Predicting gas transport in formed zeolite adsorbents from NMR studies

The self-diffusion of nitrogen, methane, and carbon monoxide within a 5A zeolitic adsorbent has been examined with use of pulsed field gradient (PFG) NMR. In all cases, the diffusion process is well-described by a refined version of the long-range diffusion model (LRDM), adapted here for use with pelletized adsorbents, which uses exclusively adsorbent porosity and isotherm data as inputs. Correlation of the experimental data with this model yields tortuosity factors that are characteristic of the adsorbate and reflect the longer diffusive path a molecule must take due to the winding nature of the pore structure. It is demonstrated that the diffusion model can be used to accurately predict the diffusion coefficients for a ternary gas mixture within a 5A zeolite. To fully characterize the diffusive process, the surface excess on the PFG NMR samples has been obtained by a novel gas-phase NMR technique that is well-suited for measuring pure and multicomponent isotherms.     

Polyfluoro-1,2-epoxy-alkanes and -cycloalkanes. Part I. Preparation of some polyfluoro-1,2-epoxycyclohexanes

Polyfluorocyclohexenes with hydrogen, bromine, and methoxy substituents yielded the corresponding 1,2-epoxides when treated with aqueous sodium hypochlorite containing some acetonitrile. 4,5-Dibromooctafluoro-1,2-epoxycyclohexane was debrominated with zinc dust to give a mixture of octafluoro- and 4H-heptafluoro-1,2-epoxycyolohex- 4-ene. Decafluoro- and 4,5-dibromo-octafluoro-1,2-epoxycyolohexane gave with potassium fluoride in acetonitrile the corresponding potassium perhalogenocyclohexyloxides; heating these gave the analogous cyclohexanones, and treatment with methyl iodide the methyl ethers. The unsaturated 1,2-epoxides also gave methyl ethers on treatment with KF, followed by methylation.     

Metal-to-ligand charge-transfer sensitisation of near-infrared emitting lanthanides in trimetallic arrays M2Ln (M=Ru, Re or Os; Ln=Nd, Er or Yb)

The new pro-ligand 4-methyl-4'-(carbonylamino(2-(tert-butoxycarbonylamino)ethyl))-2,2'-bipyridyl (L1) has been prepared and used to synthesise the complex fac-Re(I)Cl(CO)3(L1) 1 and the complex salts [M(II)(bipy)2(L1)](PF6)2 (M=RuII 8 or OsII 15). Deprotection with trifluoroacetic acid affords the amine-functionalised derivatives fac-Re(I)Cl(CO)3(L2) 2, [M(II)(bipy)2(L2)](PF6)2 (M=RuII 9 or OsII 16) which react with the dianhydride of diethylenetriamine pentaacetic acid (DTPA) to give the binuclear complex {fac-Re(I)Cl(CO)3}2(L3) 3 and the complex salts [{M(II)(bipy)2}2(L3)](PF6)4 (M = RuII 10 or OsII 17). The latter react with salts Ln(OTf)3 to afford a series of 12 heterotrimetallic compounds that contain a lanthanide (Ln) ion in the DTPA binding site; {fac-Re(I)Cl(CO)3}2(L3)LnIII (Ln=Nd 4, Er 5, Yb 6 or Y 7) and [{M(II)(bipy)2}2(L3)LnIII](PF6)(OTf)3 (M=RuII, Ln=Nd 11, Er 12, Yb 13 or Y 14; M=OsII, Ln=Nd 18, Er 19, Yb 20 or Y 21). All of these trimetallic species display absorption bands ascribed to metal-to-ligand charge-transfer (MLCT) excitations, and luminescence measurements show that these excited states can be used to sensitise near-infrared emission from LnIII (Ln=Nd, Er or Yb) ions. Single crystal X-ray structures of L1 and [RuII(bipy)2(L2H)](H2PO4)3.(CH3)2CO.0.8H2O were obtained, the latter revealing the presence of H2PO4- counter anions, the source of which is presumed to be hydrolysis of PF6- ions.     

DNA mismatch detection by resonance energy transfer between ruthenium(II) and osmium(II) tris(2,2'-bipyridyl) chromophores

Octahedral tris-chelate complexes [M(II)(bpy)(3)](2+) (M = Ru or Os, bpy = 2,2'-bipyridyl), covalently attached to the 3'- and 5'-phosphates of two oligonucleotides, are juxtaposed when hybridized contiguously to a fully complementary DNA target. Visible metal-to-ligand charge-transfer (MLCT) excitation of the [Ru(II)(bpy)(3)](2+) unit leads to resonance energy transfer to the MLCT state of the [Os(II)(bpy)(3)](2+) moiety, with the energy transfer efficiency depending on the degree of hybridization. The extent of attenuation of the intense red luminescence from the Ru(II) chromophore hence allows highly sensitive structural probing of the assembly and constitutes a novel approach to DNA sensing which is capable of detecting mutations.     

Some intramolecular rearrangements when pentofuranoses are treated with diethylaminosulphur trifluoride (DAST)

Treatment of methyl 2,3-O-isopropylidene-β- -ribofuranoside with DAST gave a goodyield of 2,3-O-isopropylidene-5-O-methyl-β- -ribofuranosyl fluoride in which the methoxygroup had migrated from C-1 → C-5 and been replaced with retention of configurationby fluorine. The corresponding aldehyde when treated under similar conditions underwenta similar migration to give 5-deoxy-5-fluoro-2,3-O-isopropylidene-5-O-methyl-β- -ribofuranosylfluoride. A similar migration occurred with methyl 2′,3′-di-O-acetyl-β- -ribofuranosideand with acetyl 2,3-O-isopropylidene - -ribofuranose but not with 1,2,3-tri-O-acetyl- -ribofuranose. Thus the migration depends upon the migratory aptitude of thesubstituent at C-1 and the conformation of the furanose ring. Two ribofuranosyl fluorideswere used as starting materials from which to make nucleosides by the method of Noyoriand Hayoshi.     

Structural and rate studies of the formation of substituted benzynes

The key elimination step for the formation of 3-substituted and 3,6-disubstituted benzynes from 2-haloaryllithiums displays a pronounced solvent-dependent regioselectivity. All 2-haloaryllithiums with electron withdrawing groups in the 6 position are shown by 6Li and 13C NMR spectroscopic studies to be monomers in THF. DFT computational studies implicate trisolvates. Rate studies reveal that LiF eliminates via monomer-based pathways requiring THF dissociation whereas LiCl eliminates via nondissociative pathways. Elimination to form 3-chloro- and 3-fluorobenzyne from 2-chloro-6-fluorophenyllithium displays a pronounced solvent-dependent regioselectivity that is traced to competing solvent-dissociative and nondissociative dissociative pathways for the elimination of LiCl and LiF, respectively.     

Synthesis of 4-azepanones and heteroaromatic-fused azepines

Two syntheses of versatile intermediate azepinones 2 and 3 are described. A 6-step intramolecular Dieckmann cyclization and decarboxylation led to the intermediate 3 while an alternate 4-step synthesis of 2 was developed and used for scale-up. The highlight of the second synthesis is the one-step per-bromination/elimination protocol from readily available azepinone 13a to provide a versatile scaffold in vinyl bromide 5, which enables SAR around the aryl moiety. An example of the elaboration of the intermediate 2 toward a heteroaryl azepinone is also described.     

Ab initio molecular dynamics of excited-state intramolecular proton transfer around a three-state conical intersection in malonaldehyde

Excited-state potential energy surface (PES) characterization is carried out at the CASSCF and MRSDCI levels, followed by ab initio dynamics simulation of excited-state intramolecular proton transfer (ESIPT) on the S2(pipi*) state in malonaldehyde. The proton-transfer transition state lies close to an S2/S1 conical intersection, leading to substantial coupling of proton transfer with electronic relaxation. Proton exchange proceeds freely on S2, but its duration is limited by competition with twisting out of the molecular plane. This rotamerization pathway leads to an intersection of the three lowest singlet states, providing the first detailed report of ab initio dynamics around a three-state intersection (3SI). There is a significant energy barrier to ESIPT on S1, and further pyramidalization of the twisted structure leads to the minimal energy S1/S0 intersection and energetic terminal point of excited-state dynamics. Kinetics and additional mechanistic details of these pathways are discussed. Significant depletion of the spectroscopic state and recovery of the ground state is seen within the first 250 fs after photoexcitation.