Structural and energetic aspects of the protonation of phenol,catechol, resorcinol,and hydroquinone

The various protonated forms of phenol (1), catechol (2), resorcinol (3), and hydroquinone (4) were explored by ab initio quantum chemical calculations at the MP2/6-31G(d) and B3LYP/6-31G(d) levels. Proton affinities (PA) of 1-4 were calculated by the combined G2(MP2,SVP) method, and their gas-phase basicities were estimated after calculation of the change in entropy on protonation. These theoretical data were compared with the corresponding experimental values determined in a high-pressure mass spectrometer. This comparison confirmed that phenols are essentially carbon bases and that protonation generally occurs in a position para to the hydroxyl group. Resorcinol is the most effective base (PA = 856 kJ mol-1) due to the participation of both oxygen atoms in the stabilization of the protonated form. Since protonation is accompanied by a freezing of the two internal rotations, a significant decrease in entropy is observed. The basicity of catechol (PA = 823 kJ mol-1) is due to the existence of an intramolecular hydrogen bond, which is strengthened upon protonation. The lower basicity of hydroquinone (PA = 808 kJ mol-1) is a consequence of the fact that protonation necessarily occurs in a position ortho to the hydroxyl group. When the previously published data are reconsidered and a corrected protonation entropy is used, a proton affinity value of 820 kJ mol-1 is obtained for phenol.     

Determination of arsenic in mussels by slurry sampling and electrothermal atomic absorption spectrometry (ETAAS)

A method for the determination of arsenic in slurries of mussel tissue using palladium-magnesium nitrate as modifier was optimized. The slurry was stabilized by a 0.015% (v/v) of Triton X-100. To achieve complete mineralization the slurries were ashed at 480 °C for 10s in an air flow (50 ml/min) and at 1200 °C for 15s in an argon flow (300 ml/min) in the presence of Pd—Mg(NO₃)₂ as modifier. The optimum atomization temperature was 2200 °C. The precision and accuracy of the method were studied using the Reference Material BCR n ° 278 Mussel Tissue (Mytilus edulis). The detection limit (LOD) of the final slurry solution was 1 g/l of arsenic corresponding to an arsenic level in the mussel of 1.3 g/g, for a 0.5% (m/v) slurry. Results of calibration using aqueous standards and the standard additions method were compared. The method was applied to the determination of arsenic in mussels from the Galician coast. The levels found lie between 2 and 9.3 g/g of arsenic.     

On-line preconcentration and determination of mercury in biological samples by flow injection vapour generation inductively coupled plasma atomic emission spectrometry

An FI-ICP-AES method for the determination of trace levels of mercury in biological samples has been described, which is based on the extraction of the mercury complex with 1,5-bis (di-2-pyridyl)methylene thiocarbonohydrazide (DPTH) on-line into isobuthyl-methyl ketone (IBMK). The organic phase (containing the complex) has been mixed on-line with SnCl₂ in N,N-dimethylformamide. Thus, mercury vapour can be generated directly from the organic phase and separated in a gas-liquid separation device. The detection limit for mercury is 4 ng/ml and the calibration curve is linear at least from 10 to 2500 ng/ml. The relative standard deviation for 10 replicate measurements is ±1% for 100 ng/ml of Hg(II). Results from the analysis of some certified biological reference materials are given.     

Quantum chemical study of several monocyclic complex β‐lactam C‐3, C‐4, and N‐derivatives,and β‐ring model molecules

A quantum chemical study of several complex monocyclic 4‐benzoyl‐4‐phenyl‐β‐lactam derivatives was carried out using cyclobutane, azetidine, 2‐azetidinone, 1‐methyl‐2‐azetidinone, and 3‐methyl‐2‐azetidinone as model compounds. The optimum geometry was obtained for the different conformations. The planarity of the ring was discussed in terms of the influence of the substituents on the amide resonance. To better analyze the amide resonance and the activity of the β‐lactam ring, a vibrational study was also carried out. To examine the influence of solvent polarity on the carbonyl bands, the Fourier transform–infrared (FT‐IR) spectra of the β‐lactam monocyclic derivatives were recorded in CCl₄, C₆H₆, and CHCl₃ solutions. The normal vibrations of the β‐lactam ring in the model compounds were characterized and used in the analysis of the β‐ring of more complex derivatives. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Reaction of tetrahydrobenz[a]acridinones with hydroxylamine hydrochloride. VII

Oximation of ortho-substituted phenylbenz[a]acridinones using hydroxylamine hydrochloride, sodium hydroxide and ethanol as the solvent gave always the benzoquinacridine N-oxide 2 . Oximation of para-substituted phenylbenz[a]acridinones, however, gave only the corresponding oximes. The structure of all products was corroborated by ir, ¹H and ¹³C-nmr and mass spectral data. Theoretical calculations support the experimental findings.     

Unsupported gold(I)-copper(I) interactions through eta1 Au-[Au(C6F5)2]- coordination to Cu+ Lewis acid sites

The reaction of the complex [Au2Ag2(C6F5)4)N[triple bond]CCH3)2]n (1) with 1 equiv of CuCl in the presence of 1 equiv of pyrimidine ligand leads to the formation of the heteronuclear Au(I)-Cu(I) organometallic polymer [Cu{Au(C6F5)2}(N[triple bond]CCH3)(mu2-C4H4N2)]n (2) through a transmetalation reaction. Complex 2 displays unprecedented unsupported Au(I)...Cu(I) interactions of [Au(C6F5)2]- units with the acid Cu(I) sites in a [Cu(N[triple bond]CCH3)(mu2-pyrimidine)]n+(n) polymeric chain. Complex 2 has a rich photophysics in solution and in the solid state.     

A comparative study on the photochemistry of two bipyridyl derivatives: [2,2'-bipyridyl]-3,3'-diamine and [2,2'-bipyridyl]-3,3'-diol.

The two isoelectronic bipyridyl derivatives, [2,2'-bipyridyl]-3,3'-diamine and [2,2'-bipyridyl]-3,3'-diol, are experimentally known to undergo very different excited-state double-proton-transfer processes, which result in fluorescence quantum yields that differ by four orders of magnitude. Herein, density functional theory (DFT), time-dependent DFT (TDDFT), and complete active space self-consistent field (CASSCF) calculations are used to study the double-proton-transfer processes in the ground and first singlet pi-->pi* excited state. The quantum-chemistry calculations indicate 1) the existence of only one energy minimum in the ground electronic state corresponding to reactants (thus avoiding the possibility of a fast fluorescent relaxation process from the photoproducts region), 2) an endoergic process of the complete double proton transfer, and 3) the presence of a conical intersection in the excited intermediate region of [2,2'-bipyridyl]-3,3'-diamine. These facts explain the very low fluorescence quantum yield in [2,2'-bipyridyl]-3,3'-diamine compared to [2,2'-bipyridyl]-3,3'-diol.     

Synthesis and structure determination of Co(HNCN)2 and Ni(HNCN)2

Well-crystallized cobalt and nickel hydrogencyanamide, Co(HNCN)₂ and Ni(HNCN)₂, were synthesized from the corresponding ammonia complexes [M(NH₃)₆]²⁺ under aqueous cyanamide conditions. The X-ray and neutron powder data evidence the orthorhombic system and space group Pnnm. The cell parameters for Co(HNCN)₂ are a=6.572(1), b=8.805(2), c=3.267(1) Å, and Z=2; for the isotypic Ni(HNCN)₂, the cell parameters arrive at a=6.457(1), b=8.768(2), c=3.230(1) Å. The octahedral coordinations of the metal ions are marginally squeezed, with interatomic distances of 4×Co-N=2.134(5) Å, 2×Co-N=2.122(9) Å, and 4×Ni-N=2.133(6) Å, 2×Ni-N=2.035(11) Å. The HNCN⁻ units appear as slightly bent (177.5(2)° for Co(HNCN)₂ and 175.7(2)° for Ni(HNCN)₂) and exhibit cyanamide shape character due to triple- and single-bond C-N distances (1.20(2) vs. 1.33(2) Å for Co(HNCN)₂ and 1.15(2) vs. 1.38(2) Å for Ni(HNCN)₂). The infrared vibration data compare well with those of the three existing alkali-metal hydrogencyanamides.     

A LASER FLASH SPECTROSCOPY STUDY OF THE KINETICS OF ELECTRON TRANSFER FROM SPINACH PHOTOSYSTEM I TO SPINACH AND ALGAL FERREDOXINS

Laser flash absorption spectroscopy has been used to investigate the kinetics of electron transfer from P700 in Photosystem I (PSI)-enriched particles from spinach to the ferredoxins from spinach and the green alga Monoraphidium braunii. Very similar behavior for the interaction of both ferredoxins with the PSI complex was observed, although the algal ferredoxin appears to be slightly more effective as an electron acceptor. For both proteins, a non-linear protein concentration dependence of the rate constant for reduction was obtained, indicating complex formation preceding electron transfer. Estimates of 3 times 10⁷M^?1 s^?1 and 140–180 s^?l were obtained from these data for the second order rate constants for complex formation, and the limiting first order rate constants for electron transfer, respectively. At neutral pH, a biphasic dependence of the rate constant for ferredoxin reduction on the concentration of NaCl or MgCl₂ was observed. This was interpreted in terms of the electrostatic interactions which occur between ferredoxin and the PSI membrane. In addition, magnesium cations appear to play a specific role in the interaction between PSI and ferredoxin. Thus, the addition of these ions under optimal conditions induces a 6-f-old increase in the electron transfer reaction rate constant, compared with a 2-f-old increase in the presence of an optimal amount of NaCI. This cannot be explained as arising from ionic strength effects. To our knowledge, this is the first time that a direct measurement of the rate constant for the reduction of ferredoxin by the PSI complex has been reported.