OH stretching vibrations and hydrogen-bonded structures of 7-hydroxyquinoline-(H2O)1–3 investigated by IR–UV double-resonance spectroscopy

Triple proton transfer of excited 7-hydroxyquinoline along a hydrogen-bonded water chain in ethers: secondary solvent effect on the reaction rate

A large secondary solvent effect on the reaction rate has been experimentally observed in the excited-state tautomerization of a 7-hydroxyquinoline (7HQ) molecule complexed cyclically with two water molecules in ethers. The proton acceptance of a water molecule from the enolic group of 7HQ is the rate-determining step while the proton donation of a water molecule to the imino group of 7HQ is followed rapidly to complete the triple proton transfer of the 7HQ.(H2O)2 complex in both diethyl ether and di-n-propyl ether. The rate constant of the tautomerization is larger in diethyl ether than in di-n-propyl ether due to the more polar environment around the complex in diethyl ether. Although the activation energies of the proton transfer are similar in both ethers, the kinetic isotope effect of the rate constant is larger in di-n-propyl ether than in diethyl ether. We attribute these kinetic differences to dissimilarity in the polarities of the two secondary solvents.

Fluorescence quenching in cyclic hydrogen-bonded complexes of 1H-pyrrolo[3,2-h]quinoline with methanol: cluster size effect

Laser induced fluorescence (LIF) excitation spectra of molecular complexes of 1H-pyrrolo[3,2-h]quinoline (PQ) with methanol (n= 1, 2, 3) as well as vibrational spectra of their electronic ground state are reported. The latter have been recorded in the mid-infrared region by fluorescence depletion (FDIR). The only PQ.methanol(n) complex clearly identified in the LIF spectrum is the triply hydrogen-bonded cyclic 1 ratio 2 aggregate. Its stoichiometry has been proven by the femtosecond multiphoton ionization detected infrared measurements [J. Am. Chem. Soc., 2006, 128, 10 000]. The structure of the 1 ratio 2 cluster is determined by means of FDIR spectroscopy in combination with ab initio and DFT calculations. No fluorescence was detected that could be attributed to the 1 ratio 1 cluster. This behaviour of the 1 ratio 1 complex is explained in terms of rapid excited state double proton transfer followed by a non-radiative relaxation. The n= 3 and heavier clusters are fluorescent. Their electronic spectra overlap, preventing the selective measurement of the FDIR spectra of individual complexes.

Structure of hydrated clusters of tetrahydroisoquinoline [THIQ–(H2O)n=1,3] investigated by jet spectroscopy

The hydrated clusters of tetrahydroisoquinoline have been investigated by laser-induced fluorescence (LIF), UV-UV hole burning, and IR-UV double-resonance spectroscopy in a seeded supersonic jet. Clusters of different sizes and isomeric structures have different 0-0 transitions (origins) in the LIF spectrum. UV-UV hole burning spectroscopy has been used to identify different cluster species and their vibrational modes. The structures of the clusters have been predicted by comparing the observed OH and NH frequencies in the IR-UV double-resonance spectra with the results calculated at different levels of sophistication. It is found that the water molecules form linear and six- and eight-membered cyclic H-bonded structures at the nitrogen center of 1:1, 1:2, and 1:3 clusters, respectively.

Molecular aspects of halide ion hydration: the cluster approach

This review provides a historical context for our understanding of the hydration shell surrounding halide ions and illustrates how the cluster systems can be used, in combination with theory, to elucidate the behavior of water molecules in direct contact with the anion. We discuss how vibrational predissociation spectroscopy, carried out with weakly bound argon atoms, has been employed to deduce the morphology of the small water networks attached to anions in the primary steps of hydration. We emphasize the importance of charge-transfer in the binary interaction, and discuss how this process affects the structures of the larger networks. Finally, we survey how the negatively charged water clusters (H2O)n(-) are providing a molecular-level perspective on how diffuse excess electrons interact with the water networks.