Azusa Muraoka

Redox-responsive molecular helices with highly condensed π -clouds

Helices have long attracted the attention of chemists, both for their inherent chiral structure and their potential for applications such as the separation of chiral compounds or the construction of molecular machines. As a result of steric forces, polymeric o-phenylenes adopt a tight helical conformation in which the densely packed phenylene units create a highly condensed π-cloud. Here, we show an oligomeric o-phenylene that undergoes a redox-responsive dynamic motion. In solution, the helices undergo a rapid inversion. During crystallization, however, a chiral symmetry-breaking phenomenon is observed in which each crystal contains only one enantiomeric form. Crystals of both handedness are obtained, but in a non-racemic mixture. Furthermore, in solution, the dynamic motion of the helical oligomer is dramatically suppressed by one-electron oxidation. X-ray crystallography of both the neutral and oxidized forms indicated that a hole, generated upon oxidation, is shared by the repeating o-phenylene units. This enables conformational locking of the helix, and represents a long-lasting chiroptical memory.

Raman spectroscopy of optically levitated supercooled water droplet

By use of an optical trap, we can levitate micrometer-sized drops of purified water and cool them below the melting point free from contact freezing. Raman spectra of the OH stretching band were obtained from those supercooled water droplets at temperatures down to -35 °C. According to the two-state model, an enthalpy change due to hydrogen-bond breaking is derived from temperature dependence of the spectral profile. The isobaric heat capacity calculated from the enthalpy data shows a sharp increase as the temperature is lowered below -20 °C in good agreement with conventional thermodynamic measurements.

Structural Evolution of the [(CO2)n(H2O)]- Cluster Anions : Quantifying the Effect of Hydration on the Excess Charge Accommodation Motif

The [(CO2)n(H2O)]- cluster anions are studied using infrared photodissociation (IPD) spectroscopy in the 2800-3800 cm(-1) range. The observed IPD spectra display a drastic change in the vibrational band features at n = 4, indicating a sharp discontinuity in the structural evolution of the monohydrated cluster anions. The n = 2 and 3 spectra are composed of a series of sharp bands around 3600 cm(-1), which are assignable to the stretching vibrations of H2O bound to C2O4- in a double ionic hydrogen-bonding (DIHB) configuration, as was previously discussed (J. Chem. Phys. 2005, 122, 094303). In the n > or = 4 spectrum, a pair of intense bands additionally appears at approximately 3300 cm(-1). With the aid of ab initio calculations at the MP2/6-31+G* level, the 3300 cm(-1) bands are assigned to the bending overtone and the hydrogen-bonded OH vibration of H2O bound to CO2- via a single O-H...O linkage. Thus, the structures of [(CO2)n(H2O)]- evolve with cluster size such that DIHB to C2O4- is favored in the smaller clusters with n = 2 and 3 whereas CO2- is preferentially stabilized via the formation of a single ionic hydrogen-bonding (SIHB) configuration in the larger clusters with n > or = 4.

Reparametrization approach of DFT functionals based on the equilibrium temperature of spin-crossover compounds.

The required approach to investigate the electronic properties of spin-crossover (SCO) compounds needs to be able to provide a reliable estimate of high-spin/low-spin (HS/LS) energy gaps while retaining an accurate and efficient computation of the ground-state energy. We propose a reparametrization approach of the density functional theory (DFT) functionals to adjust the exact exchange admixture that governs the HS/LS energy splitting. Through the investigation of the thermodynamic properties of two typical SCO compounds, we demonstrate that the computed equilibrium temperature depends linearly, like the HS/LS energy gap, on the coefficient of the exact exchange admixture. We show that by taking the experimental value of the equilibrium temperature of the studied SCO compound as a reference, different hybrid functionals converge to comparable and realistic HS/LS energy gaps as well as enthalpy and entropy differences that agree well with the prior experimental investigations.

Structures of water-CO2 and methanol-CO2 cluster ions : [H2O•(CO2)n]+ and [CH3OH•(CO2)n]+ (n=1–7)

Infrared photodissociation (IRPD) spectra of [H(2)O x (CO(2))(n)](+) and [CH(3)OH x (CO(2))(n)](+) (n=1-7) are measured in the 1100-3800 cm(-1) region. At the same time, the solvation characteristics in the clusters are investigated theoretically; the geometry optimization and the vibrational analysis are carried out for the [H(2)O x (CO(2))(n)](+) (n=1-4) and the [CH(3)OH x (CO(2))(n)](+) (n=1-3) ions at the MP2/6-31+G(*) level of theory. The IRPD spectrum of the [H(2)O x (CO(2))(1)](+) ion shows the free OH and the hydrogen-bonded OH stretching bands of the H(2)O(+) ion core and the antisymmetric CO stretching band of the solvent CO(2) molecule, indicating that the solvent CO(2) molecule is preferentially solvated to the H(2)O(+) ion core via the O-H...OCO hydrogen bond. In [H(2)O x (CO(2))(2)](+), the free OH stretching band is not observed; both of the OH groups of the H(2)O(+) ion core are hydrogen bonded to the solvent CO(2) molecules. Spectral features of the IRPD spectra of [H(2)O x (CO(2))(n)](+) (n=3-7) suggest that the third and the fourth CO(2) molecules are bound to the oxygen atom of the H(2)O(+) ion core, and that the first solvation shell of the H(2)O(+) ion core becomes filled with four CO(2) molecules. All the IRPD spectra of the [CH(3)OH x (CO(2))(n)](+) (n=1-7) ions display the hydrogen-bonded OH stretching band of the CH(3)OH(+) ion core, meaning that the solvent CO(2) molecule is preferentially bonded to the OH group of the CH(3)OH(+) ion core, similar to the case of [H(2)O x (CO(2))(n)](+). Quantum chemical calculations for the [CH(3)OH x (CO(2))(1-3)](+) ions demonstrate that the second and the third solvent CO(2) molecules are bonded to the oxygen atom of the CH(3)OH(+) ion core.

Structures of [(CO2)n(H2O)m]− (n=1–4, m=1,2) cluster anions. I. Infrared photodissociation spectroscopy

The infrared photodissociation spectra of [(CO(2))(n)(H(2)O)(m)](-) (n=1-4, m=1, 2) are measured in the 3000-3800 cm(-1) range. The [(CO(2))(n)(H(2)O)(1)](-) spectra are characterized by a sharp band around 3570 cm(-1) except for n=1; [(CO(2))(1)(H(2)O)(1)](-) does not photodissociate in the spectral range studied. The [(CO(2))(n)(H(2)O)(2)](-) (n=1, 2) species have similar spectral features with a broadband at approximately 3340 cm(-1). A drastic change in the spectral features is observed for [(CO(2))(3)(H(2)O)(2)](-), where sharp bands appear at 3224, 3321, 3364, 3438, and 3572 cm(-1). Ab initio calculations are performed at the MP2/6-311++G(**) level to provide structural information such as optimized structures, stabilization energies, and vibrational frequencies of the [(CO(2))(n)(H(2)O)(m)](-) species. Comparison between the experimental and theoretical results reveals rather size- and composition-specific hydration manner in [(CO(2))(n)(H(2)O)(m)](-): (1) the incorporated H(2)O is bonded to either CO(2) (-) or C(2)O(4) (-) through two equivalent OH...O hydrogen bonds to form a ring structure in [(CO(2))(n)(H(2)O)(1)](-); (2) two H(2)O molecules are independently bound to the O atoms of CO(2) (-) in [(CO(2))(n)(H(2)O)(2)](-) (n=1, 2); (3) a cyclic structure composed of CO(2) (-) and two H(2)O molecules is formed in [(CO(2))(3)(H(2)O)(2)](-).

An IR study of (CO2)n+ (n=3–8) cluster ions in the 1000–3800 cm–1 region

Infrared photodissociation (IRPD) spectra of carbon dioxide cluster ions, (CO(2))(n) (+) with n=3-8, are measured in the 1000-3800 cm(-1) region. IR bands assignable to solvent CO(2) molecules are observed at positions close to the vibrational frequencies of neutral CO(2) [1290 and 1400 cm(-1) (nu(1) and 2nu(2)), 2350 cm(-1) (nu(3)), and 3610 and 3713 cm(-1) (nu(1)+nu(3) and 2nu(2)+nu(3))]. The ion core in (CO(2))(n) (+) shows several IR bands in the 1200-1350, 2100-2200, and 3250-3500 cm(-1) regions. On the basis of previous IR studies in solid Ne and quantum chemical calculations, these bands are ascribed to the C(2)O(4) (+) ion, which has a semicovalent bond between the CO(2) components. The number of the bands and the bandwidth of the IRPD spectra drastically change with an increase in the cluster size up to n=6, which is ascribed to the symmetry change of (CO(2))(n) (+) by the solvation of CO(2) molecules and a full occupation of the first solvation shell at n=6.

Structures of [(CO2)n(CH3OH)m](-) (n = 1-4, m = 1, 2) cluster anions.

The infrared photodissociation spectra of [(CO 2) n (CH 3OH) m ] (-) ( n = 1-4, m = 1, 2) are measured in the 2700-3700 cm (-1) range. The observed spectra consist of an intense broad band characteristic of hydrogen-bonded OH stretching vibrations at approximately 3300 cm (-1) and congested vibrational bands around 2900 cm (-1). No photofragment signal is observed for [(CO 2) 1,2(CH 3OH) 1] (-) in the spectral range studied. Ab initio calculations are performed at the MP2/6-311++G** level to obtain structural information such as optimized structures, stabilization energies, and vibrational frequencies of [(CO 2) n (CH 3OH) m ] (-). Comparison between the experimental and the theoretical results reveals the structural properties of [(CO 2) n (CH 3OH) m ] (-): (1) the incorporated CH 3OH interacts directly with either CO 2 (-) or C 2O 4 (-) core by forming an O-HO linkage; (2) the introduction of CH 3OH promotes charge localization in the clusters via the hydrogen-bond formation, resulting in the predominance of CO 2 (-).(CH 3OH) m (CO 2) n-1 isomeric forms over C 2O 4 (-).(CH 3OH) m (CO 2) n-2 ; (3) the hydroxyl group of CH 3OH provides an additional solvation cite for neutral CO 2 molecules.

Modeling of Surface and Size Effects on Various Shape of Spin-Crossover Nanoparticles

We performed of Monte Carlo simulations using Ising-like model on two-dimensional core/shell rectangular lattice L×2L for different sizes in order to study the effect of surface and size on the thermal behavior of spin-crossover nanoparticles. The surface effect is accounted for by constraining all the atoms situated in the boundary in the high-spin state as a result of the weak ligand-field prevailing in the coordination shell. This result is similar to square lattice of spin-crossover nanoparticles, and in agreement with experimental data. Such a non-trivial change is explained as due to the competition between the negative pressures induced the high spin state surface and the bulk properties. We also described the way in which the usual occurrence condition of the first-order transition has to be adapted to the nanoscale.