Water Pentamer:  Characterization of the Torsional-Puckering Manifold by Terahertz VRT Spectroscopy

Abstract

We present the measurement and analysis of five new (D 2 O) 5 bands via vibration-rotation-tunneling (VRT) spectroscopy as well as a preliminary description of a second (H 2 O) 5 band. The vibrationally averaged rotational constants of all five fitted bands agree well with those from the two previously observed (D 2 O) 5 bands and confirm that the pentamer averages to a symmetric, quasi-planar structure on the time scale of our experiment. While the spectrum of the first two bands, located at 50.7 cm -1 (1.52 THz) and 27.3 cm -1 (0.82 THz) are indicative of unperturbed oblate rotors, the three remaining (D 2 O) 5 bands centered at 47.7 cm -1 (1.43 THz), 45.4 cm -1 (1.36 THz), and 45.0 cm -1 (1.35 THz) are severely perturbed by first-order Coriolis coupling. This represents the first observation of this perturbation in the perdeuterated water pentamer, as well as the first observation of transitions between degenerate states of the torsional-puckering manifold. Unlike transitions from the (H 2 O) 5 band observed by Brown et al. at 89.0 cm -1 and the 103.8 cm -1 band that we report here, none of the individual rovibrational transitions of any of the five (D 2 O) 5 bands demonstrate spectral splittings due to bifurcation tunneling. We conclude, through careful analysis of these water pentamer bands, that at least three torsional-puckering manifolds have been probed and that the lowest-energy manifold is highly compacted. A plausible water pentamer torsional-puckering correlation diagram is proposed, though additional experimental data are required to unambiguously establish the energies of the torsional-puckering levels.

THz and Submillimeter‐wave Spectroscopy of Molecular Complexes

“…They fit well to an oblate symmetric top energy expression, with small Coriolis interaction constants for twofold degenerated states. This is consistent with the fact that the vibrational average over the torsional-puckering motion results in an effective structure of C 5h symmetry (Harker et al 2005) …”

“…] Figure 28 Torsional-puckering dynamics of the water pentamer. [Reproduced from Harker et al (2005).  American Chemical Society, 2005.…”

“…Extensive studies of the water pentamer have been reported by TuFIR spectroscopy for both the normal species (H 2 O) 5 and the fully deuterated species (D 2 O) 5 (for example, Harker et al 2005). For the band observed at 89.0 cm −1 of (H 2 O) 5 , each rovibrational transition splits into four, separated by 4.8 MHz due to the bifurcation tunneling as in the case of the water trimer.…”

Terahertz vibration-rotation-tunneling spectroscopy of the water tetramer-d8: Combined analysis of vibrational bands at 4.1 and 2.0THz

“…For ͑D 2 O͒ 4 , two bands have been measured, including one parallel band at 2.03 THz ͑Refs. 35 This motion generates the manifolds of torsional levels in the terahertz frequency region for the water trimer and pentamer that have been characterized by VRT spectroscopy. 26 The equilibrium structure of the water tetramer is calculated to have S 4 symmetry.…”

Water vapor: An extraordinary terahertz wave source under optical excitation

“…However, isotope effects do occur in the THz absorption spectra of water vapor due to monomer rotational-vibrational excitations and in discharge plasmas have been attributed to the dissociative recombination of hydronium-ion water clusters [16]. Water clusters, especially hydronium-ion protonated [13] and deuterated [16] clusters, (H 2 O) n H + and (D 2 O) n D + , known to occur naturally in water vapor [9][10][11][12][13], might contribute to isotope-dependent THz emission because of their remarkable temperature stabilities [11] and unique THz vibrational modes (Fig 4c,d). Among these, the pentagonal dodecahedral (H 2 O) 21 H + cluster is one of the most stable "magic-number" cluster ions observed spectroscopically in water vapor [17,18].…”