Theoretical analysis of vibrational modes in uranyl aquo chloro complexes

The electronic structure of uranyl-based complexes of water and chlorine, with the general formula [UO2(H2O)xCly]2-y(y=1,2,3,4;x+y=4,5)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$[\hbox {UO}_2(\hbox {H}_2\hbox {O})_{x}\hbox {Cl}_{y}]^{2-y} \,(y = 1,\, 2,\, 3,\, 4;\, x + y = 4, 5)$$\end{document}, have been computed both in vacuo and in aqueous solution. Within the density functional theory framework, total and relative energies (including basis set superposition error corrections), equilibrium geometries, and vibrational frequencies were determined and briefly compared with available experimental and previous theoretical data. New results on vibrational modes of these complexes are emphasized. Our focus is on the trend exhibited by the frequency of the stretching mode (νs\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\nu _{\mathrm{s}}$$\end{document}) associated to the uranyl moiety at different chlorine concentrations. A charge transfer index (CT) accounting for the number of electrons transferred from the ligands to UO22+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {UO}_2^{2+}$$\end{document} is used to rationalize this trend. According to our most accurate calculations, νs\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\nu _{\mathrm{s}}$$\end{document} is red-shifted at a rate between 15 and 20cm-1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$20\,\hbox {cm}^{-1}$$\end{document} per 0.1 e transferred. Interestingly enough, a linear trend is displayed using CT as the correlating parameter.