Search

Acid–base interactions were investigated by mass spectrometry of clusters isolated from solutions containing a carboxylic acid (propionic, butyric or hexanoic acid), an aromatic base (pyridine or pyrazine) and a solvent (water, acetonitrile or propionitrile). In water, self-aggregation of the carboxylic acid molecules due to hydrophobic interaction to form carboxylic acid clusters, such as (butyric acid)n, was prominent, and acid–base interaction proceeded between the carboxylic acid clusters and the basic molecules. In acetonitrile, the acid–base interaction was sensitive to the relative strength of the base. When pyridine (relatively strong base) was used as a base, clustering proceeded through the formation of a polarized acid–base complex, (acid)δ-(base)δ+. However, when pyrazine (relatively weak base) was used as a base, self-aggregation of the acid molecules became favorable. In propionitrile, such clusters were not observed because each molecule was separated by individual solvation. This solvent effect is related to the solvation structure, which determines the balance between the self-aggregation of the acid molecules and the formation of the polarized acid–base complex. It has also been demonstrated that the balance of the above intermolecular interactions is also dependent on the size of the alkyl group of the carboxylic acid. The results of the mass spectrometry partially show the microscopic view for the effect of solvent, pKa and the size of the alkyl group on the acid–base interaction in solution.

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.

Upon crystallization from mixtures containing indole and isoquinoline, intermolecular compounds of 1:1 molecular ratio have been isolated as crystals. This crystallisation process, i.e., molecules→clusters→nucleus was studied in a specially designed mass spectrometer. From the mass spectrometric analysis of clusters isolated from liquid droplets containing indole, isoquinoline and acetonitrile, clusters composed of equimolecular indole and isoquinoline, i.e. (indole) n (isoquinoline) n : n=1, 2, 3,..., were observed as prominent species. This suggested that the hydrogen-bonded complex of indole and isoquinoline works as a unit species for the nucleation of the intermolecular compounds.

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.

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.

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.