As is well known, the Cr2+/SiO2 system is an efficient catalyst for ethylene polymerisation already at RT (even if in the industrial process the running temperature is ∼380 K). For this reason it is the ideal system for in situ spectroscopic investigations on a genuinely working catalyst (Adv. Catal., 2001, 46, 265). Many questions are still uncertain as to the initiation, propagation and termination steps of the polymerisation mechanism. On a pre-reduced sample polymerisation commences very quickly and there is no way to record initial stages. When the experiments are performed at RT the phenomenon is too fast and IR spectroscopy, even in the time-resolved mode, failed up to now in the identification of the species formed during the initiation of the ethylene polymerisation step on the Cr2+/SiO2 Phillips catalyst. We present results related to time-resolved FTIR spectroscopy at variable temperature of acetylene and methylacetylene oligomerisation on a model Phillips catalyst. These experiments have been highly informative on the nature of the active sites because we have observed that acetylene and methylacetylene result in the immediate formation of benzene and 1,3,5-trimethylbenzene, respectively, without the evidence of any measurable intermediate product. This implies that the active Cr sites are able to coordinate simultaneously three monomers and thus must exhibit a high unsaturative coordination. The results of these experiments could be an insight of chromium species active in the Phillips catalyst.
The present work reports the synthesis of trans-2-amino-5(6)-fluoro-6(5)-hydroxy-1-phenyl-2,3-dihydro-1H-indenes (4a−f, 5a−f) as a continuation of our studies to better understand the significance of the halo substituent in the trans-1-phenyl-2-aminoindane series and to extend knowledge of the monophenolic ligands of DA receptors. The affinity of the new compounds and related methoxylated precursors (10−15 and 18−23) was estimated in vitro by displacement of [3H]SCH23390 (for D1-like receptors) or [3H]YM-09-151-2 (for D2-like receptors) from homogenates of porcine striatal membranes. The results indicate that unsubstituted amines 4a, 5a, 10, and 11 are poorly effective at DA receptors. The introduction of two n-propyl groups on the nitrogen atom (compounds 14, 15, 4c, and 5c) and N-allyl-N-methyl- or N-methyl-N-propyl- substitution (compounds 20−23, 4e, 4f, 5e, 5f) increased the D2-like affinities and selectivity. The D2-like agonistic activity of selected compounds 15, 20, 21, 4e, 5c, and 5e was proved by evaluating their effects on the cyclic guanosine monophosphate (cGMP) content in rat neostriatal membranes. All tested compounds displayed a potential dopamine D2-like agonist profile decreasing basal levels of cGMP. The selective D2-like agonism of compounds 20 and 5e was proved by their effects on basal striatal adenylyl cyclase activity.
By diluting optically opaque carbon-supported Pd particles in silica Aerosil® we succeeded in observing the IR bands of adsorbed carbonyls and extracting information on the particle dispersion. Comparison with literature single crystal data and with silica–alumina supported Pd allowed us to make an assignment in terms of linear and 2-fold bridged carbonyls formed on Pd(111) and Pd(100) faces. Two Pd/C samples have been investigated. The relative intensities of the two carbonyl families observed on the two samples are consistent with the Pd dispersion independently measured with CO chemisorption, TEM and EXAFS analysis.
A systematic in situ FTIR investigation of the interaction of several molecules characterized by a different electron accepting ability (or acidity) with the basic pyridine units contained in a commercial poly(4-vinylpyridine) system characterized by a high surface area is here reported, with the final goal to fully understand the role of basicity in the catalytic processes. This approach, for the first time applied to polymeric materials, has only some similarities with the method usually adopted in the investigation of catalytic acid sites hosted inside oxidic porous materials (such as zeolites), where the probes are molecules characterized by an increased basicity. The experimental results are confirmed by ab initio theoretical calculations conducted on the 4-methylpyridine, which is the simplest molecule resembling the pyridine moiety in the P4VP system.
The IR spectra of O3 adsorbed on polycrystalline (high surface area) MgO are reported and assigned. Two distinct groups of absorptions in the 1160–970 and 1730–1170 cm−1 spectral regions were observed. The bands falling in the first region, fully reversible upon decreasing the O3 pressure, are grouped into two doublets with components at 1105–1024 cm−1 (A1–A3 doublet) and at 1140–1038 cm−1 (B1–B3 doublet). The A1–A3 doublet is assigned to the stretching modes (νsym and νasym respectively) of O3 adsorbed through a terminal oxygen atom on Mg2+5c ions exposed on the extended {001} terminations of the MgO microcrystals. The less intense B1–B3 doublet at 1140–1038 cm−1 is assigned to the stretching modes of O3 adsorbed on defective positions like edges, corners and steps. The irreversible bands in the 1730–1170 cm−1 range are assigned to oxidation products formed by reaction of O3 with carbonaceous impurities remaining on the MgO surface after the activation in vacuo at high temperature. In particular the doublet at 1690–1280 cm−1 is assigned to the νasym and νsym stretching modes of bidentate carbonate-like species. The bathochromic shifts of the A1–A3 components with increasing O3 coverage are interpreted in terms of lateral dipole–dipole interactions (of both static and dynamic nature) between the adsorbed species. The negative shift of the νasym mode of the carbonate-like species induced by the increase in the O3 coverage is ascribed to lateral interactions of the static type.
