bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of constants for weak organic bases
Authors: Chandler W. David , Lee Donald G.
Published: May 2006

A non-linear regression procedure has been developed to analyze simgoidal plots even when data at or near the plateaus are missing. The method was used to fit relative C-13 nmr chemical shifts (Δ) of five alcohols to sigmoidal functions of the excess acidity (X). With two of the functions, the regression gives [Formula: see text], the relative chemical shift of the protonated alcohol. That parameter permits calculation of the ratio, [BH+]/[B], necessary for an additional linear regression to determine [Formula: see text] and m*. With a third function, regression gives [Formula: see text], [Formula: see text] and m* directly. Comparison of [Formula: see text] values obtained from these methods with those estimated previously suggests that the sigmoidal regressions are reliable, and can be of considerable use in handling titration-type curves generally. Keywords: basicities, alcohols, regression, sulfuric aid.
Liquid assisted secondary ion mass spectrometry of [(η5-C5Me4R′)Rh(SCN)2(CNR)] (R′ = Me, Et) complexes of isocyanides
Authors: Boyd R. K. , Embree D. J. , McAlees A. J. , Taylor A.
Published: May 2006

It is shown that the primary ionization, under conditions of liquid-assisted secondary ionization mass spectrometry (LSIMS), of neutral complexes of the type [(η5-C5Me4R′)Rh(SCN)2(CNR)], involves the loss of a thiocyanate ligand. The processes leading to the observed spectra have been elucidated using techniques of accurate mass measurement and tandem mass spectrometry, so that the molecular weight and elemental composition of the isocyanide ligand RNC can be deduced with confidence. The greatest sensitivity was found when 3-nitrobenzyl alcohol was used as the matrix. Bimolecular reactions are shown to occur under LSIMS conditions, yielding ions containing two rhodium atoms but not the isocyanide. The implications of these results for the characterization and analysis of isocyanide metabolites of Trichoderma hamatum are discussed and illustrated. Key words: isocyanides, mass spectrometry, rhodium complexes, Trichoderma hamatum.
A nitrogen-15 nuclear magnetic resonance study of the tetracycline antibiotics
Authors: Curtis Ronald D. , Wasylishen Roderick E.
Published: May 2006

Natural abundance 15N NMR has been used to study several tetracycline antibiotics in the solid state and in solution. In the solid state, the 15N chemical shifts of the tetracycline free bases are sensitive to the site of protonation in the "A" ring. A high frequency shift of 14 ppm is observed for the keto form of oxytetracycline (N protonated) relative to the enol form of oxytetracycline (O protonated). Comparison of 15N chemical shifts in the solid and solution states indicates for the first time that the structural integrity (site of protonation and hence conformation) of the tetracyclines is maintained in (CD3)2SO solutions. In addition, the solid state 15N CP/MAS NMR spectra are sensitive to subtle conformational changes of the "A" ring amide substituent. Key words: antibiotics, tetracyclines, 15N NMR, molecular structure.
Theoretical calculations of chemical interactions. VII. SCF-MO conformational studies of substituted 2′-hydroxychalcones
Authors: Nudelman Norma Sbarbati , Furlong Jorge Javier P.
Published: May 2006

INDO calculations have been performed on substituted 2′-hydroxychalcones. The results show that the intramolecularly hydrogen-bonded trans-s-cis conformation is the most stable. The calculations assist in assignment of NMR signals and interpretation of kinetic determinations. The calculated energy of the intramolecular hydrogen bond is compared with IR data when available and with experimentally determined heats of formation of the chalconates. Regarding the isomerization to flavanones, calculations of the ring A rotation in the chalconates show that it is exergonic; on the contrary, complete rotation of the styryl system encounters a strong barrier. MNDO calculations on this process show similar trends. These results indicate that this last rotation is involved in the slow step, which is consistent with kinetic determinations. Key words: chemical interactions, theoretical calculations, substituted hydroxychalcones, flavanones.
The photocycloaddition of naphthoates with acetylacetone; dramatic acid promotions of product quantum yields
Authors: Chow Yuan L. , Liu Xiao-Yun
Published: May 2006

In acetonitrile and methanol, methyl 1- and methyl 2-naphthoate (1- and 2-NpCO2CH3) underwent [2 + 2] photocycloaddition with acetylacetone (acacH) from a reactive species derived from the lowest spectroscopic singlet excited state to give cyclobutane derivatives such as 9 that ring-opened to afford addition products of acetyl and acetonyl moieties at the 7,8-position for 1-NpCO2CH3 (2 and 3) and the 1,2-position for 2-NpCO2CH3 (7). Further irradiation of 3 and 7 caused a deep-seated rearrangement of 3 to give 4 and an intramolecular addition of 7 to give the caged product 8. The photocycloaddition quantum yields were increased remarkably in the presence of a trace amount (< 0.001 M) of protic acids. For example, the limiting quantum yield of 2 and 3 with Фadducts = 0.033 (in the absence) increased to Фadducts > 0.5 under comparable conditions in the presence of [H2SO4] = 0.001 M, approaching the maximum asymptotically at about this concentration; the increases of the quantum yield could not be correlated with H2SO4 concentrations. The acid promotion of the product formation elevated the photocycloaddition to a significantly useful preparative reaction. The photocycloaddition of 2-NpCO2CH3 was quenched by tributylamine, dimethylaniline, and methyl iodide, and could not be initiated by typical triplet sensitization. Strong fluorescence of 2-NpCO2CH3 at 358 nm in CH3CN was quenched by acetylacetone, tributylamine, dimethylaniline, and methyl iodide, the last of which concurrently enhanced the phosphorescence intensity of 2-NpCO2CH3. In the presence of [H2SO4] > 0.005 M the fluorescence intensity at 358 nm was quenched and a new fluorescence at 470 nm corresponding to protonated 12-NpCO2CH3 appeared. However, in the [H2SO4] < 0.002 M region, the new fluorescence was not observed, indicating that the protonation scarcely occurred. The Stern–Volmer constant, obtained by the quantum yield monitor, of the 2-NpCO2CH3 photocycloaddition with acacH was more than an order of magnitude smaller than that obtained by the quenching of 2-NpCO2CH3 fluorescence by acacH. The discrepancies outlined above clearly demonstrate that the reactive intermediate of the photocycloaddition is not the exciplex of the spectroscopic singlet excited species of 12-NpCO2CH3 and acacH, but one that is derived from the exciplex. It is speculated that this unknown reactive intermediate interacts with a proton to promote the photocycloaddition. Key words: photocycloaddition of naphthoates, photorearrangement of styryl ketones, acid catalysis of quantum yields, fluorescence of protonated naphthoates, acid enhancement of singlet quenching.
Monolayer properties of mixed chlorophyll a-digalactosyldiacylglycerol at the nitrogen–water interface
Authors: Ducharme Daniel , Shibata Osamu , Munger Gaétan , Leblanc Roger M.
Published: May 2006

Surface pressure, surface potential, and ellipsometric–area isotherms have been measured to investigate the properties of chlorophyll a – digalactosyldiacylglycerol monolayers at the nitrogen – water interface. The surface pressure – area isotherms of the mixed film show a pronounced negative deviation with respect to ideality, whereas the surface potential isotherms follow the additivity law for mixed films. As the mole fraction of Chl a increases from 0.1 to 0.5, each of the corresponding ellipsometric isotherm is shifted towards more negative values in the liquid expanded region. As the mole fraction is increased above 0.5, the ellipsometric δΔ values become more positive, with this trend continuing throughout the 0.5 to 0.9 mole fraction range. The combined effects of anisotropy in both the refractive indices and the absorption coefficient can explain the turnover observed in the ellipsometric δΔ isotherms as the mole fraction of Chl a becomes larger than 0.5.Hydrophilic interactions have been ruled out on the basis of the agreement with the additivity law, which is observed for surface potentials of the mixed monolayers. The observed deviation exhibited in the π–A isotherms is interpreted in terms of the intermolecular cavity effect. Key words: chlorophyll a, digalactosyldiacylglycerol, mixed monolayer, ellipsometry.
Mechanism of Wittig reaction with cyclic anhydrides
Authors: Kayser Margaret M. , Hatt Krista L. , Hooper Donald L.
Published: May 2006

The reactions of stabilized phosphoranes with cyclic anhydrides give enol-lactones as final products. The initial condensation, however, leads to the formation of acyclic adducts that are observable by NMR, can be easily trapped, and, in some cases, can be isolated. A study of the mechanism of these condensations by NMR spectroscopic methods and by various trapping experiments is described and the reaction pathway is proposed.
Chiral 2-acetamidoacrylates in conjugate addition – asymmetric enolate trapping reactions. Asymmetric synthesis of phenylalanine
Authors: Cativiela Carlos , Diaz-De-Villegas Maria Dolores , Galvez José Antonio
Published: May 2006

A new route to the asymmetric synthesis of α-amino acids based on the reactivity of chiral 2-acetamidoacrylates with nucleophiles through a conjugate addition followed by diastereoselective protonation of the enolate is described. Phenylalanine precursors are obtained in excellent chemical yields (80–95%) with moderate diastereomeric excess (0–44%) through the reaction of chiral 2-acetamidoacrylates with phenylmagnesium bromide in the presence of CuI followed by diastereoselective enolate protonation.
Stereoselective deprotonation of tropinone and reactions of tropinone lithium enolate
Authors: Majewski Marek , Zheng Guo-Zhu
Published: May 2006

Tropinone (6) was deprotonated with lithium diisopropylamide and with chiral lithium amides (18–24) and the resulting enolates (two enantiomers) were treated with electrophiles. The aldol reaction with benzaldehyde and deuteration were both diastereoselective. The former yielded only one isomer (exo, anti) of the aldol 8a; the latter proceeded from the exo face. This selectivity permitted us to probe the deprotonation of tropinone with lithium amides; it was concluded that the reaction involves predominantly the exo axial protons. The reaction of tropinone enolate with ethyl chloroformate led, via a ring opening, to the cycloheptenone derivative 9. The reaction with methyl cyanoformate yielded, in the presence of silver acetate and acetic acid, the β-ketoester 8b; however, in the absence of these additives, and especially when 12-crown-4 was added to the enolate, a ring opening leading to the pyrrolidine derivative 10 occurred instead. Deprotonation of tropinone with chiral lithium amides proceeded with modest enantioselectivity. A synthesis of non-racemic anhydroecgonine via this strategy allowed establishing the absolute stereochemistry of deprotonation.
On the correlation between electron mobility, free-ion yield, and electron thermalization distance in nonpolar dielectric liquids
Authors: Jay-Gerin Jean-Paul , Goulet Thomas , Billard Isabelle
Published: May 2006

The correlation between the thermal electron mobility, μe, the radiation-induced free-ion yield Gfi at zero applied electric field, and the most probable thermalization distance b of secondary electrons, is examined for 52 different pure non-polar dielectric liquids for which data have been reported in the literature. It is shown that, in liquids for which μe > 10−1 cm2 V−1 s−1, the variation of Gfi with μe is well represented by a relation of the type [Formula: see text] where n ≈ 0.31. The connection between Gfi and μe can be described through the product εsb, where εs is the static dielectric constant of the intervening liquid. In particular, 1/εsb is shown to correlate with both μe and Gfi. Analysis of these correlations allows us to estimate an upper limit of μe that can be attained in a room-temperature dielectric liquid, information of utmost importance from the point of view of application to liquid ionization detectors.
Nouveaux alcaloïdes isoquinoléiques isolés d'une Lauraceae bolivienne: Aniba canelilla H.B.K.
Authors: Oger Jean-Michel , Fardeau Agnès , Richomme Pascal , Guinaudeau Hélène , Fournet Alain
Published: May 2006

Eleven benzyltetrahydroisoquinoline alkaloids, with an original substitution pattern, were isolated from the stem bark of Aniba canelilla H.B.K. (Lauraceae). Structural studies of these compounds including 1H, 13C, nOe difference, COSY H–H and C–H, and COLOC nuclear magnetic resonance spectroscopy led to the identification of four benzylisoquinolines, monosubstituted on ring C with an hydroxyl at C-11, (−)-norcanelilline 2, (+)-canelilline 3, anicanine 4, and canelillinoxine 5; two tetrahydroprotoberberines monosubstituted on ring D at C-9, (−)-anibacanine 6 and (+)-manibacanine 8; two tetrahydroprotoberberines monosubstituted on ring D at C-11, (−)-pseudoanibacanine 7 and (+)-pseudomanibacanine 9; and three protoberberines with the same substitution pattern on rings A and D with a methyl substituent at the 8α or 8β position, (−)-α-8-methylpseudoanibacanine 10, (−)-β-8-methylpseudoanibacanine 11, and (−)-α-8-methylanibacanine 12. An examination of all isolated alkaloids suggests the existence of two enzymatic systems in A. canelilla. In the first enzymatic system, reticuline- and coclaurine-type alkaloids are involved in the bioconversion of classical proaporphine and aporphine alkaloids. In the second system, canelilline-type alkaloids are intermediate in biosynthesis of anibacanine and pseudoanibacanine protoberberine alkaloids.
Apparent molar heat capacities and volumes of some aqueous solutions of aliphatic amino acids at 288.15, 298.15, 313.15, and 328.15 K
Authors: Hakin Andrew W. , Duke Michelle M. , Klassen Sheri A. , McKay Robert M. , Preuss Kathryn E.
Published: May 2006

The thermodynamics of amino acid systems are key to the understanding of protein chemistry. We have found that many previous studies of the apparent molar volumes and heat capacities of aqueous solutions of amino acids were conducted at the standard temperature of 298.15 K. This does not allow for the fact that most biological processes occur at temperatures removed from this standard condition.In an attempt to address this imbalance we have measured densities and heat capacities for aqueous solutions of glycine, L-alanine, L-serine, and L-threonine at 288.15, 298.15, 313.15, and 328.15 K using a Picker flow microcalorimeter. Apparent molar volumes and heat capacities, and the associated standard state partial molar properties have been calculated. Constant pressure variations of revised Helgeson, Kirkham, and Flowers equations have been fitted to calculated standard state volumes and heat capacities over the temperature range 288.15 to 328.15 K. These equations may be used to estimate standard state volumes and heat capacities, and hence equilibrium constants, for aqueous amino acid systems at higher temperatures.
Activities of dilute NiO in Na2O•2SiO2 by electromotive force measurements
Authors: Tumidajski P.J.
Published: May 2006

The thermodynamic properties of dilute NiO in Na2O•2SiO2 for XNiO < 0.0125 have been investigated in the temperature range 1073–1173 K by emf measurements using the electrochemical cell[Formula: see text]where NiO denotes a solution with Na2O•2SiO2. It was found that for XNiO < 0.003 the solutions obey Henry's law. The Henrian activity coefficients and the related dilute solution thermodynamic properties based on the solid NiO standard state are reported.
Kinetics of electroreduction of Zn2+ at mercury in nonaqueous solutions
Authors: Manzini Mariléa , Lasia Andrzej
Published: May 2006

The electroreduction of Zn2+ was studied in propylene carbonate (PC), acetonitrile (ACN), and hexamethylphosphoramide (HMPA) on mercury at various concentrations of tetraethylammonium perchlorate as a supporting electrolyte using dc polarography, cyclic voltammetry, chronoamperometry, and ac polarography. It was found that in PC and ACN the electroreduction process proceeds in one step. In HMPA, however, the electroreduction proceeds through a CEE mechanism in which a chemical reaction is followed by a charge transfer in two steps. The heterogeneous rate constants, corrected for the double layer effects, decrease with increase in the solvent donor number. The electroreduction process proceeds through the cation transfer mechanism.
A 139La NMR study of the interactions between the La(III) cation and D-ribose in aqueous solution
Authors: Chen Zhigang , Morel-Desrosiers Nicole , Morel Jean-Pierre , Detellier Christian
Published: May 2006

The interactions of the La(III) cation with D-ribose and with D-arabinose in aqueous solution were investigated by 139La NMR spectroscopy. In the case of D-ribose, the formation of a La(III)-sugar complex was indicated by variations of the 139La chemical shift and linewidth with an increase of the sugar concentration in solution. In contrast, the complexation of La(III) by arabinose is very weak and almost undetectable by 139La NMR. On the basis of a 1:1 stoichiometry, the stability constant for the complex of La(III) with D-ribose was calculated from the observed 139La chemical shift values. A similar treatment was done for the viscosity corrected 139La linewidths using arabinose as an uninteractive reference. The stability constants, K, obtained independently from 139La chemical shifts and linewidths are in good agreement, 2.8 ± 0.5 and 2.2 ± 0.6 M−1 respectively at 299.0 ± 0.5 K. The thermodynamic parameters for the complexation of La(III) by D-ribose could also be obtained: ΔH0 = −12 ± 2 kJ mol−1, and ΔS0 = −31 ± 5 J K−1 mol−1. These values are in very good agreement with those obtained by calorimetric measurements.
Structure of the O-chain polysaccharide of three strains of Pseudomonas syringae ssp. savastanoi
Authors: Adinolfi Matteo , Corsaro Maria Michela , Lanzetta Rosa , Marciano Carmela Elsa , Parrilli Michelangelo , Evidente Antonio , Surico Giuseppe
Published: May 2006

The structure of the O-chain portion of the lipopolysaccharide isolated from three different strains of Pseudomonas syringae ssp. savastanoi has been determined by chemical and NMR spectral analysis. For all of the strains the O-chain consists of a tetrasaccharide repeating unit of three α-L-Rhap and one terminal nonreducing α-D-Fucp3NAc. Two rhamnosyl residues are 3-linked and the third one is 2,3-linked. LPS from an olive strain, avirulent on oleander, was shown to attenuate the response of oleander plants inoculated with a homologous strain.
Photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction. Part 7: methanol, conjugated dienes, and 1,4-dicyanobenzene
Authors: McManus Kimberly A. , Arnold Donald R.
Published: May 2006

The scope of the photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction has been extended to include conjugated dienes: 1,3-butadiene (9), 2-methyl-1,3-butadiene (10), 2,3-dimethyl-1,3-butadiene (11), and 2,5-dimethyl-2,4-hexadiene (12). Acetonitrile–methanol solutions of the dienes 9,10, and 11, and 1,4-dicyanobenzene (1), with and without codonor (biphenyl (5)), were irradiated with a medium-pressure mercury vapour lamp through Pyrex. Both 1,2- and 1,4-addition products were formed in approximately equal amounts (combined yields of photo-NOCAS products, 50–65%). In marked contrast, when an acetonitrile–methanol solution of 2,5-dimethyl-2,4-hexadiene (12), 1, and 5 was irradiated, only the 1,4-addition product, trans-2-(4-cyanophenyl)-5-methoxy-2,5-dimethyl-3-hexene (22,82%), was obtained. This photolysate also contained a small amount of another 1,4-addition product, that which had incorporated cyanide ion instead of methanol, trans-2-(4-cyanophenyl)-2,2,5-trimethyl-3-hexenenitrile (23, 2%). Irradiation of an acetonitrile solution (no methanol) of 12,1, and 5 gave 23 in good yield (68%). An excellent yield (80%) of 23 was obtained upon irradiation of an acetonitrile solution of 1,12, 5, potassium cyanide, and 18-crown-6. Addition of 2,4,6-trimethylpyridine (collidine, 25), a mild, non-nucleophilic base, to the reaction mixture diverts the reaction involving 12 from photo-NOCAS products to 1:1 substitution products; 3-(4-cyanophe-nyl)-2,5-dimethyl-1,4-hexadiene (26), trans-5-(4-cyanophenyl)-2,5-dimethyl-1,3-hexadiene (27), (Z)-1-(4-cyanophenyl)-2,5-dimethyl-2,4-hexadiene (28), and (E)-1-(4-cyanophenyl-2,5-dimethyl-2,4-hexadiene (29) were formed. The mechanisms of these reactions are discussed and an explanation for the observed regio- and stereoselectivity is given.
Control of regioselectivity in reactions of dialkylstannylene acetals. Part I. A dramatic reversal of regioselectivity in mono-p-toluenesulfonation reactions
Authors: Kong Xianqi , Grindley T. Bruce
Published: May 2006

The regioselectivities of p-toluenesulfonation reactions of dialkylstannylene acetals obtained from a number of carbohydrate-derived terminal 1,2-diols in the absence of added nucleophiles have been explored as functions of the carbohydrate structure, the nature of the alkyl group on tin, the solvent, and the reaction temperature. Virtually all dibutylstannylene acetals react preferentially on the primary oxygen atom, sometimes with excellent regioselectivity. Increasing the steric bulk of the alkyl substituents on tin increases the preference for reaction at the secondary oxygen atom, but at the expense of increased reaction times. Hexamethylenestannylene acetals react at about the same rate as dibutylstannylene acetals; in almost all cases, they react preferentially on the secondary oxygen atom, sometimes with excellent regioselectivity.
The biosynthesis of antibiotic F-244 in Fusarium sp. ATCC 20788: origin of the carbon, hydrogen, and oxygen atoms
Authors: Saepudin Endang , Harrison Paul
Published: May 2006

Biosynthetic incorporation experiments were performed with carbon-13 labelled precursors including sodium [1-13C]-, [2-13C], and [1,2-13C2]-acetate as well as [methyl-13C]methionine into antibiotic F-244 (1) in growing cultures of Fusarium sp. ATCC 20788. After conversion to the methyl ester 2, analysis by NMR showed that the carbon skeleton of 1 derives from seven intact acetate units; the remaining four carbons are from methionine. Hence, the pathway is similar to that reported for 1 in Scopulariopsis. The biogenesis of the hydrogen atoms in 1 was also investigated. Incorporation of sodium [1-13C, 18O2]acetate gives 2, which exhibits 18O-induced isotope shifts at C-1 and C-3. The labelling pattern is consistent with formation of the β-lactone ring by nucleophilic attack of a C-3 hydroxyl group in the nascent polyketide precursor onto the C-1 carbonyl. Keywords: biosynthesis, polyketide, F-244, β-lactone, Fusarium.
The effect of meta- or para-cyano substitution on the reactivity of the radical cations of arylalkenes and alkanes. Radical ions in photochemistry, Part 34
Authors: Arnold Donald R. , Du Xinyao , Chen Jing
Published: May 2006

The effect of electron-withdrawing substituents, meta- or para-cyano, on the reactivity of the radical cation of arylalkenes and alkanes has been determined. The radical cations were generated by single electron transfer (set) to an electron-accepting photosensitizer. Three reactions were studied: (i) the addition of nucleophile to the radical cation of arylalkenes, (ii) cleavage of the benzylic carbon–carbon bond of the radical cation of arylalkanes; and (iii) the deprotonation of the benzylic carbon–hydrogen bond of the radical cation of arylalkanes. The radical cations of 4-(1-phenylethenyl)benzonitrile (1b), 3-(1-phenylethenyl)benzonitrile (1c), 4-(2-methoxy-1-phenylethyl)benzonitrile (2b), 3-(2-methoxy-1-phenylethyl)benzonitrile (2c), cis- and trans-5-cyano-2-methoxy-1-phenylindane (6b-cis and -trans), and 6-cyano-3-phenylindene (7b) were generated, by single electron transfer to the lowest excited singlet state of 1,4-dicyanobenzene (3), in acetonitrile–methanol. The radical cations of 1b, 1c, and 7b react with methanol to yield the anti-Markovnikov adducts (2b, 2c, and 6b-cis and 6b-trans). The radical cations of 2b, 2c, and 6b-trans cleave at the benzylic carbon–carbon bond to give products derived from the radical and carbocation fragments. The radical cation of 6b-cis deprotonates from the benzylic position with subsequent formation of the diastereomer, 6b-trans. This behaviour can be explained/predicted on the basis of the proposed mechanisms for these reactions. Molecular orbital calculations (AM1) support the conclusions. Keywords: photosensitized, electron transfer, radical ions, radicals, molecular orbital calculations (AM1).
A solid-state 13C NMR and theoretical investigation of carbonyl and thiocarbonyl carbon chemical shift tensors
Authors: Kirby Christopher W. , Lumsden Michael D. , Wasylishen Roderick E.
Published: May 2006

The carbon chemical shift tensors of the carbonyl and thiocarbonyl groups of acetamide, thioacetamide, thioacetanilide, 4′-methoxyacetanilide, and 4′-methoxythioacetanilide have been experimentally determined using dipolar – chemical shift solid-state 13C NMR spectroscopy. The magnitudes of the three principal components of the carbon chemical shift tensors are found to exhibit marked variations between the carbonyl and thiocarbonyl functionalities. However, in contrast to the conclusions of an earlier comparative investigation involving benzophenone and thiobenzophenone, the orientations of the principal axis systems of these chemical shift tensors are found to be similar. These experimental results represent the first complete characterizations of the carbon chemical shift tensor in organic thiocarbonyls. The results of our ab initio GIAO and LORG calculations of carbon chemical shielding tensors in formaldehyde, thioformaldehyde, formamide, and thioformamide as well as in acetamide and thioacetamide are in qualitative agreement with experiment. The findings of the present investigation provide conclusive evidence that the well-known isotropic deshielding of the carbon nucleus in the C=S group relative to C=O is primarily attributable to the decreased energy associated with the σ ↔ π* excitation within the thiocarbonyl fragment. This result is in contrast with the conventional interpretation that the deshielding originates from a red shift in the C=S HOMO–UMO n → π* transition. Keywords: chemical shift tensors, solid-state 13C NMR, carbonyls, thiocarbonyls, ab initio calculations.
Synthesis and photoisomerization of provitamin D analog with 11β,19-oxide bridge
Authors: Siciński Rafal R.
Published: May 2006

Triol diester 1 was converted into the B-ring 5,7-diene 6 representing the first example of the provitamin D analog where the 10β angular methyl group is connected to ring C by a C11/C19 ether linkage. Ultraviolet light irradiation of 6 resulted in the formation of stereoisomeric 9β, 10α-compound 8a. The structure of the photoproduct was established by analysis of vicinal 1H–1H coupling constants and by molecular mechanics. Keywords: 11β, 19-oxido steroids, lumisterol analog, 5,7-androstadienes, electrocyclic reactions, photochemistry.
Synthesis of heptamethylindenyl rhodium(III) complexes: molecular structures of [(η5-C9Me7)RhCl(µ-Cl)]2 and [(η5-C9Me7)Rh(PMe2Ph)Cl2]
Authors: Kakkar Ashok K. , Stringer Graham , Taylor Nicholas J. , Marder Todd B.
Published: May 2006

The synthesis of a series of permethylindenyl (Ind*) rhodium(III) complexes, [(η5-C9Me7)RhCl(μ-Cl)]2, [(η5-C9Me7)RhLCl2] (L = PMe3, PMe2Ph, PPh3), and [(η5-C9Me7)Rh(PMe3)2Cl]+Cl−, is reported. The structure of [(η5-C9Me7)RhCl(μ-Cl)]2, (4) was determined, confirming the dimeric nature of the compound. Ruby red crystals of 4 are monoclinic, space group C2/c, with four molecules per unit cell of dimensions a = 18.598(5), b = 10.520(3), c = 17.376(3) Å and β = 115.10(2)°. The indenyl rings exhibit minimal distortion from perfect η5 coordination to the Rh centres. A single crystal X-ray structure determination of [(η5-C9Me7)Rh(PMe2Ph)Cl2] (6) indicates a more distorted η5-coordination of the indenyl ligand, with the auxiliary ligand having the greatest trans influence, namely, the PMe2Ph group, being trans to the indenyl ring junction C—C bond. Yellow prismatic crystals of 6 are orthorhombic, space group P212121, with four molecules per unit cell of dimensions a = 8.819(1), b = 15.303(3), c = 17.296(3) Å. Keywords: rhodium, indenyl, permethylindenyl, heptamethylindenyl, phosphine complex. X-ray structure.
Évaluation des ruptures de liaison α(C—H) centrale et terminale dans la photolyse du propène gazeux à 184,9 nm
Authors: Maré George R. De , Collin Guy J. , Deslauriers Hélène
Published: May 2006

The minor, α(C—H) photofragmentation processes in the far-UV photolysis of gaseous propene at λ = 184.9 nm are investigated. At this wavelength the β(C—H) and α(C—C) photofragmentation processes are of major importance, accounting for more than 95% of the primary reaction process. The total quantum yield of the three α(C—H) bond cleavage processes is close to 0.03. However [Formula: see text] and the sum of the two [Formula: see text] cleavages is 0.005 ± 0.003. Thus [Formula: see text] whereas statistically one expects 1:2. Thus the energy of the bond that is breaking is important, even though the photon energy is much higher than that required to break it. The geometrical parameters, total energies, and fundamental frequencies of the cis- and trans-propen-1-yl and propen-2-yl radicals have been determined by abinitio methods at various levels of theory (STO-3G, 3-21G, 6-31G, 6-31G*, and 6-31G** basis sets; UHF, ROHF, and GVB methods). cis-Propen-1-yl is predicted to be 0.6–1.0 kJ mol−1 lower in energy than trans-propen-1-yl, except with the minimal STO-3G basis set. The optimizations predict that propen-2-yl is more stable than the cis- and trans-propen-1-yl radicals by 14 ± 3 kJ mol−1. Keywords: propene, unimolecular reactions, photochemistry, α(C—H) cleavage, propenyl radicals.
Association between halothane and oxygenated solvents by 1H NMR spectroscopy
Authors: Tkadlecová Marcela , Havlíček Jaroslav , Dohnal Vladimír
Published: May 2006

Using 1H NMR spectroscopy the complex-formation equilibria between halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) and methyl tert-butyl ether or tetrahydrofuran in various inert solvents (hexane, heptane, decane, cyclohexane) were measured as a function of temperature. For two different association models (ideal solution and athermal solution), assuming only the formation of a 1:1 H-bonded complex, the equilibrium constants and the standard enthalpies of the complex-formation reaction were calculated. The ideal solution model provides values of the equilibrium constant that differ for different inert solvents. The athermal solution model makes this false solvent effect much smaller. For the low halothane concentration used, its dimerization was neglected. This assumption was verified experimentally. Keywords: 1H NMR, association, complex formation, halothane.
Unsymmetrical dienophiles and their reactions with ortho-quinodimethanes
Authors: Charlton James L. , Bogucki David , Guo Peng
Published: May 2006

Three unsymmetrical chiral dienophiles have been synthesized and their reactivities with α-hydroxy-ortho-quinodimethanes have been assessed. Thus, the methyl (S)-lactyl esters of trans-4-bromocrotonate and trans-4-acetoxycrotonate (1 and 2) and mono (methyl (S)-lactyl) fumarate 3 were synthesized. Each was reacted with α-hydroxy-o-quinodimethane and α-hydroxy-α-phenyl-o-quinodimethane. While reaction with α-hydroxy-o-quinodimethane gave synthetically useful yields of cycloadducts with high regioselectivity and diastereoselectivity, the corresponding reactions with α-hydroxy-α-phenyl-o-quinodimethane were less successful. The reaction of 3 with α-hydroxy-α′-phenyl-o-quinodimethane also gave a mixture of cycloadducts. Keywords: ortho-quinodimethanes, lignans, Diels–Alder, asymmetric, lactate.
Kinetics and mechanism of the oxidation of the sulphite ion by the Mn(III)–cydta complex ion
Authors: Chandrawat Uttra , Prakash Aditya , Mehrotra Raj N.
Published: May 2006

The reinvestigated oxidation of S(IV), HSO3−/SO32−ions, by [Mn(cydta)(OH)]− confirmed that S(IV) is oxidized in two parallel paths; the order with respect to [S(IV)] is one in one of the paths and two in the other. The nature of the dependence of the rate on [H+] is also confirmed. However, the rapid scan of the reaction mixture and measurement of the initial absorbance of the reaction mixture at different wavelengths at the beginning of the reaction suggest an outer-sphere mechanism. The rate parameters are of the same order as obtained in known reactions of an outer-sphere mechanism and this mechanism is further supported by the Marcus cross relation. Keywords: kinetics, outer-sphere mechanism, [Mn(cydta)]−, SO32−.
A two-dimensional copper(I) polymer containing both molecular rod and anionic bridges: synthesis and crystal structure of (µ-chloro)-(µ-pyrazine)copper(I)
Authors: Moreno J.M. , Suarez-Varela J. , Colacio E. , Avila-Rosón J.C. , Hidalgo M.A. , Martin-Ramos D.
Published: May 2006

The crystal structure of bright-red crystals of the bidimensional polynuclear diamagnetic (µ-chloro)(µ-pyrazine)copper(I) complex has been analyzed from X-ray diffraction data. The structure was solved by the heavy-atom method. Full-matrix least-squares refinement based on 673 reflections with F > 4σ(F) converged to a final R = 0.043 and Rw = 0.048. Crystals of this complex are monoclinic, space group P2/c, a = 3.814(1), b = 6.356(1) and c = 11.497(5) Å; β = 96.04(2)°, Z = 2. The structure consists of planes of copper(I) atoms linked by pyrazine and Cl− bridges, each Cu(I) being in a distorted tetrahedral environment formed by two chloride ions and two nitrogen atoms belonging to two pyrazine molecules, with bond distances of Cu—Cl = 2.349(1) and Cu—N = 2.038(4) Å. Vibrational and electronic spectroscopy of the title compound is also reported. Keywords: two-dimensional copper(I)–pyrazine complexes.
Studies on the transannular Diels–Alder reaction of 15-membered macrocyclic trienes containing a trans-trans diene. Part I: Synthesis and isomerization of various model cyclopentadecatrienes
Authors: Hall Dennis G. , Müller Renate , Deslongchamps Pierre
Published: May 2006

The transannular Diels–Alder reaction of 15-membered macrocyclic trienes with a trans-trans-cis (TTC) olefin geometry can lead to A.B.C.[6.6.7] tricyclic products of trans-syn-trans (TST) and cis-syn-cis (CSC) stereochemistry whereas the TTT isomers can produce the trans-anti-cis (TAC) and cis-anti-trans (CAT) tricycles. In order to study the influence of the position (pro-6.6 or pro-6.7) and the nature (alkyl, alkoxymethyl, or formyl) of the dienophile substituent, a set of 10 model cyclopentadecatrienes was prepared. The synthesis of appropriately functionalized trisubstituted dienophile and diene synthons, as well as their coupling reactions affording acyclic precursors, is described in this paper (first in a series of two). A mild and efficient macrocyclization protocol yielded the required methyl- or alkoxymethyl-substituted cyclic substrates. Further transformations led to the formyl-substituted ones. In particular, the TTT macrocycles containing an enal as a dienophile could be completely isomerized to the corresponding TTC thermodynamic isomers in acidic aqueous media. Keywords: transannular, Diels–Alder, 15-membered ring, A.B.C.[6.6.7] tricycles, diterpene.
Liquid chromatographic analysis and isolation of diastereoisomers obtained by an aldolization reaction
Authors: Lafosse Michel , Morin Philippe , Anceau Christophe , Coudert Gérard , Guillaumet Gérald , Dreux Michel
Published: May 2006

A new route using an aldolization reaction has been investigated in order to synthesize a precursor of pilocarpin, an alkaloid having sympathomimetic properties. To study the stereoselectivity of this route and to propose a mechanism, liquid chromatography has been used to analyse and to isolate each diastereoisomer for a structural determination by NMR. To further investigate the reaction mechanism, seven different aldehydes (or ketones) were used in the aldolization reaction, producing seven different mixtures. This paper describes the improvement of LC conditions with diol- and amino-bonded silica and the nonaqueous mobile phases required for the isolation of each isomer. Keywords: diastereoisomers, pilocarpin precursor, normal phase liquid chromatography, diol- and amino-bonded silicas.
Photodegradation mechanism of phenyl isopropenyl ketone – methyl methacrylate copolymers in the solid phase and their performance as a deep UV resist
Authors: Sugita Kazuyuki , Ishida Tomohito , Kushida Masahito , Harada Kieko , Saito Kyoichi , Ueno Nobuo
Published: May 2006

Phenyl isopropenyl ketone (PIPK) copolymer films were irradiated with a low-pressure mercury lamp, and benzoyl side-chain cleavage and main-chain scission at various temperatures were followed by UV spectral changes and gel permeation chromatography, respectively. Quantum yields of both reactions and the ratio of the latter to the former reaction increased markedly above their glass transition temperatures. The main-chain scission continued to proceed by irradiation even after the side chain ceased to split. These experimental results suggest a new reaction mechanism of photosplitting of the methyl methacrylate ester side chain on the carbon adjacent to the C=C bond followed by β-scission, in addition to the known benzoyl side-chain cleavage followed by β-scission or C=C bond formation. The enhanced quantum yield for the main-chain scission of the PIPK copolymer at 130 °C realized sensitivity of 100 mJ/cm2 as a DUV resist, which was 1200 times as high as that of poly(methyl methacrylate) exposed at room temperature and developed under similar conditions. Keywords: Norrish type I cleavage, β-scission of main chain, temperature dependence, photosplitting of ester side chain, conjugated polymer radical.
Radiolysis studies on the corrosion inhibitors 1 -hexyn-3-ol, cinnamonitrile, and 1,3-diethyl-2-thiourea
Authors: Elliot A.J. , Chenier M.P. , Ouellette D.C.
Published: May 2006

In this publication we report: (i) the rate constants for reaction of the hydrated electron with 1-hexyn-3-ol ((8.6 ± 0.3) × 108 dm3 mol−1 s−1 at 18 °C), cinnamonitrile ((2.3 ± 0.2) × 1010 dm3 mol−1 s−1 at 20 °C), and 1,3-diethyl-2-thiourea ((3.5 ± 0.3) × 108 dm3 mol−1 s−1 at 22 °C). For cinnamonitrile and diethylthiourea, the temperature dependence up to 200 °C and 150 °C, respectively, is also reported; (ii) the rate constants for the reaction of the hydroxyl radical with 1-hexyn-3-ol ((5.5 ± 0.5) × 109 dm3 mol−1 s−1 at 20 °C), cinnamonitrile ((9.2 ± 0.3) × 109 dm3 mol−1 s−1 at 21 °C), and diethylthiourea ((8.0 ± 0.8) × 108 dm3 mol−1 s−1 at 22 °C). For cinnamonitrile, the temperature dependence up to 200 °C is also reported; (iii) the rate constant for the hydrogen atom reacting with 1-hexyn-3-ol ((4.3 ± 0.4) × 109 dm3 mol−1 s−1 at 20 °C). Keywords: radiolysis, corrosion inhibitors, rate constants.
Single-crystal polarized spectra in the near-infrared region: a local-mode analysis of the spectra of M2MnCl4•2X2O (M = Cs, Rb; X = H, D)
Authors: Walker Ian M. , McCarthy Paul J.
Published: May 2006

The polarized crystal spectra of M2MnCl4•2X2O (M = Cs, Rb; X = H, D) have been measured at 10 K in the near-infrared region. The data have been analyzed using a local-mode model that includes the bending mode. The resulting parameters are related to hydrogen bonding in the crystals. The spectra of partially deuterated species show a large number of bands due to HOD vibrations. The parameters from these spectra are compared to those from the H2O and D2O spectra. Arguments as to the importance of Fermi resonance in these spectra are presented. Key words: near-infrared, single crystal, hydrogen bond, Fermi resonance.
Expedient synthesis of unsaturated amide alkaloids from Piper spp: Exploring the scope of recent methodology
Authors: Strunz George M. , Finlay Heather J.
Published: May 2006

The Sakai aryl aldehyde – cyclic ketone aldol – Grob fragmentation sequence was extended to cinnamaldehyde and cyclohexanone, and the product was elaborated to analogues of the alkaloid piperstachine. The effects of substituents on the reaction involving cinnamaldehyde were studied. The aldol-fragmentation sequence failed with benzaldehyde when cyclooctanone or cyclobutanone was substituted for cyclohexanone or cyclopentanone, and the reasons for this failure were examined. Four-carbon Wittig homologation of the piperonal–cyclobutanone aldol-fragmentation product, a hypothetical route to alkaloids such as retrofractamide A, was thus not viable. Instead, three-carbon homologation of the readily available piperonal–cyclopentanone product, using alkyne chemistry recently disclosed by Lu and Trost, afforded these alkaloids in excellent overall yield. The alkyne isomerization was also used to effect efficient syntheses of pellitorine and several other non-aromatic 2E,4E-dienoic Piper amide alkaloids. Key words: Piper, amides, alkaloids, insecticides, aldol, fragmentation, cinnamaldehydes, alkyne, redox, isomerization.
1995 R.U. Lemieux Award Lecture Ketenes and bisketeness: organic chemistry in microcosm
Authors: Allen Annette D. , Colomvakos Jim D. , Egle Ian , Liu Ronghua , Ma Jihai , Marra Romeo M. , McAllister Michael A. , Tidwell Thomas T.
Published: May 2006

Silyl substituents stabilize ketenes, and permit the preparation of persistent bisketenes, which show characteristic 13C, 17O, and 29Si NMR chemical shifts. The structures and conformations of bisketenes have been examined, as well as their interconversion with cyclobutenediones. A tetraketene has been prepared, and the hydration reactivity of carbon suboxide has been measured. Key words: bisketenes, structures and conformations; cyclobutenedione interconversion.
A unified approach to dynamic NMR based on a physical interpretation of the transition probability
Authors: Bain Alex D. , Duns G.J.
Published: May 2006

A general theory of the effect of dynamics (relaxation and (or) exchange) on NMR spectra is presented. This theory is based on a reexamination of the transition probability. The classic expression for this is as the square of the transition moment, but we feel it is useful to separate the square into two separate terms. In the generalization presented here, we show that one of these terms corresponds to the share of the initial magnetization that each spin coherence receives at the start of the experiment. The second term is how much that coherence contributes to the total detected signal. The final intensity is the product of these two factors. For a static spectrum, these two terms are complex conjugates, so the product is real and we recover the standard transition probability. When there is dynamics, the product becomes complex, so the time evolution includes oscillatory and dispersive terms. This means that a dynamic spectrum is still a sum of individual transitions, but the lineshapes are distorted in phase, intensity, position, and linewidth by the dynamic process. In this paper we develop the general theory, and illustrate it with a calculation of the classic problem of mutual exchange in an AB spin system. Key words: NMR spectroscopy, transition probability, chemical exchange, kinetics.
Atoms in molecules from the exact one-electron wave function
Authors: Hunter Geoffrey
Published: May 2006

Use of the electron amplitude (i.e., the square root of the electron density) in place of the electron density in Bader's topological theory of atoms in molecules, is shown to lead to identical definitions of bond paths and interatomic surfaces. The concepts of regions of charge concentration and depletion, derived from the Laplacian of the electron density, become the concepts of classically allowed and forbidden regions of space when derived from the Laplacian of the electron amplitude, because the latter is the wave function in the exact Schrödinger equation for (any) one electron in the many-electron molecule. However, the domains of corresponding regions are different, the size and depth of a classically allowed region always being larger than the corresponding region of charge concentration; in some cases there is no region of charge concentration corresponding to a classically allowed region. The surface between the outermost allowed and forbidden regions of space provides a definition of the size and shape of the molecule. This definition of the outermost surface of the electronic charge cloud (the molecular envelope) is exact apart from quantum mechanical tunelling. Hence it is proposed as a basis for molecular graphics; i.e., as the unique, non-arbitrary, and, in principle, exact definition of the size and shape of a molecule. Key words: electron density, quantum chemistry, molecular graphics.
Molecular charge density analysis
Authors: Hinze Juergen , Biegler-Konig F. , Lowe A.G.
Published: May 2006

It is proposed to analyse the first-order reduced density matrix of a molecular wave function in terms of the first-order reduced density matrices of different states of the constituent atoms. With this an unambiguous partitioning of the molecular charge distribution in terms of the atomic charge distributions is obtained. Simple practical formulae are derived, such that in many ab initio molecular wave function calculations the analysis proposed can be carried out routinely. The results obtained should be useful for the interpretation of molecular wave functions in terms of their atomic constituents, as well as for the determination of atomic form factors to be used in X-ray molecular structure determination. Some simple examples are given, and the results obtained are compared with those obtained using other methods of analysis. Key words: charge density, density matrix, goodness-of-fit, correlation coefficient, standard deviation.
Quantum chemical valence indices from the one-determinantal difference approach
Authors: Nalewajski Roman F. , Mrozek Janusz , Mazur Grzegorz
Published: May 2006

The recently introduced quadratic (two-electron) valence indices, ionic and covalent, derived from the Hartree–Fock finite-difference approach, are applied to selected organic and inorganic molecules to demonstrate their utility in monitoring chemical bonding patterns in molecular systems. The indices are defined in terms of differerences between simultaneous probabilities of finding two electrons on specified atoms, calculated from the molecular and separated-atom-limit (SAL) wave functions, respectively, in the UHF approximation. The total quadratic valence number represents the overall number of chemical bonds in the system under consideration; it is interpreted as the molecular expectation value of the difference operator of the molecular and SAL density operators. This interpretation leads to a new set of ionic atomic and diatomic valence components; these modified valence numbers are discussed using the two-orbital model in the UHF scheme. A new procedure is proposed for dividing the one-center contributions to the bond valences; it generates effective bond orders in qood agreement with chemical expectations. The new valence quantities are tested on selected typical molecules and prototype hydrogen-bonded dimers. A more extensive study has been carried out on small-ring propellanes, to examine changes in bond valences between bridgehead atoms in selected systems. Key words: chemical valence: UHF difference approach; chemical bond: two-electron model; bond multiplicities; ionic/covalent bond components; propellanes: valence study.