Terahertz laser vibration–rotation–tunneling spectrum of the water pentamer–d10.: Constraints on the bifurcation tunneling dynamics

The vibration–rotation–tunneling (VRT) spectrum of a low-frequency intermolecular vibration of (D2O)5 was recorded near 0.9 THz (30.2 cm−1). From an analysis of the relative intensities in the compact Q-branch region, the ground-state C-rotational constant is estimated to be 975±60 MHz, consistent with ab initio structural predictions. The precisely determined B-rotational constant (B=1750.96±0.20 MHz) agrees well with previous results. Efforts to resolve possible bifurcation tunneling fine structure, such as that observed in VRT spectra of (D2O)3, revealed no such effects. This constrains the splittings to be less than 450 kHz, or roughly 3 times smaller than required by previous results.