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

Abstract

It is not clearly known whether H þ (H 2 O) 9 has a 2-dimensional (2D) net-like structure or 3-dimensional (3D) structure. Thus, calculations have been carried out based on density functional theory [DFT], Moller-Plesset second-order perturbation theory (MP2), resolution of identity MP2 theory (RIMP2), and coupled cluster theory with single, double and perturbative triple excitations (CCSD(T)). For the complete basis set (CBS) limit at the CCSD(T) level of theory, two nearly isoenergetic 3D closed structures are the most stable in zero-point energy (ZPE) corrected binding energy at 0 K. However, at 150 K, there are three competing 2D net-like structures. The structure with two acceptor (A) type dangling water molecules is the most stable. This structure is in good agreement with the experimental symmetric and asymmetric OH stretching peaks for the free acceptor (A) type of dangling water molecule(s) at 3650 and 3742 cm À1 . On the other hand, another structure shows a better spectral intensity pattern. Thus, these three 2D net-like structures together would be present at $150 K.

Structure and thermodynamics of H3O+(H2O)8 clusters: A combined molecular dynamics and quantum mechanics approach

“…Karthikeyan and Kim [48], whose computational investigation is the most similar to ours, employed Halkier's [100,101] two-point inverse cubic extrapolation scheme using aVDZ-aVTZ and aVTZ-aVQZ basis sets to obtain RI-MP2/CBS energies. Since the convergence of the binding energy with increasing basis sets is not monotonic, a two-point extrapolation with an inverse cubic function can lead to erroneous CBS limits.…”

“…Unlike Karthikeyan and Kim [48], we did not include higher-order electron correlation corrections with a small basis set,…”

“…Many groups have used quantum mechanics (QM) calculations to optimize structures and energies of H 3 O + (H 2 O) n with n = 1-21 [4,. Most of these calculations have been on species of particular interest, such as H 3 20 (a magic number of stable water molecules), with fewer on medium sized clusters ranging in size from H 3 O + (H 2 O) 4 to H 3 O + (H 2 O) 10 [4,35,37,39,41,43,48,49,51,54,56].…”

“…more than $5 molecules), sampling configurational space becomes increasingly difficult. To address this, several research groups have used Monte Carlo, molecular dynamics (MD), and basin hopping techniques to sample and generate different configurations [46][47][48]54,[61][62][63][64][65][66][67]. Karthikeyan and Kim [48] performed the highest level calculations on H 3 O + (H 2 O) 8 ; they employed the Resolution-ofthe-identity (RI) second-order Møller-Plesset perturbation theory (MP2) [68] method, extrapolated to the complete basis set limit, and corrected for higher-order correlation using CCSD(T).…”

Disentangling the Contribution of Multiple Isomers to the Infrared Spectrum of the Protonated Water Heptamer

“…21 In the region of the hydrogen-bonded OH stretches between 3300 and 3700 cm −1 , the complexity of the IR spectra increases with n. For n = 7, the observed features are due to the symmetric and antisymmetric stretches of AD-H 2 O's in the first and second solvation shell. 11 The broad absorption in between 3100 and 3300 cm −1 for n = 6−8 can be attributed to the four terminal hydrogen-bonded O−H stretches of a Zundel core.…”