Theoretical studies on the mechanism of acid-promoted hydrolysis of N-formylaziridine in comparison with formamide

[reaction: see text] We present an ab initio study of the acid-promoted hydrolysis reaction mechanism of N-formylaziridine in comparison with formamide. Since the rate of amide hydrolysis reactions depends on the formation of the tetrahedral intermediate, we focused our attention mainly on the reactant complex, the tetrahedral intermediate, and the transition state connecting these two stationary points. Geometries were optimized using the density functional theory, and the energetics were refined using ab initio theory including electron correlation. Solvent effects were investigated by using polarizable continuum method calculations. The proton-transfer reaction between the O-protonated and N-protonated amides was investigated. In acidic media, despite that the N-protonated species is more stable than the O-protonated one, it is predicted that both N-protonated and O-protonated pathways compete in the hydrolysis reaction of N-formylaziridine.     

Ruthenium(II)/(III) complexes of bidentate acetyl hydrazide Schiff bases

A series of octahedral Ru^II/Ru^III complexes of the type [Ru(Y)(CO)(BAX)(PPh₃)₂] and [RuCl₂(BAX)(PPh₃)₂] (Y = H or Cl; BAX = benzaldehydeacetylhydrazone anion; X = H, Me, OMe, OH, Cl or NO₂) have been prepared and characterised by spectral, magnetic and cyclic voltammetric studies. The Ru^II complexes are low spin diamagnetic (S = 0) whereas the Ru^III complexes are low spin and paramagnetic (S = 1/2). These Ru^II and Ru^III complexes absorb in the visible region respectively at ca. 16,000 and 28,000 cm^–1 which bands are assigned to the MLCT. The correlation of the _max values of the Ru^III complexes with the ⁺ Hammett parameter, is linear, indicating the profound effect of substituents on the electron density of the central metal. I.r. spectral data reveals that the hydrazone is chelated to ruthenium through the hydrazinic nitrogen and the deprotonated enolic oxygen. The rhombic nature of the e.s.r. spectra of the Ru^III complexes indicates an asymmetry in the electronic environment around the Ru atom. Ru^II complexes in CH₂Cl₂ show an irreversible Ru^II/III redox couple at ca. 0.9–0.5 V, while the Ru^III complexes show two reversible redox couples in the –0.1–0.1 and 0.8–0.6 V range, indicating that the higher oxidation state of ruthenium is stabilised by hydrazones.     

Investigation of the use of B3LYP zero-point energies and geometries in the calculation of enthalpies of formation

The use of B3LYP/6–31G^* zero-point energies and geometries in the calculation of enthalpies of formation has been investigated for the enlarged G2 test set of 148 molecules [J. Chem. Phys. 106 (1997) 1063]. A scale factor of 0.96 for the B3LYP zero-point energies gives an average absolute deviation nearly the same as scaled HF/6–31G^* zero-point energies for G2, G2(MP2), and B3LYP/6–311 + G(3df,2p) enthalpies. A scale factor of 0.98, which has been recommended in some studies, increases the average absolute deviation by about 0.2 kcal/mol. Geometries from B3LYP/6–31G^* are found to do as well as MP2/6–31G^* geometries in the calculation of the enthalpies of formation.     

A gravimetric and an X-ray fluorescence method for the determination of rubidium in Rb2U(SO4)3

Chemical characterization of rubidium uranium(IV) trisulfate, Rb₂U(SO₄)₃, a new chemical assay standard for uranium requires accurate analysis of rubidium. A gravimetric and an X-ray fluorescence method (XRF) for the determination of rubidium in this compound are described. In the gravimetric method, rubidium is determined as Rb₂Na[Co(NO₂)₆].H₂O without separating uranium with a precision of the order of ±0.5%. In the XRF method, the concentration ratio of rubidium to uranium, C_Rb/C_U, is determined in the solid samples by the binary ratio method using calibration between intensity ratios (I_Rb/I_U) and concentration ratios (C_Rb/C_U). The concentration of rubidium is derived using the uranium value which is known with a precision better than ±0.05%. The XRF method has a precision better than ±0.8% for rubidium determination.     

The ground-state singlet potential surface for C2H4

Calculations with an extended polarized basis set and Møller-Plesset perturbation theory including triple substitution correlation corrections in the fourth-order treatment indicate that singlet ethylidene (CH₃CH:) is not a local minimum on the C₂H₄ potential energy surface. Rearrangement to ethylene occurs without actuation. Barriers for hydrogen scrambling and for 1,1-hydrogen elimination are estimated.     

Electron spin resonance study of copper(II) complexes with square pyramidal and compressed trigonal bipyramidal symmetries in different solvents at 295 K and 77 K

An electron spin resonance (ESR) study has been carried out on six copper(II) complexes. Two of these, Cu(den)(SCN)₂ and [Cu(den)(SCN)](ClO₄), have a square pyramidal CuN₄S coordination in the solid state while the remaining four complexes, Cu(pib)tu, Cu(bipy)tu, Cu(bipy) and Cu(bipyam), have compressed trigonal bipyramidal Cu(N₂S)N₂ Cu(N₂S)N₂, Cu(N₂Cl)N₂ and Cu(N₂I)N₂, respectively, type of coordination in the solid state. Excepting Cu(pib)tu, all complexes show characteristic isotropic fourline hyperfine structure in water, methanol, ethanol and dimethylformamide at 295 K. At 77 K, owing to the poor glass-forming ability of water, the spectral features are not properly resolved and are also not consistent with the lineshape requirements. Therefore, evaluation of spin Hamiltonian parameters is made from the frozen dimethylformamide spectra. Some interesting observations have been made. In highly viscous 2-methyl-benzoxazole, the spectral lineshape is considerably modified. Quantitative evaluation has been made of metal-ligand bonding and various contributions to linewidths.     

Ultracentrifugation Method to Measure Water-Soluble Monomer Incorporation in Latex

Emulsion copolymerizations involving water-soluble functional monomers such as N-methylolacrylamide often result in the formation of water-soluble polymer that remains in the aqueous phase of the latex. A simple procedure is developed to determine the amount of water-soluble polymer in the aqueous phase of a high-solids latex using ultracentrifugation. Dilution of the latex with water is usually found necessary for good separation during ultracentrifugation. If the latex contains a surface-active species (surfactant) in amounts comparable to the amount of water-soluble polymer, calculation of the amount of water-soluble polymer in the serum of the original latex will require information about dilution dependent partitioning of the surfactant between the surface of the particles and the aqueous phase. This paper outlines a procedure that enables determination of the amount of water-soluble polymer in the aqueous phase of the original latex, by ultracentrifugation of the latex diluted to different concentrations. The procedure also gives the Henrys constant for the adsorption of the surfactant on the surface of the polymer particles in the latex.     

Singlet quenching of tetraphenylporphyrin and its metal derivatives by iron(III) coordination compounds

The quenching of fluorescence of the free-base tetraphenylporphyrin, H₂TPP, and its metal derivatives, MgTPP and ZnTPP by diverse iron(III) complexes, [Fe(CN)₆]³⁻, Fe(acac)₃, [Fe(mnt)₂]⁻, Fe(Salen)Cl, [Fe₄S₄(SPh)₄]²⁻·, FeTPPCl and [Fe(Cp)₂]⁺ has been studied both in homogeneous medium (CH₃CN) and micellar media, SDS., CTAB and Triton X-100. The quenching efficiencies are analysed in terms of diffusional encounters and it has been possible to separate static quenching components. The quenching constants are dependent on the nature of the ligating atoms around iron(III) and also on the extent of π-conjugation of the ligands. The quenching mechanism has been investigated using steady-state irradiation experiments. Evidence for oxidative quenching by iron(III) complexes was obtained, though the spin multiplicities of the excited electronic states of iron(III) complexes permit both energy and electron transfer mechanisms for quenching of the singlet excited state of the porphyrins.     

Hafnium oxide and zirconium oxide atomic layer deposition: initial precursor and potential side-reaction product pathways with H/Si(100)-2 x 1

Hybrid density functional calculations have been carried out using cluster models of the H/Si(100)-2 x 1 surface to investigate the mechanistic details of the initial surface reactions occurring in the atomic layer deposition of hafnium and zirconium oxides (HfO2 and ZrO2). Reaction pathways involving the metal precursors ZrCl4, Zr(CH3)4, HfCl4, and Hf(CH3)4 have been examined. Pathways leading to the formation of a Zr-Si or Hf-Si linkage show a significant sensitivity to the identity of the leaving group, with chloride loss reactions being both kinetically and thermodynamically less favorable than reactions leading to the loss of a methyl group. The energetics of the Zr(CH3)4 and Hf(CH3)4 reactions are similar with an overall exothermicity of 0.3-0.4 eV and a classical barrier height of 1.1-1.2 eV. For the reaction between H2O and the H/Si(100)-2 x 1 surface, the activation energy and overall reaction enthalpy are 1.6 and -0.8 eV, respectively. Due to contamination, trace amounts of H2O may be encountered by metal precursors, leading to the formation of minor species that can lead to unanticipated side-reaction pathways. Such gas-phase reactions between the halogenated and alkylated metal precursors and H2O are exothermic with small or no reaction barriers, allowing for the possibility of metal precursor hydroxylation before the H/Si surface is encountered. Of the contaminant surface reaction pathways, the most kinetically favorable corresponds to the surface -OH deposition. Interestingly, for the hydroxylated metal precursors, a unique reaction pathway resulting in the direct formation of Si-O-Zr and Si-O-Hf linkages has been identified and found to be the most thermodynamically stable pathway available, being exothermic by approximately 1.0 eV.