biuret analogues

Biuret analogues, dithiobiuret and thiobiuret, form six-membered ring structures <i>via</i> intramolecular hydrogen bonds. The proton donor and acceptor atoms differ between these molecules, leading to varying energy barriers for proton transfer. We performed path integral molecular dynamics (PIMD) simulations for dithiobiuret and thiobiuret to investigate the correlation between proton transfer and changes in the bond alternation pattern in the backbone structure. In addition, we performed PIMD simulations for deuterated species. The results indicate that the fluctuations of the backbone originate not from secondary nuclear quantum effects (NQEs) of the transferring hydrogen nucleus, but from the NQEs of the heavy nuclei in the backbone structure.