TY - JOUR
T1 - Theoretical Modeling of Electronic Structures of Polyiodide Species Included in α-Cyclodextrin
AU - Okuda, Masaki
AU - Hiramatsu, Takato
AU - Yasuda, Mitsuru
AU - Ishigaki, Mika
AU - Ozaki, Yukihiro
AU - Hayashi, Michitoshi
AU - Tominaga, Keisuke
AU - Chatani, Eri
N1 - Funding Information:
The calculations were performed using the supercomputers at the Research Center for Computational Science in Okazaki. This work was funded by the Ministry of Science and Technology (MOST) of Taiwan under Grant Number MOST 108-2113-M-002-003 (M.H.) and JSPS KAKENHI under Grant Number JP16H04778 (E.C.).
Publisher Copyright:
© 2020 American Chemical Society.
PY - 2020/5/21
Y1 - 2020/5/21
N2 - The molecular mechanism of blue color formation in an iodine-starch reaction is studied by employing the iodine-α-cyclodextrin (α-CD) complex as a practical model system that resembles the structural properties of the blue amylose-iodine complex. To this end, we construct, using the quantum chemistry method, a molecular model of the complex (I5-/Li+/2α-CD) that consists of one I5-, two molecules of α-CD, and a lithium cation, and this model is employed as a basic unit in constructing the structural models of polyiodide ions (I5-)n. The initial structure in the geometry optimization is adopted from the α-CD-iodine complex structure obtained from the X-ray crystallography study. The structural models of (I5-)n are built by adding the basic unit n times along the crystal axis and by optimizing the structure using quantum mechanics/molecular mechanics (QM (iodine)/MM (α-CD)) calculations. The electronic absorption spectra of the resulting model structures are calculated by time-dependent density functional theory (TD-DFT). We find that I5- acts as a basic unit of coloration in the visible region. The visible color originates from the electronic transition within the I5- molecule, and any charge transfer between the I5- ion and either of α-CD or a coexisting counter cation is not involved. We also reveal that the electronic transitions of (I5-)n are delocalized, which accounts for the well-known observation that the color of the iodine-starch reaction becomes bluish with an increase in the chain length of amylose. Furthermore, the preresonance Raman spectra calculated from the model suggest that the vibrational motions are localized in the I5- subunit dominantly. A comparison between an experimental absorption spectrum feature of the α-CD-iodine complex and the calculated ones of (I5-)n ions with various n values suggests that (I5-)4 polyiodide ions tend to be populated dominantly in the α-CD-iodine complex under aqueous conditions.
AB - The molecular mechanism of blue color formation in an iodine-starch reaction is studied by employing the iodine-α-cyclodextrin (α-CD) complex as a practical model system that resembles the structural properties of the blue amylose-iodine complex. To this end, we construct, using the quantum chemistry method, a molecular model of the complex (I5-/Li+/2α-CD) that consists of one I5-, two molecules of α-CD, and a lithium cation, and this model is employed as a basic unit in constructing the structural models of polyiodide ions (I5-)n. The initial structure in the geometry optimization is adopted from the α-CD-iodine complex structure obtained from the X-ray crystallography study. The structural models of (I5-)n are built by adding the basic unit n times along the crystal axis and by optimizing the structure using quantum mechanics/molecular mechanics (QM (iodine)/MM (α-CD)) calculations. The electronic absorption spectra of the resulting model structures are calculated by time-dependent density functional theory (TD-DFT). We find that I5- acts as a basic unit of coloration in the visible region. The visible color originates from the electronic transition within the I5- molecule, and any charge transfer between the I5- ion and either of α-CD or a coexisting counter cation is not involved. We also reveal that the electronic transitions of (I5-)n are delocalized, which accounts for the well-known observation that the color of the iodine-starch reaction becomes bluish with an increase in the chain length of amylose. Furthermore, the preresonance Raman spectra calculated from the model suggest that the vibrational motions are localized in the I5- subunit dominantly. A comparison between an experimental absorption spectrum feature of the α-CD-iodine complex and the calculated ones of (I5-)n ions with various n values suggests that (I5-)4 polyiodide ions tend to be populated dominantly in the α-CD-iodine complex under aqueous conditions.
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U2 - 10.1021/acs.jpcb.0c01749
DO - 10.1021/acs.jpcb.0c01749
M3 - Article
C2 - 32343576
AN - SCOPUS:85085263681
SN - 1520-6106
VL - 124
SP - 4089
EP - 4096
JO - Journal of Physical Chemistry B
JF - Journal of Physical Chemistry B
IS - 20
ER -