DFT study of the structure of hydroxybenzoic acids and their reactions with OH and radicals

B3LYP/6-31++G(d) method was used for the structural study of 3,4,5-trihydroxybenzoic acid (3,4,5-THBA), 3,4-dihydroxybenzoic acid (3,4-DHBA), and 4-hydroxybenzoic acid (4-HBA). Calculated structures agree with available X-ray experimental data within 2%. The phenolic OH bond dissociation enthalpy (BDE) of all sites in each benzoic acid were determined and compared with those of phenol (for 4-HBA) and catechol (for 3,4-DHBA). The consistency between the calculated values and experimental ones are within 5.4% and 9.2%, respectively, for 4-HBA and 3,4-DHBA. The reactions of benzoic acids with and OH radicals were studied and it turns out that benzoic acids react differently with both radicals. We have shown that the reaction of hydroxybenzoic acids with the hydroxyl radical was governed by a phenolic hydrogen (H⁺ + e⁻) transfer from the acid to the radical, while for the superoxide radical, the reaction is governed by a proton (H⁺) transfer from the acid to the radical. The first reaction is evidenced by the delocalization of the radical on the entire quinone moiety, and the second reaction is evidenced by the negative NBO charge on the phenoxide moiety as well as the localization of the radical on the hydroperoxy (O₂H) moiety.     

On restarted and deflated block FOM and GMRES methods for sequences of shifted linear systems

Numerical Algorithms - The problem of shifted linear systems is an important and challenging issue in a number of research applications. Krylov subspace methods are effective techniques for...     

Theoretical study of the C3S molecule

For the most stable linear isomer of C₃S in its X¹Σ⁺ state a six-dimensional potential energy surface (PES) has been calculated ab initio by coupled cluster – connected triples (CCSD(T)) method. The analytic form of the PES has been transformed in a quartic force field in dimensionless normal coordinates and employed in calculations of spectroscopic constants using second-order perturbation theory. The PES and the full kinetic energy operator in internal coordinates have been used to calculate variationally the anharmonic ro-vibrational energies for J=0 and J=1. The two experimental band origins of C₃S observed in the gas phase, ν₁ and ν₁+ν₅−ν₅, agree very well with the theoretical values. The anharmonic ro-vibrational levels, including the bending modes up to 2200 cm⁻¹, are reported. The singlet ground state PES has a saddle point at about 1.25 eV above the linear minimum and two other higher lying cyclic local minima. The only dipole- and spin-allowed electronic transition between 0 and 5 eV is calculated to be the ¹Π−X¹Σ⁺ transition with a vertical transition energy of 353.2 nm in good agreement with the matrix value of 378 nm. The dissociative paths C + C₂S, C₂ + CS and C₃ + S of low lying singlet and triplet states have been investigated. Electronic Supplementary Material: Supplementary material is available in the online version of this article at dx.doi.org/10.1007/s00214-005-0683-7 Dedicated to Professor H. Stoll.     

Structural and optical properties of a-Si1−xCx:H films synthesized by dc magnetron sputtering technique

Hydrogenated amorphous SiC films (a-Si_1−xC_x:H) were prepared by dc magnetron sputtering technique on p-type Si(1 0 0) and corning 9075 substrates at low temperature, by using 32 sprigs of silicon carbide (6H-SiC). The deposited a-Si_1−xC_x:H film was realized under a mixture of argon and hydrogen gases. The a-Si_1−xC_x:H films have been investigated by scanning electronic microscopy equipped with an EDS system (SEM-EDS), X-ray diffraction (XRD), secondary ions mass spectrometry (SIMS), Fourier transform infrared spectroscopy (FTIR), UV-vis-IR spectrophotometry, and photoluminescence (PL). XRD results showed that the deposited film was amorphous with a structure as a-Si_0.80C_0.20:H corresponding to 20 at.% carbon. The photoluminescence response of the samples was observed in the visible range at room temperature with two peaks centred at 463 nm (2.68 eV) and 542 nm (2.29 eV). In addition, the dependence of photoluminescence behaviour on film thickness for a certain carbon composition in hydrogenated amorphous SiC films (a-Si_1−xC_x:H) has been investigated.     

Nitrobenzoxadiazoles and related heterocycles: a relationship between aromaticity, superelectrophilicity and pericyclic reactivity

A study of the dual electrophilic and pericyclic reactivity of 4,6-dinitrobenzofurazan (DNBZ, 2), 4,6-dinitro-2,1,3-benzothiadiazole (DNBS, 3), 4,6-dinitro-2,1,3-benzoselenadiazole (DNBSe, 4) is reported. Kinetic and thermodynamic measurements of the ease of covalent hydration of 2-4 to give the corresponding hydroxy sigma-adducts C-2-C-4 have been carried out over a large pH range in aqueous solution. Analysis of the data has allowed a determination of the rate constants k1(H2O) pertaining to the susceptibility of 2-4 to water attack as well as the pKa values for the sigma-complexation processes. With pKa values ranging from 3.92 for DNBZ to 6.34 for DNBSe to 7.86 for DNBS, the electrophilic character of the three heteroaromatics is much closer to that of the superelectrophilic reference, i.e. 4,6-dinitrobenzofuroxan (DNBF, 1; pKa = 3.75), than that of the standard Meisenheimer electrophile 1,3,5-trinitrobenzene (TNB, pKa = 13.43). Most importantly, water is found to be an efficient nucleophile which contributes strongly to the formation of the adducts C-2 and C-4. This confirms a previous observation that a pKa value of ca. 8 is a primary requirement for having H2O competing effectively as a nucleophile with OH- in the formation of hydroxy sigma-adducts. On the other hand, 2-4 are found to exhibit dienophilic and/or heterodienic behaviour on treatment with isoprene, 2,3-dimethylbutadiene, cyclopentadiene or cyclohexadiene, affording Diels-Alder mono- or di-adducts which have all been structurally characterized. A major finding is that the order of Diels-Alder reactivity follows clearly the order of electrophilicity, pointing to a direct relationship between superelectrophilic and pericyclic reactivity. This relationship is discussed.     

A theorem of Riesz type with Pettis integrals in topological vector spaces

Let X be a locally convex Hausdorff space and let C₀(S,X) be the space of all continuous functions f:S→X, with compact support on the locally compact space S. In this paper we prove a Riesz representation theorem for a class of bounded operators T:C₀(S,X)→X, where the representing integrals are X-valued Pettis integrals with respect to bounded signed measures on S. Under the additional assumption that X is a locally convex space, having the convex compactness property, or either, X is a locally convex space whose dual X^′ is a barrelled space for an appropriate topology, we obtain a complete identification between all X-valued Pettis integrals on S and the bounded operators T:C₀(S,X)→X they represent. Finally we give two illustrations of the representation theorem proved, in the particular case when X is the topological dual of a locally convex space.     

4-Nitrobenzodifuroxan: a highly reactive nitroolefin in Diels-Alder reactions

The reactions of 4-nitrobenzodifuroxan (NBDF) with a series of common dienes are found to proceed very readily to afford stable cycloadducts, which are the result of highly stereoselective normal electron demand (NED) Diels-Alder reactions. Due to the additional activation provided by the two adjacent furoxan rings, the nitroolefinic double bond of NBDF is also prone to undergo NED reactions with less reactive dienic structures such as the enol form of ethoxymethyleneacetylacetone and the in situ generated 2-ethoxy-4-(2-furfuryl)buta-1,3-diene. A number of X-ray structures could be obtained, which leave no doubt as to the stereochemistry of the resulting cycloadducts. A rationalization of the reactions in terms of the electrophilicity parameter ω defined by Parr suggests that all cycloadditions proceed with a notable polar character.