Ab Initio Studies on the Photophysics of Uric Acid and Its Monohydrates: Role of the Water Molecule

The photophysical behavior of three lowest-energy tautomers of uric acid and seven most stable isomers of uric acid monohydrate is comprehensively studied by ab initio calculations. Ground-state energies are calculated with the CCSD­(T) method, while excitation and ionization energies as well as excited-state potential energy profiles of photoinduced processes are calculated with the CC2 method. For the ¹ππ* state, it is found that the excitation energy of the monohydrate cluster is significantly lower than that of isolated uric acid when the water molecule is hydrogen-bonded at a specific carbonyl group. The calculated excited-state potential energy profiles suggest that some monohydrate isomers can undergo a migration of the water molecule from one site to another site in the ¹ππ* state with a small energy barrier. It is also found for both uric acid and its monohydrate that nonradiative decay via the NH bond dissociation in the ¹πσ* state is likely to occur at higher excitation energies. On the basis of the computational results, possible mechanisms for the absence of specific isomers of uric acid monohydrate from the resonant two-photon ionization spectrum are discussed

The Topmost Water Structure at a Charged Silica/Aqueous Interface Revealed by Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

Despite recent significant advances in interface-selective nonlinear spectroscopy, the topmost water structure at a charged silica surface is still not clearly understood. This is because, for charged interfaces, not only interfacial molecules at the topmost layer but also a large number of molecules in the electric double layer are probed even with second-order nonlinear spectroscopy. In the present study, we studied water structure at the negatively charged silica/aqueous interface at pH 12 using heterodyne-detected vibrational sum frequency generation spectroscopy, and demonstrated that the spectral component of the topmost water can be extracted by examining the ionic strength dependence of the Imχ⁽²⁾ spectrum. The obtained Imχ⁽²⁾ spectrum indicates that the dominant water species in the topmost layer is hydrogen-bonded to the negatively charged silanolate at the silica surface with one OH group. There also exists minor water species that weakly interacts with the oxygen atom of a siloxane bridge or the remaining silanol at the silica surface, using one OH group. The ionic strength dependence of the Imχ⁽²⁾ spectrum indicates that this water structure of the topmost layer is unchanged in a wide ionic strength range from 0.01 to 2 M

Water Structure at the Buried Silica/Aqueous Interface Studied by Heterodyne-Detected Vibrational Sum-Frequency Generation

Complex χ⁽²⁾ spectra of buried silica/isotopically diluted water (HOD-D₂O) interfaces were measured using multiplex heterodyne-detected vibrational sum frequency generation spectroscopy to elucidate the hydrogen bond structure and up/down orientation of water at the silica/water interface at different pHs. The data show that vibrational coupling (inter- and/or intramolecular coupling) plays a significant role in determining the χ⁽²⁾ spectral feature of silica/H₂O interfaces and indicate that the doublet feature in the H₂O spectra does not represent two distinct water structures (i.e., the ice- and liquid-like structures) at the silica/water interface. The observed pH dependence of the imaginary χ⁽²⁾ spectra is explained by (1) H-up oriented water donating a hydrogen bond to the oxygen atom of silanolate, which is accompanied by H-up water oriented by the electric field created by the negative charge of silanolate, (2) H-up oriented water which donates a hydrogen bond to the neutral silanol oxygen, and (3) H-down oriented water which accepts hydrogen bonds from the neutral silanol and donates hydrogen bonds to bulk water molecules. The broad continuum of the OH stretch band of HOD-D₂O and a long tail in the low frequency region represent a wide distribution of strong hydrogen bonds at the silica/water interface, particularly at the low pH

Structural Anomality of the Adsorbed Water on Al-Doped Silica Revealed by Heterodyne-Detected Vibrational Sum-Frequency Generation Spectroscopy

Water adsorption on glasses is known to change their macroscopic surface properties, such as wettability and friction coefficient; however, the mechanism by which it changes them is still poorly understood. Furthermore, the surface structure of water adsorbed on glasses has not been explored except for pure silica (SiO2). This is presumably because of the experimental difficulty involved in selective measurements under ambient conditions. Although SiO2 is the main constituent of glasses, most glasses contain various minor elements that affect the structure of adsorbed water. In this study, heterodyne-detected vibrational sum-frequency generation spectroscopy was applied to aluminum (Al)-doped (0–75%) SiO2 and the spectra were analyzed by using singular value decomposition. The results clearly indicated that the adsorbed water drastically changes its structure only on adding a small amount of Al (<13%), which is contrary to the previous notion that the water structures simply change from those on pure SiO2 to Al2O3. The anomaly was discussed from the viewpoint of a characteristic water structure around the boundaries of silicon and Al nanodomains in the materials