Ali A. El-Rayyes

Acidity of Zeolite Y—Probed by Adsorption of 1-Naphthylamine and Studied by Laser-Induced Fluorescence Spectroscopy

Proton transfer reactions between zeolite Y surface and 1-naphthylamine (NA) in the ground and excited states have been studied by laser-induced picosecond spectroscopy. The acidic form of zeolite Y readily protonates NA in the ground state. At low acid strength of the zeolite the excited state of the protonated NA transfers back the proton to the zeolite surface as indicated by the fluorescence spectra. At high acid strength of the zeolite the fluorescence comes from the protonated form of NA and from an adduct “X” previously found in highly concentrated HClO4 solutions. The concentrations of the protonated NA and X increase with the reduction in the unit cell size. The presence of these species is discussed in terms of the next nearest neighbor “NNN” theory of zeolite Y acidity and the role of the non-framework aluminum. The acidity of the zeolite is estimated, based on the fluorescence lifetimes of X, to vary from 3.7 to 17 M HClO4 depending on the unit cell size. Low loading levels of NA in the zeolite pores are best in studying the proton transfer reaction and for the estimation of the surface acidity of zeolite Y.

Solid state NMR study of 1,3‐imidazolidine‐2‐thione, 1,3‐imidazolidine‐2‐selenone and some of their N‐substituted derivatives

Solid-state NMR spectra were recorded for 1,3-imidazolidine-2-thione, 1,3-imidazolidine-2-selenone and some of their N-substituted derivatives. Spinning side-bands of thione and selenone carbons were analysed to yield chemical shift anisotropies for these carbons. The NMR spectrum of imidazolidine-2-thione (Imt) showed some evidence for the presence of thiol tautomer. Molecular computations were carried out for Imt and its N-methyl derivative to yield relative energies of various tautomers.

Acidity of All-Silica MCM-41—Studied by Laser Spectroscopy of Adsorbed Fluorescent Probe Compounds

The polarity within the channel environment of all-silica MCM-41 and the surface acidity were probed by the adsorption of rhodamine-B lactone (RhB-L) and 1-naphthylamine (NA) respectively. The photochemical properties of these probe compounds were studied by laser-induced fluorescence spectroscopy and compared with their properties in solution. The environment within the channels was found to be as polar as methanol. The fluorescence wavelength and decay rate have shown that when adsorbed on all-silica MCM-41, the lactone ring in RhB-L was opened to form the zwitterion (RhB-Z). It was not possible to assess whether RhB-L was protonated by the surface of MCM-41 because the fluorescence of both the RhB-Z and the cationic form RhB-C are almost identical. NA was protonated at the ground state on the surface of all-silica MCM-41. The proton, however, was donated back to the surface upon excitation with laser. From the pKa of the conjugate acid of NA and the fluorescence decay time of the excited state of NA (NA*), the acidity of the surface of the all-silica MCM-41 was estimated to be equivalent to an aqueous perchloric acid with pH 1.8–2.5, depending on the level of loading of NA. The origin of the acid surface was concluded to be the silanol groups known to be present on the surface. The nature of these groups and the influence of the polarity and the solvation effect of the framework on the acidity were discussed.

Ground and Excited States Proton Transfer Reactions of 1,8-Diaminonaphthalene in Perchloric Acid Solutions

The proton-transfer reaction of 1,8-diaminonaphthalene (1,8-DAN) in acidic medium was studied by means of fluorescence and picosecond spectroscopic techniques. It has been found that there are three different forms of 1,8-DAN in the ground state, but only two different forms in the excited state. The absorption of the mono-cation form of 1,8-DAN is found to be a mixture of the neutral form and the di-cation form. However, the emission is found to be the same as the neutral form, due to the fast dissociation of the mono-cation form once it is excited. The fluorescence of the mono-cation form of 1,8-DAN shows a small shift under different excitation wavelengths. The di-cation form only fluoresces if no free water cluster is available as a proton acceptor. The reaction in the excited state is shown to be a diabatic quenching reaction. With the help of quantum yields and fluorescence lifetime measurements these results are interpreted in terms of a new photochemical scheme. All dissociation and quenching rate constants, pKa and kq, have been determined.