Metal-heptalodide interactions in cyclomaltoheptaose (beta-cyclodextrin) polyiodide complexes as detected via Raman spectroscopy

摘要

The Raman spectra of the cyclomaltoheptaose (beta-cyclodextrin, beta-CD) polyiodide complexes (beta-CD)(2)center dot NaI7 center dot 12H(2)O, (beta-CD)(2)center dot RbI7 center dot 18H(2)O, (beta-CD)(2)center dot SrI7 center dot 17H(2)O, (beta-CD)(2)center dot BiI7 center dot 17H(2)O and (beta-CD)(2)-VI(7 center dot)14H(2)O (named beta-M, M stands for the corresponding metal) are investigated in the temperature range of 30 - 140 degrees C. At room temperature all systems show an initial strong band at 178 cm(-1) that reveals similar intramolecular distances of the disordered I-2 units (similar to 2.72 angstrom). During the heating process beta-Na and beta-Rb display a gradual shift of this band to the final single frequency of 166 cm(-1). In the case of beta-Sr and beta-Bi, the band at 178 cm(-1) is shifted to the final single frequencies of 170 and 172 cm(-1), respectively. These band shifts imply a disorder-order transition of the I-2 units whose I - I distance becomes elongated via a symmetric charge-transfer interaction I-2 <- I-3(-)-> I-2. The different final frequencies correspond to different bond lengthening of the disordered I-2 units during their transformation into well-ordered ones. In the Raman spectra of beta-V, the initial band at 178 cm(-1) is not shifted to a single band but to a double one of frequencies 173 and 165 cm(-1), indicating a disorder-order transition of the I-2 molecules via a non-symmetric charge-transfer interaction I-2 <- I-3(-)-> I-2. The above spectral data show that the ability of I-3(-) to donate electron density to the attached I-2 units is determined by the relative position of the different metal ions and their ionic potential q/r. The combination of the present results with those obtained from our previous investigations reveals that cations with an ionic potential that is lower than similar to 1.50 (Cs+, Rb+, Na+, K+ and Ba2+) do not affect the Lewis base character of I-3(-). However, when the ionic potential of the cation is greater than similar to 1.50 (Li+, Sr2+, Cd2+, Bi3+ and V3+), the Mn+center dot center dot center dot I-3(-) interactions become significant. In the case of a face-on position of the metal (Sr2+, Bi3+) relative to I-3(-) the charge-transfer interaction is symmetric. On the contrary, when the metal (Li+, Cd2+, V3+) presents a side-on position relative to I-3(-), the charge-transfer interaction is symmetric. On the contrary, when the metal (Li+, Cd2+, V3+) presents a side-on position relative to I-3(-), the charge-transfer interaction is non-symmetric. (c) 2007 Elsevier Ltd. All rights reserved.