bluuebunny/crossref_metadata_2025 huggingface dataset:
The photochemistry of EDA complexes in micellar solutions. II. The stilbene–methylviologen system in the presence of oxygen
Authors: Hamity M. , Lema R. H.
Published: May 2006

The photolyses of electron donor–acceptor (EDA) complexes formed between both cis-stilbene (cS) and trans-stilbene (tS) as donors and mefhylviologen (MV+2) as acceptor have been carried out in homogeneous ethanolic and in micellar sodium dodecyl sulfate (SDS) solutions, in the presence of oxygen. The stilbene loss quantum yields (Φ−S) have been determined in both media. Quantum yields were observed to be dependent upon methylviologen concentration and the acidity of media. A reaction scheme is proposed, which accounts for experimental results and can be related to the photochemistry of MV+2 and its cation radical MV•+. Key words: EDA complexes, micelles, methyl viologen, stilbene.
Synthesis and X-ray crystal structure determination of 3,4-diphenyl-6-(diphenylmethyl)-1,5-dioxa-2-thiane-2-oxide
Authors: Breau L. , Sharma N. K. , Butler I. R. , Durst T
Published: May 2006

Erythro and threo-1,2-diphenyl-2-tert-butylsulfinylethanols 5 and 8 were prepared from the condensation of the α-lithio derivative of benzyl-tert-butyl sulfoxide with benzaldehyde. Reaction of 5 with sulfuryl chloride gave the title compound, (2R*,3S*,4R*,6R*)-3,4-diphenyl-6-(diphenylmethyl)-1,5-dioxa-2-thiane-2-oxide 6, the first representative of a new heterocyclic ring system. Similar treatment of 8 resulted in the formation of 7, a stereoisomer of 6. This same compound was also obtained by photolysis of 6. In both 6 and 7 the S=O bond is in an axial orientation. Key words: sulfuryl chloride, reaction with β-hydroxy sulfoxides; 1,2-diphenyl-tert-butylsulfinylethanol; β-hydroxysulfoxides; β-sultines; N-chlorosuccinimide, reaction with β-hydroxysulfoxides.
An ab initio study of symmetry breaking in calculations on the first excited singlet state of N2H2
Authors: Del Bene Janet E. , Kim Kyungsun , Shavitt Isaiah
Published: May 2006

Calculation of the vertically excited singlet n → π* state of trans-N2H2 carried out in Cs symmetry at the RHF and RHF-CISD levels results in a broken symmetry solution in which the wave function does not reflect the C2h symmetry of the molecular framework. The excited state RHF energy computed in Cs symmetry is lower than the corresponding energy computed in C2h symmetry, but the single-reference CISD energy based on the Cs RHF configuration is higher than the corresponding C2h result. The symmetry breaking leads to difficulties in the description of the excited state potential energy surface, including the location of stationary points and the calculation of vibrational frequencies, and in the treatment of complexes of excited N2H2 with other molecules. The molecular symmetry of the vertically excited state can be restored in Cs calculation by the use of a four-configuration MCSCF wave function. The MCSCF procedure provides consistent data and leads to the prediction that the relaxed excited state has C2 molecular symmetry. Unlike the RHF case, the UHF wave function does not display symmetry breaking at the C2h geometry, and agrees with the MCSCF model in predicting an equilibrium structure of C2 symmetry. A low-energy structure of C2 symmetry has also been found with the UMP2 model, although it could not be positively confirmed that it corresponds to an equilibrium structure. The symmetry-breaking dilemma is not unique to the excited n → π* state of N2H2, but arises in other molecules in which the two chromophores are similarly symmetry related. Key words: symmetry breaking, excited state, MCSCF, N2H2.
Étude conformationnelle et structurale de 1,5-diacétoxy-3,4-bis(bicyclophosphoranylato)quinate de méthyle
Authors: Baltas Michel , Chahoua Latifa , Gorrichon Liliane , Tisnès Pierre , Houalla Douraid , Wolf Robert , Jaud Joël
Published: May 2006

In connection with our work on quinic acid derivatives, which are supposed to regulate the biosynthesis of aromatic amino acids, we have prepared the methyl 1,5-di-O-acetyl-3,4-bis-O-(bicyclophosphoranylato)quinate 7. The compound obtained has been characterized by NMR (1H and 31P), IR, elemental analysis, and mass spectrometry. Its molecular structure has been determined by X-ray diffraction and an exhaustive conformational study has been accomplished. The quinic ring adopts an almost perfect chair form. Both bicyclophosphoranyl groups present a trigonal bipyramidal geometry with an axial–equatorial–axial type annelation. The pentagonal rings adopt an envelope form in which the tetrasubstituted carbon atom is at the top of the fold. Key words: shikimic acid pathway, methyl 1,5-di-O-acetyl-3,4-bis-O-(bicyclophosphoranylato)quinate, conformational analysis, 1H and 31PNMR, crystal structure.
Isobaric vapour–liquid equilibria of 1-bromobutane + chloroethane systems
Authors: Rambabu K. , Venkateswarlu P. , Raman G. K. , Kumar Y. V. L. Ravi
Published: May 2006

The boiling point data for the binary liquid mixtures of 1-bromobutane with 1,2-dichloroethane, 1,1,1-trichloroethane, and 1,1,2,2-tetrachloroethane were obtained over the entire composition range by using a standard Swietoslawski type ebulliometer at 715 Torr. The experimental t–x data were used to estimate Wilson parameters and also to calculate the equilibrium vapor compositions for these systems. These Wilson parameters were used to calculate activity coefficients and these in turn were used to compute excess Gibbs free energy. Key words: Wilson parameters, 1-bromobutane + chloroethane systems.
Excess properties for binary liquid mixtures of propionic acid with aniline derivatives
Authors: Garcia Begoña , Alcalde Rafael , Leal José M.
Published: May 2006

Excess viscosities, excess volumes, and excess free energies of activation for flow of the binary mixtures containing propionic acid with aniline, o-toluidine, o-anisidine, and o-chloroaniline were determined at five different temperatures from density and viscosity measurements. Based on the Regular Solutions Theory, the empirical parameter θ was calculated; this parameter is related to the viscosity, and also denotes the extent of the non-ideality of the system. The results obtained suggest the existence of acid–base interactions, as well as the formation of 1:2 complexes for the three first mixtures, and 2:1 complexes for o-chloro-aniline + propionic acid. The extent of the specific interaction decreases with an increase in the strength of the base, and increases with an increase in the dipole moments. The equatioin [Formula: see text] is proposed for predicting the mole volume of the mixtures, V, at any composition, Xi, as a function of the mole volumes of the pure components, Vi. Key words: binary mixture, excess properties, predicting mole volumes, propionic acid.
The conformational behaviour of 2-aryloxytetrahydropyrans and 2-acetoxytetrahydropyran, and barrier to ring inversion
Authors: Ouedraogo Adama , Lessard Jean
Published: May 2006

The 13C nuclear magnetic resonance data of a series of 2-(4-substituted-phenoxy)tetrahydropyrans at 156 K and in CF2Br2 and CHFCl2 solvents show that the axial preference increases with electron withdrawal in the aryloxy group: from 79% (ΔG°E→A = −0.4 kcal mol−1) (4-OCH3) to 90% (ΔG°E→A = −0.7 kcal mol−1) (4-NO2) in CF2Br2. The axial preference (anomeric effect) is smaller in the more polar CHFCl2 solvent, as expected, and the substituent effect is smaller also: change in ΔG°E→A from −0.3 (4-OCH3) to −0.5 (4-NO2) kcal mol−1. However, the axial preference of 2-acetoxytetrahydropyran is shown to be smaller than that of 2-phenoxytetrahydropyran in CF2Br2 solvent although the acetoxy group is expected to be more electronegative than the phenoxy group. Furthermore, the polarity of the solvent has no effect on the conformational equilibrium of 2-acetoxytetrahydropyran. The results are interpreted in terms of both orbital and electrostatic interactions. The ring-inversion barrier of 2-alkoxy and 2-aryloxytetrahydropyrans, determined by the temperature-coalescence method, is lower than that of tetrahydropyran by about 1.5 kcal mol−1. Key words: 2-substituted tetrahydropyrans, conformational analysis, anomeric effect, ring inversion.
The proximate coupling constant, 5J(H,CH3), and the torsional mobility of the thiomethyl group in some thioanisole derivatives
Authors: Schaefer Ted , Sebastian Rudy , Salman Salman R. , Baleja James D. , Penner Glenn H. , McKinnon David M.
Published: May 2006

The proximate coupling constants, 5J, between ortho and methyl protons in thioanisole and 18 of its derivatives are discussed as conformational indicators. On the assumption that 5J varies as cos4θ, for 0° ≤ θ ≤ 90°, θ being the angle by which the methyl group twists out of the aromatic plane, 5J for θ = 0° follows as −0.43 (2) Hz from the known internal barrier in thioanisole in solution. A measurement of 5J in meta- or para-substituted thioanisole derivatives then yields an approximate value for the twofold barrier to rotation about the Csp2—S bond. For derivatives containing an ortho substituent, 5J yields an estimate of the torsion angle for the thiomethyl moiety. In some instances these angles are compared with those derived from long-range 1H, 13C and 13C, 13C coupling constants. The size of the ortho substituent appears to have only a small effect on the magnitude of 5J. The major determinant of the latter appears to be the manner in which the substituent perturbs the mobile bond order of the Csp2—S bond. Key words: spin–spin coupling constants, thioanisole derivatives; 1H NMR, thioanisole derivatives; conformations, thioanisole derivatives; conformations, torsional motion of SCH3 group.
Dimerization and continuous association including formation of cyclic dimers
Authors: Apelblat Alexander
Published: May 2006

New aspects of the theory of ideal associated mixtures related to a differentiation between formed cyclic and linear dimers are discussed. A mathematical analysis is presented for the dimerization model, A + B + A2, the continuous association (Mecke–Kempter) model, [Formula: see text], both coupled with the cyclic dimmer → linear dimer transformation and for the unsymmetrical (mixed) dimer formation model, A + B + A2 + B2 + AB. Introduction of the standard reaction enthalpies and volumes associated with transformations of dimers leads to a considerable change in behavior and symmetry properties of the excess thermodynamic functions. In terms of the modified Mecke–Kempter model, a consistent representation of the excess Gibbs energy of mixing GE, heat of mixing HE, and excess molar volume VE is reported for the acetic acid – water system at 298.15 K. Key words: association, dimerization, linear dimers, cyclic dimers, hydrogen bonding, carboxylic acids, alcohols, aqueous solutions.
Influence électronique et stérique du ligand tétraméthylcyclopentadiényle sur les propriétés physiques et sur la réactivité de complexes du titane et du zirconium
Authors: Courtot P. , Pichon R. , Salaun J. Y. , Toupet L.
Published: May 2006

The complexes (C5Me4H)2MR1R2 (M = Ti, Zr) and (C5Me4H)(C5H5)MR1R2 (M = Ti) have been prepared (R1 = R2 = Cl, CH3, C6H5, p-C6H4CH3, CO; R1 = Cl, R2 = CH3, C6H5, p-C6H4CH3). The NMR and IR spectra of these complexes indicate that C5Me4H exhibits an electron-donor effect more important than C5H5 and a little bit weaker than that of C5Me5. A structural investigation of the complex (C5Me4H)2Zr(C6H5)Cl and the study of the 1H or 13C dynamic NMR properties of the compounds (C5Me4H)2MR1R2 (M = Ti, Zr; R1 = R2 = C6H5, p-C6H4CH3; R1 = Cl, Br, R2 = C6H5, p-C6H4CH3) show clearly that C5Me4H has a steric effect intermediate between C5H5 and C5Me5. The steric or electronic effects of C5Me4H have been used to induce specific isomerization reactions of various alkenes by using (C5Me4H)2Ti(CH3)2 as catalyst. Key words: zirconium, titanium, tetramethylcyclopentadienyl, photo-assisted, isomerization.
Total synthesis of d,l-quassin
Authors: Stojanac Nada , Valenta Zdenek
Published: May 2006

The completion of the total synthesis of d,l-quassin 1 is described. Key steps are a facile intramolecular acylation of a protected cyanohydrin carbanion (4 → 5), a selective deprotonation with trioctyl methyl ammonium hydroxide leading to the removal of HCN from 8, and the unusual sequence 5 → 12 in which the ring C enol ether of 5 is converted selectively into ring C diosphenol of 12. Key words: quassin, synthesis, selective base, diosphenols, acylation.
Comparison of chemical and electrochemical reduction of cyanamides: an example of cathodic decyanation
Authors: Cariou Michel , Simonet Jacques
Published: May 2006

Most disubstituted cyanamides exhibit a very low cathodic activity and are therefore difficult to reduce. However, the cathodic cleavage of diarylcyanamides can be carried out directly and leads to the corresponding diarylamine; the action of sodium causes the same cleavage. Alkylarylcyanamides display no cathodic step, but can be reduced indirectly by redox catalysis; this allows the determination of the standard potential corresponding to the first electron transfer. The macroscale electrolysis, in the presence of the catalyst, leads to the alkylarylamine by cleavage, whereas the action of sodium gives mainly a cyanoguanidine. Dialkylcyanamides are electroreducible neither directly, nor indirectly, whereas by action of sodium they trimerize almost quantitatively into triazines. Key words: cyanamide, decyanation, electroreduction, redox catalysis.
Vibrational spectra of two new organic semiconductors: tetrathiafulvalene (TTF) and tetramethyltetraselenafulvalene (TMTSF) salts of paranitrophenylmalononitrile (PNMA)
Authors: Truong K. D. , Bandrauk A. D. , ZAUHAR J. , Carlose C.
Published: May 2006

Two new complexes of stoichiometry 2:1 are reported for the donors tetrathiafulvalene (TTF) and tetramethyltetraselenafulvalene (TMTSF) with the acceptor paranitrophenylmalononitrile (PNPMA). Both compounds are semiconductors with a resistivity of about 4 × 10−4 Ω m for (TMTSF)2PNPMA and 0.58 Ω m for (TTF)2PNPMA. The larger conductivity of the first complex can be attributed to the disorder of the PNPMA anions. Vibrational spectra were obtained by FTIR and Raman spectroscopy, in order to determine the degree of charge transfer in these systems. Both complexes have the electron distribution (D+0.4)2A−0.8. As a result the donors D stack in tetramerized units and exhibit vibronic activation of certain symmetric monomer modes, thus indicating the presence of strong electron–vibrational interactions in the donor stacks. Key words: TTF and TMTSF salts, charge transfer complexes, IR and Raman spectra, degree of charge transfer, paranitrophenylmalononitrile (PNPMA).
Conformation and dynamics in solution of an N-quaternarized benzophenone derivative: a molecule active as UV filter
Authors: Anselmi Cecilia , Centini Marisanna , Scotton Mirella , Sega Alessandro
Published: May 2006

The dynamics and conformation of N,N-dimethyl-N-|3-(benzoyl-4-phenoxy)|-N-n-dodecylammonium bromide, 1, have been established in two solvents (CDCl3 and DMSO-d6) by the use of 13C spin-lattice relaxation rates, non-selective and selective proton spin-lattice relaxation rates, and 1H–{1H} nuclear Overhauser enhancement (nOe) experiments. The data obtained are consistent with two main mean conformations for compound 1: a "linear" conformation in CDCl3 and a folded conformation in DMSO-d6 where the alkyl chain forms a loop toward the aromatic moiety. Key words: UV filter, carbon and proton relaxation rates, nuclear Overhauser enhancement experiments, solvent dependent conformations.
The structure of the CC1n− (n = 1–4) anions and electronic affinities of the corresponding neutrals
Authors: Gutsev G. L. , Ziegler Tom
Published: May 2006

Electronic and geometrical structures for the ground states of the singly charged anions CCln−, n = 1–4, and Clm−, m = 2, 3, as well as the corresponding neutral species have been studied by Approximate Density Functional Theory (DFT). A full geometry optimization has been carried out on both anionic and neutral species. Vertical and adiabatic electron affinities were evaluated for the neutral species along with an estimate of the first ionization potential (FIP) for the anions. A discussion is given of the different dissociation channels for CCl4−. It follows from the present set of calculations that the anion CCl4− is non-rigid with a global minima corresponding to an elongated tetrahedron of C3v symmetry. Key words: Density Function Theory, electronic affinities.
Ab initio studies of isocyanatoborane and difluoroisocyanatoborane; bent or linear BNCO chains?
Authors: Ellis Susan , Livingstone Edward G. , Westwood Nicholas P. C.
Published: May 2006

Ab initio calculating at the 3-21G, 6-31G*, 6-31G**, 6-311G*, and 6-311G** Hartree–Fock levels, have been performed on the unknown H2BNCO and F2BNCO molecules in order to establish the geometries of these isoelectronic propadienone analogues. For H2BNCO the double split valence basis sets lead to linear BNCO chains, whereas either a triple split valence basis set, or the inclusion of correlation to second order (MP2/6-31G*) gives trans-bent structures. These have angles at nitrogen of 153.6° (6-311G*) or 149.9° (MP2/6-31G*), with the potential surface for angle bending extremely flat, and 0.5 kJ mol−1 (6-311 G*) or 1.12 kJ mol−1 (MP2/6-31 G*) separating the C2v and Cs structures. For the bent structures there is a small trans-bend (4–6°) at the carbon atom. The F2BNCO molecule is also linear at the 3-21G level, but is, however, already trans-bent (145.6° at nitrogen, 175.9° at carbon) at the 6-31G* Hartree–Fock level; 1.47 kJ mol−1 separates the bent and linear structures. The triple split valence basis set 6-311G* leads to a further decrease in the angle at nitrogen (141.0°), and a similar NCO angle (175.9°), with the bent structure favoured by 3.85 kJ mol−1. MP2/6-31G* calculations give a minimum with an angle at nitrogen of 140.2°, and a bent-linear energy difference of 3.58 kJ mol−1. Key words: ab initio calculations, isocyanatoboranes, structures, quasilinearity, propadienone analogues.
Rate of the OH + C6H6 + He reaction in the fall-off range by discharge flow and OH resonance fluorescence
Authors: Goumri A. , Pauwels J. F. , Devolder P.
Published: May 2006

The rate constant k1 of the reaction[Formula: see text]has been investigated by discharge flow resonance fluorescence of OH in the fall-off pressure range. From systematic measurements at five pressures between 0.5 and 9.5 torr, the Troe parameters k0 and k∞ (with Fc = 0.6) have been derived at two temperatures: room temperature and 353 K. For room temperature, (297 ± 3) K, these parameters are k0 = (1.7 ± 0.5) × 10−29 cm6 molecule−2 s−1, k∞, = (10 ± 2) × 10−13 cm3 molecule−1 s−1. Our experimental results are consistent with addition as the dominant path, in agreement with flash photolysis investigations and with the existence of a fast reaction with NO2 of the (OH—benzene) radical adduct. A numerical simulation shows that this latter reaction should have a rate constant of (4 ± 2) × 10−11 cm3 molecule−1 s−1 at 353 K. Key words: gas phase kinetics, discharge flow, resonance fluorescence, reaction of OH with benzene, tropospheric chemistry.
The crystal structure of the "White Salt" trisilver(I) amidoselenate-3-ammonia-2-water [Ag(NH3)2]Ag2SeO3N•3NH3•2H2O
Authors: Žák Zdirad , Růžička Antonín , Głowiak Tadeusz
Published: May 2006

Trisilver(I) amidoselenate-3-ammonia-2-water, Ag3SeO3N•3NH3•2H2O, Mr = 551.70, is orthorhombic, space group P212121 with a = 11.864(3), b = 14.550(3), c = 6.433(2) Å, Z = 4, Dm = 3.25, Dx = 3.30 Mg m−3, radiation [Formula: see text], λ = 0.71069 Å, μ = 8.62 mm−1, F(000) = 1024, T = 295 K. The structure was solved by direct methods and refined to R = 0.036 for 1759 observed reflections. In the structure, only two independent Ag atoms are directly bonded to the N(Se) atom of the amidoselenic group (NSeO3) and these groups are joined by Ag into an infinite polymeric chain via [—N(Se)—Ag] bridges. The third Ag atom is present as a complex cation [Ag(NH3)2]+. Each silver atom is coordinated by two N atoms: Ag(1) by N(Se) and a NH3 molecule, Ag(2) by two N(Se) atoms, and Ag(3) by two NH3 molecules. Important blood lengths are Ag(1)—N(Se), NH3 2.111 and 2.110(8); Ag(2)—N(Se), N(Sei) 2.106 and 2.096(7); and Ag(3)—NH3, NH3 2.115 and 2.150(10); N(Se)—Se 1.727(8); and Se—O 1.641, 1.647, and 1.639(7) Å. Key words: Silver amidoselenate, silver–nitrogen bond, nitrogen–selenium bond.
Catalysis by β-cyclodextrin in the reaction of p-nitrophenyl acetate with α-amino acids
Authors: Barra Mónica , Rossi Rita H. De
Published: May 2006

The reactions of p-nitrophenyl acetate, 1, with both enantiomers of the following α-amino acids: alanine (2a), methionine (2b), leucine (2c), and tryptophan (2d), were studied in the presence of p-cyclodextrin (β-CD). All the reactions were catalyzed by β-CD and no chiral discrimination was observed. The reactions of 1 with the D isomers of 2a and 2d were also studied in the presence of α- and γ-CD. In both cases the catalysis increased in the order α < γ < β-CD. Also, to analyze the influence of the polar nature of the nucleophile, the reactions of 1 with both enantiomers of the methyl esters of 2a, 2c, and 2d were studied in the presence of β-CD. The catalysis observed was significantly smaller than that for the reactions with the amino acids. It is suggested that the catalysis by CDs is due to the formation of a ternary hydrogen-bonding complex that results from the association of the amino acid with the complexed substrate. The effect of the size of the CD cavity on the observed catalysis is attributed to the geometrical requirements for the reaction. The decrease in catalysis in the reactions with the methyl esters is due to a weaker hydrogen-bonding association with these neutral nucleophiles. Key words: cyclodextrins, catalysis, ester aminolysis.
Chemistry of chromotropic acid. 1H and 13C NMR spectroscopy of chromotropic acid and its derivatives
Authors: Georghiou Paris E. , Ho Chi Keung (Jimmy) , Jablonski Chester R.
Published: May 2006

The 1H and 13C NMR spectra of chromotropic acid (CTA) (4,5-dihydroxy-2,7-naphthalenedisulphonic acid) have been unambiguously assigned. Proton NOED spectra were used to show the proximity of both H-3 and H-6 and the hydroxyl groups. Two-dimensional 1H–13C NMR correlation spectra of CTA, of its corresponding diacetoxy derivative, and of 3-bromo- and 3,6-dibromo-CTA support the assignments. A regioselective deuterium exchange reaction of the C-3 and C-6 protons of CTA with deuterium oxide was observed during the NMR experiments. This latter finding is strongly indicative of the mode of formation, and of the nature of the chromogen formed in the reaction of CTA with formaldehyde in the well-known CTA-formaldehyde analytical reaction. Key words: chromotropic acid, 3-bromochromotropic acid, 3,6-dibromochromotropic acid.
Structural studies of organoboron compounds. XLVII. 6,8-Bis(1-cyano-1-methylethyl)-2,4-diphenyl-1,3,5-trioxa-6,8-diaza-2,4-diboracyclooctane
Authors: Kliegel Wolfgang , Lubkowitz Gottfried , Rettig Steven J. , Trotter James
Published: May 2006

The reaction of N,N′-bis(1-cyano-1-methylethyl)-N,N′-dihydroxymethanediamine with phenylboronic acid gives 6,8-bis(1-cyano-1-methylethyl)-2,4-diphenyl-1,3,5-trioxa-6,8-diaza-2,4-diboracyclooctane in good yield. Crystals of the product are monoclinic, a = 9.2681(9), b = 15.946(1), c = 15.491(1) Å, β = 95.946(8)°, Z = 4, space group P21/n. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.047 and Rw = 0.064 for 2936 reflections with I ≥ 3σ(I). The central eight-membered O(BON)2C ring has a boat–boat ("saddle") conformation and features cross-ring non-bonding [Formula: see text] distances of 2.475 and 2.755 Å, which are significantly shorter than the sum of van der Waals radii (~3.15 Å). Features of the molecular geometry are consistent with weak dative N → B interactions, particularly in the case of the shorter transannular contact. The formally trigonal boron atom involved in the short interaction is displaced 0.098 Å from the plane of its three substituents towards the nitrogen atom while the corresponding displacement for the other boron atom is 0.017 Å. The C(phenyl)—B distances are equal within experimental error at the two B atoms (1.545(3) and 1.547(3) Å), but the (B)O—B and (to a lesser extent) the (N)O—B bonds to the boron atom involved in the shorter transannular interaction are longer than the others (1.371(3) and 1.392(3) vs. 1.354(3) and 1.388(3) Å). The structure may be thought of as an intermediate in the transition from an eight-membered monocylic system to a bridged ring system. Key words: crystal structure, organoboron compound, boron compound.
The synthesis of some novel N-chloro-Δ1-4-azasteroids by efficient N-chlorination of azasteroid lactams with trichloroisocyanuric acid
Authors: Back Thomas G. , Chau Joseph H.-L. , Dyck Brian P. , Gladstone Patricia L.
Published: May 2006

The preparation of N-chloro-Δ1-4-azasteroids 2a–2c from lactams 1a–1c was achieved conveniently and in high yield with trichloroisocyanuric acid. The treatment of 20β-hydroxy-4-aza-5α-pregn-1-en-3-one (1d) with this reagent produced the less stable N-chloro compound 2d, which reacted further to afford 4-aza-5α-pregn-1-en-3,20-dione (4) as the principal product. The similar chlorination of 4-azacholest-5-en-3-one (5) occurred at C-6, giving 6-chloro-4-azacholest-5-en-3-one (6). Photolysis of N-chloro-4-aza-5α-cholest-1-en-3-one (2a) in methanol produced the transient N-acyl imine 7, which was trapped by the solvent to afford 5-methoxy-4-azacholest-1-en-3-one (8), along with 4-azacholesta-1,5-dien-3-one (9) and its 6-chloro derivative 10. Key words: N-chloroazasteroid, Δ1-4-azasteroid, N-chloro-Δ1-4-azasteroid, trichloroisocyanuric acid, N-chlorination.
The photochemistry of RDX in solid argon at 10 K
Authors: Alix Jeff , Collins Susan
Published: May 2006

The photochemistry of RDX was studied in argon matrices at 10 K and examined by FTIR spectroscopy. The spectra and kinetics of product growth indicate that there are two decomposition pathways: [I]        RDX → 4NO + 2CH2O + N2 + CH2 → 4NO + 2CO + N2 +CH4 + H2

[II]        RDX → N2O + [O3 + N2] or [N2O3] + CO2 + C2H2 +N2 + 2H2 The mechanism II nitrogen products can easily be rationalized for the chair configuration of RDX with axial NO2 groups adjacent (Cs symmetry). When considering the carbon products of II, it may be necessary to postulate intermolecular reactions. Our studies gave no evidence for the symmetric triple fission path, which leads to N2O and CH2O, nor for the NO2 stripping mechanism reported for the gas-phase thermal reactions, which leads to the formation of NO2 and HCN. Rather, our findings were more consistent with the known condensed phase products. The production of CH4, C2H2, N2O3, and O3 are unique to the matrix study. Key words: RDX, photochemistry, argon matrices.
Thermal diffusion with hydrolysis in aqueous solutions of sodium carbonate
Authors: Lü Hui , Leaist Derek G.
Published: May 2006

A conductimetric technique is used to measure thermal diffusion in aqueous solutions of sodium carbonate. In dilute solutions hydrolysis produces significant amounts of sodium bicarbonate and sodium hydroxide: Na2CO3 + H2O = NaHCO3 + NaOH. The applied temperature gradient causes the various solutes to migrate to the cooler parts of the solution. NaOH is found to diffuse more rapidly than NaHCO3, leading to the accumulation of excess NaOH (relative to NaHCO3) at the cold plate. Binary Na2CO3(m) + H2O mixtures therefore separate into ternary Na2CO3(m1) + NaOH(m2) + H2O mixtures under nonisothermal conditions. The steady-state molality gradients dm1/dT and dm2/dT and the ternary heats of transport of aqueous Na2CO3 and NaOH are reported. Key words: aqueous sodium carbonate, hydrolysis, mixed electrolytes, Soret coefficient, thermal diffusion.
The infrared emission spectra of LiF and HF
Authors: Hedderich H. G. , Frum C. I. , Engleman Jr. R. , Bernath P. F.
Published: May 2006

The high resolution infrared spectrum of LiF has been measured in emission with the McMath Fourier transform interferometer at Kitt Peak. A total of 800 lines with ν = 1 → 0 to ν = 8 → 7 of the main isotopomer, 7LiF, and 250 lines with ν = 1 → 0 to ν = 3 → 2 of the minor isotopomer, 6LiF, were observed. These ro-vibrational transitions and the pure rotational transitions from the literature were fit to a set of Dunham coefficients Yij and a set of mass-reduced Dunham coefficients Uij. The same spectrum shows 13 pure rotational emission transitions of HF in the vibrational ground state with J = 13 → 12 to J = 25 → 24. These transitions were used to determine an improved set of rotational constants for HF. Key words: infrared spectra, LiF, HF.
Infrared spectra of tautomers and rotamers of 9-methylguanine. An experimental and theoretical study
Authors: Szczepaniak K. , Szczesniak M. , Szajda W. , Person W. B. , Leszczynski J.
Published: May 2006

Both amino-oxo and amino-hydroxy tautomeric forms of 9-methylguanine have been identified in approximately equal abundance in. infrared studies of these molecules isolated in the hydrophobic environment of an argon matrix at 12 K. The amino-hydroxy tautomer occurs in two different rotamers correlated with the rotation of the OH group. The ratio of concentrations of the two rotamers is sensitive to UV irradiation, and this ratio then relaxes to an equilibrium value after irradiation is stopped. This sensitivity allows us to separate the experimental spectra related to the oxo tautomer and to each of the rotamers of the hydroxy tautomer. The relative concentrations of the amino-oxo and amino-hydroxy tautomers ([a-o]/[a-h] = K(o-h) = 1.0 ± 0.3) and of the two rotamers (K(h1-h2) = 0.31 ± 0.10 in an argon matrix at 12 K and about 30 ± 15 in the vapor at 470 K) are estimated from the observed relative infrared absorbances. From these relative concentrations the differences between the free energies of the tautomers (ΔG470 (o-h) = 0 ± 0.5 kJ mol−1) and of the two rotamers (ΔG(h1-h2) = 0.12 ± 0.03 kJ mol−1 in the argon matrix at 12 K and ΔG470 between +2 and −13 kJ mol−1 in the vapor at 470 K) have been estimated. The electronic absorption spectrum of 9-methylguanine isolated in the argon matrix at 12 K and the effect of brief ultraviolet irradiation on it have also been studied. In an effort to interpret the experimental results, ab initio calculations of the infrared spectra have been made for 9-methylguanine at the 3-21G//3-21G level. Comparison with the experimental spectra is of some help with the assignment of the infrared spectra for the different tautomers. Key words: 9-methylguanine, tautomerism, infrared and ultraviolet spectra, matrix isolation, ab initio calculation.
Polarized and depolarized continuum resonance Raman scattering of molecular iodine: accurate determination of repulsive states
Authors: Strempel J. , Kiefer W.
Published: May 2006

We have recorded high-resolution polarized and depolarized resonance Raman spectra of molecular iodine with excitation by argon-ion laser lines at 496.5, 488.0, and 476.5 nm up to the seventh, and with 457.9 nm up to the fifth, overtone. These spectra were also calculated numerically applying second-order perturbation theory. In contrast to earlier publications on this subject, special emphasis was put on obtaining highly reliable polarized and depolarized spectra. The variations of the spectral band shapes and depolarization ratios obtained at different excitation wavelengths could be explained by means of the specific contributions of the two excited electronic states [Formula: see text] and 1П1u. About 60 different experimentally obtained spectra were finally used to derive the positions of the two excited-state potentials involved in this type of light scattering. The derivation was done by comparison of the experimental and theoretically simulated spectra where parameters of the 1П1u as well as of the [Formula: see text] state potentials were varied. Key words: resonance Raman scattering, diatomic molecules, potentials, excited electronic states.
Phase transitions in adamantane derivatives: 1-fluoroadamantane
Authors: Kawai Nancy T. , Gilson Denis F. R. , Butler Ian S.
Published: May 2006

The solid-state phase behaviour of 1-fluoroadamantane (1-C10H15F) has been investigated by differential scanning calorimetry, and by infrared and Raman spectroscopy. The phase transition occurs at 227 K on cooling and at 231 K upon heating, with average ΔHt and ΔSt of 1.65 kJ mol−1 and 7.3 J K−1 mol−1. The transition was also apparent in the low-temperature IR and Raman spectra by a sudden line narrowing, and characteristic spectral features of each phase were identified. Both phases I and II were disordered, and further cooling of phase II resulted in the formation of a glass. Attempts to induce a glass transition (predicted to occur at ~90 K) by slow heating were unsuccessful. Vibrational spectroscopic studies at high pressures showed a phase transition at 3.3 ± 1.0 kbar, upon compression. The phases formed under pressure were the same as those induced by lowering temperature. Key words: high-pressure infrared and Raman spectroscopy, phase transition, plastic crystal, orientational disorder.
Kinetics and mechanism of oxidation of DL-α-alanine by permanganate ion in acid perchlorate media
Authors: Hassan Refat M.
Published: May 2006

The kinetics of permanganate oxidation of DL-α-alanine in aqueous perchloric acid solution at a constant ionic strength of 2.0 mol dm−3 has been investigated spectrophotometrically. The reaction was found to show second-order kinetics overall with respect to each of the reactants in the slow initial stage; the second-order kinetics are not, however, maintained throughout the relatively fast final stage of reaction. The added salts lead to the prediction that Mn(III) and (or) Mn(IV) play a very important role in the reaction kinetics. A tentative mechanism consistent with the kinetics is discussed. Key words: kinetics, oxidation, reduction, amino acids, permanganate.
Preparation of Cr2(CO)8(NO)2 and X-ray crystal structures of Cr2(CO)8(NO)2 and Cr2(CO)9NO−. Stereochemical comparisons to Cr2(CO)102−
Authors: Masters A. P. , Parvez M. , Sorensen T. S.
Published: May 2006

Cr2(CO)8(NO)2 (1) has been prepared and characterized, and the crystal structure determined, along with that of the related salt Cr2(CO)9NO− monoanion (2) as the CH2Cl2-solvated PPN+ salt (where PPN+ denotes the (PPh3)2N+ monocation). Both 1 and 2 possess a metal–metal bonded structure analogous to that of Mn2(CO)10 and the electronically equivalent Cr2(CO)102− dianion. Although Cr2(CO)8(NO)2 (1) is slowly formed in poor yield from Cr(CO)4NO− and Cr(CO)4NOBr, it can be prepared in an almost quantitative yield by the one-electron oxidation of the monoanion with trityl hexafluorophosphate. The slowness of the former reaction makes Cr(CO)4NO− useful as a two-electron reductant of organohalide bonds. Triclinic crystals of 1 incorporate two distinct molecules in die [Formula: see text] space group, although the geometries of each are almost identical. Triclinic crystals of monoanion (2) also belong to the [Formula: see text] space group. The Cr—Cr bond length is 3.00–3.02 Å in 1 and 2.995 Å in 2, compared to 2.904 Å for the central Mn—Mn bond in Mn2(CO)10. Key words: chromium nitrosyl, chromium carbonyl, X-ray crystal structure, metalate anion, metal–metal dimer.
High-resolution amplified spontaneous emission (ASE) gain spectroscopy to study excited state complexation
Authors: Yip R. W. , Wen Y-X.
Published: May 2006

Weakly bound excited state complexes involving 7-diethylamino-4-methylcoumarin have been studied by ASE gain spectroscopy to explore the applicability of the method for studying excited state complexation. Comparative steady state spectrofluorometric and ASE gain measurements showed that ASE gain measurements are capable of giving information on excited state complexation of the laser dye but with greater ease and frequency resolution. Key words: excited state complex, amplified spontaneous emission, laser gain, ASE gain spectroscopy
Photoelectrochemical and optical properties of porphyrin thin films prepared by the electrolytic micelle disruption method
Authors: Takeda Kazuhiko , Harima Yutaka , Yamashita Kazuo
Published: May 2006

Photoresponses for various porphyrin films prepared by the electrolytic micelle disruption (EMD) method have been investigated in the indium-tin oxide / porphyrin / I3, I  / Pt cell. The EMD film of an n-type porphyrin, 5, 10, 15, 20-tetra(4-pyridyl)porphyrin (T(4-py)P), exhibited a clear optical filtering effect (OFE), while not for the EMD films of p-type porphyrins such as (5, 19, 15, 20-tetraphenylporphyrinato)zinc(II) (ZnTPP). For the EMD-ZnTPP film, the photocurrents at 500700 nm were remarkably enhanced by increasing the film thickness. These photoelectrochemical properties were explained in terms of the electrolyte-permeability of the EMD films, which were confirmed by the electrochemical measurements and electron microscopic observations. The high conversion efficiency of the EMD-ZnTPP film was explained on the same basis. The difference in the thickness dependence of OFE between T(4-Py)P films prepared by the EMD and physical vapor deposition method was elucidated by the differences in diffusion lengths of excitons and in widths of photoactive layers in the films. In addition, a modified version of the EMD method was described in conjunction with a morphology control of the porphyrin solids.

Key words: porphyrin, organic thin film, photoresponse, micelle, electrochemistry.
Réduction électrochimique de 4H-1,3-thiazines: obtention de 6H-1,3-thiazines et (ou) de pyrroles substitués
Authors: Abouelfida Abdesselam , Pradère Jean Paul , Jubault Michel , Tallec André
Published: May 2006

Controlled potential electroreduction (protic medium, mercury cathode) of substituted 2-ethoxy and 2-phenyl-4H-1,3-thiazines leads to 6H-1,3-thiazines and (or) pyrroles. The nature of the isolated products appears strongly dependent on pH of the medium and type of substitution: pyrrole formation takes place in acidic medium (0.5 mol L−1 H2SO4) and is favoured by phenyl and alkoxycarbonyl groups at positions 2 and 4, respectively; formation of 6H-1,3-thiazines and their reduction products (substituted thiobenzamides, carbamates and 2-thiazoline) occurs in weakly acidic (acetate buffer) or basic (ammoniacal buffer) medium. Keywords: electrochemical reductions, substituted 4H- and 6H-1,3-thiazines, pyrroles, 2-thiazoline, cyclic regression.
Tin-119 NMR of 1,3,2-dioxastannolanes and a 1,3,2-dioxastannane in the solid state
Authors: Grindley T. Bruce , Wasylishen Roderick E. , Thangarasa Rasiah , Power William P. , Curtis Ronald D.
Published: May 2006

The cross-polarized static and high-resolution magic angle spinning 119Sn NMR spectra of a number of 2,2-dialkyl-1,3,2-dioxastannolanes and one 1,3,2-dioxastannane have been measured in the solid state. For the four compounds on which X-ray studies had been performed, the numbers and positions of the isotropic peaks in the high-resolution spectra were related to the number of tin sites present and the state of oligomerization of the compounds. The chemical shifts of hexacoordinate Sn nuclei are 35–80 ppm larger in polymeric solids than for the same compounds in solution where the compounds exist as trimers and tetramers. States of oligomerization for solids that had not been previously studied by X-ray crystallography were determined using CP/MAS 119Sn NMR spectroscopy. The principal components of the 119Sn chemical shift tensors were obtained from the static spectra and used to calculate chemical shift anisotropies and asymmetry parameters. The values of the chemical shift anisotropies ranged from 600 to 800 ppm for 1,3,2-dioxastannolanes but the value for a 1,3,2-dioxastannane was larger, 919 ppm. The chemical shift anisotropies measured directly from the solid-state powder patterns are in excellent agreement with the values derived from previous variable temperature spin-lattice relaxation measurements in solution when the same oligomer was present in both states. Our results support our previous conclusion that the antisymmetric terms of the chemical shift tensor make a small or negligible contribution to the rate of 119Sn spin-lattice relaxation in these compounds. Keywords: 1,3,2-dioxastannolanes, stannylene acetals, 119Sn NMR, 119Sn NMR of solids, 119Sn chemical shift an-isotropy.
Molecular recognition X. A novel procedure for the detection of the intermolecular hydrogen bonds present in a protein•oligosaccharide complex
Authors: Nikrad P. V. , Beierbeck H. , Lemieux R. U.
Published: May 2006

The relative potencies of both the monodeoxy and mono-O-methyl derivatives of the Leb-OMe tetrasaccharide (α-L-Fuc-(1 → 2)-β-D-Gal-(1 → 3)-[α-L-Fuc-(1 → 4)]-β-D-GlcNAc-OMe) as inhibitors of the complexation of a Leb artificial antigen by the lectin IV of Griffoniasimplicifolia are interpreted in terms of the X-ray crystal structure at 2.5 Å resolution of the GS-IV • Leb-OMe complex. Both kinds of derivatives maintain high potencies when the hydroxyl groups involved appear, in the crystal structure, to be in contact with the aqueous phase. Hydroxyl groups situated at the periphery of the combining site and hydrogen bonded to the protein can also be deoxygenated without important loss in potency. However, their methylation leads to a strong decrease in the stability of the complex, because the steric bulk of the introduced methyl group causes loss of complementarity. In contrast, the hydroxyl groups that form hydrogen bonds with the protein along the base of the shallow amphiphilic cleft of the combining site can neither be deoxygenated nor methylated without virtually complete loss of binding activity. Thus, the procedure can provide an appreciation of the various kinds of hydrogen bonds that are present in a protein • oligosaccharide complex. Hard-sphere calculations supported these contentions since an energetically favorable orientation was indicated for a methoxy group at any one of the five positions that were expected to remain in contact with the aqueous phase. However, the calculations, as expected, showed the introduction of strong destabilizing nonbonded interactions when the methylation involved hydroxyl groups that are hydrogen bonded to the protein in the complex. The results are in accord with the previously made rationalization of the near linear enthalpy–entropy compensation found for the active deoxy congeners. Keywords: molecular recognition, lectin IV of Griffoniasimplicifolia, O-methyl derivatives of the Lewis b-OMe tetrasaccharide, detection of intermolecular hydrogen bonds, hard-sphere calculations.
Cycloaddition de la C-p-anisyl-N-phénylnitrilimine sur deux énamines cycliques dérivées de la 1,2-dihydro-6,7-diméthoxyisoquinoléine-3-carboxylate d'éthyle. Régiochimie, stéréochimie et évolution des cycloadduits
Authors: Moustaid Khadija , An Nguyen Dinh , Laude Bernard , Mercier Marie-France , Sedqui Ahmed
Published: May 2006

Two ethyl-1,2-dihydro-6,7-dimethoxyisoquinoline-3a-carboxylate derivatives were synthesized. These cyclic enamines in which the enaminic carbon atom was bonded with an ester group react regio- and stereospecifically with a diarylnitrilimine. The regiochemistry was specified by chemical and spectroscopic methods. The radiocrystallographic structure of one cycloadduct allows us to determine the stereochemistry of the reaction and to compare it with that of the reaction of a diarylnitrilimine with a 1,2-dihydro-1,2-dialkylisoquinoline without an ester group at the enaminic carbon atom. Keywords: 1,3-dipolar cycloaddition, diarylnitrilimine, cyclic enamine, regiochemistry, stereochemistry.
Prototropic equilibria of 2-pyridylbenzimidazoles in aqueous solution
Authors: Novo Mercedes , Mosquera Manuel , Prieto Flor Rodriguez
Published: May 2006

In this paper we report the acid–base behaviour of 2-(n′-pyridyl)benzimidazoles (n′ = 2,3,4) and some methyl derivatives in aqueous solution over a wide range of acidity, involving interesting features with respect to competition in protonation between the benzimidazole N3 and the pyridyl N, with the existence of the corresponding two monocations in equilibrium in the case of n′ = 4. The influence of pH on the electronic absorption spectra of all compounds has been studied in order to identify the ground-state species present as a function of acidity and to obtain the acidity constants of the various prototropic equilibria occurring in the wide range of acidity studied. Keywords: 2-pyridylbenzimidazoles, prototropic equilibria, protonation, acidity constants.
Kinetic analysis of thermal decomposition reactions. VII. Effect of radiation and doping on the thermal decomposition of BaCO3–TiO2 and SrCO3–TiO2 crystalline mixtures
Authors: Basahel Suliman N. , Diefallah El-Hussieny M.
Published: May 2006

DTA–TG techniques were applied to study the thermal decomposition and reactivity in intimately mixed powders of barium or strontium carbonate and titanium dioxide. The results showed that the temperature for the thermal decomposition of BaCO3–TiO2 mixtures precedes the decomposition of pure BaCO3 by about 250 °C, whereas the decomposition of SrCO3–TiO2 mixtures precedes the decomposition of pure SrCO3 by about 60 °C. Kinetic analysis of the isothermal data in view of various solid-state reaction models showed that the reaction is best described by the phase boundary models. The effects of 60Co γ irradiation and of doping the metal oxide with Li+ or Cu2+ ions on the thermal decomposition reactions were investigated. Keywords: titanates, thermal decomposition, doping, irradiation effects.
Mono- and difunctional furan-based 1,2,3,5-dithiadiazolyl radicals; preparation and solid state structures of 2,5-[(S2N2C)OC4H2(CN2S2)] and 2,5-[(S2N2C)OC4H2(CN)]
Authors: Cordes A. Wallace , Chamchoumis Charles M. , Hicks Robin G. , Oakley Richard T. , Young Kelly M. , Haddon Robert C.
Published: May 2006

The preparation and solid state characterization of the bifunctional radical furan-2,5-bis(1,2,3,5-dithiadiazolyl) 2,5-[(S2N2C)OC4H2(CN2S2)] and the related monofunctional radical 2-cyanofuran-5-(1,2,3,5-dithiadiazolyl) 2,5-[(S2N2C)OC4H2(CN)] are described. The crystal structure of 2,5-[(S2N2C)OC4H2(CN2S2)] is orthorhombic, space group Pna21, and consists of interleaved arrays of dimers, for which the mean interannular [Formula: see text] contact is 3.137 Å. The crystal structure of the monofunctional radical 2,5-[(S2N2C)OC4H2(CN)] is monoclinic, space group P21/n, and consists of a ribbon-like network of dimers (mean interannular [Formula: see text] interconnected by close head-to-tail [Formula: see text] contacts. The dimer units form stacks parallel to z, with a mean interdimer [Formula: see text] separation of 3.956 Å. The similarities and differences between these two crystal structures and those of related benzene-substituted systems are discussed. Keywords: dithiadiazolyl radicals, furan-based diradicals, cyanofuran-based radicals, radical dimers, crystal structures.
Fragmentation pathways in the mass spectra of isomeric phenylazoxypyridine-N-oxides
Authors: Bunce Nigel J. , McKinnon H. Stewart , Schnurr Randy J. , Keum Sam R. , Buncel Erwin
Published: May 2006

The mass spectral fragmentation pathways of a series of phenylazoxypyridine-N-oxides have been studied under electron impact conditions using tandem mass spectrometry. Besides simple C—N cleavages, the azoxypyridine-N-oxides undergo deep-seated rearrangements directly from the molecular ion. In addition, the spectra are complicated by a purely chemical reduction of the N—O functionalities that occurs in the ion source prior to ionization.
Metabolites of the fungus Arthropsistruncata
Authors: Ayer William A. , Craw Peter A. , Neary John
Published: May 2006

The structures of 12 new polyketide metabolites produced in liquid cultures of the fungus Arthropsistruncata have been determined by a combination of chemical and spectroscopic methods. Four of these metabolites possess an unusual 2-oxaspiro[4.5]decane skeleton, and the structure assigned to one of these metabolites 20 has been confirmed by an X-ray crystallographic study of the dibenzoyl derivative 21. The absolute stereochemistry of these compounds has been predicted by application of the Harada–Nakanishi dibenzoate rule to arthropsolide D tribenzoate (32).