Solvent‐Dependent Structure of the I₃⁻ Ion Derived from Photoelectron Spectroscopy and Ab Initio Molecular Dynamics Simulations

Abstract

Ab initio molecular dynamics (MD) simulations of the solvation of LiI3 in four different solvents (water, methanol, ethanol, and acetonitrile) are employed to investigate the molecular and electronic structure of the I3(-) ion in relation to X-ray photoelectron spectroscopy (XPS). Simulations show that hydrogen-bond rearrangement in the solvation shell is coupled to intramolecular bond-length asymmetry in the I3(-) ion. By a combination of charge analysis and I 4 d core-level XPS measurements, the mechanism of the solvent-induced distortions has been studied, and it has been concluded that charge localization mediates intermolecular interactions and intramolecular distortion. The approach involving a synergistic combination of theory and experiment probes the solvent-dependent structure of the I3(-) ion, and the geometric structure has been correlated with the electronic structure.

Photoelectron shake-ups as a probe of molecular symmetry: 4d XPS analysis of I3− in solution

“…As an important component in dye-sensitized solar cells, the electronic, structural and dynamic properties of I 3 À in solution have been studied in great detail both with experimental [5][6][7] and theoretical methods. [8][9][10][11][12] These studies all demonstrate how the structure of the molecular ion and, in particular, its degree of nuclear asymmetry can be directly correlated with the varied strength of solvent-solute interaction in different solutions. In aqueous solution, the strong hydrogen bonding drives large amplitude asymmetrical stretches and charge localization, leading to an electronic structure more akin to an I 2 Á Á ÁI À configuration.…”

“…3c) evidences dominant photo-ionization from the ionic site 4d(i) at 1.3 eV to 2.0 eV higher (À53.4 eV and À55.0 eV) rather than from the terminal 4d(t) (À54.8 eV and À56.3 eV) and central site 4d(c) (À55.4 eV and 56.9 eV), as explained by localization of surplus negative charge on the I À site. 11,12 The single exception is a weak feature (less than 0.05 of maximal intensity) near À51.6 eV, ascribed to ionization from the central site 4d(c), rationalized in ESI, † Section S2.3. We note that while the characters and the order of the 5p z orbitals are essentially preserved upon ionization in the symmetric geometry, this is not the case for the asymmetric geometry.…”

“…For the symmetric geometry, the main features of the 4d spectrum can be assigned to ionization from orbitals localized mainly on either the terminal 4d(t) or central 4d(c) sites in accordance with previous calculations. 11,12 Each type of orbital gives rise to two features separated by spin-orbit splitting of 1.6 eV between the 4d 3/2 and 4d 5/2 levels, seen as two parallel vertical lines in the occupations (Fig. 3a) near À53.6 eV and À55.2 eV (terminal) and, respectively, near À55.1 eV and À56.7 eV (central).…”

“…The influence of hydrogen bonding is less pronounced in, e.g., ethanol and acetonitrile where the charge stays fairly delocalized over the whole moiety, [I-I-I] À , and structural symmetry is comparatively preserved. In previous work 11,12 it has been demonstrated how the solvent-induced nuclear asymmetry may be evidenced from altered features of the main signal in 4d XPS measurements. Thus, I 3 À constitutes an ideal model system to explore not only the possible effects of nuclear asymmetry on electrochemical functionality, but also its interplay with XPS intensities.…”

Quasi-classical approaches to vibronic spectra revisited

“…Importantly each value of l defines a particular PES and thus a particular realization of the ring polymer, undergoing different dynamics as will become clear later, see Eq. (21). An example of such a realization of the ring polymer is illustrated in Fig.…”