Infrared Spectroscopic Evidence for Protonated Water Clusters Forming Nanoscale Cages

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Science  21 May 2004:
Vol. 304, Issue 5674, pp. 1134-1137
DOI: 10.1126/science.1096037


Size-dependent development of the hydrogen bond network structure in largesized clusters of protonated water, H+(H2O)n (n = 4 to 27), was probed by infrared spectroscopy of OH stretches. Spectral changes with cluster size demonstrate that the chain structures at small sizes (n ≲ 10) develop into two-dimensional net structures (∼10 < n < 21), and then into nanometer-scaled cages (n ≥ 21).

Because of the fundamental importance of protonated water, H+(H2O)n cluster cations have been studied extensively as a microscopic model of protonated water in the condensed phase (111). These studies, however, have been based mainly on mass spectrometry measurements. Although the thermodynamics of H+(H2O)n has been extensively investigated (3, 4), structural information is still very limited, except for the small clusters, n ≤ 8. Lee, Chang, and their co-workers carried out a detailed structural analysis of protonated water clusters using infrared (IR) spectroscopy and ab initio calculations (57). They determined the most stable structures for the clusters, n ≤ 6, and they also suggested that the hydrogen bond network would develop from a chain-type structure at small cluster sizes (n ≤ 6) to a two-dimensional (2D) net-type structure at n ∼ 7 to 8 (7). Such hydrogen bond network structures in the protonated water clusters are substantially different from those of the neutral water clusters, (H2O)n, which form 3D cage structures even at n = 6 (1113).

Very few experimental studies on structures of protonated water of n > 8 have been carried out. The only exception is the cluster n = 21, which is the well-known “magic number” in the mass distribution of H+(H2O)n. A regular dodecahedral cage structure encaging one water molecule in the cavity has been proposed (1, 2) with support from ab initio calculations (9, 10), but no direct experimental evidence has been obtained.

We report IR spectra of size-selected protonated water clusters, H+(H2O)n, from n = 4 to n = 27. The OH stretching vibrational region of the clusters was probed, and the formation of hydrogen-bonded 3D cage structures was demonstrated for the large-sized protonated water of n ≥ 21.

The IR spectra of the H+(H2O)n cluster ions in the gas phase were measured with the tandem quadrupole mass filter–type spectrometer, described in (14), with some modifications for the present work (15). Cluster cations of a specific size, which were size-selected by the first of two quadrupole mass filters, were introduced into an octopole ion guide. The cluster cations were irradiated by counter-propagating IR light in the ion guide. When the IR light was resonant with a vibrational transition of the cluster cation, vibrational predissociation resulted in fragmentation of the cluster cation. A second quadrupole mass filter was tuned to pass only the (n – 1) or (n – 2) fragment cluster cations. IR spectra were recorded by scanning the IR wavelength while monitoring the intensity of the fragment cations. IR light was generated by difference-frequency generation (DFG) between the fundamental outputs of a Nd:YAG (Nd:yttrium-aluminum-garnet) laser and a dye laser.

The IR spectra of H+(H2O)n (n = 4to27) in the 3-μm region (Fig. 1) show broad features below 3600 cm–1 that we attribute to hydrogen-bonded OH stretching vibrations. We assigned the relatively sharp bands above 3600 cm–1 to free OH stretching vibrations (expanded scale, Fig. 2). The spectral gap in the region of 3460 to 3520 cm–1 is caused by the depletion of the IR laser power by water impurities in the DFG crystal. Additionally, sharp dips in the spectra above 3520 cm –1 (more noticeable in Fig. 2) resulted from IR absorptions due to atmospheric water in the optical path. In the spectra of the clusters larger than n = 21, fluctuation of the parent-cluster ion intensity caused sharp dips throughout the whole spectral region. IR spectra of H+(H2O)n (n = 4 to 8) clusters have been reported by Lee, Chang, and co-workers (57), and the present spectra show essentially the same features except for somewhat broader bandwidths arising from hot bands and structural isomers. Lee et al. have estimated their cluster beam temperature to be about 170 K (7). The broader bandwidths observed in the present spectra suggest a much higher internal temperature (energy) of the clusters, and many structural isomers may contribute to the present spectra (16). With an increase in the cluster size, however, the IR spectra show gradual changes in both the free and hydrogen-bonded OH stretch regions. Such spectral changes reflect a general trend toward a hydrogen bond network structure that strongly depends on cluster size.

Fig. 1.

IR spectra of size-selected protonated water-cluster cations, H+(H2O)n (n = 4 to 27), in the OH stretching vibrational region.

Fig. 2.

Free OH stretching vibrational region of the IR spectra of H+(H2O)n (n = 4 to 27). Four different types of free OH stretching vibrations are seen. Bands at 3640 and 3740 cm–1 (indicated by green lines) are ν1 and ν3 vibrations of a terminal (A-) water molecule of a hydrogen bond chain. Bands at 3695 cm–1 (red line) and 3715 cm–1 (blue line) are attributed to dangling OH stretches of three-coordinated (AAD-) and two-coordinated (AD-) water molecules, respectively (see text). Sharp dips in the spectra are due to the absorption of the atmospheric water in the optical path.

Detailed cluster structures up to n = 6 were determined by Jiang et al.(7). For these cluster sizes, the H3O+ or H5O+2 ion core is located at the center of the cluster, and radial hydrogen bond chains originate from this three- or four-coordinated ion core, as schematically shown in Fig. 3A. The water molecule that terminates the hydrogen bond chain is a single proton acceptor (A-water), and its symmetric (ν1) and antisymmetric (ν3) stretching vibrations of free OH bonds appear at ∼3640 and ∼3740 cm–1, respectively. A hydrogen bond chain is composed of two-coordinated water molecules of single acceptor–single donor (AD-water molecules). The hydrogen-bonded OH stretches in such an AD-water site occur at ∼3400 cm–1, whereas the dangling OH stretch appears at 3715 cm–1 (7, 17, 18).

Fig. 3.

A schematic representation of the development of hydrogen bond network structure with increasing cluster size. (A) Chain structure in n ≲ 10. (B) Two-dimensional net structure (∼10 < n < 21). (C) Three-dimensional cage structure in n ≥ 21.

Above n = 6, an obvious spectral change occurs in the free-OH stretch region (Fig. 2). The ν1 and ν3 bands of the terminal A-water show a gradual high-frequency shift with increasing cluster size, reflecting the increase in distance from the charge center. Moreover, the intensity of these bands decreases with cluster size, and the bands finally disappear at n ∼ 10. At the same time as this decrease, a new band starts to appear at ∼3690 cm–1 that has been assigned to the dangling OH stretching vibration in a three-coordinated water molecule of double acceptor–single donor (AAD-water molecule) (1721). This assignment has been made on the basis of IR spectroscopy and ab initio calculations of smaller-sized protonated water clusters, and it has also been proposed in ice surface studies. Such a three-coordinated water molecule occurs at a bridging site of hydrogen bond chains. The disappearance of A-water bands and simultaneous appearance of AAD-water bands for clusters n = 7 to 10 indicates the development of the hydrogen network from the chain type to a 2D net structure (Fig. 3, A and B). With a donation of one proton, a terminal A-water molecule is bound to an AD-water molecule in a hydrogen bond chain. Then, the terminal A-water molecule is transformed to an AD-water molecule in a chain, and the AD-water molecule results in an AAD-water molecule at the bridging site of the chains. When all of the terminal water molecules are bound, the A-water bands disappear, and the hydrogen bond network is rearranged into a 2D net structure that consists only of AD- and AAD-water molecules. In accordance with this change, a new band appears in the spectral region 3200 cm–1 for the clusters n ≥ 7. This band can be assigned to the hydrogen-bonded OH stretch of the AAD-water molecule, which is expected to appear at ∼3050 cm–1 (2022). However, alternative assignments, such as the overtone of an OH bending vibration or a hydrogen-bonded OH stretch of the H5O+2 ion core (7), are also possible for this band.

In the clusters from n = 10 to n = 19, the IR spectra of the free-OH stretch region show two bands of almost equal intensity at 3695 and 3715 cm–1 due to the dangling OH stretches of AAD- and AD-water molecules, respectively. The hydrogen-bonded OH region shows only a small modulation in the band feature. These spectral features show that the hydrogen bond network preserves a 2D net structure for these cluster sizes.

From n ≳ 19, new spectral features representing the next stage of network growth were observed. A shoulder at ∼3580 cm–1 starts to appear as a new absorption band, which is assigned to hydrogen-bonded OH stretching vibrations of three-coordinated water molecules of single acceptor–double donor (ADD-water molecule) (20, 21). At the same time as the new band appears, the intensity of the dangling OH stretch band of AD-water at 3715 cm–1 gradually decreases with increasing cluster size. Although the intensity of the AD-water OH stretch remains as strong as that of the AAD-water for n ≤ 19, it is substantially weaker than that of the AAD-water OH stretch for n ≥ 21. Such spectral changes indicate a decrease in the number of AD-water sites and a corresponding increase in the number of ADD- and AAD-water sites. That is, an AD-water molecule is transformed to an ADD-water molecule by donation of the remaining dangling OH bond to the other AD-water molecule to form an AAD-water molecule with the acceptance of one more proton. As schematically shown in Fig. 3C, when all of the water molecules are three-coordinated (AAD and ADD), the hydrogen bond network should have a 3D cage structure in which each water molecule is located at the vertex of the polyhedral cages, because of the tetrahedral coordination nature of the water hydrogen bond. On the basis of mass spectrometry and ab initio calculations, the structure of cluster n = 21 has been suggested to be a regular dodecahedron cage that includes one water inside the cage (1, 2, 9, 10). Many nearly isoenergetic structural isomers, including the 2D net types, may contribute to the present IR spectra because of the internal energy of the clusters. However, the dominance of the AAD dangling OH band intensity over the AD dangling OH band intensity in n ≥ 21 clearly indicates that the 2D net types are converted to 3D cage structures in this size region. Such nanometersized cage structures have not previously been experimentally confirmed for hydrogen-bonded cluster systems. The central H3O+ or H2O+5 ion core prefers the planar coordination by nature, and it allows 3D cage formation only in such large-sized clusters.

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