ANGEWANDTE CHEMIE, Issue 29 2010
Claudia Gerdes
Elegante Synthesen: Fortschritte in der Synthesemethodik ermöglichten die Herstellung und Isolierung neuer Siliciumverbindungen mit Mehrfachbindung. Ein cyclisch konjugiertes Isomer 1 des schwer zu fassenden Hexasilabenzols sowie ein Silylidin-Komplex 2 mit einer echten Molybdän-Silicium-Dreifachbindung wurden synthetisiert. [source]

Regioselecitive Huisgen Cycloaddition within Porous Coordination Networks,

ANGEWANDTE CHEMIE, Issue 13 2010
Takehide Kawamichi
Poren als Reaktionsräume: 1,3-Dipolare Huisgen-Cycloadditionen von 2-(Azidomethyl)triphenylen und Alkinen (siehe Schema) in einem porösen Koordinationsgerüst laufen als Einkristall-Einkristall-Umwandlungen ab. ,-Stapel und Nanoporen im Gerüst richten die Reaktionspartner so aus, dass das 1,4-substituierte Isomer des 1,2,3-Triazols selektiv als Produkt gebildet wird. [source]

Addition of Carbene to the Equator of C70 To Produce the Most Stable C71H2 Isomer: 2,aH -2(12)a-Homo(C70 - D5h(6))[5,6]fullerene,

ANGEWANDTE CHEMIE, Issue 5 2010
Bao Li Dr.
Endlich ist es gelungen, das schwer fassbare thermodynamisch stabilste Isomer von C71H2 (siehe Struktur) zu synthetisieren, und zwar durch Addition von CH2 an eine äquatoriale Bindung von C70,, und damit an eine Stelle, an der eine solche Reaktion kinetisch ungünstig ist. Rechnungen zufolge ist die verwendete pyrogene Synthesemethode der Grund, dass die Reaktanten die höhere Aktivierungsbarriere bei äquatorialer Addition überwinden können. [source]

Kristallstrukturen von C94(CF3)20 und C96(C2F5)12: Nachweis der Bindungsmuster in C94 -(61)- und C96 -(145)-Fullerenkäfigen,

ANGEWANDTE CHEMIE, Issue 48 2009
Nadezhda
Die Perfluoralkylierung von Gemischen höherer Fullerene ermöglichte den ersten direkten Nachweis der Bindungsmuster in C94 - (Isomer 61) und C96 -Fullerenkäfigen (Isomer 145). Grundlage war die Bestimmung der Additionsmuster der RF -Gruppen an diesen größten bisher untersuchten Fullerenkäfigen durch Röntgenstrukturanalysen von C94(CF3)20 (siehe Struktur; grau C, grün F) und C96(C2F5)12. [source]

Innentitelbild: Momentaufnahmen bei der Bildung einer Al-Al-,-Bindung aus {AlR2}-Einheiten , experimentelle und quantenchemische Befunde (Angew. Chem.

ANGEWANDTE CHEMIE, Issue 43 2009
43/2009)
Schnappschüsse der Bildung einer ,-Bindung in Al2R4 -Isomeren liefern quanten- und strukturchemische Untersuchungen, die H. Schnöckel, W. Klopper et,al. in der Zuschrift auf S.,8285,ff. beschreiben. Mit R=PtBu2 konnte als erster Schritt ein Triplettmolekül mit einem großen Al-Al-Abstand isoliert werden. Der Grundzustand ist ein bicyclisches Schmetterlingsmolekül, kein unverbrücktes R2AlAlR2 -Isomer. Die Ringinversionsisomere sind durch einen ,Flügelschlag" und einen Übergangszustand getrennt. [source]

Photoisomerization of All- cis -1,6-diphenyl-1,3,5-hexatriene in the Solid State and in Solution: A Simultaneous Three-Bond Twist Process,

ANGEWANDTE CHEMIE, Issue 43 2009
Jack Saltiel Prof.
Disrotatorische Fahrradpedale: Beim Bestrahlen der Titelverbindung im festen Zustand entsteht in einer Kristall-Kristall-Reaktion direkt das all- trans -Isomer. Für diese dreifache cis-trans -Photoisomerisierung wird ein zweistufiger Mechanismus vorgeschlagen, der zwei simultanen disrotatorischen Fahrradpedalprozessen um die zentrale Bindung entspricht. [source]

Reversible 100,% Linkage Isomerization in a Single-Crystal to Single-Crystal Transformation: Photocrystallographic Identification of the Metastable [Ni(dppe)(,1 -ONO)Cl] Isomer,

ANGEWANDTE CHEMIE, Issue 31 2009

Mal N, mal O: Raman-spektroskopische und photokristallographische Studien an Einkristallen identifizierten den Komplex [Ni(dppe)(,1 -NO2)Cl] als das erste System, das im Festkörper eine reversible und vollständige Umwandlung in ein metastabiles Isomer, in diesem Fall [Ni(dppe)(,1 -ONO)Cl], eingeht. [source]

ChemInform Abstract: Synthesis of Bent [4]Phenylene (Cyclobuta[1,2-a:3,4-b,]bisbiphenylene) and Structure of a Bis(trimethylsilyl) Derivative: The Last [4]Phenylene Isomer.

CHEMINFORM, Issue 23 2002
Dennis T.-Y.
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]

ChemInform Abstract: A Catalytic Synthesis and Structural Characterization of a New [84]Fullerene Isomer.

CHEMINFORM, Issue 45 2001
Nikos Tagmatarchis
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]

ChemInform Abstract: Pyrrole-Inverted Isomer of 5,10,15,20-Tetraaryl-21-selenaporphyrin.

CHEMINFORM, Issue 11 2001
Ewa Pacholska
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]

Synthesis and Characterization of the D5h Isomer of the Endohedral Dimetallofullerene Ce2@C80: Two-Dimensional Circulation of Encapsulated Metal Atoms Inside a Fullerene Cage

CHEMISTRY - A EUROPEAN JOURNAL, Issue 37 2009
Michio Yamada Dr.
Abstract Herein we show the synthesis and characterization of the second known Ce2@C80 isomer. A 13C,NMR spectroscopic study revealed that the structure of the second isomer has D5h symmetry. Paramagnetic NMR spectral analysis and theoretical calculation display that the encapsulated Ce atoms circulate two-dimensionally along a band of ten contiguous hexagons inside a D5h -C80 cage, which is in sharp contrast to the three-dimensional circulation of two Ce atoms in an Ih -C80 cage. The electronic properties were revealed by means of electrochemical measurements. The D5h isomer of Ce2@C80 has a much smaller HOMO,LUMO gap than cluster fullerenes (M3N@C80, M=Sc, Tm, and Lu) with the same D5h -C80 cages. The chemical reactivity was investigated by using disilirane as a chemical probe. The high thermal reactivity toward 1,1,2,2-tetramesityl-1,2-disilirane is consistent with the trends of the redox potentials and the lower LUMO level of the D5h isomer of Ce2@C80 compared with that of C60. [source]

C80Cl12: A Chlorine Derivative of the Chiral D2 -C80 Isomer,Empirical Rationale of Halogen-Atom Addition Pattern

CHEMISTRY - A EUROPEAN JOURNAL, Issue 8 2009
Kalin
Abstract Welcome to the family! The constitution of the chiral D2 -C80 fullerene has been confirmed through single-crystal X-ray analysis of the chlorinated C80Cl12. The addition pattern of the chlorine atoms in the structure of C80Cl12 together with other structures of halogenated higher fullerenes is discussed. A stepwise principle of higher fullerene reactivity is proposed. Unusual short intermolecular chlorine,chlorine contacts are reported. [source]

The Total Synthesis and Biological Properties of the Cytotoxic Macrolide FD-891 and Its Non-Natural (Z)-C12 Isomer

CHEMISTRY - A EUROPEAN JOURNAL, Issue 18 2007
Jorge García-Fortanet Dr.
Abstract A total, stereoselective synthesis of the naturally occurring, cytotoxic macrolide FD-891 and of its non-natural (Z)-C12 isomer is described. Three fragments of the main carbon chain were stereoselectively prepared by using asymmetric aldol and allylation reactions as the key steps. The molecule was then assembled by using two Julia,Kocienski olefinations to connect the three fragments and a Yamaguchi reaction to close the macrolactone ring. Some specific biological properties (cytotoxicity, binding to tubulin) have been determined for both macrolides. The E configuration of the C12,C13 olefinic bond seems to be an important feature in determining the cytotoxicity but the precise biological mechanism of the latter still remains to be cleared. [source]

G,G Base-Pairing in Nucleobase Adducts of the Anticancer Drug cis -[PtCl2(NH3)(2-picoline)] and Its trans Isomer

CHEMISTRY - A EUROPEAN JOURNAL, Issue 15 2005
Geraldine McGowan
Abstract cis -[PtCl2(NH3)(2-picoline)] (AMD473) is a sterically-hindered anticancer complex with a profile of chemical and biological activity that differs significantly from that of cisplatin. Adducts of AMD473 with neutral 9-ethylguanine (9-EtGH) and anionic (N1-deprotonated) 9-ethylguanine (9-EtG) as perchlorate and nitrate salts, and also a nitrate salt of the trans isomer (AMD443), were prepared and their structures determined by X-ray crystallography: cis -[Pt(NH3)(2-pic)(9-EtGH)2](ClO4)2 (1),2,H2O,Me2CO, cis -[Pt(NH3)(2-pic)(9-EtGH)2](NO3)2 (2),2,H2O, cis -[Pt(NH3)(2-pic)(9-EtGH)(9-EtG)]NO3 (3),3.5,H2O, trans -[Pt(NH3)(2-pic)(9-EtGH)(9-EtG)]NO3 (4),8,H2O. In all cases, platinum coordination is through N7 of neutral (1, 2) and anionic (3, 4) guanine. In each complex, the guanine bases are arranged in the head-to-tail conformation. In complex 1, there is an infinite array of six-molecule cycles, based on both hydrogen bonding and ,,, stacking of the 2-picoline and guanine rings. Platinum(II) coordinated at N7 acidifies the N1 proton of neutral 9-ethylguanine (pKa=9.57) to give pKa1=8.40 and pKa2=8.75 for complex 2, and pKa1=7.77 and pKa2=9.00 for complex 4. In complexes 3 and 4, three intermolecular hydrogen bonds are formed between neutral and deprotonated guanine ligands involving O6, N1 and N2 sites. Unusually, both of the platinated guanine bases of complexes 3 and 4 participate in this triple GG hydrogen bonding. This is the first report of X-ray crystal structures of nucleobase adducts of the promising anticancer drug AMD473. [source]

Identification of the D3h Isomer of Carbon Trioxide (CO3) and Its Implications for Atmospheric Chemistry

CHEMPHYSCHEM, Issue 12 2006
Corey S. Jamieson
Abstract The CO3 molecule is considered an important reaction intermediate in the atmospheres of Earth and Mars for quenching electronically excited oxygen atoms and in contributing to the anomalous 18O isotope enrichment. The geometry of the CO3 intermediate plays an important role in explaining these effects; however, only the cyclic (C2v) isomer has been experimentally confirmed so far. Here, we report on the first spectroscopic detection of the acyclic (D3h) isomer of carbon trioxide (12C16O3) via its ,1 and ,2 vibrational modes centered around 1165 cm,1 under matrix isolation conditions; the identification of the 12C18O3, 13C16O3, 13C18O3, 16O12C18O2, and 18O12C16O2 isotopomers of the acyclic isomer confirms the assignments. [source]

Differentiation of structural isomers in a target drug database by LC/Q-TOFMS using fragmentation prediction

DRUG TESTING AND ANALYSIS, Issue 6 2010
Elli Tyrkkö
Abstract Isomers cannot be differentiated from each other solely based on accurate mass measurement of the compound. A liquid chromatography/quadrupole time-of-flight mass spectrometry (LC/Q-TOFMS) method was used to systematically fragment a large group of different isomers. Two software programs were used to characterize in silico mass fragmentation of compounds in order to identify characteristic fragments. The software programs employed were ACD/MS Fragmenter (ACD Labs Toronto, Canada), which uses general fragmentation rules to generate fragments based on the structure of a compound, and SmartFormula3D (Bruker Daltonics), which assigns fragments from a mass spectra and calculates the molecular formulae for the ions using accurate mass data. From an in-house toxicology database of 874 drug substances, 48 isomer groups comprising 111 compounds, for which a reference standard was available, were found. The product ion spectra were processed with the two software programs and 1,3 fragments were identified for each compound. In 82% of the cases, the fragment could be identified with both software programs. Only 10 isomer pairs could not be differentiated from each other based on their fragments. These compounds were either diastereomers or position isomers undergoing identical fragmentation. Accurate mass data could be utilized with both software programs for structural elucidation of the fragments. Mean mass accuracy and isotopic pattern match values (SigmaFit; Bruker Daltonics Bremen, Germany) were 0.9 mDa and 24.6 mSigma, respectively. The study introduces a practical approach for preliminary compound identification in a large target database by LC/Q-TOFMS without necessarily possessing reference standards. Copyright © 2010 John Wiley & Sons, Ltd. [source]

Direct Simultaneous Determination of ,- and ,-Naphthol Isomers at GC-Electrode Modified with CNTs Network Joined by Pt Nanoparticles Through Derivative Voltammetry

ELECTROANALYSIS, Issue 5 2006
Xiao-Gang Wang
Abstract The semi-derivative technique was adopted to improve the resolution and surfactant was added to sample solution to enhance the sensitivity, , - and , -naphthol isomers could be determined directly and simultaneously at glassy carbon electrode modified with carbon nanotubes network joined by Pt nanoparticles. In 0.1,mol,L,1 HAc-NaAc buffer solution (pH,5.8), the linear calibration ranges were 1.0×10,6 to 8.0×10,4 mol,L,1 for both , - and , -naphthols, with detection limits of 5.0×10,7 for , - and 6.0×10,7,mol,L,1 for , -naphthol. The amount of naphthol isomers in artificial wastewater has been tested with above method, and the recovery was from 98% to 103%. [source]

Configurational Isomers of a Stilbene-Linked Bis(porphyrin) Tweezer: Synthesis and Fullerene-Binding Studies

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 35 2009
Maher Fathalla
Abstract A new stilbene-tethered bis(porphyrin) tweezer 5 has been synthesized through a Sonogashira cross-coupling reaction. The tweezer exists as two configurational isomers [(Z) + (E)], which have distinct cavity sizes. Fullerene-binding studies show that the (Z) isomer of the tweezer has a significantly higher affinity toward both C60 and C70 compared to the (E) congener. In addition, the (Z) , (E) photoisomerization of tweezer 5 is also discussed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

Synthesis and Structural Characterization of Four Isomers of C70(n -C3F7)8

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 19 2008
Tatyana Mutig
Abstract The first four isomers of C70(n -C3F7)8 synthesized by the ampoule reaction of C70 with n -C3F7I, isolated by HPLC, and characterized by X-ray crystallography possess unprecedented addition patterns among C70X8 compounds. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

Chemo-Enzymatic Synthesis of All Isomers of 2-Methylbutane-1,2,3,4-tetraol , Important Contributors to Atmospheric Aerosols

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 8 2007
Anders Riise Moen
Abstract By a combination of stereospecific osmium catalyzed oxidation of dimethyl citraconate and lipase catalysed enantioselective resolution of the formed dimethyl (2R*,3S*)-2,3-dihydroxy-2-methylbutanedioate, followed by reduction, (2R,3S)- and (2S,3R)-2-methylbutane-1,2,3,4-tetraol were isolated. Similar reactions starting with dimethyl mesaconate gave the isomers, (2R,3R)- and (2S,3S)-2-methylbutane-1,2,3,4-tetraol. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source]

syn - and anti -Isomers of a O -Methyloxime-Substituted Tricyclic Ozonide

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 8 2006
Karl Griesbaum
Abstract The ozonolysis of bicyclo[9.4.0]pentadec-1(11)-en-12-one (4) in pentane, followed by the treatment of the ozonolysisproduct with O -methylhydroxylamine afforded the stable ozonides anti -(8) and syn -2- O -methyloxime-16,17,18-trioxatricyclo[10.3.2.1]octadecane (9). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source]

The Synthesis of the Dimethyl Ester of Quino[4,4a,5,6- efg]-Annulated 7-Demethyl-8-deethylmesoporphyrin and Three of Its Isomers with Unprecedented peri -Condensed Quinoline Porphyrin Structures.

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 19 2004
Molecules with Outstanding Properties as Sensitizers for Photodynamic Therapy in the Far-Red Region of the Visible Spectrum
Abstract The mesoporphyrin dimethyl ester nickel complex has been formylated via the Vilsmeier method. The four possible mono meso-formyl derivatives were isolated and characterized. Wadsworth,Emmons coupling with the anion of (diethylphosphono)acetonitrile converted these aldehydes into the four novel meso acrylonitriles. Brief treatment of these acrylonitrile systems in hot trichloroacetic acid resulted in the formation of four achiral porphyrin derivatives with unprecedented nickel complexes of quino-fused porphyrins. Subsequent removal of the nickel gave four quino-porphyrin free bases: quino[4,4a,5,6- efg]-annulated 7-demethyl-8-deethylmesoporphyrin dimethyl ester 6a, 2,-(methoxycarbonyl)quino[4,4a,5,6- jkl]-annulated 12-demethyl-13-de[2,-(methoxycarbonyl)ethyl]mesoporphyrin dimethyl ester 6b, 2,-(methoxycarbonyl)quino[4,4a,5,6- qrs]-annulated 18-demethyl-17-de(2,-methoxycarbonylethyl)mesoporphyrin dimethyl ester 6c and quino[4,5,6,7- abt]-annulated 2-demethyl-3-deethylmesoporphyrin dimethyl ester 6d. The structures of these systems were unambiguously determined via mass spectroscopy and a plethora of NMR techniques. In the same way, etioporphyrin and octaethylporphyrin were converted into the corresponding peri -condensed quinoporphyrins as products, which shows that the formation of novel pericondensed quino-porphyrins is a general reaction in the porphyrin series and will have a wide scope in this field. Also, a plausible reaction mechanism for the formation of the quinoporphyrin systems was derived. As a first test for the use of these systems as sensitizers in far-red phototherapy, the quantum yield of singlet oxygen generation by 6a in toluene was studied. This quantum yield is 0.77, which is even higher than the singlet oxygen generation by sensitized meso-tetraphenylporphyrin. Secondly, when Chinese Hamster ovary (CHO) cells were incubated in medium which contained up to 15 ,g/ml of 6a, the survival rate of the cells in the dark is complete within experimental error, showing that under these conditions, 6a is not toxic to CHO cells. When CHO cells incubated in medium containing 6a in concentrations of 1 ,g/ml and higher were treated with white light of intensity 30 mW/cm2 for 15 minutes, complete cell death was observed. Based on these facts, we expect that all four achiral systems will show very promising properties to form the basis of a photodynamic therapy in far-red light. The fact that these systems are achiral is an additional bonus for medical applications. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

Computational Interpretation of Vibrational Optical Activity: The ROA Spectra of (4S)-4-Methylisochromane and the (4S)-Isomers of Galaxolide®

HELVETICA CHIMICA ACTA, Issue 9 2004
Gérard Zuber
The reliable computation of Raman- optical-activity (ROA) spectra of molecules of the size of the title compounds has, until now, not been possible. We show that our rarefied basis sets yield results in good agreement with the experimental data for (4S)-4-methylisochromane (=(4S)-3,4-dihydro-4-methyl-1H -2-benzopyran; 1), provided the equilibrium between the pseudo -equatorial and the pseudo -axial conformers is taken into account. Comparison between the measured and the computed ROA back-scattering spectra allows the unequivocal assignment of the absolute configuration of the molecule. Comparison with more-approximate calculations for the larger (4S)-isomers of Galaxolide® (2), which contain the (4S)-4-methylisochromane moiety, shows large-scale group frequencies on the same chiral fragments of the two molecules. The data confirm that ROA can be generated by interactions extending over several bonds, i.e., over larger distances than can be probed by NMR spectroscopy. Thus, the absolute configuration at C(7) of Galaxolide® is assignable independently of that at C(4). The computation of ROA for forward-scattering, which will soon be measurable for Galaxolide®, suggests that this scattering geometry provides additional stereochemical information that will be valuable in situations where absolute configurations at several stereogenic centers have to be assigned. [source]

Stereoselective Synthesis of Three Isomers of tert -Butyl 5-Hydroxy-4-methyl-3-oxohexanoate through Alcohol Dehydrogenase-Catalyzed Dynamic Kinetic Resolution

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 1-2 2009
Steffen Lüdeke
Abstract Regioselective reduction of the 5-keto group of tert -butyl 4-methyl-3,5-dioxohexanoate (1) leads to a stereodiad of tert -butyl 5-hydroxy-4-methyl-3-oxohexanoate (2). Alcohol dehydrogenases from Lactobacillus brevis (LBADH), Rhodococcus sp. (RS 1-ADH) and Saccharomyces cerevisiae (YGL157w) reduce 1 under dynamic kinetic resolution conditions, thereby establishing two chiral carbons with a single reduction step. While it had been shown previously that LBADH reduction of 1 stereoselectively leads to syn -(4S,5R)- 2, alcohol dehydrogenase-mediated dynamic kinetic resolution now allows easy access to syn -(4R,5S)- 2 (RS 1-ADH; 97.6% ee, syn:anti=92:8, 66% conversion, 37% isolated yield) and anti -(4S,5S)- 2 (YGL157w; 90% ee, anti:syn=93:7, 64% conversion, 42% isolated yield), as well. Thus three out of four possible stereoisomers were formed selectively upon reduction of 1. [source]

Isomers of C20: An energy profile III

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 3 2004
Jennifer I. Chavez
Abstract Semiempirical calculations, at the PM3 level provided within the Winmopac v2.0 software package, are used to geometrically optimize and determine the absolute energies (heats of formation) of a variety of C20 isomers that are predicted to exist in and around the ring and cage isomers. Using the optimized Cartesian coordinates for the ring and the cage isomers, a saddle-point calculation was performed. The resulting energy profile, consisting of a series of peaks and valleys, is used as a starting point for the identification and location of fifteen additional isomers of C20 that are predicted to be energetically stable, both via geometry optimizations and force constant analysis. These additional isomers were subsequently determined to lie adjacent to one another on the potential surface and establish a step-wise transformation between the ring and the cage. Transition-state optimization of the Cartesian coordinates at the saddle point between adjacent isomers was performed to quantify the energy of the transition state. The step-wise process from one isomer to another, which extends out over the three-dimensional surface, is predicted to require ,15% less energy than that of the direct, two-dimensional transformation predicted in the bowl-cage profile. However, the net atomic rearrangement for the step-wise process is about four times greater than that of the direct process. Although less in energy, the amount of atomic rearrangement in the step-wise process would make the occurrence of such a route prohibitive. Utilizing the direct distance separating the three primary isomers (ring, bowl, cage), the method of triangulation is performed to quantitatively position other C20 structures on the potential surface, relative to the ring, bowl, and cage isomers. © 2003 Wiley Periodicals, Inc. J Comput Chem 25: 322,327, 2004 [source]

Isomers of C20: An energy profile II

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 11 2003
Kyle A. Beran
Abstract Semi-empirical calculations, at the PM3 level provided within the Winmopac v2.0 software package, are used to geometrically optimize and determine the absolute energies (heats of formation) of a variety of C20 isomers that are predicted to exist in and around the bowl and cage isomers. Using the optimized Cartesian coordinates for the bowl and the cage isomers, a saddle-point calculation was performed. The output file generated, containing energy, distance, and geometry information, is then organized into a graphical format. The resulting graph, which plots the energy of the 20-atom system as a function of the distance from the geometric midpoint, is a two-dimensional energy profile. This profile illustrates an estimation of the contours on the potential energy surface, showing energy minima and maxima that are encountered as the bowl evolves into the cage structure, or vice-versa. To expand the surface into three dimensions, geometry optimizations were performed on the sets of Cartesian coordinates that correspond to energy minima in the bowl-cage profile. Based on these optimizations, eight additional isomers of C20 have been identified and are predicted to be energetically stable. These additional isomers were subsequently subjected to saddle-point calculations in order to identify those isomers that lie adjacent to one another on the three-dimensional surface. Two isomers that are adjacent to each other will exhibit an energy profile that progresses smoothly from the potential well of each isomer up to the saddle point separating them. Consequently, these adjacent pairs of isomers establish a step-wise transformation between the bowl and the cage. This process, which extends out over the three-dimensional surface, is predicted to require less energy than that of the direct, two-dimensional transformation predicted in the bowl-cage profile. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1287,1290, 2003 [source]

The Kinetic Modeling of Strong Diffusion-Limited Reaction with Cyclization for Three Isomers , Diallyl Ortho-, Iso- and Tere - Phthalate in a Bulk

MACROMOLECULAR REACTION ENGINEERING, Issue 3 2007
Iztok Hace
Abstract Free radical polymerization kinetic in a bulk for three diallyl phthalate isomers , diallyl orthophthalate, diallyl isophthalate and diallyl terephthalate was investigated in a temperature range from 50 to 70,°C initiated with dicyclohexan peroxydicarbonate as initiator at three different initiator concentrations. Conversion points were measured using Fourier Transform Raman measurements. A new kinetic model for polymerization of three diallyl phthalate isomers was developed. It demands the inclusion of only two new kinetic parameters kDegC and kpc which were obtained as a ratio kDegC/kpc from an additional set of experiments conducted. Computed conversions from the proposed kinetic model show good agreement with the conversion and molecular weight measured data for all three investigated diallyl phthalate isomers. [source]

Diastereochemical differentiation of bicyclic diols using metal complexation and collision-induced dissociation mass spectrometry

RAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 12 2009
Jaana M. H. Pakarinen
Metal complex formation was investigated for di- exo -, di- endo - and trans -2,3- and 2,5-disubstituted trinorbornanediols, and di- exo - and di- endo - 2,3-disubstituted camphanediols using different divalent transition metals (Co2+, Ni2+, Cu2+) and electrospray ionization quadrupole ion trap mass spectrometry. Many metal-coordinated complex ions were formed for cobalt and nickel: [2M+Met]2+, [3M+Met]2+, [M,H+Met]+, [2M,H+Met]+, [M+MetX]+, [2M+MetX]+ and [3M,H+Co]+, where M is the diol, Met is the metal used and X is the counter ion (acetate, chloride, nitrate). Copper showed the weakest formation of metal complexes with di- exo -2,3-disubstituted trinorbornanediol yielding only the minor singly charged ions [M,H+Cu]+, [2M,H+Cu]+ and [2M+CuX]+. No clear differences were noted for cobalt complex formation, especially for cis -2,3-disubstituted isomers. However, 2,5-disubstituted trinorbornanediols showed moderate diastereomeric differentiation because of the unidentate nature of the sterically more hindered exo -isomer. trans -Isomers gave rise to abundant [3M,H+Co]+ ion products, which may be considered a characteristic ion for bicyclo[221]heptane trans -2,3- and trans -2,5-diols. To differentiate cis -2,3-isomers, the collision-induced dissociation (CID) products for [3M+Co]2+, [M+CoOAc]+, [2M,H+Co]+ and [2M+CoOAc]+ cobalt complexes were investigated. The results of the CID of the monomeric and dimeric metal adduct complexes [M+CoOAc]+ and [2M,H+Co]+ were stereochemically controlled and could be used for stereochemical differentiation of the compounds investigated. In addition, the structures and relative energies of some complex ions were studied using hybrid density functional theory calculations. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Phosphatidylinositol Synthase of Tetrahymena: Inositol Isomers as Substrates in Phosphatidylinositol Biosynthesis and Headgroup Exchange Reactions

THE JOURNAL OF EUKARYOTIC MICROBIOLOGY, Issue 2 2007
BRIDGET M. RIGGS
ABSTRACT. Phosphatidylinositol (PtdIns) synthase in microsomal fractions derived from Tetrahymena vorax was studied to determine its activity requirements. The suitability of inositol isomers as substrates for the synthase and in headgroup exchange reactions also was investigated. Tetrahymena PtdIn synthase activity was optimum in the presence of 2 mM MgCl2 plus 2 mM MnCl2, a pH of 7.8, and a temperature of 30°C. The enzyme retained approximately 80% of its activity after incubation at 70°C for 10 min. PtdIns headgroup exchange activity was maximal in the presence of cytidine monophosphate. By following either the accumulation of radiolabeled reaction products or the loss of radiolabel from precursors, each of the inositol isomers tested appeared to serve as substrates for both the PtdIns synthase and PtdIns:inositol phosphatidyl transferase activities. In each case, myo -inositol and scyllo -inositol were the preferred substrates. The data suggest two routes for the formation of phosphatidyl-non- myo -inositols in Tetrahymena and the potential for the production of novel, non- myo -inositol-containing second messengers. [source]

Gold(I)-Catalyzed Addition of Diphenyl Phosphate to Alkynes: Isomerization of Kinetic Enol Phosphates to the Thermodynamically Favored Isomers,

ANGEWANDTE CHEMIE, Issue 38 2010
Prof.
Gold, Gold oder , Gold? Der Gold- Katalysator bestimmt, ob bei der im Schema gezeigten Hydrophosphoryloxylierung das Produkt der thermodynamischen oder der kinetischen Steuerung entsteht. Ein dritter Katalysator, [(C6F5)3PAuOTf], erwies sich als enorm effektiv in der bislang unbekannten Isomerisierung kinetischer Enolphosphate in die thermodynamisch begünstigten Isomere. Tf=Trifluormethansulfonyl; R=Alkyl, Cyclohexyl, Ph. [source]