bluuebunny/crossref_metadata_2025_split huggingface dataset:
Degradation Thermique D'Imidazole‐N‐Oxydes dans la Source D'Un Spectrometre de Masse
Authors: Maquestiau A. , van Haverbeke Y. , Flammang R. , Chua S. O. , Cook M. J. , Katritzky A. R. 
Published: September 2010

AbstractThe mass spectral characteristics of the products described previously as triphenylimidazole‐N‐oxide and N‐hydroxytriphenylimidazole can be interpreted on the basis of polymorphic forms of the same tautomer ( probably the N‐oxide structure ). Intense thermal degradation is observed in the source of the mass spectrometer.
Structure Electronique des Bandes de Valence du Polyethylene et de Ses Derives Fluores
Authors: Delhalle J. , Delhalle S. , Andre J. M. 
Published: September 2010

AbstractLes bandes de valence des dérivés fluorés du polyéthylène ont été calculées par la méthode de Hückel généralisée. Les modifications dues à la substitution chimique sont examinées.
Application de la Spectrométrie de Masse en Serie Hétérocyclique. Système N‐Phényl‐3,6‐Dihydro‐2H‐1,2‐Oxazine. Mise en Évidence D'Une Réaction de Substitution Intramoléculaire au Niveau des Cations Radicalaires Phényl et Pyridylbutadiényles
Authors: Maquestiau A. , van Haverbeke Y. , De Meyer C. 
Published: September 2010

AbstractThe mass spectra of α−γ− and β‐pyridylbutadienes contain[M‐l]+ peaks which are probably formed via an intramolecular substitution reaction leading respectively to azonianaphtalene, quinolinium and isoquinolinium ions. These structures can also be formed, by ring expansion from suitable alkylated heterocyclic systems like 2‐methylindolizine and methylindoles. Abundance ratios including metastable and normal ions are in agreement with the proposed mechanism. The same type of reaction, observed in the case of chlorophenylbutadienes is discussed.
Étude par RPE de la Structure Électronique et Moléculaire de Complexes du Cuivre(II) Dans des Zéolites de Type Y
Authors: Derouane Eric G. , Vedrine Jacques C. , Taarit Younès Ben 
Published: September 2010

AbstractL'analyse du spectre RPE de complexes du cuivre(II) avec l'ammoniac et la pyridine, piégés dans des zéolites de type Y, permet d'obtenir certains renseignements sur la structure électronique et géométrique de tels complexes, à savoir le degré de covalence relatif de la liaison Cu‐N, le nombre de ligands coordinés, et leur disposition autour de l'ion central.Une étude en fonction de la température de la constante de couplage hyperfin de l'ion Cu2+ dans ces complexes montre que celle‐ci diminue pour des températures croissantes. Ce comportement s'explique en admettant une modulation de ce couplage hyperfin par une vibration bien définie, celle des liaisons Cu‐N. Il est possible, à partir des variations observées en RPE, de déterminer pour les divers types de complexes les fréquences de vibration Cu‐N. Celles‐ci sont en accord parfait avec celles mesurées par spectroscopie IR dans l'infrarouge lointain. Ces fréquences, très sensibles à l'interaction des complexes avec leur environnement, permettent d'obtenir certaines informations sur leur localisation dans la structure zéolitique.
Contribution à L'Étude des Tellurures de Cobalt et de Nickel
Authors: De Meester P. , Naud J. 
Published: September 2010

AbstractThe tellurides Co1‐xTe (0.1< x<0.43) and Ni1‐xTe (0.04< x <0.42) precipitated by hydrogen telluride have a composition which is a function of the pH of the precipitation medium. This variation of composition is due to the interaction of the two redox systems Te−/Te− on one hand and H+/H2 on other hand.
Mise en Évidence D'Un Composé BiBo3 et de Son Polymorphisme par Spectroscopie Vibrationnelle
Authors: Pottier M. J. 
Published: September 2010

AbstractBismuth orthoborate BiBO3, a non‐existent compound after the Bi2O‐B2O3 phase diagram, has been obtained by melting a mixture of H3BO3 and Bi2O3 corresponding to the BiBO3 composition and subsequent cooling.Two different phases are obtained, depending on the cooling conditions. Infrared and Raman spectroscopy, together with X‐ray powder diffractometry, lead to good combined evidence that both phases are polymorphs of bismuth orthoborate BiBO3. At 600°C, this compound is rapidly decomposed into a mixture of the stable phases predicted by the phase diagram, namely 2 Bi2O3.B2O3 and 3 Bi2O3. 5 B2O3.The metastable character of BiBO3 near its melting point is thus clearly established.The structures of these phases, and their stabilization by various cation substitutions are still under investigation.
Equilibre Prototropique des Aryl‐ et Alkylisoxazoline‐5‐Ones. Influence de la Basicité des Milieux Aprotiques
Authors: Maquestiau A. , van Haverbeke Y. , Muller R. N. 
Published: September 2010

AbstractAlkyl‐ and arylisoxazolin‐5‐ones can exist as three prototropic forms. Percentage of ketonic CH structure decreases as hydrogen bond accepting power of the solvent raises. The equilibrium shifts then toward NH tautomer; nevertheless it seems necessary to consider participation of a 5‐hydroxyisoxazolic species in highly basic media; this is particularly evident for 3‐monosubstituted compounds.
NMR experiments on acetals. 51. On the origin of the conformational behaviour of the trimethylammonium substituent: ‐N+Me3substituted 1,3‐dioxanes and ‐CH2N+Me3substituted 1,3‐dioxolanes and 1,3‐dioxanes
Authors: van Cauwenberghe R. , Anteunis M. , Valckx L. 
Published: September 2010

AbstractBy a systematic investigation of ‐CH2N+Me3‐substituted 1,3‐dioxolanes and 1,3‐dioxanes, together with other conformational peculiarities (axial preference of a 5‐N+Me3grouping in 1,3‐dioxane) it is concluded that a C‐N+bond prefers to become parallel with (the) p‐lobe(s) of a β‐heteroatom.
NMR experiments on acetals. Part 52. Chair chair inversion in some symmetrically substituted gem‐dimethoxy‐cyclohexanes
Authors: De Beule H. , Anteunis M. 
Published: September 2010

AbstractBarriers for ring inversion in 4,4‐diø, 4,4‐di c.propyl‐, and 4,4‐pentamethylene‐1,1‐dimethoxycyclohexane have been evaluated using different methods (Table 1). The close parallelism of the observed effect on ΔG≠ caused by gem. substitution in the present compounds as compared with correspondingly substituted 1,3‐dioxanes let to suppose that strain in rotations along C‐C bonds neighbouring the substitution site may contribute to the observed trends.
Stereochemistry of the mercurideboronation at a saturated primary carbon atom: Inversion of configuration
Authors: Gielen M. , Fosty R. 
Published: September 2010

AbstractThe mercurideboronation of tris‐erythro‐1,2‐dideuterio‐3,3‐dimethylbutylborane occurs with inversion of configuration in THF. The bromodemercuration of threo‐1,2‐dideuterio‐3,3‐dimethylbutylmercury chloride occurs with retention of the configuration in pyridine.
Composés oléfiniques et acétyléniques. III. Synthèse et stéréochimie de dérivés polybromés hexadieniques; obtention du 1,5‐hexadiyne‐3‐ene.
Authors: Figeys H. P. , Gelbcke M. 
Published: September 2010

AbstractThe bromination of 1,5‐hexadiyne at ‐40° in chloroformic solution gives 1,2,5,6‐tetrabromo‐1,5‐hexadiene (5) with a yield of 948; at higher temperatures (e. g. ‐15°), bromination of the double bonds occurs. The stereochemistry of (5) has been studied by N. M. R. ‐spectroscopy; the ratio of cis to trans double bonds (defined with respect to the two bromine atoms) is shown to be 1/9. When (5) is treated repeatedly with N‐bromo‐succinimide (NBS) in CCl4 solution, 1,1, 2,5, 6,6‐hexabromo‐2,4‐hexadiene (7) (trans, trans or cis, cis) is formed, implying a double allylic bromine migration; this structure is assigned on the basis of U. V.‐ and N. M. R.‐ evidence, in comparison with the spectroscopic properties of 1,2,3‐tribromopropene. Treatment of (7) with zinc powder in ethanol produces mainly trans‐1,5‐hexadiyne‐3‐ene (yield 57%), together with a small amount (2,8%) of the cis‐derivative.
Composés Oléfiniques et Acétyléniques. IV. Nouvelles Synthèses du 1,3,5‐Hexatriene; Électrodimérisation de L'Acroléine sur un Couple Zinc‐Cuivre
Authors: Figeys H. P. , Gelbcke M. 
Published: September 2010

AbstractThe electrodimerisation of acroleine on a zinc‐copper couple in the presence of acetic acid gives 1,5‐hexadiene‐3,4‐diol 5 in 76% yield, the ratio meso/dl being 60/40. Gas chromatographic analysis of the reaction mixture reveals the presence of threo‐ and erythro‐1‐hexene‐3,4‐diol and of the mono‐ and di‐acetates (meso/dl) of 4, especially when large quantities of acetic acid are used. Optimum conditions are described in order to minimize these side‐reactions. Starting from 4, two new routes to 1,3,5‐hexatriene 1 has been developped, namely by treating the bistosylate derivative of 4 with sodium iodide in 2‐(2‐ethoxy‐ethoxy) ethanol (adaptation of Linstead's reaction) and by a modification of Corey's method, where 4 is treated with N′, N′‐thiocarbonyl‐diimidazole and the resulting cyclic thiocarbonate decomposed in triphenylphosphite. Hexatriene is obtained respectively in 52 and 47% overall yield, the cis/trans ratios being 10/90 and 20/80. Although these reaction schemes appear to be non‐stereospecific, they constitute two new valuable methods to cis‐1,3,5‐hexatriene.
New Routes to Some New Partially Reduced Myrcenes
Authors: Anteunis M. , De Smet A. 
Published: September 2010

AbstractWe have tested out some preparations of regio‐selectively reduced myrcenes. New routes to the preparation of seq. trans‐2,6‐diMe‐2,6‐octadiene (Wilputte‐Steinert's method3 with Cr(CO)6 as the catalyst) and 2,6‐diMe‐2‐octane (Ru/C in MeOH:H2O) are reported. The Cr (CO)6 procedure needs only such mild conditions that epoxide rings are not cleaved. The preparation in moderate yields but unique fashion of a new dihydromyrcene (3‐Methylidene‐7‐methyl‐l‐octene) is also described.
German II ‐ Décomposition Thermique De L'hydroperoxy‐2 Méthyl‐2 Propionate De T. Butyle Dans Les Solvants Possédant Des Hydrogènes Labiles ‐ Cas Des Alcools Primaires et Secondaires
Authors: Rafii E. , Lena L. , Metzger J. 
Published: September 2010

AbstractLa décomposition thermique en phase liquide de l'hydroperoxy‐2 méthyl‐2 propionate de t.butyle 1, dans une série d'alcools secondaires et primaires a été étudiée. Dans ces solvants, la décomposition est plus rapide que dans les solvants ne possédant pas d'hydrogène labile. L'etude cinétique montre que lors de la décomposition thermique de l'hydroperoxyde l. dans les alcools secondaires et primaires linéaires, l'ordre par rapport à l'hydroperoxyde est respectivement 1 et 1/2. L'énergie d'activation est voisine de 20 kcal.mole−1L'influence des inhibiteurs, les résultats de R. P. E., l'analyse qualitative et quantitative des produits des réactions, nous permettent de proposer trois réactions en chaîne de décomposition de l'hydroperoxyde 1. Le mécanisme “MH, YH” appliqué généralement lors de la pyrolyse des alcanes pour expliquer l'influence accélératrice de certains corps hydrogénés (YH), peut être applique à la décomposition en phase liquide de cet hydroperoxyde.
Fluorescence Quenching of Anthracene by Dimethyl‐ and Diphenylmercury
Authors: Donckt E. Vander , Lootens M. , Swinnen D. , Haquin C. 
Published: September 2010

AbstractThe fluorescence quenching of anthracene by dimethyl‐(DMM) and diphenylmercury (DPM) has been examined and compared in toluene and 95% ethanol as solvent. The rates of quenching by DMM and DPM are similar at room temperature. The application of the Arrhenius relationship shows that this is due to a fortuitous compensation of pre‐exponential and activation energy terms.It is shown that the fluorescence quenching by DPM results only from an increased rate of intersystem crossing induced by the heavy‐atom molecule.
Chemical Studies of Marine Invertebrates. XII(1). Lemnacarnol, a Novel Sesquiterpene From the Soft Coral Lemnalia Carnosa
Authors: Tursch B. , Colin M. , Daldze D. , Losman D. , Karlsson R. 
Published: September 2010

AbstractA novel, non isoprenic sesquiterpene alcohol, lemnacarnol, has been isolated from the Indo‐Pacific soft coral Lemnalia carnosa. Its structure and absolute configuration have been determined by single‐crystal X‐ray diffraction.
German Irradiation De L'Hexane et Du Decane Par Des Électrons Acceleres Effet De La Température Sur Les Produits Légers
Authors: Fontaine R. , Tilquin B. , Claes P. 
Published: September 2010

AbstractThe influence of the temperature on the yields of light radiolysis products of n‐hexane (‐70 to 60°C) and n‐decane (38° to 150°C) irradiated oy electrons at high dose rates has been investigated. Relative yields of unsaturates reach a maximum at a temperature which is a function of the irradiated alkane and of the gaseous products in the irradiated hydrocaroon. These temperature effects may be explained by a lowering of the solubility and by a competition between organic radicals recombination and hydrogen abstraction from the solvent molecule by these radicals as the temperature is raised.
Étude Par Spectrométrie Infrarouge De L'Interaction Entre Le Bromure De Tetrabutylammonium Et Quelques Dérivés Phenoliques
Authors: Bulinda J. B. , Zeegers‐Huyskens Th. 
Published: September 2010

AbstractThe hydrogen bonded complexes formed between tetrabutylammonium bromide and phenol derivatives are studied by infrared spectrometry in CCl4. The logarithm of the stability constants of complexes of 1:1 and 2:1 stoechiometry, and the frequency shifts of the stretching vibration IVOH are linearly related to the sum of the Hammett substitution constants. The influence of the proton donor concentration on the infrared spectrum and the structure of the complexes are discussed.
CNDO/2 Study of Polar Interactions in Acetonitrile
Authors: Paoloni L. , Hauser S. 
Published: September 2010

AbstractThe interaction of two acetonitrile molecules has been studied by the CNDO/2 approximation of the MO method. The antiparallel dipoles model obtains a 4.5 kcal/mole energy gain at a contact distance of 2.35 A. The skewed‐T interaction configuration gives an energy gain about ten times larger at a much shorter contact distance, 1.6 A. This latter model seems however much more affected than the other by the shortcomings of the CNDO/2 approximation.The reported calculations indicate the skewed‐T interaction to be stronger than that of two antiparallel molecules, and suggest the possible presence of polymeric clusters in the liquid. Both interaction models evolve towards the formation of a (CN)2 unsaturated ring, which in turn might go into a more stable nearly‐tetrahedral cage structure.
Secondary Structures in Protected Homo‐Oligo‐L‐Norvalines. Comparison with Isomeric Oligo‐L‐Valines
Authors: Toniolo C. , Bonora G. M. , Fontana A. 
Published: September 2010

AbstractThe secondary structures of a homologous series of chemically and optically pure protected L‐norvaline homo‐oligomers, the synthesis and characterization of which is described in the preceding paper, were examined in a number of solvent systems using essentially CD*, and the results compared with those obtained in the isomeric L‐valine series. All protected norvaline and valine oligopeptides exist as statistical coil conformations in HFIP and HFA. In TFE, however, BOC‐(‐L‐Val‐)7‐OMe exists in a β‐associated structure in contrast to BOC(‐L‐Nva)7‐OMe. This finding points to the influence of β‐branching in directing peptide conformations. The addition of water to solutions of the protected norvaline and valine peptides in TPE forces the higher oligomers to assume intermolecular β‐type conformations. The aggregated structures could be disrupted by dilution or heating.
H Bonding Properties of Hydroxyl Held on the Surface of Silica
Authors: Sempels R. E. , Rouxhet P. G. 
Published: September 2010

AbstractThe H bonding properties of silica gel surface hydroxyl have been studied by adsorbing series of organic compounds which are weak H bond acceptors and by observing the OH stretching band. While the data are consistent with the charge transfer model of H bond, a critical examination shows that their support to this theory, as well as the support of similar data reported for solutions, is not really significant. All the results, and particularly relationships found between absorption coefficients, show a complete similarity between the characteristics of H bonds formed on the solid surface and in solutions.
Synthesis of Brominated Isophorone Derivatives
Authors: Verhé R. , Schamp N. , De Buyck L. , Vanloocke R. 
Published: September 2010

AbstractBromination of isophorone with excess of bromine in carbon tetrachloride at 0° gave 2,4,6‐tribromo‐3‐bromomethyl‐5, 5‐dimethyl‐2‐cyclohexenone which on further treatment with bromine afforded 2,4,6, 6‐tetrabromo‐3‐bromomethyl‐5, 5‐dimethyl‐2‐cyclohexenone. Bromination of isophorone and 4‐bromoisophorone at 25° gave rasp. 2,6,6‐tribromo‐ and 2,4,6‐tribromo‐3‐dibromomethyl‐5,5‐dimethyl‐2‐cyclohexenone.The possible rearrangement in dimethylformamide and the catalytic hydrogenation of the bromo compounds were investigated. The reaction of isophorone oxide with hydrogen bromide did not give brominated products.
Reactivity of α,α‐Dihalogeno‐Imino Compounds Part VII: Reactivity of N‐1‐(2,2‐dichloroalkylidene)t. Butylamines
Authors: De Kimpe Norbert , Verhé Roland , De Buyck Laurent , Schamp Niceas 
Published: September 2010

AbstractTreatment of N‐1‐(2, 2‐dichloroalkylidene)t.butylamines 1a‐c with excess methanolic sodium methoxide under reflux afforded mainly a mixture of α‐methoxy‐α,β‐unsaturated aldimines 2a‐c and the rearrangement products 3, besides small amounts of N‐t.butyl 2‐chloroamides 4 and N‐t.butyl 2‐chloro imidates 6. On the other hand, N‐1‐(2, 2‐dichlorobutylidene)t.butylamines 1d‐e gave α‐chlo‐ro‐α, β‐unsaturated aldimines, while α, α‐dichloropropylidene t.butylamine 9 provided mainly the α‐methoxy‐α,β‐unsaturated aldimine 11 next to the α‐me‐thoxyamide 12, α, α‐dimethoxyaldimine 10 and the rearrangement product 13. The reaction mechanisms are discussed.
Constituents of Peppers Part VII. Spectroscopic Structure Elucidation of Piperine and its Isomers
Authors: De Cleyn R. , Verzele M. 
Published: September 2010

AbstractPiperine and its cis‐trans isomers isopiperine, chavicine and isochavicine are separated on preparative scale. Their spectral data are recorded and the structures are derived mainly from NMR data obtained at 300 MHz.
Etude des Metabolites De l'aspergillus Versicolor (Vuillemin) Tiraboschi
Authors: Berger Yves , Jadot Joseph 
Published: September 2010

AbstractThe modified “Prins reaction” between 4‐hydroxy‐l‐butene and 1,3,6,8‐tetramethoxyanthraquinone‐2‐carboxaldehyde gives mainly 1,3,6,8‐tetramethoxy‐2,2′e‐(4′e‐acetoxy‐tetrahydropyran) anthraquinone. The conformation of the reaction product was established by 1H‐NMR spectroscopy. There is non‐identity between this product and tetramethylversicorufin.
On the Relation Between Isomerization Modes for Idealized and Distorted Skeleta
Authors: Brocas J. , Willem R. , Fastenakel D. , Buschen J. 
Published: September 2010

AbstractPermutational isomerization reactions[1][2] may occur in two extreme situations. If the n ligands do not distort the symmetry of the skeleton, then the permutations of the full permutation group Sn have to be classified [1,3] according to the symmetry group G of the idealized skeleton. If the ligands are sufficiently different, they will be blocked and will distort the skeletal framework. In this case, the allowed permutations belong to S′n, a subgroup of Sn and have to be classified [2] according to G′, the symmetry group of the distorted skeleton. In this paper, the relation between these two descriptions is analysed and illustrated on examples of chemical interest.
Surface analysis by secondary ion emission
Authors: Blaise G. 
Published: September 2010

AbstractIn a first part, the applicability of secondary ion emission to surface analysis is discussed by considering the ejection and ionization processes of atoms and taking into account the progressive erosion of the sample.In a second part, the general features of the secondary ion emission induced by the adsorption of a gas are presented.The effect of oxygen adsorption on M+ metal ion production is studied as a function of oxygen coverage for pure metals, alloys and single crystals. In particular, an incorporation process which takes place at a critical coverage is described on a nickel single crystal (100).
Standard Free Energy of Formation of CmCl3 in (Li‐K)Cl Eutectic
Authors: Martinot L. , Reul J. , Duyckaerts G. , Muller W. 
Published: September 2010

AbstractFrom e.m.f. measurements in dilute solutions, we have calculated the standard free energy of formation of CmCl3 in (Li‐K)Cl eutectic between 400 and 600°C: ΔG°f = (‐ 753 000 + 315 T) J. mol−1.
Dosage d'Elements A l'etat de Traces dans les Eaux Souterraines au Moyen de l' Analyse par Activation Neutronique. I. Preconcentration au Moyen de la Resine Echangeuse d'Ions Dowex A1
Authors: Bury F. , Levert J. M. 
Published: September 2010

AbstractL'article présente une méthode de préconcentration de traces d'éleménts lourds (Zn, Hg, Cr, Cd, Cu) dans les eaux en vue de leur dosage par activation neutronique. Le choix s'est porté sur la résine complexante Dowex Al. Les rendements de fixation ont été déterminés pour les éléments cités. L'influence du pH et de la force ionique a été étudiée et les conditions optima de fixation ont été établies. Cette étude montre que la résine Dowex A1 s'avère satisfaisante.
The Determination of Trace Oxygen in Aluminium and Aluminium‐Silicon Alloy by Helium‐3 Activation Analysis
Authors: Vandecasteele C. , Goethals P. , Kieffer P. , Hosted J. 
Published: September 2010

AbstractThe determination of oxygen in aluminium and aluminium‐silicon alloy by helium‐3 activation is studied. The 18 F formed from oxygen is separated by distillation followed by precipitation of leadfluorochloride. The chemical yield is determined by activation in an isotopic neutron source. Concentrations of resp. 27 and 64 ng. g−1 with a precision for a single determination of resp, 30 and 13% are found in 99. 5% aluminium and in aluminium‐silicon (3%) alloy.
Conformation et Stabilite des Differents Cycles Lactames Formes A Partir de l'acide 2(S), 5(S)‐Diamino 3(R)‐Carboxyadipique
Authors: Welter A. 
Published: September 2010

AbstractThe hydrolysis of a new naturally occuring γ lactam aminoacid has been studied.Information is reported about the conformation and the stability of the 5 and 6 membered lactam rings which are present in the equilibrium during the hydrolysis.
Studies on Organophosphorus Compounds XIV. HMPA‐Induced rearrangements of N‐Benzyl‐carboxamides into 3‐phenylpropionitriles
Authors: Meyer H. J. , Goldman J. , Pedersen E. B. , Lawesson S.‐O. 
Published: September 2010

AbstractN‐benzylacetamides, when refluxed in HMPA, undergo a rearrangement reaction by which 3‐phenylpropionitriles are produced.
Molecular Geometry of Trans, Trans‐1,2‐Dimethyl‐ and of Trans, cis‐1,2‐Dimethyl‐Ethylenesulfite; a Gas Diffraction Study
Authors: Geise H. J. , van Laere E. 
Published: September 2010

AbstractThe configuration of trans, trans‐1,2‐dimethyl and of trans, cis‐1, 2‐dimethylethylene‐sulfite, as suggested by NMR, is in agreement with electrondiffraction results in the gas phase. The best fit to the observed diffraction intensities was obtained with a non‐planar sulfite ring, ϕmax = 42.6°. In the trans, cis derivative the methyl groups are pseudo‐equatorial, whereas in the trans, trans compound one methyl is pseudo‐axial, the other pseudo‐equatorial. Although these are the best, static representations of the molecules, mixtures containing several conformations cannot be excluded.
Etude Structurale de 2‐Phenyl‐4‐[1′‐Heteroalkylidene]Oxazoline‐5‐Ones
Authors: van Haverbeke Y. , Maquestiau A. , Muller R. N. , Vanden Eynde J. J. , Zeegers‐Huyskens Th. , Rulinda J. B. , Leroy G. , Louterman‐Leloup G. 
Published: September 2010

AbstractFifteen 2‐pheny1‐4‐[1‐heteroalkylidene]oxazolin‐5‐ones are examined by IR and NMR spectroscopy. The only tautomeric form observed is the one exhibiting an exocyclic double bond. The XH exo E structure is confirmed by CNDO/2 calculations.