Search

Solvation of perfluorooctanoic acid (PFOA) in methanol–water mixed solvents was investigated by means of the mass spectrometric analyses of clusters in the solutions. The observed clusters were composed of PFOA, methanol, and water molecules. The molecular composition of the clusters showed that PFOA was preferentially solvated by methanol in the methanol–water mixed solvents, and that the ratio of methanol molecules in the PFOA solvation sphere was beyond 0.6 at the bulk methanol mole fraction (xMeOH) = 0.045. The microscopic environment of PFOA was almost occupied by the methanol from such lower xMeOH (xMeOH ≥ 0.045), and hence the ratio of methanol in the PFOA solvation sphere did not change at xMeOH ≥ 0.045 as much as the bulk xMeOH. This preferential solvation of PFOA by methanol suggested a potential overestimation on the dissociation constant (pKa) of PFOA extrapolated from a Yasuda–Shedlovsky plot.

We showed that there is a close relation between evaporation properties and cluster-level structures in alcohol–water binary mixtures, on the basis of the mass spectrometric analyses of clusters generated through fragmentation of liquid droplets. The azeotropy for alcohol (ethanol, 1-propanol or 1-butanol)–water mixtures is caused by the change of the evaporation properties attributed to the cluster-level structures in the mixtures. When the hydrogen-bonding network of water molecules is stable at the cluster level in the water-rich mixtures, the alcohol molecules included in the water-rich clusters are evaporated primarily. On the other hand, when the alcohol self-association clusters are formed with an increase of the alcohol concentration in the mixtures, evaporation of the water existing around the alcohol self-association clusters is increased.

Molecular clustering in the liquid state is controlled by the relativity of interaction energies. This means that the clustering is strongly promoted by the coexistence of relatively weakly interacting molecules in the solution. This relativity-controlled clustering was observed for 1-pentanol clustering through the mass spectrometry for clusters isolated from liquid droplets via adiabatic expansion in a vacuum chamber. The 1-pentanol clustering through hydrogen-bonding interaction was significantly promoted by the mixing with water, methanol, acetonitrile or dichloromethane, but it was not promoted by the mixing with 1-propanol or 1, 2-dichloroethane. This solvent effect on the 1-pentanol clustering is explained by the relativity of the 1-pentanol–1-pentanol interaction energy to the 1-pentanol–solvent interaction energy. Thermodynamic analysis on this solvent-induced clustering also supports that the clustering controlled by the relativity of interaction energy is inherent in the liquid state.

The neutralization of CH 3 COOH by NaOH in water has been studied by the mass spectrometric analysis of clusters generated via fragmentation of liquid droplets. In water, CH 3 COOH molecules form self-association clusters at higher acid concentrations like the molar ratio of CH 3 COOH:H 2 O=1:10. At these concentrations, the neutralization takes place via interaction between the CH 3 COOH clusters and NaOH to afford Na + (CH 3 COOH) a (CH 3 COONa) b clusters. Furthermore, Na + is preferentially solvated by CH 3 COOH in water to form Na + (CH 3 COOH) α clusters. In consideration with these cluster structure, the reaction mechanism in solution can be realized more precisely.

The relationship between liquid-liquid phase separation and microheterogeneity in water-primary alcohol mixtures was examined by analysing the mass spectra of clusters generated through the fragmentation of liquid droplets. By comparing the cluster structures of water-ethanol, -1-propanol, and -1-butanol binary mixtures at various alcohol concentrations, we discovered differences in the molecular clusters that control phase separation. We also studied the role of water in alcohol self-association. Alcohol self-association is promoted in the presence of a small amount of water (ca. 10 approximately 20 wt%), in which the water-water hydrogen-bonding network is weak and does not contribute to alcohol self-association. We have demonstrated that alcohol self-association is also promoted by non-ideal mixing with other alcohols. The self-association of alcohol molecules complements the loss of stabilization energy caused by the relatively weak coexisting interactions. This complementary relationship among intermolecular interactions is an inherent property of solutions, and plays a key role in the phase separation process.

Molecular clusters formed by fragmentation of liquid droplets of alcohol (methanol or n-butanol)-acetonitrile mixtures have been detected and analyzed by means of a specially designed mass spectrometer. In the methanol-acetonitrile mixture, methanol clusters retain a sizable magnitude through most of the composition range, whereas acetonitrile clusters decrease in intensity upon increasing the concentration of methanol. Hydrogen bonding among methanol molecules controls the clustering. On the other hand, in n-butanol-acetonitrile mixtures, self-association of n-butanol through hydrogen bonding is remarkably promoted by the mixing with acetonitrile. With decreasing the acetonitrile contents, however, n-butanol self-associated clusters disintegrate completely. The interaction among n-butanol molecules changes from hydrogen bonding to dispersive, depending on the mixing ratio. When phenol is added as a solute to these binary mixtures, the solvation of phenol is found to be controlled by the solvent molecular clustering.

The cluster structures observed by means of mass spectrometry for binary mixtures-tert-bulyl alcohol (TBA)-H 2 O and tetrahydrofuran (THF)-D 2 O-with varying mixing ratios exhibit striking contrast, even though both TBA andTHF are miscible with water at any mixing ratio. In the TBA-H 2 O mixtures at TBA mole fractions of (X T B A ) ≤ 0.01-0.025, some of the H 2 O molecules in the H 2 O clusters are replaced by TBA molecules. For 0.01-0.025 ≤ X T B A ≤ 0.2-0.3, the self-aggregation of TBA forms dominant cluster structures, and the hydrogen-bonded water clusters are disintegrated with increasing X T B A . This TBA self-aggregation is reduced with further increasing TBA at X T B A ≥ 0.3. However, in the THF-D 2 O mixtures, THF molecules have a weak additional interaction with D 2 O clusters, and the self-aggregation of THF is not promoted in the THF-D 2 O mixtures. The D 2 O clusters still exist, even at a THF mole fraction of X T H F = 0.3. On the basis of the observed cluster structure, the mechanism for the mixing between water and the organic solvent and the controlling factors in the self-aggregation are proposed.

Mass spectrometric study on the cluster structure of methanol solutions containing lithium halides (LiX = LiCl, LiBr, and LiI) is reported. Solvated ions: Li+(CH3OH)n and X-(CH3OH)k, and salt clusters: Li+(Li+X-)s(CH3OH)m and X-(Li+X-)p(CH3OH)r, were observed in the mass spectra. The number of methanol molecules around Li+, especially in Li+(Li+X-)s(CH3OH)m clusters, increased when changing the anions from Cl- to I-, which suggested that there was a complementary relation between a Li+−CH3OH interaction and a Li+−X- interaction. In the case of X- = I-, the Li+−CH3OH interaction was enhanced in comparing with the case of X- = Cl-, because a Li+−I- interaction is weaker than a Li+−Cl- interaction. This observed complementary relation is a kind of intrinsic property of a liquid phase. Furthermore, mass distribution of the solvated ions and the salt clusters had correlations with physicochemical properties such as solvation energies and molar conductivities.

A part of microscopic structure in aqueous HNO3, and H2SO4 solutions was directly observed as ionic clusters isolated from these aqueous solutions by means of a specially designed electrospray mass spectrometer. The difference in the hydration structure for these acids was partially visualized on the basis of the molecular composition in the observed ionic clusters. For aqueous HNO3 solutions, the protonated water clusters, H+(H2O)n: n = 1, 2, 3 ..., which have similar mass distribution to the inherent water clusters, were observed predominantly. This is in good agreement with microscopic picture that the protons released from HNO3 are hopping and delocalized among water clusters. Such a cluster structure was independent of the HNO3 concentrations of [HNO3] < 1 mol/dm3. On the other hand, for aqueous H2SO4 solution, cluster structure was drastically changed with varying H2SO4 concentration. For diluted H2SO4 solution, clusters that consisted of only water molecules were mainly observed. This means that t...

Density functional theory (DFT) B3LYP calculations characterize the structure and stability of the clusters of tropylium ion (Tr+) coordinated by methanol molecules Tr+(MeOH)n with n=1–7. Methanol molecules are bound together through strong O–H⋯O type hydrogen bonds, resulting in a cyclic structure when n≥3, and the methanol cluster thus formed coordinates to Tr+ through weak C–H⋯O type hydrogen bonds. Thus, the formation of the Tr+(MeOH)n clusters is mediated by two kinds of hydrogen bonds. Calculated distances of the O–H⋯O hydrogen bonds lie in the range 1.570–1.991 A (1.712 A in average) while those of the C–H⋯O hydrogen bonds lie in the range 2.083–2.319 A (2.160 A in average). Mass spectroscopic experiments demonstrated that Tr+(MeOH)4 is a dominant, magic-numbered species and that Tr+(MeOH)3 and Tr+(MeOH)5 are minor [Chemical Communication, (2001) in press]. The experimental result is analyzed from the viewpoint of energetics. The specific size effect on the stability of Tr+(MeOH)n is a direct consequence of the stability of the (MeOH)n fragment itself.

The self-associations of butan-1-ol, butan-2-ol, and 2-methylpropan-2-ol ( tert-butanol) in the pure liquid state and in carbon tetrachloride solutions have been studied mainly through near-infrared spectroscopic observation at various temperatures. A new analysis assuming a successive association process for the alcohol molecules was applied to the sharp band around 1410 nm (the first-overtone band of the OH stretching vibration mode attributed to free OH monomer and partly to OH polymer); it became clear that the mean association number for each alcohol increases with increasing concentration and decreases with increasing temperature. Comparisons of the association numbers at various constant temperatures for the three kinds of alcohols show that the association abilities are on the order butan-1-ol > butan-2-ol > 2-methylpropan-2-ol.

The solvent effect on the acid–base interaction between acetic acid and pyridine has been studied by the mass spectrometric analysis of clusters isolated from liquid droplets. The clusters resulting from the acetic acid–pyridine, acid–base, interaction in water are quite different from those in acetonitrile solvent. In water (acetic acid∶pyridine∶water = 1∶1∶10) the acid–base interaction occurs through the intercluster interaction between acetic acid and pyridine clusters. On the other hand, in acetonitrile (acetic acid∶pyridine∶acetonitrile = 1∶1∶10), the acid–base interaction proceeds through an intermonomer interaction between an acetic acid molecule and pyridine molecule, and the clusters are produced through the aggregation of a polar (acetic acid)δ–(pyridine)δ+ complex. This solvent effect is mainly attributed to the cluster structure of acetic acid and pyridine in water and acetonitrile solvent.

We here report a comparative experimental and theoretical study of dipole-bound electron attachment on four polar hydrogen-bonded dimers containing one water molecule (water-water, ammonia-water, phenol-water and pyridine-water). When the water molecule is the proton acceptor in the neutral complex, with a trans-linear hydrogen bond (water-water and phenol-water), it is shown that the equilibrium geometry of the dipole-bound anion tends to a cis-linear hydrogen bond for which the total dipole moment is larger. On the contrary, when the water molecule is the proton donor to a nitrogen atom (ammonia-water and pyridine-water), dipole-bound electron attachment does not lead to subsequent modifications of the complex geometry.

Very-low-energy electron attachment to homogeneous and mixed pyridine, phenol and cyclohexanol dimers is studied by means of collisions with laser-excited Rydberg atoms and several new dipole-bound anions are observed. The corresponding structures and binding energies of their neutral parents are determined by model calculations of the intermolecular interactions between these polar closed-shell molecules. In the case of the phenol–pyridine dimer, a valence negative ion is observed. The geometry of its neutral parent is obtained from both abinitio and model calculations.

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.

Clusters, ensemble of atoms and/or molecules with less number enough to form stable bulk, are thought to play important roles in various phenomena such as, crystallization, phase separation, chemical reactions, etc. Several techniques to produce clusters have been developed and studies in this field are becoming active. Two new methods to produce clusters and the investigations on their characteristics are described in this paper. One is the characteristics of clusters formed from liquid (solution), and the other is the formation and the stabilization of clusters on the surface and their structural investigation.

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.

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.

Preferential solvation of Na+ by N,N-dimethylformamide (DMF) in a DMF−water binary mixture is observed at the cluster level through mass spectrometry. Since DMF is highly electron donating, the Na+−DMF interaction is more favorable than the DMF−H2O interaction. The observed clusters in a DMF−water mixture with and without NaCl, show that once a DMF molecule interacts with Na+, the DMF molecule hardly forms a hydrogen bond with a water molecule or cluster.

Ethanol, acetonitrile, acetone etc. are miscible with water at any mixing ratio; however, the mixtures are not homogeneous in view of the molecular level owing to the hydrogen-bonding interaction between water molecules and the hydrophobic interaction between water and organic solvent molecules. Through these interactions, molecular clusters are easy to be formed in the aqueous organic solvents. The clustering structure of the aqueous organic solvents influences the microscopic environment of substrates in these solvents and controls various chemical reactions such as the hydrolysis of tert-butyl chloride (SN1 reaction).This review will focus on the solvent effects studied in terms of the molecular clusters which reflect the microscopic environment around the substrates. Furthermore, it will be described that the fluctuation of solvent molecules in the solvation cluster is another important factor to trigger a chemical reaction.

Ethanol, acetonitrile, acetone etc. are miscible with water at any mixing ratio; however, the mixtures are not homogeneous in view of the molecular level owing to the hydrogen-bonding interaction between water molecules and the hydrophobic interaction between water and organic solvent molecules. Through these interactions, molecular clusters are easy to be formed in the aqueous organic solvents. The clustering structure of the aqueous organic solvents influences the microscopic environment of substrates in these solvents and controls various chemical reactions such as the hydrolysis of tert-butyl chloride (SN1 reaction).This review will focus on the solvent effects studied in terms of the molecular clusters which reflect the microscopic environment around the substrates. Furthermore, it will be described that the fluctuation of solvent molecules in the solvation cluster is another important factor to trigger a chemical reaction.

Raman scattering and mass spectra of n-propanol and p-dioxane mixtures have been measured in order to study diluted effects on n-propanol. The results of both high frequency Raman spectra and mass spectra show the rapid decrease of clusters of hydrogen bonded molecules with decreasing n-propanol mole fractions ( x np ). In the low frequency Raman spectra, line shapes were analyzed using the mode coupling theory. It is found that the minimum frequency of the imaginary part of dynamic susceptibility, ω min , becomes lower with decreasing x np . These concentration dependencies suggest a strong relation between the slow structural relaxation and the network structure of molecules in n-propanol.

C 60 aggregates generated by sublimation were characterised using scanning mobility particle sizer (SMPS) and cavity ring-down (CRD) spectroscopy. Sublimation under dried nitrogen atmosphere resulted in aggregates with a mode diameter ranging from 30 to 60 nm. The authors found the size reduction of the aggregates upon humidification of the carrier gas. CRD spectroscopy enabled on-line monitoring of C 60 structure that composed particles and showed no chemical modification of the sublimation product in the presence of water molecules. The authors concluded that the heat balance was modified in the presence of water, thereby the size of the aggregates was reduced.