Akihiro Wakisaka

Non-ideality of binary mixtures Water[ndash ]methanol and water[ndash ]acetonitrile from the viewpoint of clustering structure

Water–methanol and water–acetonitrile, which show exothermic and endothermic mixing, respectively, represent good contrast in non-ideality of a binary mixture. The microscopic structure observed through the mass-spectrometric analysis of clusters isolated from solution also shows good contrast between these binary mixtures as follows: (1) methanol molecules have substitutional interaction with water clusters, while acetonitrile molecules have additional interaction with water clusters; (2) the clustering of methanol molecules are promoted in the presence of water; on the contrary, the acetonitrile clusters are disintegrated in the presence of water. Such findings could partially explain the non-ideality of these binary mixtures on the basis of the cluster structures.

Interaction of hydrophobic molecules with water influenced by the clustering conditions of acetonitrile–water mixtures

In acetonitrile–water mixtures, dynamical processes of hydrophobic substrates, such as deprotonation of excited-state 2-naphthol and hydrolysis of tert-butyl chloride, hardly occur at water mole fractions (xw) less than 0.8. In contrast, the reaction rates increased markedly with increasing xw for xw > 0.8. These results were correlated with properties which reflect the solvent clustering conditions, such as partial vapour pressure and IR absorption of acetonitrile–water mixtures. IR absorption of water molecules due to the antisymmetric O—H stretching vibration showed a step-wise change of the absorption peak position suggesting the presence of four different solvent structures in the regions 0 < xw < 0.2, 0.2 < xw < 0.5, 0.5 < xw < 0.8 and 0.8 < xw < 1.0. Mass spectrometric analyses of liquid fragments of acetonitrile–water mixtures and the solutions with phenol, benzyl alcohol and 2-n-butoxyethanol showed clustering conditions characteristic of the water mole fractions in the mixtures. In 0.8 < xw < 1.0 the hydrate clusters of phenol and benzyl alcohol increased very quickly with increasing xw. In xw < 0.8, the signals of the hydrate clusters of the solutes were very weak, while the acetonitrile hydrates increased with decreasing xw. The observed mass spectral change with varying xw is discussed in relation to the structure of acetonitrile–water solutions and its effect on the dynamical processes of hydrophobic substrates.

Energetics for carbon clusters produced directly by laser vaporization of graphite: dependence on laser power and wavelength

Carbon-cluster positive ions C+n have been produced by laser vaporization of a graphite target by using three wavelengths (1064, 532 and 266 nm). When the graphite target was placed between the acceleration electrodes of a time-of-flight mass spectrometer and irradiated, the mass distribution of C+n immediately after the laser vaporization was recorded as a mass spectrum. C+n with n= 1, 3, 5, 7 and 11 atoms were produced predominantly, and their distribution was found to be dependent on the laser power and its wavelength. When the laser power was not too high, the logarithm of the concentration of carbon-cluster ions is linearly dependent on the logarithm of the laser power density. This logarithmic relation is well explained by the mechanism that C+n with 1 ⩽n⩽ 20 is vaporized directly from the graphite through conversion of the multiphoton energy into thermal energy. Furthermore, contributions of photoelectronic excitation to the formation of carbon clusters become important for 266 nm laser irradiation.

Preferential solvation controlled by clustering conditions of acetonitrile–water mixtures

Experimental evidence for the preferential solvation of phenol in a mixed solvent of acetonitrile and water has been obtained by mass spectrometric analysis of the clusters isolated from liquid droplets by the adiabatic expansion. The effect of temperature on the formation of phenol–hydrate clusters, (C6H5OH)(H2O)n : n= 1,2,3, …, showed that phenol molecules are solbated preferentially by acetonitrile molecules at xw(water mole fraction) < 0.85, the phenol–hydrate clusters were hardly observed at temperatures lower than 50 °C but appeared at higher temperatures. On the contrary, at xw 0.85, hydrate formation became preferable at lower temperatures. The observed temperature effect confirmed microscopically inhomogeneous clustering of the solvent and solute molecules in the mixtures. A. similar temperature effect was also observed in the emission spectra of 2-naphthol in the same mixtures.

Solvation-controlled clustering of a phenol–pyridine acid–base pair

Solvent effects on acid–base interactions between phenol and pyridine have been observed via mass spectrometry of solutions containing phenol, pyridine and water, alcohol and nitrile solvents. In the solvents having sufficiently long alkyl chains to solvate phenol and pyridine, such as propan-1-ol and propionitrile, the phenol–pyridine interaction was limited to the inter-unimolecular interactions. However, in the solvents having short alkyl chains such as methanol and acetonitrile, and in water, clustering between phenol and pyridine was observed as a result of inter-cluster interactions. The phenol molecules also formed clusters in methanol and acetonitrile, but not in propan-1-ol or propiontrile. This indicates that the observed clusters are controlled by the solvation structure depending on the solvent alkyl-chain length.

Molecular self-assembly controlled by acid–base non-covalent interactions: a mass spectrometric study of some organic acids and bases

Molecular clusters generated from vacuum adiabatic expansion of liquid droplets including acid and base molecules provide an insight into molecular self-assembly through non-covalent interactions. The mass spectrometric analysis for the resulting clusters indicates a systematic structure change which is dependent on the acid–base interaction: a multilayer stacking structure for relatively strong acid–base pairs (phenol–pyridine, phenol–N,N-dimethylaniline, etc.), and a monolayer structure for relatively weak acid–base pairs (phenol–pyrazine, cyclohexanol–pyridine, etc.). As another viewpoint, mass spectrometry of the molecular clusters composed of acid and base molecules can be presented as a new method to characterise the acid–base interaction.

Growth of carbon clusters : the simplest process, 2C1→C2, observed via spectrometry and chemical reaction

A graphite sample has been irradiated with a YAG laser under an argon atmosphere. The emission due to the C2(d 3Πg→ a 3Πu) Swan band was observed with a decay time 20 times the intrinsic lifetime of C2(d 3Πg). In the presence of benzene, which reacts with C1, the observed decay time of the C2 Swan band decreased with increasing benzene around the graphite. Furthermore, the reaction products between benzene and C1 or C2 were identified and their distribution varied remarkably with benzene concentration. From these results, the simplest growth process of carbon clusters, 2C1→ C2, was confirmed experimentally. Comparing the emission spectra from the irradiated graphite surface under an argon atmosphere and under vacuum, the collisional relaxation of C1 with argon was found to be indispensable to the growth process.

Reorganization of clusters through hydrophobic and hydrogen-bonding interaction in pyridine–phenol–water solution

Clusters composed of pyridine and phenol molecules have been grown in aqueous solution by increasing the temperature.

Molecular self-assembly composed of aromatic hydrogen-bond donor[ndash ]acceptor complexes

Molecular self-assembling systems derived from the clustering of acid and base molecules have been investigated by mass spectrometric analysis of clusters isolated from liquid droplets. N–H···N and O–H···N hydrogen-bonded acid–base systems were compared. When heteroaromatic N–H···N hydrogen-bonding acid–base systems, such as 7-azaindole dimer, the indole–quinoline pair, etc. were used as acid–base pairs, the clusters composed of equimolar acid and base molecules were generated. This means that the hydrogen-bonding acid–base complex, N–H···N, behaves like a single molecule in cluster formation. On the other hand, clustering of the aromatic O–H···N hydrogen-bonding systems, such as phenol–pyridine, phenol–pyrazine, etc., was controlled by the acid–base interaction determined by the pKa values, giving a multilayer structure for a relatively strong acid–base pair and a monolayer structure for a relatively weak acid–base pair. Molecular self-assembling systems containing hydrogen-bond donor and acceptor molecules have been systematically described here.

Energetics for preferential solvation of a chromium complex in aqueous organic solvent

Line broadening of NMR signals of solvent components induced by a paramagnetic chromium complex has been measured at varying temperatures to reveal the energetics for preferential solvation of the complex by a component of a mixture of an organic solvent (acetonitrile or acetone) and water. At water mole fractions lower than 0.8, [Cr(SCN)6]3– is preferentially solvated by the organic solvent molecules, mostly owing to lowering of the enthalpy of the organic solvent in the solvation layer compared to that of the bulk solution. However, at water mole fractions higher than 0.8, the population of water molecules in the solvation layer increases markedly because of the increase in entropy. Such thermodynamic properties of the solvation are correlated with the microscopic structure of mixed solvents. In contrast, in an acetonitrile (or aceteone)–benzene mixture, no preferential solvation was observed for Cr(acac)3.