bluuebunny/crossref_metadata_2025 huggingface dataset:
Reactions of the methylmercury cation with xanthine and crystal structures of μ-(xanthinato-N3,N7,N9)tris(methylmercury(II)) nitrate and μ-(xanthinato-N1,N3,N7,N9)tetrakis(methylmercury(II)) nitrate
Authors: Allaire François , Beauchamp André L.
Published: May 2006

Compounds of the types [(CH3Hg)3X], [(CH3Hg)4X]Z, and [(CH3Hg)3(HX)]Z (where Z = NO3−, ClO4−) were obtained by reacting the methylmercury cation with xanthine (H3X) in aqueous media. The three acidic protons of H3X at N(1), N(3), and N(7) are substituted by CH3Hg+ groups in [(CH3Hg)3X]•2H2O. Crystals of [(CH3Hg)3(HX)]NO3•H2O belong to the triclinic [Formula: see text] space group, with a = 7.319(3), b = 9.466(8), c = 12.004(3) Å, α = 96,06(5), β = 95.00(3), γ = 97.69(5)°, and Z = 2. The structure was refined on 1610 nonzero reflections to an R factor of 0.038. The CH3Hg+ groups are linearly coordinated to N(3), N(7), and N(9) of a dianionic xanthine retaining its N(1)—H proton (mean Hg—N = 2.07(3) Å). The [(CH3Hg)4X]NO3 compound is monoclinic, space group C2/c, a = 24.55(2), b = 17.016(4), c = 19.108(9) Å, β = 110.26(5)°, Z = 16. The structure was refined on 1418 nonzero reflections to R = 0.046. The two independent [(CH3Hg)4X]+ ions possess methylmercury groups linearly coordinated to the four nitrogen atoms of the X3− anion (mean Hg—N = 2.07(5) Å). The compounds prepared were characterized by ir spectroscopy, and by 1H and 13C nmr. The [(CH3Hg)3X]•2H2O and [(CH3Hg)4X]Z compounds were found to remain undissociated in DMSO solution. The [(CH3Hg)3(HX)]+ salts disproportionated to some extent into [(CH3Hg)4X]+ and a lower complex which was identified as the N(7),N(9)-bonded [(CH3Hg)2(H2X)]+ ion.
Gas phase chemistry of α-thio carbanions
Authors: Ingemann Steen , Nibbering Nico M. M.
Published: May 2006

Proton abstraction from CH3SCH3 and C6H5SCH3 in the gas phase has been studied by the use of Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry. The gas phase acidity of CH3SCH3 is reported to be [Formula: see text]. The methyl group in C6H5SCH3 is observed to be the most acidic position. The gas phase acidity of this group is determined to be [Formula: see text]. The structure and reactivity of the [M – H]− ions from CH3SCH3 and C6H5SCH3 are probed by studying their reactions with various molecules. Rate constants are reported for the reactions of CH3SCH2− – and several other nucleophiles – with CH3Cl, CH3Br, and HCO2CH3.
New complexes of the hydronium ion with crown ethers: synthesis and vibrational spectra
Authors: Chénevert Robert , Rodrigue André , Beauchesne Pierre , Savoie Rodrigue
Published: May 2006

The charged-component complexes H3O+•18-crown-6.CF3SO3−, H3O+•18-crown-6.SbF6−, and (H3O+•18-crown-6)2•PtCl62− have been isolated and studied by infrared and Raman spectroscopy, along with the previously reported H3O+•18-crown-6•C1O4− and H3O+•18-crown-6•PF6− adducts. Also synthesized were the corresponding derivatives with BF4− and PtCl62− anions, using dicyclohexano-18-crown-6 as the host for the hydronium ion.
Addition initiated ring closure: synthesis of substituted spiro[2.4]hepta-4,6-dienes via cyclopentadiene intermediates from fulvene precursors
Authors: Antczak Kazimierz , Kingston John F. , Fallis Alex G.
Published: May 2006

The reactivity of the epoxy-fulvene 1, prepared by pyrrolidine catalyzed condensation of cyclopentadiene and epoxybutanone, has been examined with various nucleophiles. It is a versatile intermediate for the preparation of spiro[2.4]hepta-4,6-diene synthons via nucleophilic addition to the C6 position of the fulvene followed by intramolecular cyclization of the substituted cyclopentadiene anion generated insitu.
Organic sulfur mechanisms. 27. A reexamination of the reaction of thiirane 1,1-dioxide with aqueous hydroxide
Authors: King James Frederick , Hillhouse John Henry , Khemani Kishan Chand
Published: May 2006

Reaction of thiirane 1,1-dioxide (1) with aqueous barium hydroxide gives chiefly barium ethanesulfonate (3) together with some ethylene and barium sulfite, and only a small amount (<5%) of barium 2-hydroxyethanesulfinate (2), the material previously described by Hesse etal. as the major product of this reaction. Reaction of 1 with potassium hydroxide proceeds analogously except for the formation of barium 2-sulfinatoethanesulfonate (6) from reaction of potassium sulfite with unreacted 1. This revision, when taken with what is already known of the final step of the Ramberg–Bäcklund reaction, provides a consistent picture of the reaction of thiirane 1,1-dioxides with hydroxide. For this reaction we suggest a common intermediate with a pentacoordinated sulfur for formation of both the sulfonate anion and the olefin, though an attempt to confirm the existence of such a species using H218O was unsuccessful.
The radiation chemistry of 2-mercaptoethanol
Authors: Lal Manohar , Armstrong David A.
Published: May 2006

Hydrogen, hydrogen sulphide, mercaptoethanol disulphide, and ethanol yields from the radiolysis of 2-mercaptoethanol are consistent with the same mechanism postulated for other sulphydryl-containing molecules. In all cases studied gH in neutral solution appears to be enhanced (gH = 0.7–0.9 molecule per 100 eV). However, there is no basis for attributing this to reaction [10] which was postulated in earlier work:[Formula: see text]but is not supported by independent data from H atom studies.
Packing analysis of carbohydrates and polysaccharides. 16. The crystal structures of celluloses IVI and IVII
Authors: Gardiner Eric S. , Sarko Anatole
Published: May 2006

The crystal structures of cellulose polymorphs IVI and IVII have been determined by X-ray fiber diffraction analysis combined with stereochemical model refinement. Both structures crystallize in a two-chain unit cell of essentially identical parameters. The most probable space group in both cases is P1. The chain conformations, although close to two-fold helical, are marked by unequal rotational positions of the O(6) hydroxyl groups in adjacent residues. Despite identical unit cell parameters, the structures differ in chain polarity: in cellulose IVI both chains of the unit cell are parallel, whereas in cellulose IVII they are antiparallel. The difference in polarity is further substantiated by the results of chemical conversions which show that cellulose IVI is converted to cellulose I, and cellulose IVII is converted to cellulose II, via parallel and antiparallel cellulose triacetates, respectively. The reliability of the structure analyses is indicated by the residual R″ = 0.115 for cellulose IVI and 0.094 for cellulose IVII, for data sets of 41 and 43 reflections, respectively.
Cationic polymerization of 1,3-dioxolane and 1,3-dioxepane. Application to graft and block copolymer synthesis
Authors: Reibel Léonard C. , Durand Claude P. , Franta Emile
Published: May 2006

The cationic polymerization of 1,3-dioxolane and 1,3-dioxepane has been investigated to prepare "living" polymers and then to use them for copolymerization. Carbenium hexafluoroantimonate salts that are good initiators for tetrahydrofuran polymerization have been tested but produce unsatisfactory results with these acetals. In contrast, triflic anhydride, another good initiator for tetrahydrofuran polymerization, allows molecular weight control. Attempts to prepare block copolymers of two acetals by adding the second one to the first one under living conditions were unsuccessful because of randomization through transacetalization. Block copolymers containing a central block of polydioxepane and outer blocks of 1,2-dimethoxyethylene could be prepared by addition of the latter to living polydioxepane. Active polydioxolane reacts readily with polystyrene and leads to the formation of graft copolymers in the absence of gel.
The high molecular weight polyphloroglucinols of the marine brown alga Fucusvesiculosus L.: degradative analysis
Authors: Ragan Mark A.
Published: May 2006

The high-molecular-weight polyphloroglucinol tannin of Fucusvesiculosus has been islated in pure form as judged by 1H and 13C nmr. Acetate and methyl derivatives were prepared and examined by uv and ir spectroscopy. Calcium – liquid ammonia degradation of the methyl ether derivative liberated numerous aryl compounds including fragments arising from ether-linked phloroglucinol end groups, internal ortho-, meta-, and para-oriented diethers, vic- and (tentatively identified) sym-triethers, internal phloroglucinol-derived biphenyls (three isomers), probable terminal biphenyls, terphenyl(s), and from qua-terphenyl(s). No evidence was found for vic-trihydroxylated end groups. In all, 63 fragments were identified by gc–ms, and abundances of the major fragments were estimated by quantitative gc. The data indicate that the F. vesiculosus tannins – or at least the portion represented by these fragments – are highly branched structures consisting entirely of phloroglucinol units.
The high molecular weight polyphloroglucinols of the marine brown alga Fucusvesiculosus L. 1H and 13C nuclear magnetic resonance spectroscopy
Authors: McInnes A. Gavin , Ragan Mark A. , Smith Donald G. , Walter John A.
Published: May 2006

1H and 13C nuclear magnetic resonance spectroscopy of the high-molecular-weight (> 104) polyphenol fraction from Fucusvesiculosus, before and after acetylation or methylation, demonstrates that it is composed of phloroglucinol units (48.9 ± 3.5%) and 2,2′,4,4′,6,6′-hexahydroxybiphenyl, 2,2′,2″,4,4′,4″,6,6′,6″-nonahydroxy-1,1′:3′1″-terphenyl, and related quater-and polyphenyl groups (together, 51.1 ± 3.5%) linked by ether bonds, each bond involving the carbinol and methine carbons of different units and the formal loss of a molecule of hydrogen. The polymer is highly branched, with ca. 20 – 22% of the constituent units being chain termini; there is no evidence for large rings of units. Approximately 5 – 10% of the phloroglucinol and 54 – 82% of the directly bonded units occur on the exterior of the molecule, whereas the interior backbone consists predominantly of ether-linked phloroglucinol units.
Cyclotetraphosph(III)azanes: donor properties and ylide formation
Authors: Gallicano Keith D. , Paddock Norman L.
Published: May 2006

The ligand (MeNPMe)4 (L) forms complexes [Formula: see text] [Formula: see text] L.M(CO)3 (M = Cr, Mo, W, Mn+), and a nickel complex L.Ni2(CO)3, which is easily oxidised to the phosphonimide [MeNP(0)Me]4. Quaternary complexes [LM(CO)3R]+I− have been prepared both by quaternisation of the neutral complex (R = Me, CH2CN, CH2C(O)NH2, M = Cr, Mo, W) and (R = Me) by reaction of the simple quaternary with the metal carbonyl. The quaternary complexes have been deprotonated to ylidic complexes, L.M(CO)3CH2 (M = Mo, W) in which the ylidic group has displaced one phosphine function. Chemical and structural features are discussed in terms of d-orbital contraction by acceptor groups. The phosphazane (MeNPPh)4 has been prepared.
Autoxidation in micelles. Synergism of vitamin C with lipid-soluble vitamin E and water-soluble Trolox
Authors: Barclay Lawrence Ross. Coates. , Locke Steven Jeffrey. , MacNeil Joseph Mark
Published: May 2006

A study was made of the effect of the inhibitors ascorbic acid (C), α-tocopherol (E), and 6-hydroxy-2,5,7,8-tetramethyl-chroman-2-carboxylate (Trolox, T) on the autoxidation of linoleic acid in 0.50 M sodium dodecyl sulfate (SDS) micelles at pH 7.0 in phosphate buffer. Reactions were thermally initiated at 30 °C in the SDS micelles by a micelle-soluble initiator, di-tert-butylhyponitrite (DBHN). Although water-soluble C alone is an inefficient inhibitor, when combined with micelle-soluble E, it acts synergistically with the latter to extend the efficient antioxidant action of E beyond the sum of the induction periods of C and E acting separately. Similarly C acts synergistically with the water-soluble antioxidant, T. Quantitative studies of these effects under controlled rates of initiation (Ri,) reveal that C functions to regenerate a mole of E (or T) per mole of C used. Kinetic studies show that the rate of autoxidation is first order in micellar linoleic acid and one-half order in micellar DBHN concentrations. Therefore, the classical rate law, −dO2/dt = kp[R—H] (Ri)1/2/(2k1)1/2 is followed. The higher oxidizability (kp/2kt1/2 = 4.48 × 10−2 M−1/2 s−1/2) of linoleate in micelles compared to that in homogeneous solution in chlorobenzene (kp/2kt1/2 = 2.30 × 10−2 M−1/2 s−1/2) is interpreted in terms of the effect of the polar interfacial region of the micelles on a dipolar transition state, R—OŌ: H•R, of the propagation reaction.
The formation of branched-chain deoxypentofuranosides by ring contraction in the reductive desulfonyloxylation of hexopyranoside p-toluenesulfonates
Authors: Baer Hans H. , Astles David J. , Chin Ho-Chi , Siemsen Lisa
Published: May 2006

Several methyl 6-deoxy-, 6-deoxy-6-halo-, and 6-O-p-tolylsulfonylhexopyranoside 2- and 4-p-toluenesulfonates reacted readily with lithium triethylborohydride in boiling tetrahydrofuran under reductive desulfonyloxylation at C-2 and C-4, respectively, and with reduction at C-6 where applicable. The desulfonyloxylations proceeded with ring contraction to furnish, in 40–60% isolated yields, new branched-chain pentofuranoside derivatives, namely, methyl 2,5-dideoxy-2-C-(hydroxymethyl)pentofuranosides having the α-L-xylo, α-D-xylo, and α-D-ribo configurations (from 2-tosylates), and methyl 3,5-dideoxy-3-C-(hydroxymethyl)pentofuranosides having the α-D-arabino and α-D-ribo configurations (from 4-tosylates). In some of the reactions, reductively desulfonyloxylated but unrearranged products were formed in addition. Methyl 4,6-O-benzylidene-2,3-di-O-p-tolylsulfonyl-α-D-galactopyranoside reacted by partial de-O-sulfonylation and partial desulfonyloxylation, with and without ring contraction, to give methyl 3,5-O-benzylidene-2-deoxy-2-C-(hydroxymethyl)-α-D-lyxo-pentofuranoside, methyl 4,6-O-benzylidene-3-deoxy-α-D-lyxo-hexopyranoside, and the parent 2,3-diol.
Enzymes in organic synthesis. 33. Stereoselective pig liver esterase-catalyzed hydrolyses of meso cyclopentyl-, tetrahydrofuranyl-, and tetrahydrothiophenyl-1,3-diesters
Authors: Jones J. Bryan , Hinks R. Scott , Hultin Philip G.
Published: May 2006

Preparative-scale pig liver esterase-catalyzed hydrolyses of five-membered ring meso-1,3-diesters are enantiotopically selective. While pro-S enantiotopic selectivity is exhibited in each case, the absolute configuration sense of the hydrolysis in the cyclopentyl series is opposite to that of both the tetrahydrofuranyl and tetrahydrothiophenyl diesters. The enantiomeric excess levels induced are in the 34–46% range.
Synthesis and resolution of R(−)-5-hexyn-3-ol
Authors: Roy Bernard L. , Deslongchamps Pierre
Published: May 2006

The resolution of (±)-5-hexyn-3-ol (1) was effected by high performance chromatography of its O-methyl mandelic esters 3 and 4. The absolute configuration of (−) and (+)-5-hexyn-3-ol were determined by comparison with an authentic sample prepared from D(+)-glucose.
Régiosélectivité et stéréosélectivité de l'hydroxylation homolytique des hydrocarbures par le peracide benzoïque
Authors: Fossey Jacques , Lefort Daniel , Massoudi Massoud , Nedelec Jean-Yves , Sorba Jeanine
Published: May 2006

Cyclohexane, methylcyclohexane, and adamantane are hydroxylated by a radical process using perbenzoic acid. A regioselectivity of 60–90% in favour of tertiary alcohols is noted. In the case of cis and trans decalins, stereoselection leading to 9-decalols can reach 97%. Such a stereoselectivity for hydroxylation by use of a peracid does not necessarily indicate lack of a radical pathway.
Conformation of methyl 3,6-dideoxy-α-D-arabino-hexopyranoside, the immunodominant sugar of Salmonella serogroup D1: crystal structure, 1H nmr analysis, and semi-empirical calculations
Authors: Birnbaum George I. , Bundle David R.
Published: May 2006

Methyl 3,6-dideoxy-α-D-arabino-hexopyranoside (methyl tyveloside) crystallizes in the orthorhombic space group P212121 and the cell dimensions are a = 7.478(1), b = 7.933(1), c = 14.064(1) Å. X-ray intensity data were measured with a diffractometer and the structure was solved by direct methods. Least-squares refinement, which included all hydrogen atoms, converged at R = 0.038. The pyranose ring exists as an almost perfect 4C1 chair and the conformation adopted by the glycosidic methyl group is in agreement with the requirements of the exo-anomeric effect. Both acetal oxygen atoms act as hydrogen-bond acceptors, and the [Formula: see text] bond distances are in agreement with this feature. The 1H nmr spectrum shows that the conformation of the pyranose ring in aqueous solution is indistinguishable from that in the crystal. The minimum energy conformation of a pentasaccharide fragment of a Salmonella O-antigen, calculated with tyvelose coordinates obtained by bond modification, is in good agreement with the conformation which was calculated with tyvelose coordinates obtained from the X-ray analysis.
A formal [1,3] sigmatropic reaction involving free radical intermediates: a mechanistic investigation
Authors: Bates Gordon S. , Ramaswamy S.
Published: May 2006

The quantitative isomerization of 2,2-bis(ethylthio)-3,3-dimethylpent-4-enal to 2,2-bis(ethylthio)-5-methylhex-4-enal was studied over the temperature range 130–170 °C. An investigation of the generality and specific mechanism of this formal [1,3] sigmatropic shift was conducted with six related compounds. The rearrangements were found to obey first-order kinetics, and on the basis of significant positive entropies of activation (52–106 J deg−1 mo−1), crossover and trapping experiments, and the lack of a solvent effect (decane vs. DMF), an intermolecular, free-radical chain pathway has been proposed for the isomerization. During the rearrangement of several of the compounds esr signals were observed that were consistent with the presence of the proposed free-radical intermediates. These esr signals have been computer simulated.
The reaction of the hexacyanoferrate(III) ion with hydrogen peroxide
Authors: Eaton Donald R. , Pankratz Marianne
Published: May 2006

In basic solution hydrogen peroxide reduces the hexacyanoferrate(III) ion to the Fe(II) complex. The stoichiometry and kinetics of this reaction have been examined. Both a stoichiometric reduction and a catalytic reaction leading to decomposition of the hydrogen peroxide occur. The latter is largely, but not completely, photochemical in origin. The data are consistent with the involvement of a common intermediate for the stoichiometric and the thermal catalytic reactions. It is suggested that this intermediate is formed by nucleophilic attack of the hydroperoxide ion or the superoxide radical on the carbon of a cyanide ligand.
Free energies and entropies of transfer of hydrogen halides from water to aqueous 2-methoxy ethanol and structuredness of the solvents
Authors: Guha Prabir K. , Kundu Kiron K.
Published: May 2006

Standard free energies (ΔGt0) and entropies (ΔSt0) of transfer of HBr and HI from water to some aqueous solutions of 2-methoxy ethanol (ME) have been determined from emf measurements of the cells: Pt, H2 (g, 1 atm)/HBr (m), solvent/AgBr–Ag and Pt, H2 (g, 1 atm)/KOH (m1), KI (m2), solvent/AgI–Ag, respectively, at seven equidistant temperatures ranging from 15 to 45 °C. ΔGt0 values of HBr and HI as well as of HCl obtained from literature, and particularly that of the individual ions obtained by tetraphenylarsonium tetraphenylboron (TATB) assumption, suggest that while H+ is increasingly stabilized by cosolvent-induced larger "basicity", halide ions (X−) are increasingly destabilized by cosolvent-induced decreased "acidity" and the dielectric constant of the mixed solvents compared to that of water. Analysis of the variation of the observed TΔSt0(HX) and particularly of ΔY (= TΔSt0(H+) + TΔS0t.ch (X−), with composition, in the light of Kundu etal's semi-quantitative theory reveals that ME induces breakdown of three dimensional (3D) tetrahedral structures of water at water-rich compositions. This is being followed by an ordered region due to possible H-bonded cosolvent–water complexation and then the usual disordered region due to packing imbalance. Comparison of ΔY(HI)–composition profiles for aqueous mixtures of t-butanol (ButOH), ethylene glycol (EG), and 1,2-dimethoxy ethane (DME) also demonstrates that the remarkable enhancement of 3D water structures by the well known structure promoter ButOH gets succintly diminished when cosolvent ButOH is replaced by EG, ME, and DME, as is expected from structural and electronic considerations of the cosolvents.
Photochemical reactions of quinonoid compounds with phosphorus derivatives
Authors: Alberti Angelo , Hudson Andrew , Pedulli Gian Franco , McGimpsey W. Grant , Wan Jeffrey K. S.
Published: May 2006

The photoreactions of tetraethylpyrophosphite, tetraphenylbiphosphine, diphenylphosphine oxide, and tetraethylbiphosphine disulphide with five quinones or quinonoid compounds have been studied by esr spectroscopy; [Formula: see text] and [Formula: see text] radicals add to both carbon–carbon and carbon–oxygen double bonds, whilst with [Formula: see text] and [Formula: see text] radicals only the species resulting from addition to a carbonyl group are observed. It is also reported that the photogenerated quinone triplets react with tetraethylpyrophosphite leading to the formation of diethoxyphosphonyl radicals, which in turn add to other ground state quinone molecules, the diethoxyphosphinyl adduct to a carbonyl group being also formed in the process.
Stéréochimie des complexes du chlorure d'iode avec les bases carbonylées
Authors: Berthelot Michel , Helbert Maryvonne , Laurence Christian
Published: May 2006

The splitting of the ν(I—Cl) and ν(C=O) bands of ICl complexes with carbonyl bases in highly diluted heptane solutions is explained by the existence of two 1:1 stereoisomeric complexes: a linear complex (ICl along the axis of the carbonyl bond) and an angular complex (ICl in the direction of a lone pair). In the series RCOOEt, RCOPh, RCOt-Bu, R2CO, RCONPh2 and RCONMe2, where R = Me, Et, i-Pr, or t-Bu, the angular complex for R = Me transforms into the linear complex for R = t-Bu, since bulky substituents on the carbonyl group destabilize the angular arrangement. In the series RCOH and RCOMe the complex is angular whatever the steric effect of R, since ICl is located on the lone pair near the small H or Me substituents. When ICl is added in excess, a 1:2 complex appears.
Mécanisme des réactions du chlorite et du dioxyde de chlore. 3. La dismutation du chlorite
Authors: Schmitz Guy , Rooze Henri
Published: May 2006

The disproportionation of chlorite was studied in 0.01 to 1 M perchloric acid solutions at 25 °C and an ionic strength of 1 M. The results suggest at least three reaction paths. The first is catalysed by Cl− ions, the second gives a second-order rate law, and the third is catalysed by iron. Its rate law is[Formula: see text]This can be interpreted by the reversible reaction [Formula: see text] followed by two rate-determining reactions Fe2+ + HClO2 → products, [Formula: see text] From this study and the former, made with added ortho-tolidine, we conclude that the second-order reaction proceeds by a radical chain mechanism.
Compounds of W(VI) with l(−)malic acid: a polarimetric and 1H nuclear magnetic resonance study of the formation and interconversion equilibria in excess of W(VI)
Authors: Cervilla Antonio , Ramírez José A. , Llopis Elisa
Published: May 2006

Complex formation occurring between l(−)malic acid and W(VI) has been investigated by polarimetric and cryoscopic measurements, and also by proton nmr spectroscopy. The complexing ability of malic acid depends on the metal–ligand ratio. With excess W(VI), four different complexes are formed. The pH of the medium is the principal variable controlling complex formation and the various interconversion equilibria. At high pH ( > 7.0), the stable complex is a monomer, while at lower pH values dimers predominate. Conditional dissociation constants of some species have been evaluated and they show that the complexes formed with excess W(VI) have a higher stability than those found when malic acid is in excess. Comparison of the proton coupling constants shows that the conformation of malic acid in its metal complexes depends on which reagent is in excess, and leads to a number of conclusions about the structure of the dimeric species.
Nuclear magnetic resonance studies of molecular motion in guanidinium chloride, bromide, and iodide
Authors: Ratcliffe Christopher I.
Published: May 2006

Guanidinium [Formula: see text] chloride, bromide, and iodide salts have been studied by 1H and 2H nmr as a function of temperature. The 2H powder lineshapes show conclusively that reorientation of the guanidinium ion occurs about the principal three-fold axis in the chloride, bromide, and high temperature phase of the iodide. Activation energies for this process have been obtained from 1H spin-lattice relaxation results. The question of whether or not there are concurrent two-fold flips of the —NH2 units is discussed, but must presently remain unresolved. It was found that the high temperature phase of the iodide can be supercooled.
Synthesis and vibrational spectra of complexes of small carboxylic acids with 18-crown-6
Authors: Savoie Rodrique , Rodrigue André , Pigeon-Gosselin Marie , Chênevert Robert
Published: May 2006

The neutral-component complexes (HCOOH)3•(18-crown-6)2, (CH3COOH)2•18-crown-6, and various halogenated CH3COOH/18-crown-6 adducts have been isolated and studied by infrared and Raman spectroscopy. The halogenated (F, Cl, and Br) derivatives, which occur as (acid•water)x•18-crown-6(x = 1 or 2) adducts, probably have a structure which is similar to that of the known CH2(CN)COOH•H2O•18-crown-6 complex.
Reaction ergodography for dehydrogenation reaction of methanethiol
Authors: Yamabe Tokio , Zhao Cheng-Da , Koizumi Masahiko , Tachibana Akitomo , Fukui Kenichi
Published: May 2006

The dehydrogenation reaction paths of methanethiol CH3SH were analyzed by abinitio MO calculations. The geometries and energies of the reactants, transition states, and products have been determined on the singlet potential energy surface of the ground state. We also analyze the similar dehydrogenation reactions of methanol (CH3OH) and ethanol (CH3CH2OH) for the purpose of comparison. The activation energy and the heat of reaction calculated by incorporating the effect of electronic correlation shows that the product H2C=S is chemically more unstable than the product H2C=O. "Reaction ergodography" along the intrinsic reaction coordinate (IRC) for these dehydrogenation paths clarifies the distinct differences in atomic movement. The path of methanethiol has two characteristic steps before the transition state, while neither paths of the two carbinols have such multisteps. The electronic intramolecular interaction between subsystems of the reactant molecule is investigated in terms of Interaction Hybrid Molecular Orbital (IHMO).
Violation of Hund's multiplicity rule in the electronically excited states of conjugated hydrocarbons
Authors: Koseki Shiro , Nakajima Takeshi , Toyota Azumao
Published: May 2006

Violation of Hund's multiplicity rule in the electronically excited states of conjugated hydrocarbons is studied by using the Pariser–Parr–Pople type SCF MO method and the abinitio MO method with STO-3G basis set, both methods being augmented by CI-type treatments. It is shown that for symmetrical structures (D2h) of the nonalternant hydrocarbons, propalene, pentalene, and heptalene, the lowest excited singlet state is energetically lower than the corresponding triplet state. This is mainly due to the spin polarization (SP) effects. For D2h, structures of pentalene and heptalene the open-shell excited singlet state is predicted to be lower in energy than the closed-shell state, with the result that the former is really the ground state. Further, calculations made by including electron correlation effects reveal that in linear polyenes and polyacenes, the lowest excited singlet "minus" state (using Pariser's classification of the alternancy symmetry species) is lower in energy than the corresponding triplet state. The energy lowering of the singlet "minus" state in linear polyenes is due mostly to the mixing with the doubly excited configurations (mm → nn), while the considerable part of it in polyacenes is due to the SP effects.
Electrostatic lock-and-key model for the analysis of inhibitor recognition by dihydrofolate reductase
Authors: Nagy Péter , Náray-Szabó Gábor
Published: May 2006

We propose a simple electrostatic model for the analysis of ligand binding to proteins. Besides a geometric fit electrostatic complementarity is also needed to allow favourable hydrogen bonding and ion-pair interactions. Nonpolar regions of the ligand and the biomacromolecule active site should match to reduce unfavourable hydrophobic effects, i.e., to lower the Gibbs free energy of the complex in aqueous solution. These principles are applied to the inhibition of dihydrofolate reductase by methotrexate. The enzyme active site is represented by an electrostatic lock which is derived from the macromolecular electrostatic potential and field at certain reference points. The key, defined similarly for the ligand, should fit into the lock to ensure complementarity, i.e., recognition. Hydrogen bonding and ion-pair interactions can be studied by the "potential key" while the "field key" accounts for hydrophobic complementarity. Analyzing the dihydrofolate reductase–methotrexate interaction in the light of the above principles the relative importance of various molecular fragments in binding can be determined.
Hydrogen sulphide – halogen complexes studied by matrix isolation vibrational spectroscopy
Authors: Agarwal U. P. , Barnes A. J. , Orville-Thomas W. J.
Published: May 2006

Infrared spectra are reported of mixtures of hydrogen sulphide with chlorine or bromine in low-temperature argon or nitrogen matrices. The H2S … Cl2 and H2S … Br2 complexes were identified from the perturbed halogen stretching vibration. Absorptions due to the corresponding hydrogen halides provided evidence that reaction was occurring between the hydrogen sulphide and halogen (despite the use of a twin-jet deposition), and other reaction products were tentatively identified from the observed absorptions.
Hartree–Fock stability and symmetry breaking: oxygen doubly negative ion
Authors: Paldus Josef , Čížek Jiří
Published: May 2006

The Hartree–Fock stability problem, its relationship to the broken symmetry solutions, and the implications for a molecular nuclear framework distortion are briefly reviewed. The case where no nuclear framework distortion is possible is studied for the oxygen doubly negative ion, using an abinitio Slater-type orbital analytical approach, in order to illustrate other possible implications of the symmetry breaking in the independent particle models. It is shown that the existing pure singlet broken symmetry solutions for this ten electron ion, which were obtained earlier by adhoc procedures, follows systematically and straightforwardly from the HF stability problem for the corresponding symmetry adapted solution which shows a strong singlet instability. The chemical and physical implications of this type of symmetry breaking are briefly discussed.
Resonance Raman study of phenylhydrazonopropanedinitriles
Authors: Merlin Jean Claude , Thomas Emrys W. , Petit Guislaine
Published: May 2006

Resonance Raman spectra and band assignments of some phenylhydrazonopropanedinitriles, including the 3-chloro and the 2-carboxy derivatives, in the 900–2300 cm−1 spectral range are presented and discussed. I5N and 2H isotopic shifts have been used to clarify the assignments, which have been made for both protonated and deprotonated forms of the compounds. The resonance Raman spectra of the protonated forms show strong coupling between NH deformation and hydrazone vibrations. This coupling is lost in the anionic forms, which are considered to have considerable carbanion character. By using the drastic changes between the resonance Raman spectra of protonated and deprotonated forms, the pKa value of the 2-carboxy derivative has been determined.
Photochemical reactions of bacteriorhodopsin in Triton X-100 solution studied by low temperature spectrophotometry
Authors: Iwasa Tatsuo , Tokunaga Fumio , Yoshizawa Tỏru
Published: May 2006

The photochemical reaction of purple membrane solubilized with Triton X-100 (T-BR) was investigated by low temperature spectrophotometry. The batho- and meta-intermediates of T-BR were observed to resemble bacteriorhodopsin in native purple membrane. Two photoproducts characteristic of the T-BR system were found, which were named the "490-nm complex" and the "380-nm complex". The 490-nm complex was in thermal equilibrium with T-BR in the dark. Cooling T-BR to low temperature favoured the 490-nm complex, which was photoinsensitive. On the other hand, the 380-nm complex was produced by warming the batho-intermediate and reverted to the original T-BR. The meta-intermediate of T-BR may possibly be in thermal equilibrium with the 380-nm complex. On the basis of the above results, the possible role of the membrane structure was discussed
Magnetic circular dichroism spectra of cobalt(III) amine complexes of photochemical importance
Authors: Hollebone Bryan R. , Langford Cooper H. , Malkhasian Aramice Y. S.
Published: May 2006

Magnetic circular dichroism spectra of several acidoamine cobalt(III) complexes are reported which reveal B terms for the spin allowed bands and A terms for the spin forbidden bands. It is shown that ligand field parameters derived from fitting of a full set of spin forbidden bands, including the quintet, gives Racah B and C electron repulsion parameters closer to those anticipated from theory than has been common using mainly the singlets and a single weak low energy absorption feature assigned to a triplet. The spectra indicate that the quintet which has figured in numerous photochemical speculations lies at relatively high energy. This conclusion is a consequence of both the recognition of an A term assignable to the quintet and the best fit ligand field parameters for all bands.
Molecular and electronic structure of ozone and thiozone from LCAO local density calculations
Authors: Morin Mario , Foti Aniko E. , Salahub Dennis R.
Published: May 2006

LCAO local density calculations for ozone yield a ground state geometry in good agreement with experiment (R = 1.27 Å vs. 1.278 Å (exp.), θ = 117.5° vs. 116.8° (exp.)). A second local minimum is found about 45 kcal/mol higher for a cyclic geometry (R = 1.44 Å, θ = 60°). For S3 the calculations predict a bent ground state (R = 2.00 Å, θ = 116°) with the cyclic geometry (R = 2.125 Å, θ = 58°) about 15 kcal/mol higher.
Molecular conformation of ozone and thiozone: the transition state method
Authors: Laidlaw William G. , Trsic Milan
Published: May 2006

The relative energy of the closed and open forms of ozone and thiozone are calculated using the HFS DVM transition state method. For ozone the open form is lower by some 21 kcal/mol and for thiozone the open form is some 11 kcal/mol lower.
Azulenium cations. An experimental and theoretical study of the cations derived from hydrocarbon derivatives of azulene
Authors: Lewis J. W. , Nauman R. V.
Published: May 2006

The ambient temperature ultraviolet absorption and the 77 K luminescence spectra of a series of four hydrocarbon derivatives of azulene in sulfuric acid solution are presented and discussed in detail. CNDO/S calculations have been carried out and the calculational results are compared with the experimental results. The phosphorescences of these cationic species are reported for the first time. The present results suggest that an inversion in the order of electronic states is perhaps responsible for the normal emissions of the protonated species and the anomalous emissions of the parent molecules.
Organic reactions in liquid crystalline solvents. 1. The thermal cis–trans isomerization of a bulky olefin in cholesteric liquid crystalline solvents
Authors: Leigh William J. , Frendo Debbie T. , Klawunn Paul J.
Published: May 2006

The effect of cholesteric liquid crystalline solvents on the energetics of rotational thermal cis–trans isomerization of olefins has been examined. Rate constants have been obtained over a 70-degree temperature range for isomerization of trans-1,2-di-(4-cyanophenyl)-1,2-diphenylethylene in two isotropic solvents and three cholesteric liquid crystals and the Arrhenius parameters determined. The rates of isomerization are found to be consistently slower in the liquid crystalline phases compared to the isotropic solvents. The Arrhenius parameters for isomerization of the olefin in the isotropic solvents (Ea = 34.8 ± 0.3 kcal/mol; ΔS≠ = −1.5 ± 0.5 e.u.) compare favourably with reported values for its isomerization in benzene solution. In the cholesteric phases, Ea is consistently 1–1.5 kcal/mol higher and ΔS≠ slightly more positive than the corresponding values for the isotropic solvents. The results are tentatively rationalized in terms of disruption of liquid crystalline order as the olefin twists from its pseudo-planar, ground state geometry through the globular, twisted transition state. The magnitude of this effect is proposed to depend on both the difference in steric bulk of the ground and transition states and the "tightness" of the solvation shell seen by the isomerizing molecule. It is believed that in the present case the observed effects are somewhat truncated as a result of rather poor solvation of the bulky olefin in the liquid crystalline phases.
The preparation of bis(perfluoroethyl) tellurium difluoride and dichloride, trans perfluoroethyl tellurium monochloride tetrafluoride, trans bis(perfluoroethyl) tellurium tetrafluoride; and the preparation and X-ray crystal structure of perfluoroethyl tellurium trifluoride
Authors: Lau Clement , Passmore Jack , Richardson E. Keith , Whidden Tom K. , White Peter S.
Published: May 2006

The reaction of (C2F5)2Te and XeF2 in a slurry of SO2ClF yielded (C2F5)2TeF2 essentially quantitatively. Chlorine and (C2F5)2Te gave (C2F5)2TeCl2. Both (C2F5)2TeF2 and (C2F5)2TeCl2 were assigned a trigonal bipyramidal geometry, on the basis of their 19F nmr and vibrational spectra, with the lone pair and C2F5 groups in equatorial, and the halogens in the axial positions. Perfluoroethyl tellurium trifluoride was prepared essentially quantitatively by the reaction of C2F5TeTeC2F5 and XeF2 in liquid SO2F2. The generally inert SO2ClF was found to react with C2F5TeTeC2F5 to give C2F5TeClxF3−x, and sulphur dioxide. The structure of C2F5TeF3 was determined by X-ray diffraction. The crystals are tetragonal with a = 10.129(4), c = 25.561(6) Å, and Z = 16. The structure was refined in space group I41/a to a conventional R factor of 0.051 for 901 observed reflections with I ≥ 3σ(I). Each tellurium atom is surrounded by two terminal fluorine atoms and two bridging fluorine atoms and a C2F5 group in an axial position around the apex of a distorted square pyramid. The square-pyramidal units are linked by symmetrical cis bridging atoms into endless chains with bridging angles of 180° and 177°. The geometry of the (C2F5TeF4) group is consistent with steric activity of the non-bonded electron pair. The 19F nmr and Raman spectra of C2F5TeF3 were recorded and the Raman spectrum assigned. Trans-C2F5TeClF4 was prepared by the reaction of C2F5TeTeC2F5, and an excess of ClF. The trans octahedral geometry of C2F5TeClF4 was unambiguously assigned from its 19F nmr and vibrational spectra. A mixture of trans-(C2F5)2TeF4 and trans-C2F5TeClF4 was obtained from the reaction of (C2F5)2Te and an excess of ClF. Trans geometry of (C2F5)2TeF4 was unambiguously assigned from its 19F nmr spectrum.
The mechanism of the oxidative deamination and decarboxylation of serine and threonine by periodate in acid medium
Authors: Pascual Rosa , Herráez Miguel A.
Published: May 2006

The kinetics of oxidation of serine and threonine by periodate have been investigated in acid medium at 10 °C. The reaction rate is first order in both periodate and amino acid, and the overall reaction follows second-order kinetics. The rates decrease with increase in [H+]. A catalytic effect of the buffers was not observed in the oxidation process. An analysis of the dependence of the rate on [H+] reveals that the reactive species under the experimental conditions are periodate monoanion and dianion and the dipolar form of the amino acid. The mechanism proposed and the derived rate law are consistent with the observed kinetics. The rate constants predicted using the derived rate law are in agreement with the observed rate constants, thus justifying this rate law and hence the proposed mechanistic scheme.
Oriented collisions in the recombination and disproportionation of free radicals
Authors: Cohen L. K. , Pritchard H. O.
Published: May 2006

It is shown that there will be strong orienting effects in the collision of two radicals which will invalidate attempts to calculate disproportionation–recombination ratios by simple geometric arguments.
Substituent effects on benzylic radical hydrogen hyperfine coupling constants. Part 4. The effect of branching of the alkyl substituent
Authors: Wayner Danial D. M. , Arnold Donald R.
Published: May 2006

The effects of substituents in the meta and para positions of the benzyl radical on the α-hydrogen hyperfine coupling constants (hfc's) are discussed. The electron spin resonance (esr) spectra of the para-methyl, ethyl, isopropyl, and tert-butyl substituted benzyl and cumyl radicals are analysed. Hyperconjugation involving the C—C bond is 40–60% as effective as C—H hyperconjugation for delocalizing spin density. This conclusion is supported by INDO molecular orbital calculations. Similar analysis of the 13Cmr spectra of the para-alkyl substituted cumyl carbocations provides evidence that C—C hyperconjugation is 75–90% as effective as C—H hyperconjugation for delocalizing charge density.
Synthesis and nuclear magnetic resonance spectra of platinum compounds with thiourea derivatives
Authors: Rochon F. D. , Bariyanga J. , Kong P. C.
Published: May 2006

Compounds of the type trans-[PtL2T2]Cl2 where L = NH3, methylamine, dimethylamine, pyridine, and 2-aminopyrimidine and T = N-methylthiourea and N,N′-dimethylthiourea and where L = NH3 and T = N,N,N′,N′-tetramethylthiourea have been synthesized. The 1H nmr spectra have shown that the thiourea derivatives are bonded to platinum by the sulfur atom. The methyl protons of N-methylthiourea and one methyl group protons of N,N′-dimethylthiourea in the complexes, where L is aromatic, showed resonance at higher field after coordination to the metal. This was explained by an intramolecular interaction between the methyl group and the aromatic ring. The nmr spectra of the [PtT4]Cl2 compounds were also measured.
Charge distributions and chemical effects. XXXVII. Note on tetracyclo[4.1.0.02,4.03,5]heptane
Authors: Bridet J. , Béraldin M.-T. , Fliszár S.
Published: May 2006

Full geometry and ζ optimizations are a must in the study of small cycles, as in tetracyclo[4.1.0.02.4.03.5]heptane. Indeed, nonoptimized STO-3G calculations suggest the 4-carbon to be electron richer than the carbon in 3 position, whereas the reverse is shown by optimized calculations ([Formula: see text] vs. −138 me forC-3 andC-4, respectively). The latter result is in accord with the general trends indicating that a gain in local 2s charge is accompanied by a downfield nmr shift (δ 22.1 for C-3 and −4.0 ppm for C-4, from tetramethylsilane). These charge and shift results can be directly compared to one another because the local geometries (namely, the angles) about these two carbon atoms are very similar. This, however, is not the case with other carbon atoms of 3-membered cycloalkanes, a situation restricting the validity of simple charge–shift correlations in this class of compounds because of the changes in local geometry affecting the hybridization of carbon.
The products of hydrolysis of cyclic orthoesters as a function of pH and the theory of stereoelectronic control
Authors: Deslongchamps Pierre , Lessard Jean , Nadeau Yves
Published: May 2006

The acid hydrolysis of cyclic orthoesters 1, 3–6 (R = Me), and 2 (R = Me and Et) as a function of pH was studied. The bicyclic orthoester 5 yields mainly the hydroxy-ester (less than 5% lactone), and this result is essentially independent of pH. For the other orthoesters, the relative percentage of products differs for each case and varies with pH. At pH ≤ 3, the percentage of lactone is always larger than at pH > 3. These results are explained on the basis of the stereoelectronic theory for the cleavage of tetrahedral intermediates.
Réactivité du nucléophile azoture vis-à-vis de cations hétérocycliques aromatiques. VIII. Réarrangement de β-tétrazolo-trans-benzalacétophénones
Authors: Cherton Jean-Claude , Bazinet Marc , Bolze Marie-Madeleine , Lanson Marc , Desbene Paul-Louis
Published: May 2006

β-Tetrazolyl-trans-benzalacetophenones (T) isomerize upon heating into the corresponding azido-azomethines (A). These non-isolable products undergo various transformations depending on the reaction conditions. With organic bases such as triphenylphosphine or pyridine, their interception can occur. In aromatic solvents, a rearrangement involving loss of nitrogen leads to five-membered diazoted heterocycles:-N-benzoyl imidazoles and pyrazoles, via intermediate diazirines and oxazepines. Protic solvents have been found to facilitate the isomerisation of tetrazoles into azidoazomethines. Solvolysis of the azidooxazine and in some cases of the oxazinium species resulting from the equilibrium and reaction:[Formula: see text]are then observed.
Organic reactions in liquid crystalline solvents. 2. An investigation into the use of liquid crystalline solvents to effect stereochemical control in the Diels–Alder reaction
Authors: Leigh William J.
Published: May 2006

The possibility of using liquid crystalline solvents to control the stereospecificity of bimolecular reactions as a result of differences in liquid crystalline solvation of the various possible transition states has been examined. The Diels–Alder reactions of 2,5-dimethyl-3,4-diphenylcyclopentadienone with four dienophiles of varying steric size (cyclopentene, cycloheptene, indene, and acenaphthylene) have been carried out in benzene, cholesteryl nonanoate (isotropic), cholesteryl propionate (cholesteric), and 4-ethyl-4′-(4-pentylcyclohexyl)biphenyl (smectic) at 105 °C. In spite of very large differences in steric solvation requirements for the endo- (globular in shape) and exo- (plate-like in shape) transition states in these reactions, no variation in product ratio with solvent phase is observed in any case.The inability of the ordered liquid crystalline phases to differentiate between endo- and exo-transition states in the Diels–Alder reactions investigated is rationalized as being the possible result of several factors. The most important of these are believed to be free volume effects, owing to both inefficient steric solvation of the bulky diene and volume contraction in the transition states for cycloaddition.
Novel stereochemical equilibria in crown ethers. Detection of additional conformations of cis-syn-cis- and cis-anti-cis-dicyclohexano-18-crown-6 by 100.6-MHz 13C nuclear magnetic resonance at 173 K
Authors: Buchanan G. W. , Bourque K. , Bovenkamp J. W. , Rodrigue A.
Published: May 2006

High field, 13C nmr has been utilized for the analysis of new conformational equilibria in each of the title crown ethers at low temperature. Evidence indicates that these equilibria arise from restricted rotation in the macrocyclic portions of these molecules. The conformational free energy differences between the isomers are small (0.1–0.3 kcal/mol) while barriers to their interconversion are of the order of 8.5–9.0 kcal/mol. Comparisons are made with the degenerate equilibria involving inversion of the cyclohexane rings.