Despite the importance of ultrafast (time scale exceeding 10(-11) s) intramolecular proton transfer (PT) events between electronic ground states in solution, experimental determination of the rates of such reactions has not yet been accomplished because of the limitations of the utilized methods. The objective of this study was to evaluate the PT rates of intramolecular O···H···O hydrogen-bonded systems in solution through the (1)H spin-lattice relaxation times of the hydroxyl protons, induced by the (1)H-(17)O dipolar interactions (T(1dd)(OH)), taking into account the contribution of the OH reorientational motion to T(1dd)(OH). Solutions of the benzoic acid dimer (BA dimer), 1-benzoyl-6-hydroxy-6-phenylfulvene (Fulvene), and dibenzoylmethane (DBM) were chosen as test systems. For Fulvene in CCl(4), the PT time, τ(PT), was deduced to be 7 × 10(-11) s. In the case of the BA dimer in CCl(4), the τ(PT) value was considerably greater than the OH reorientational correlation time, τ(R(OH)) = 4.3 × 10(-11) s. In contrast, the experimental results for DBM in CCl(4) indicated that the proton is located about midway between the two oxygen atoms, that is, the PT potential energy surface is a single well or a double well with a PT barrier near or below the zero-point energy.
The proton location and dynamics in a hydrogen bond in solution are fundamentally important for understanding the phenomenon of proton transfer (PT). In the present study, the proton location and its dynamics were explored for the NH form of the two PT tautomers of the Schiff base by analyzing the fluctuation of the (15)N-(1)H magnetic dipolar coupling by the PT as well as the NH reorientational motion. For this purpose, the (15)N and (13)C spin-lattice relaxation times were measured in dichloromethane or acetonitrile solutions of three Schiff bases with different substituents on the benzene moieties, N-(4,6-dimethoxysalicylidene)methylamine (compound 1), N-(1-methylnitrilomethylidyne)-2-naphthalenomethylamine (compound 2), and N-(3,5-dibromosalicylidene)-methylamine (compound 3). For the NH form of compound 2 in dichloromethane, the proton location shifted to the center between the nitrogen and oxygen atoms, as compared with the minimum of the PT potential surface derived from molecular orbital calculations. For the NH form of compound 3 in dichloromethane, the proton location shift was not observed, and the PT rate was significantly lower than the reorientation rate of the NH bond. The results are discussed in terms of the electronic effect of the substituents and the static and dynamic solvent effect.
The photodimerization of acenaphthylene in liquid crystalline media exhibiting cholesteric or nematic phase was studied from the view point of the stereoselectivity of the products. In the cholesteric phase trans-isomer formation increased remarkably and the ratio of the trans-isomer to cis-isomer depended significantly upon the acenaphthylene concentration. These results suggest a steric effect of the cholesteric phase structure on stereoselectivity on photodimerization compared with the predominant cis-isomer formation in the nematic phase.
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