Protonated clathrate cages enclosing neutral water molecules: H+(H2O)21 and H+(H2O)28

Abstract

This paper describes a systematic study on the clathrate structure of (H+)(H2O)21 using tandem mass spectrometry, vibrational predissociation spectroscopy, Monte Carlo simulations, and density functional theory calculations. We produced (H+)(H2O)n from a continuous corona-discharged supersonic expansion and observed three anomalies simultaneously at the cluster temperature near 150 K, including (1) the peak at n=21 is more intense than its neighboring ions in the mass spectrum, (2) the size-dependent dissociation fractions show a distinct drop for the 21-mer, and (3) the infrared spectrum of (H+)(H2O)21 exhibits only a single feature at 3699 cm(-1), corresponding to the free-OH stretching of three-coordinated water molecules. Interestingly, the anomalies appear or disappear together with cluster temperature, indicating close correlation of these three observations. The observations, together with Monte Carlo simulations and density functional theory calculations, corroborate the notion for the formation of a distorted pentagonal dodecahedral (5(12)) cage with a H2O molecule in the cage and a H3O+ ion on the surface for this "magic number" water cluster ion. The dodecahedral cage melts at higher temperatures, as evidenced by the emergence of a free-OH stretching feature at 3717 cm(-1) for the two-coordinated water in (H+)(H2O)21 produced in a warmer molecular beam. Extension of this study to larger clusters strongly suggests that the experimentally observed isomer of (H+)(H2O)28 is most likely to consist of a distorted protonated pentakaidecahedral (5(12)6(3)) cage enclosing two neutral water molecules.

Infrared Spectra and Hydrogen‐Bonded Network Structures of Large Protonated Water Clusters H+(H2O)n (n=20–200)

“…[2] One possible cause of this difference is the lower temperature of neutral (H 2 O) hni , cooled in a cell at 4.2-140 K. [2] H + (H 2 O) n are estimated to have higher temperatures (ca. 150-200 K), [9,17,18] and this temperature range is much closer to those in condensed-phase experiments.…”

“…21 clusters. [5][6][7][9][10][11] By comparison with the previous IR study on H + (H 2 O) 21 formed under various conditions and theoretical studies, we estimate that clusters observed here have finite temperatures of 150-200 K. [9,17,18] The behavior of the free OH band shows that closed cage (neither chain nor net) networks are maintained at least up to n = 200 and that clusters with n ! 21 consist mainly of 3-or 4-coord water molecules.…”

“…[ [5][6][7]9] This network development of H + -(H 2 O) n has been discussed by analyzing free (non-H-bonded) OH stretching bands corresponding to the 1-3-coord water molecules. In larger clusters, 4-coord water is expected to be dominant; however, no direct observations have been reported.…”

“…[5][6][7]9] They are models for the hydrated proton and provide structural information about water networks around an excess proton. Studies on H + (H 2 O) n (n 9 30) have shown that clusters larger than n % 10 form closed net (multiple ring) structures, and they develop into closed cage structures with a dramatic decrease in 2-coord water molecules at n = 21.…”

Comprehensive genetic algorithm forab initioglobal optimisation of clusters

“…[94] Several successive studies revealed that the lowest energy structure of H + (H 2 O) 21 is a distorted pentagonal dodecahedral (5 12 ) cage with a H 2 O molecule in the centre and a H 3 O + ion on the surface, which has nine dangling OH bonds on the surface. [95][96][97][98] Figure 2(f) shows the evolution for structure prediction of H + (H 2 O) 21 cluster using CGA. With a population of 16 candidates, CGA search takes about 1200 iterations to locate this global minimum configuration without any symmetry setting.…”