The geometries and interaction energies of complexes of pyridine with C6F5X, C6H5X (X=I, Br, Cl, F and H) and RFI (RF=CF3, C2F5 and C3F7) have been studied by ab initio molecular orbital calculations. The CCSD(T) interaction energies (Eint) for the C6F5X–pyridine (X=I, Br, Cl, F and H) complexes at the basis set limit were estimated to be −5.59, −4.06, −2.78, −0.19 and −4.37 kcal mol−1, respectively, whereas the Eint values for the C6H5X–pyridine (X=I, Br, Cl and H) complexes were estimated to be −3.27, −2.17, −1.23 and −1.78 kcal mol−1, respectively. Electrostatic interactions are the cause of the halogen dependence of the interaction energies and the enhancement of the attraction by the fluorine atoms in C6F5X. The values of Eint estimated for the RFI–pyridine (RF=CF3, C2F5 and C3F7) complexes (−5.14, −5.38 and −5.44 kcal mol−1, respectively) are close to that for the C6F5I–pyridine complex. Electrostatic interactions are the major source of the attraction in the strong halogen bond although induction and dispersion interactions also contribute to the attraction. Short‐range (charge‐transfer) interactions do not contribute significantly to the attraction. The magnitude of the directionality of the halogen bond correlates with the magnitude of the attraction. Electrostatic interactions are mainly responsible for the directionality of the halogen bond. The directionality of halogen bonds involving iodine and bromine is high, whereas that of chlorine is low and that of fluorine is negligible. The directionality of the halogen bonds in the C6F5I– and C2F5I–pyridine complexes is higher than that in the hydrogen bonds in the water dimer and water–formaldehyde complex. The calculations suggest that the CI and CBr halogen bonds play an important role in controlling the structures of molecular assemblies, that the CCl bonds play a less important role and that CF bonds have a negligible impact.

WaterÈmethanol and waterÈacetonitrile, 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 Ðndings could partially explain the non-ideality of these binary mixtures on the basis of the cluster structures.

We have studied the microscopic solvent structure of dimethyl sulfoxide−water mixtures and its influence on the solvation structure of solute from a clustering point of view, by means of a specially designed mass spectrometric system. It was observed that the propensity to the cluster formation is nonlinearly dependent on the solvent composition, exhibiting the existence of a critical value of mixing ratios where drastic changes in the microscopic solvent structure occur. It was also demonstrated that in such a solvent mixture the solvation structure of solutes such as 2-butanol, cyclopentanol, cyclohexanol, and phenol is greatly related to the microscopic solvent structures, implying that solute species interact with already established solvent clusters, rather than with individual solvent molecules.

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