Emanuele Ortoleva

Temperature-programmed desorption study of ammonia desorption-diffusion in molecular sieves: II. Application to partially decationated Y-zeolites

Abstract An analysis of TPD data relating to the desorption of ammonia from two different samples of partially decationated Y-zeolite is reported, assuming various chemical or physical steps to be rate-determining. The process is shown to be controlled by intracrystalline surface diffusion, with values of the apparent activation energy E a = 30.8 and 27.1 kcal/mol, respectively, for the two zeolite samples. These values include the retardation effect associated with the hindering interaction of the diffusing ammonia molecules with the acid sites of the zeolite. Values of the apparent effective diffusion coefficient ranging from ca. 3 10 −18 to ca. 4 10 −14 cm 2 /s have been calculated for the 423–573 K temperature range.

Temperature-programmed desorption-diffusion of ammonia in molecular sieves V. ZSM-5 zeolite

Abstract Some ZSM-5 zeolite samples with an SiO 2 Al 2 O 3 ratio of 10.3 have been analysed by temperature-programmed desorption (TPD) of ammonia to investigate how the rate-determining step of the desorption-diffusion process depends on some experimental parameters and on the physicochemical characteristics of the zeolite. By comparison with previous results on a ZSM-5 sample with SiO 2 Al 2 O 3 = 33.6 , a shift of the probable rate-determining step from intracrystalline diffusion of the desorbed gaseous base to desorption of the latter without appreciable readsorption has been noticed. On the other hand, both the change of the temperature ramp from 10 to 2 K/min and the substitution of Na+ for H+ by ion exchange shift the rate-determining step to desorption with free readsorption of the desorbed base.

Geometry and molecular parameters of 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione and its isomer bis(dimethylamino)squaraine. Combined study by IR spectroscopy, XRD and ab initio MO calculations

Absorption FTIR spectra have been recorded for solutions and solid phases of 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione and its isomer bis(dimethylamino)squaraine. The crystal structures of the two compounds have been established by XRD and compared with those of other bis(dialkylamino) analogues already studied by the same technique. Ab initio calculations for the squaraine led to a 6–31G** geometry substantially similar to the experimental one, indicating that crystal packing only marginally perturbs the equilibrium configuration of the isolated molecule. In contrast, the geometry of the 3-cyclobutene-1,2-dione derivative shows a degree of planarity considerably greater in the crystal than in the free molecule at the MP2/6–31G* minimum-energy nuclear configuration. The squaraine is predicted to be more stable, at all basis set levels here adopted, than its isomer. Differences and similarities between experimental IR frequencies and their ab initio estimates are presented.

Desorption—diffusion of ammonia in molecular sieves: IV. Decationated ZSM5 zeolite

Abstract The analysis of t.p.d.a. spectra from ZSM5 is reported, assuming various chemical or physical steps as rate-determining. After saturation, quite long isothermal desorption times (up to 50 h) were needed, before starting the temperature ramp, to obtain reproducible data. Intracrystalline diffusion was found to be the probable rate-determining step with apparent activation energy of ca. 13 kcal/mol. A comparison of the present results with the ones obtained with HNaY zeolites provided information on the most probable hindering phenomena characterizing the overall diffusive process.

Naphthalene and azulene adsorption on Pt(111). Extended Hückel calculations

Abstract Current interest on the chemisorption of aromatics on metal surfaces has both theoretical and applied aspects. No determination of adsorption energies

An ab initio study of the NCl bond

Closed-shell SCF MO calculations and geometry optimization were performed with the STO-3G, 3-21G and 6-31 G* basis sets for ten molecules containing the NCl group: ClNC, ClNO, ClNO2, ClNCO, ClNNN, Cl3N, Cl2NH, ClNH2, ClNCH2 and ClNCH(NH2). The hydrogenation energies were evaluated and qualitatively correlated with the RNCl bond angle and the Cl gross atomic population. For the molecules considered here, the NCl bond is rather weak and its stability - larger for longer than for shorter NCl distances - increases as the RNCl angle narrows. There appears to be no significant double-bond character for the shortest NCl bonds.

Aloeresin H, a new polyketide constituent of Cape aloe

A new constituent, aloeresin H (1), was isolated from Cape aloe, a bittering and laxative agent. Its structure was elucidated by degradation experiments combined with 1D and 2D NMR data. Aloeresin H represents the first C,C-diglucoside discovered in commercial samples of the drug and its polyketide origin can be interpreted in terms of two-chain condensation. Possible conformations of the virtual aglycone were obtained as energy minima by quantum mechanical calculations and were found to be consistent with particular NOE associations observed in the original glucoside.

Geometry and Conformation of Thietanium Ions from Diffraction Data and Ab Initio Calculations

Four-membered ring thiosulfonium ions may be obtained quantitatively and under mild conditions by anionotropic rearrangement of C-(tert-butyl)-substituted thiiranium ion precursors. Thus, t-4-(tert-butyl)-r-1,2,2,c-3-tetramethylthietanium tetrafluoroborate or hexachloroantimonate (2a or 2b, resp.) were formed from thiiranium ion 1. The thietanium salts 2a and 2b were characterized by X-ray crystal-structure analysis. Their cation geometry was also optimized by ab initio calculations at the RHF/6-31G*//RHF/6-31G* level, as were those of its stereoisomer 3 and of the unsubstituted S-methylthietanium ion 5. Comparison of 2, 3, and 5 with 4 – the only other thietanium ion studied by XRD, where the C-atoms of the thioniacyclobutane ring are part of a trinorbornane skeleton – indicates that, in these systems, relief from substituent overcrowding is easily achieved by a folding of the four-membered ring along the line connecting the two opposite C-atoms. The corresponding ring-deformation normal mode has a calculated frequency as low as 109 cm−1 in ion 5, to be compared with a frequency of 138 cm−1 in methylcyclobutane. For thietanium ion 2, the frequencies of the two normal modes involving such ring deformation have calculated values of 61 and 85 cm−1.

Substituted 1-amino-cyclopentenes from 2,3-diamino-1,3-butadienes and nitroalkenes

Abstract 2,3-Diamino-1,3-butadienes ( 1 ) reacted with β-nitrostyiene or nitroethene to afford a mixture ofisomeric 1,5-diaminoo-5-methyl-4-nitro-cyclopentenes ( 3 and 4 ). The configuration of the main isomers ( 3 ) was established by X-ray diffraction analysis which also revealed the great steric crowding of this molecule. The conformation of the cyclopentene ring in 3 in the solid state is also discussed. The enamines ( 3 and 4 ) were hydrolyzed to the corresponding aminoketones ( 5 and 6 ) which, on reduction, gave the same diaminoketone ( 7 ).

Extended Hckel calculations on the chemisorption of acetylene on tungsten

The chemisorption of acetylene on various faces of bcc tungsten, modelled by clusters of various sizes, has been studied by the Extended Hückel method. Relative binding energies on the various sites have been obtained, and the bonding modes of acetylene are discussed, also in comparison with experiment. It is concluded that, whenever possible, acetylene is di-σ bonded to the surface, and sp2hybridization is considered likely in all cases.