Vibrational frequency fluctuations of poly(N,N-diethylacrylamide) in the vicinity of coil-to-globule transition studied by two-dimensional infrared spectroscopy and molecular dynamics simulations.

Poly(N,N-diethylacrylamide) (PdEA), one of the thermoresponsive polymers, in aqueous solutions has attracted much attention because of its characteristic properties, such as coil-to-globule (CG) transition. We performed two-dimensional infrared spectroscopy and molecular dynamics (MD) simulations to understand the hydration dynamics in the vicinity of the CG transition at the molecular level via vibrational frequency fluctuations of the carbonyl stretching modes in the side chains of PdEA. Furthermore, N,N-diethylpropionamide, a repeating monomer unit of PdEA, is also investigated for comparison. From decays of the frequency-frequency time correlation functions (FFTCFs) of the carbonyl stretching modes, we consider that inhomogeneity of the hydration environments originates from various backbone configurations of PdEA. The degree of the inhomogeneity depends on temperature. Hydration water molecules near the carbonyl groups are influenced by the confinements of the polymers. The restricted reorientation of the embedded water, the local torsions of the backbone, and the rearrangement of the whole structure contribute to the slow spectral diffusion. By performing MD simulations, we calculated the FFTCFs and dynamical quantities, such as fluctuations of the dihedral angles of the backbone and the orientation of the hydration water molecules. The simulated FFTCFs match well with the experimental results, indicating that the retarded water reorientations via the excluded volume effect play an important role in the vibrational frequency fluctuations of the carbonyl stretching mode. It is also found the embedded water molecules are influenced by the local torsions of the backbone structure within the time scales of the spectral diffusion.

Effect of acidic conditions on decomposition of methylene blue in aqueous solution by air microbubbles.

Methylene blue (MB) has recently been proposed as a new chemical probe to colorimetrically verify the presence of hydroxyl radicals and was applied to the microbubble technique in this report. The degradation of MB in water was observed by air microbubbles under acidic condition or adding H2O2. The discoloration of MB induced by air microbubbles supports that hydroxyl radicals are generated by the collapse of microbubbles in water. The decomposition rate of MB increases with increasing the concentration of acids (HCl, HNO3, H2SO4) and H2O2 up to ab. 2 mM, and becomes almost constant above ab. 2 mM. Note that the decomposition rate of MB mainly depends on pH, not the kind of acid and that adding salt slightly enhanced the decomposition of MB. The fact that the blue-shift of the absorbance band of MB was observed for not H2O2 but acids proposed that the methyl groups of MB are preferentially dissociated under acidic conditions due to the protonation of dimethylamino group.

Anharmonic calculations of frequencies and intensities of OH stretching vibrations of (R)-1,3-butanediol conformers in the fundamentals and first overtones by density functional theory.

The frequencies and absorption intensities of the five kinds of conformers of 1,3-butanediol with the same carbon skeleton (GG') were calculated by anharmonic calculation for the fundamentals and first overtones of OH stretching vibrations. The four kinds of conformers form intramolecular hydrogen bonds and one conformer did not. Intramolecular hydrogen bond formation shifted the frequency of fundamental and first overtone of H-bonding OH stretching vibration to the lower frequency. The absorption intensities of the fundamentals as well as the vibrational anharmonicities increased upon hydrogen bond formation, while the intensities of first overtones decreased. The differences of conformers were clearly seen in the frequencies of the first overtones of free OH.

Self-assembly of a tetrapodal adamantane with carbazole branches into hollow spherical aggregates in organic media.

A hydrophobic tetrapodal molecule is composed of carbazole units at the periphery linked by a phenyl spacer on an adamantane core. Tetrapodal adamantane self-assembles into hollow spherical aggregates with a multilayer membrane in organic media. The spherical assembly size is dependent on the organic solvent used. Hollow spheres can entrap guest molecules within their internal spaces. By increasing the concentrations of tetrapodal molecules, hollow spheres fused into necklace-shaped nanostructures and two-dimensional networks were obtained.