Comparative ab initio study of the structures, energetics and spectra of X−⋅(H2O)n=1–4 [X=F, Cl, Br, I] clusters

Abstract

X − ⋅(H 2 O) n=1–4 [X=F, Cl, Br, I] have been studied using high level ab initio calculations. This extensive work compares the structures of the different halide water clusters and has found that the predicted minimum energy geometries for different cluster are accompanied by several other structures close to these global minima. Hence the most highly populated structures can change depending on temperature due to the entropy effect. As the potential surfaces are flat, the wide-ranging zero point vibrational effects are important at 0 K, and not only a number of low-lying energy conformers but also large amplitude motions can be important in determining structures, energies, and spectra at finite temperatures. The binding energies, ionization potentials, charge-transfer-to-solvent (CTTS) energies, and the O–H stretching frequencies are reported, and compared with the experimental data available. A distinctive difference between F−⋅(H2O)n and X−⋅(H2O)n (X=Cl, Br, I) is noted, as the former tends to favor internal structures with negligible hydrogen bonding between water molecules, while the latter favors surface structures with significant hydrogen bonding between water molecules. These characteristics are well featured in their O–H spectra of the clusters. However, the spectra are forced to be very sensitive to the temperature, which explains some differences between different spectra. In case of F−⋅(H2O)n, a significant charge transfer is noted in the S0 ground state, which results in much less significant charge transfer in the S1 excited state compared with other hydrated halide clusters which show near full charge transfers in the S1 excited states. Finally, the nature of the stabilization interactions operative in these clusters has been explained in terms of many-body interaction energies.

Nature of the Aqueous Hydroxide Ion Probed by X-ray Absorption Spectroscopy

“…Similar results have been found when the Löwdin charge populations, as determined from ab initio calculations, were instead considered. 46,47 (Note that when the ECP is used to describe the iodide anion, q(X -) differs significantly from that calculated for chloride or bromide, whereas, when the allelectron basis set is used, q(X -) on Cl -, Br -, and I -are very similar, consistent with ab initio calculations. 46 Therefore, for the calculations of the ground state q's we will focus primarily on the results from the all-electron calculations.)…”

“…46,47 (Note that when the ECP is used to describe the iodide anion, q(X -) differs significantly from that calculated for chloride or bromide, whereas, when the allelectron basis set is used, q(X -) on Cl -, Br -, and I -are very similar, consistent with ab initio calculations. 46 Therefore, for the calculations of the ground state q's we will focus primarily on the results from the all-electron calculations.) For these ground-state ion-water dimers, a notable asymmetry between the q(H)'s of the two water-molecule hydrogens (q(∆H)) is found, wherein the hydrogen located between the oxygen and the halide anion is more positive than the free hydrogen, consistent with Löwdin charge populations from previous ab initio calculations.…”

“…Taken together with the finding that there is a significant difference in q(∆H) between OHand the larger halide ions, this suggests that hydrogen charge asymmetry on the ground-state solvation-shell water molecules may be particularly important in determining the observed changes in the XA spectrum. Although differences in q(X -) between OH -and the halide anions in small water clusters have been previously noted 46,47 and a hydrogen charge asymmetry for the I -(H 2 O) dimer has been identified, 48 to our knowledge the importance of q(∆H) in determining the differing halide versus hydroxide ion behavior in water has not yet been clearly established. It could well be important in explaining the high mobility of aqueous hydroxide.…”

Polarization and charge transfer in the hydration of chloride ions

“…The resulting structures are similar to the 1(C s ), 2(C 1 ), 3(C 3 ), and 4(C 4 ) structures in Figure 1 of Ref. 73 . Table III lists the computed charges from the ChelpG method (one charge only on the chloride ion), ChelpG with an added point dipole on the chloride ion, and the QTAIM method.…”

“…First, this level of theory should generate relatively accurate charge distributions for analysis of local polarization around the ions. 73 Second, MP2 calculations include many-body dispersion interactions at a decent level of accuracy, and we believe this may be important for modeling ion-water and water-water interactions accurately. 7,82 Third, we plan to couple this QM/MM approach to computations of solvation free energies via quasi-chemical theory.…”