Enols of 2‐nitro‐ and related 2‐substituted malonamides

The structures of 2‐substituted malonamides, YCH(CONR¹R²)CONR³R⁴ (Y = Br, SO₂Me, CONH₂, COMe, and NO₂) were investigated. When Y = Br, R¹R² = R³R⁴ = HEt; Y = SO₂Me, R¹–R⁴ = H and for Y = CONH₂ or CONHPh, R¹–R⁴ = Me, the structure in solution is that of the amide tautomer. X‐ray crystallography shows solid‐state amide structures for Y = SO₂Me or CONH₂, R¹–R⁴ = H. Nitromalonamide displays an enol structure in the solid state with a strong hydrogen bond (O^…O distance = 2.3730 Å at 100 K) and d(OH) ≠ d(O^…H). An apparently symmetric enol was observed in solution, even in appreciable percentages in highly polar solvents such as DMSO‐d₆, but K_enol values decrease on increasing the solvent polarity. The N,N′‐dimethyl derivative is less enolic. Acetylmalonamides display a mixture of enol on the acetyl group and amide in non‐polar solvents, and only the amide in DMSO‐d₆. DFT calculations gave the following order of pK_enol values for Y: H > CONH₂ > COMe ≥ COMe (on acetyl) ≥ MeSO₂ > CN > NO₂ in the gas phase, CHCl₃, and DMSO. The enol on the C?O group is preferred to the aci‐nitro compound, and the N? O? H^…O?C is less favored than the C?O? H^…O?C hydrogen bond. Copyright © 2009 John Wiley & Sons, Ltd.     

Morphological fluctuations in discharge-produced μc-Si:H studied by small angle X-ray scattering

Structural inhomogeneities of μc-Si:H in the range 20≈500 A are observed for the first time by small angle X-ray scattering. Inhomogeneous distributions of bonded hydrogen in the amorphous tissue are suggested to be a possible origin of the SAXS yield. Such inhomogeneities can be reduced when the average grain size and volume fraction of microcrystallites are simultaneously increased.     

Pentakis(trifluoromethyl)phenyl, a sterically crowded and electron-withdrawing group: synthesis and acidity of pentakis(trifluoromethyl)benzene, -toluene, -phenol, and -aniline

A general route to functionalized pentakis(trifluoromethyl)phenyl (C6(CF3)5) derivatives, promising building blocks for designing novel stable carbenes, radical species, superacids, weakly coordinating anions and other practically and theoretically useful species, is presented. This pertrifluoromethylation route proceeds via conveniently pregenerated (trifluoromethyl)copper (CF3Cu) species in DMF, stabilized by addition of 1,3-dimethyl-2-imidazolidinone (DMI). These species react with hexaiodobenzene at ambient temperature to give the potassium pentakis(trifluoromethyl)phenoxide along with hexakis(trifluoromethyl)benzene and pentakis(trifluoromethyl)benzene in a combined yield of 80%. A possible reaction pathway explaining the formation of pentakis(trifluoromethyl)phenoxide is proposed. Pentakis(trifluoromethyl)phenol gives rise to easily functionalized pentakis(trifluoromethyl)chlorobenzene and pentakis(trifluoromethyl)aniline. Pertrifluoromethylation of pentaiodochlorobenzene and pentaiodotoluene allows straightforward access to pentakis(trifluoromethyl)chlorobenzene and pentakis(trifluoromethyl)toluene, respectively. XRD structures of several C6(CF3)5 derivatives were determined and compared with the calculated structures. Due to the steric crowding the aromatic rings in all C6(CF3)5 derivatives are significantly distorted. The gas-phase acidities (Delta Gacid) and pKa values in different solvents (acetonitrile (AN), DMSO, water) for the title compounds and a number of related compounds have been measured. The origin of the acidifying effect of the C6(CF3)5 group has been explored using the isodesmic reactions approach.     

Structural effects on interconversion of oxygen-substituted bisketenes and cyclobutenediones

Cyclobutenediones 5 disubstituted with HO (a), MeO (b), EtO (c), i-PrO (d), t-BuO (e), PhO (f), 4-MeOC6H4O (g), 4-O2NC6H4O (h), and 3,4-bridging OCH2CH2O (i) substituents upon laser flash photolysis gave the corresponding bisketenes 6a-i, as detected by their distinctive doublet IR absorptions between 2075 and 2106 and 2116 and 2140 cm-1. The reactivities in ring closure back to the cyclobutenediones were greatest for the group 6b-e, with the highest rate constant of 2.95 x 10(7) s-1 at 25 degrees C for 6e (RO = t-BuO) in isooctane, were less for 6a (RO = OH, k = 2.57 x 10(6) s-1 in CH3CN), while 6f-i were the least reactive, with the lowest rate constant of 3.8 x 10(4) s-1 in CH3CN for 6h (RO = 4-O2NC6H4O). The significantly reduced rate constants for 6f-i are attributed to diminution of the electron-donating ability of oxygen to the cyclobutenediones 5f-h by the ArO substituents compared to alkoxy groups and to angle strain in the bridged product cyclobutenedione 5i. The reactivities of the ArO-substituted bisketenes 6f-h in CH3CN varied by a factor of 50 and gave an excellent correlation of the observed rate constants log k with the sigma p constants of the aryl substituents. Computational studies at the B3LYP/6-31G(d) level of ring-closure barriers are consistent with the measured reactivities. Photolysis of squaric acid (5a) in solution provides a convenient preparation of deltic acid (7).     

Experimental gas-phase basicity scale of superbasic phosphazenes

Seventeen superbasic phosphazenes and two Verkade's bases were used to supplement and extend the experimental gas-phase basicity scale in the superbasic region. For 19 strong bases the gas-phase basicity values (GB) were determined for the first time. Among them are such well-known bases as BEMP (1071.2 kJ/mol), Verkade's Me-substituted base (1083.8 kJ/mol), Et-N=P(NMe2)2-N=P(NMe2)3 (Et-P2 phosphazene, 1106.9 kJ/mol), and t-Bu-N=P(NMe2)3 (t-Bu-P1 phosphazene, 1058.0 kJ/mol). For the first time experimental GB values were determined for P2 phosphazenes. Together with our previous results self-consistent experimental gas-phase basicity scale between 1020 and 1107 kJ/mol is now established. This way an important region of the gas-phase basicity scale, which was earlier dominated by metal hydroxide bases, is now covered also with organic bases making it more accessible for further studies. The GB values for several superbases were calculated using density functional theory at the B3LYP/6-311+G** level. For the phosphazene family the standard deviation of the correlation between the experimental and theoretical values was 6.5 kJ/mol.     

Aminolysis of Y-substituted phenyl X-substituted cinnamates and benzoates: effect of modification of the nonleaving group from benzoyl to cinnamoyl

A kinetic study is reported for reactions of Y-substituted phenyl X-substituted cinnamates (1a-e and 3a-g) and benzoates (2a-e and 4a-g) with a series of alicyclic secondary amines in 80 mol % H2O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. Reactions of 2,4-dinitrophenyl X-substituted cinnamates (1a-e) and benzoates (2a-e) with amines result in linear Yukawa-Tsuno plots. The rho(X) values are much smaller for the reactions of 1a-e than for those of 2a-e. A distance effect and the nature of the reaction mechanism (i.e., a concerted mechanism for 1a-e) have been suggested to be responsible for the small rho(X) values. The Br?nsted-type plots for the reactions of 2,4-dinitrophenyl X-substituted cinnamates (1a, 1c, and 1e) with amines are curved with a decreasing betanuc value from 0.65 to 0.3-0.4. The reactions of Y-substituted phenyl cinnamates (3a-g) with morpholine also result in a curved Br?nsted plot, while the corresponding reactions of Y-substituted phenyl benzoates (4a-e) exhibit a linear Br?nsted plot. It has been concluded that the curved Br?nsted plots found for the reactions of the cinnamates (1a, 1c, 1e, and 3a-g) are not due to a change in the rate-determining step (RDS) but due to a normal Hammond effect for a concerted mechanism, that is, an earlier transition state (TS) for a more reactive amine or substrate.     

Effects of Y (Ⅲ) Ions on the Fluorescence Properties of Ce(Ⅲ) Ion and the Organic Ligands in the Complex

The enhancement effects of Y ( Ⅲ) ions on the fluorescence of Ce ( Ⅲ) in Ce ( Ⅲ)-Y ( Ⅲ)-PMMA (polymethylmethacrylate ) or Ce ( Ⅲ)-Y ( Ⅲ)-PVC (polyvinyl chloride ) complex systems were observed. The influence of Y ( Ⅲ) ions on the emission spectra of PMMA ligands in PMMA-Y ( Ⅲ) and the fluorescent enhance- ment of Y( Ⅲ) on Ce( Ⅲ) emission in PMMA-Ce-Y by Y( Ⅲ) ion were studied. It was also of interest to note that when Y ( Ⅲ) ions were added into PMMA and into bpy(bipyridine ), respectively, the emission spectrum of PMMA ligands was split into fine structure bands by Y ( Ⅲ), and the fluorescence intensities of bpy ligands in bpy-Y ( Ⅲ) complexes were considerably increased.     

Stacking of multilayer InAs quantum dots with combination capping of InAlGaAs and high temperature grown GaAs

We are reporting the growth of multilayer stacks of quantum dots (10 periods) with a combination capping of In_0.21Al_0.21Ga_0.58As (30 Å) and GaAs (70–180 Å) grown by solid source molecular beam epitaxy (MBE). Reflection high energy electron diffraction (RHEED) has been used for the insitu monitoring of quantum dot (QD) formation in heterostructure samples. The samples were also characterized by other exsitu techniques like cross sectional transmission electron microscopy (XTEM) and photoluminescence measurements (PL). For a heterostructure sample with thin barrier thickness (<100 Å), an XTEM image showed the stacking of QDs only up to the 5th layer and in the upper layers there was hardly any formation of dots. We presume the stoppage of dot formation is due to the uneven surface of the InAlGaAs alloy overgrown on the InAs QDs, as a result of the local compositional deviations of the Group-III atoms. Samples grown with thicker barriers (>100 Å of GaAs) showed good stacking of islands until the tenth layer. The thick GaAs layer overgrown on the InAlGaAs at 590 ^°C is believed to remove the surface modifications of the quaternary layer thereby creating a smoother surface front for the growth of subsequent QD layers.