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Formation of a hydrogen bond between molecules of a proton donor (phenol, pyrrole, N-methylanilines) and a solvent decreases the sensitivity of the XH stretching frequency to the polarity/polarizability of solvents. A change in the bond configuration in the amine moiety of N-methylaniline and related compounds upon formation of a solvation H complex is manifested in that the absolute terms of the solvatochromic equations for inert and protophilic media are different. The spectroscopic effect from the geometric reorganization of molecules is determined by their structure and the capability to act as hydrogen bond donors. Multicentered hydrogen bond with π bases affects the geometry of the amine fragment of N-substituted anilines to a lesser extent than does two-centered H bond with onium bases. [ABSTRACT FROM AUTHOR]

(4-Nitrobenzoyloxymethyl)trifluorosilane containing a five-coordinate silicon atom ( = O→Si) is specifically solvated in a protophilic medium to form a complex in which the solvent molecule donates an electron pair and the coordination number of the silicon atom increases to six. Formation of such solvation complexes gives rise to a pronounced positive specific solvatochromic effect in the UV spectra. According to dielectrometric, 19F NMR, and IR data, and also to ab initio calculations (HF/6-31G*), the solvent molecule occupies an equatorial position. [ABSTRACT FROM AUTHOR]

The solvatochromism of the long-wave UV absorption band of the contact ion pairs of the 4-nitropyrazolide anion with alkali metal cations in aprotic protophilic solvents with ℇ ≤ 24 was studied and quantitatively described using the Kamlet-Taft empirical solvent parameters. Of two possible types of tight ion pairs differing in the murtual arrangement of the counterions, the solvent stabilizes the pair in which the cation is located in the σ plane of the anion in the vicinity of its nitrogen atoms. Among the factors affecting the quantitative characterisitcs of the solvatochromism of such ion pairs, the major factors are the polarity/polarizability of the solvent and the capability of its molecules for coordination with cations. The peripheral specific solvation of the cation in an ion pair stabilizes its Franck-Condon excited state relative to the ground electronic state. In the quantitative respect, this effect is similar to that characteristic of solvation H complexes. [ABSTRACT FROM AUTHOR]