Structure and spectral features of H+(H2O)7: Eigen versus Zundel forms

Abstract

The two dimensional (2D) to three dimensional (3D) transition for the protonated water cluster has been controversial, in particular, for H+(H2O)7. For H+(H2O)7 the 3D structure is predicted to be lower in energy than the 2D structure at most levels of theory without zero-point energy (ZPE) correction. On the other hand, with ZPE correction it is predicted to be either 2D or 3D depending on the calculational levels. Although the ZPE correction favors the 3D structure at the level of coupled cluster theory with singles, doubles, and perturbative triples excitations [CCSD(T)] using the aug-cc-pVDZ basis set, the result based on the anharmonic zero-point vibrational energy correction favors the 2D structure. Therefore, the authors investigated the energies based on the complete basis set limit scheme (which we devised in an unbiased way) at the resolution of the identity approximation Møller-Plesset second order perturbation theory and CCSD(T) levels, and found that the 2D structure has the lowest energy for H+(H2O)7 [though nearly isoenergetic to the 3D structure for D+(D2O)7]. This structure has the Zundel-type configuration, but it shows the quantum probabilistic distribution including some of the Eigen-type configuration. The vibrational spectra of MP2/aug-cc-pVDZ calculations and Car-Parrinello molecular dynamics simulations, taking into account the thermal and dynamic effects, show that the 2D Zundel-type form is in good agreement with experiments.

Infrared Spectroscopy and Ab Initio Theory of Isolated H5O2+: From Buckets of Water to the Schrödinger Equation and Back

“…However, only the theoretically predicted Zundel-like structures can be brought into agreement with the experimentally determined IR spectra. [19,21] The experimental and theoretical investigation of so-called "magic" cluster sizes (as for example, n = 21) have been covered in the literature [22,23] and will not be discussed here.…”

Structure, stability, thermodynamic properties and IR spectra of the protonated water cluster H+(H2O)9

“…It is now known that even at 0 K the 2D structures are more stable than the 3D structures for n ¼ 7 and 8 [10]. However, the controversial issue over the 2D vs. 3D structure for n ¼ 9 needs to be resolved, because both 2D net-like structure [3] and 3D extended cubic structure [7] have been proposed.…”

“…Despite a number of experimental data for various sizes of protonated water clusters, the theoretical information for the mid-sized clusters which still cannot be treated as liquid water is scarce. On the other hand, the theoretical information for small protonated water clusters H þ (H 2 O) n (n 8) has been gained from a number of studies [5][6][7][8][9][10]. This is because as n increases, the number of possible configurations increases exponentially, while the structures still have the characteristics of clusters which cannot be treated statistically simply like liquid.…”

“…This has raised the question of which form would be more stable for a given n. A number of theoretical studies on the structures of protonated water clusters with increasing cluster size have been made [6][7][8][9][10], because these structures are quite different from neutral water clusters [13], anionic water clusters [14], and water clusters containing a cation [15]. In the case of neutral nonamer water clusters, the 3D extended cubical structure [16] is the most stable at 0 K. Therefore, one might expect that the protonated water nonamer would have a similar structure.…”

A comprehensive search of topologically distinct local minimum structures of protonated water octamer and the classification of O H topological types

“…There can be many isomers and many local minima even for medium-sized PW clusters. Since structural properties of PW clusters are relevant to the understanding of thermodynamic properties and proton transfer mechanisms in aqueous solutions and biological systems, PW clusters have been extensively studied experimentally [1][2][3][4][5][6][7][8][9][10] as well as theoretically [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. Since the number of possible structural isomers increases rapidly with the cluster size, it is difficult to present all of them even for medium-sized PW clusters.…”