Combined deuterium labeling and appearance potential measurements to uncover competing reaction mechanisms in the electron-impact-induced loss of water from cyclohexanol

The appearance potentials for loss of H₂O and DOH from the constitutional isomers 3,5-dideuterocyclohexanol and 4,4- dideuterocyclohexannol show, in agreement with proposals based on stereoselectivites, that 1,3 loss of water is energetically more costly than 1,4 loss of water.     

Rhodium‐Catalyzed Synthesis of Chiral Spiro‐9‐silabifluorenes by Dehydrogenative Silylation: Mechanistic Insights into the Construction of Tetraorganosilicon Stereocenters

Mechanistic insight into the construction of quaternary silicon chiral centers by rhodium‐catalyzed synthesis of spiro‐9‐silabifluorenes through dehydrogenative silylation is reported. The C₂‐symmetric bisphosphine ligand, BINAP, was effective in controlling enantioselectivity, and axially chiral spiro‐9‐silabifluorenes were obtained in excellent yields with high enantiomeric excess. Monitoring of the reaction revealed the presence of a monohydrosilane intermediate as a mixture of two constitutional isomers. The reaction proceeded through two consecutive dehydrogenative silylations, and the absolute configuration was determined in the first silylative cyclization. Competitive reactions with electron‐rich and electron‐deficient dihydrosilanes indicated that the rate of silylative cyclization increased with decreasing electron density on the silicon atom of the starting dihydrosilane. Further investigation disclosed a rare interconversion between the two constitutional isomers of the monohydrosilane intermediate with retention of the absolute configuration.     

Isoporphycene: The Fourth Constitutional Isomer of Porphyrin with an N4 Core—Occurrence of E/Z Isomerism

Liberation of the ligand from the nickel complex 1 obtained by template synthesis yielded isoporphycene (as the octaethyl derivative 2 ), the first constitutional isomer of porphyrin with an N₄ core for which E/Z isomerism is involved: Compound 2 is present as the E isomer, which is in rapid, presumably acid-catalyzed equilibrium with a small amount (2 %) of the Z isomer. The remaining unknown constitutional isomers of porphyrin are considerably higher in energy than the already rather labile isoporphycene, so that the latter should mark the border of existence for this type of structural variant of porphyrin.     

Beiträge zur Chemie des Phosphors. 223. Hexaisopropyl-icosaphosphan(6), P20i-Pr6 — Darstellung und kernresonanz-spektroskopische Strukturbestimmung von zwei Konstitutionsisomeren

Contributions to the Chemistry of Phosphorus. 223. Hexaisopropylicosaphosphane(6), P₂₀i? Pr₆ — Preparation and Structure Determination of Two Constitutional Isomers by Nuclear Magnetic Resonance Hexaisopropyl-icosaphosphane(6) has been obtained by reaction of i-PrPCl₂ with P₄ and magnesium and subsequent thermolysis of the crude reaction product. The compound is formed as a mixture of two constitutional isomers 1 and 2 of equal abundance, which have been almost purely isolated by HPLC as a mixture of the diastereomers 1 a , 1 b and in the form of the separate configurational isomers 2 a and 2 b , respectively. According to NMR-spectroscopic investigations, the new conjuncto-phosphane skeletons of 1 and 2 consist of a P₁₃(5)- and a P₉(5)-structural element analogous to that of brexane and of two P₁₁(5)-partial skeletons, respectively, joined in each case through a common P₂-bridge. Thus, 1 is 6,7,9,16,17,20-hexaisopropyloctacyclo[10.8.0.0^2,14.0^3,11.0^4,8.0^5,10.0^13,18.0^15,19]icosaphosphane and 2 is 7,9,15,17,19,20-hexaisopropyl-octacyclo[14.2.1.1^5,8.0^2,14.0^3,12.0^4,10.0^6,11.0^13,18]icosaphosphane. The phosphorus hydrogen compound P₂₀H₆ [22, 2c] should exhibit the same constitutional isomerism.     

Linkage Isomerism Leading to Contrasting Carboboration Chemistry: Access to Three Constitutional Isomers of a Borylated Phosphaalkene

We describe the reactivity of two linkage isomers of a boryl-phosphaethynolate, [B]OCP and [B]PCO (where [B]=N,N’-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl), towards tris- (pentafluorophenyl)borane (BCF). These reactions afforded three constitutional isomers all of which contain a phosphaalkene core. [B]OCP reacts with BCF through a 1,2 carboboration reaction to afford a novel phosphaalkene, E-[B]O{(C₆F₅)₂B}C=P(C₆F₅), which subsequently undergoes a rearrangement process involving migration of both the boryloxy and pentafluorophenyl substituents to afford Z-{(C₆F₅)₂B}(C₆F₅)C=PO[B]. By contrast, [B]PCO undergoes a 1,3-carboboration process accompanied by migration of the N,N’-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl to the carbon centre.     

Synthesis and characterization of liquid crystalline copolymethacrylates,copolyacrylates, and copolysiloxanes containing 4-methoxy-4′-hydroxy-α-methylstilbene and 4-hydroxy-4′-methoxy-α-methylstilbene constitutional isomers as side-groups

The synthesis of methacrylates and acrylates containing 4-methoxy-4′-hydroxy-α-methylstilbene and 4-hydroxy-4′-methoxy-α-methylstilbene constitutional isomers attached to the polymerizable group through flexible spacers containing 11, 8, 6, 3, and respectively 2 methylenic units is described. The radical copolymerization of a 1/2 or 2/1 mole ratio of the two constitutional isomeric monomers led to thermotropic side-chain liquid crystalline polymers in all cases. The synthesis of copolysiloxanes based on the same constitutional isomeric mesogens as side groups, and flexible spacers containing 11, 8, 6, 5, and respectively 3 methylenic units is also described. All polymers were characterized by differential scanning calorimetry and optical polarization microscopy. The polymers containing 11 methylenic units in the spacer exhibit S_c mesomorphism, while the other polymers are nematic. Copolymethacrylates do not undergo side-chain crystallization. Only the copolyacrylate containing 11 methylenic units in the spacer exhibits side-chain crystallization. All the copolysiloxanes display side-chain crystallization. The number of melting transitions seen for these polymers decreases with increasing spacer length. Copolysiloxanes containing dissimilar spacer length were also prepared. Only the copolymer synthesized with highly dissimilar spacer lengths, i.e., containing 3 and 11 methylenic units, does not undergo side-chain crystallization. These results have demonstrated that while the type of mesophase is dictated only by the spacer length, the degree of decoupling of the motion of the side-groups from the motion of the main chain is strongly dependent on the nature of the polymer backbone. For the same mesogenic unit and spacer length, the thermal stability of the mesophase is also dictated by the nature of the polymer backbone. The use of constitutional isomers of mesogenic units as side groups in liquid crystalline polymers provides at least qualitative information on the degree of decoupling of the side groups from the polymer main chain.     

Topological polymer chemistry by electrostatic self‐assembly

Recent developments in topological polymer chemistry are outlined. First, nonlinear polymer topologies are systematically classified on the basis of topological considerations of constitutional isomerism in a series of alkanes (C_nH_2n+2), monocycloalkanes (C_nH_2n), and polycycloalkanes (C_nH_2n?2, C_nH_2n?4, etc.). Various pairs of topological isomers are identified in randomly coiled, flexible polymer molecules with cyclic and branched structures. An electro‐ static self‐assembly and covalent fixation strategy has subsequently been developed for the efficient synthesis of a variety of topologically unique polymers, including monocyclic and polycyclic polymers, topological isomers, and topological block copolymers. In this process, new telechelics with moderately strained cyclic onium salt groups carrying multifunctional carboxylate counteranions have been designed as key polymeric precursors. Further extensions of topological polymer chem‐ istry have been achieved by the use of cyclic telechelics (kyklo‐telechelics) and cyclic macromonomers, obtainable also by means of the electrostatic self‐assembly and covalent fixation process. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2905–2917, 2003     

Enumeration of isomers of alkylcyclopropanes by means of alkyl 1,1‐biradicals

Henze and Blair [2] have successfully derived algorithms for enumerating the number of constitutional isomers of aliphatic compounds using alkyl radicals; however, the algorithms cannot be extended to enumerate constitutional isomers of cyclic compounds. Similarly, Read [4] has advocated the use of alkyl biradicals to enumerate constitutional isomers of aliphatic compounds, but not cyclic compounds. Apparently, the use of alkyl biradicals in the enumeration of constitutional isomers of cyclic compounds has been neglected. In this communication, an algorithm using alkyl biradicals to enumerate the number of constitutional isomers of cyclic compounds, namely alkylcyclopropanes, is described. This revised version was published online in July 2006 with corrections to the Cover Date.     

Conformational Analysis of the cis- and trans-Isomers of FK506 by NMR and Molecular Dynamics

The potent immunosuppressant drug FK506 ( 2 ) has been examined by ¹H- and ¹³C-NMR spectroscopy and NOE-restrained molecular dynamics to elucidate the conformation in solution. A combination of two- and three-dimensional NMR techniques was used to completely assign the ¹H- and ¹³C-NMR chemical shifts of the two configurational isomers resulting from the cis-trans isomerization about the single amide bond. Hetero- and homonuclear coupling constants were measured to assign the diastereotopic methylene protons at C(16), C(18), and C(23). Intramolecular H? H distances were defined from NOESY spectra recorded at ?30° in CDCl₃ and used as constraints in molecular-dynamics simulations. The conformational preferences of 2 in solution are discussed in light of the constitutional features recently proposed to be necessary for binding and activity.     

Enantiomeric and Diastereomeric Relationships of Ethylene Derivatives. Restructuring Stereochemistry by a Group-Theoretical and Combinatorial Approach

To restructure stereochemistry into a systematic format, enantiomeric and diastereomeric relationships have been investigated by using ethylene derivatives as examples in the light of a new group-theoretical and combinatorial approach. On one hand, enantiomeric relationship for ethylene derivatives has been characterized by means of a point group of order 8 (D _2h ), where chirality fittingness based on the sphericity concept has been applied to the enumeration of stereoisomers. On the other hand, diastereomeric relationship for ethylene derivatives has been examined by a permutation group of order 8 (S ₉ ^[4]), which is a subgroup of the symmetric group of order 4 (S ^[4]) and isomorphic to a point group D _2d . The subgroups of S ₉ ^[4] have been classified into stereogenic and astereogenic ones. A stereogenic subgroup corresponds to a pair of diastereomers, while an astereogenic subgroup is assigned to a self-diastereomer. The relationship between diastereomers and constitutional isomers have been also discussed.     

A Computational Study of the Combinatorial Addition of Oxygen to Buckminsterfullerene

All possible isomers of C₆₀O _x have been investigated for x=1, 2, and 3. The method used is a modified extended Hückel method. There is a single C₆₀O isomer, 8 unique C₆₀O₂ isomers, and 47 unique C₆₀O₃ isomers. It is found that a single C₆₀O₂ isomer is much more stable than the remaining 7 and that 3 C₆₀O₃ isomers are more stable than the remaining 44. A qualitative reason for these results is proposed. The results indicate that the three lowest-energy C₆₀O₃ isomers should exist in equilibrium at room temperature.     

Use of ion-molecule reaction equilibria to identify structural isomers

A new approach is described for identifying structural isomers which cannot be distin-guished on the basis of their electron-impact mass spectra. Using a pulsed ion cyclotron resonance mass spectrometer, equilibrium constsnts are measured for ion-molecule reactions involving various C₈H₁₀ isomers and C₇H₈O isomers. The equilibrium constants are shown to be highly sensitive to the structural features of the various isomers.     

Der massenspektrometrische Zerfall isomerer Diacetamido-cyclo-hexane,ihrer N-Phenäthyl-Derivate und Bis (acetamidomethyl)cyclohexane. 32. Mitteilung über massenspektrometrische Untersuchungen,,

The Mass Spectral Decomposition of Isomeric Diacetamido-cyclohexanes, their N-Phenethyl-Derivatives and Bis(acetamidomethyl)cyclohexanes In the mass spectra of the six isomeric diacetamidocyclohexanes 2--4 (cis and trans each, Scheme 2) as well as of the six isomeric bis(acetamidomethyl)cyclohexanes 6--8 (cis and trans each, Scheme 5) are clear differences between the constitutional isomers, whereas cis/trans isomers show very similar spectra. The lack of stereospecific fragmentations is explained by loss of configurational integrity of the molecular ion before fragmentation. However, the mass spectral fragmentation of epimeric diamidocyclohexanes becomes very stereospecific by the introduction of a phenethyl group on one of the nitrogen atoms: this group avoids epimerization of the molecular ion prior to fragmentation. In the N-phenethyl derivatives 10, 11, 13 and 14 (Scheme 8) the typical fragmentations of the cis-isomer after loss of ·C₇H₇ from the molecular ion are the elimination of CH₂CO by formation of cyclic ions, and the loss of p-toluenesulfonic acid or benzoic acid, respectively, with subsequent elimination of CH₃CN (Scheme 9). In the trans-isomer the typical fragmentations are the loss of the side chain bearing a tertiary nitrogen atom, and the elimination of the tosyl or benzoyl radical, respectively, with subsequent loss of CH₃CONH₂ (Scheme 10).     

The structure of small carbon clusters

New non linear isomers of C _n forn=4, 5, 6 have been observed using the combination of the laser photodetachment technique and the Coulomb Explosion Imaging method. Electron affinities of these isomers were found to be lower than the corresponding known linear isomers. The structure of low electron affinity C₄ isomer was found to be rhombic in accordance with recent theory. Indications for the non linearity of the low electron affinity isomers of C₅ and C₆ are also presented.     

用密度泛函理论研究一种新型有机硅化合物异构体的振动光谱

在B3LYP/6-31＋G(d)的水平上, 对两种含有手性Si原子的新型有机硅单体Si₂(CH₃)₂H₂N₂(C₂H₅)₄和Si₄(CH₃)₄H₄N₂(C₆H₅)₂的几种异构体进行了研究, 在全参量几何构型优化的基础上, 进行了简谐振动频率计算, 同时对所研究的体系进行了热力学性质和低能激发态的含时密度泛函理论(TDDFT)计算. 理论计算表明, 构象异构体之间的红外光谱差异不大, 热力学和低能激发态性质也相似; 顺/反结构相似的异构体之间红外光谱差异不大, 但热力学和低能激发态性质却呈现差异; 旋光异构体或顺/反结构相差较大的异构体之间, 红外光谱和热力学及低能激发态性质有明显的差异. 从理论上解释了实验红外光谱中Si—H振动峰的裂分是由异构体的存在所致, 并找到裂分峰所对应的异构结构. Si—H键振动频率与其键长相关.     

Distribution of chain defects and microstructures of melt crystallized polyethylene. II. Influence of defect size and of plastic deformation

Unit cell expansion data for (a) melt-crystallized polyethylene (PE) containing known amounts of methyl, ethyl, and butyl branches and for (b) plastically deformed samples, are examined in the light of a model which takes into account the penetration of constitutional defects (branches) at interstitial crystal sites by means of a generation of 2g1 step chain defects (kink isomers). The present analysis complements previous results obtained for melt-crystallized PE samples with a widely varying number concentration ? of butyl or longer branches. An estimation of the concentration of chain defects incorporated into the crystal lattice is carried out. The results reveal that the fraction χ_c of defects which are accomodated within the lattice depends on both the amount and size of the chain defects and on the mechanical deformation of the sample. For PE chains with methyl and ethyl groups, χ_c ≈ 50%, whereas for butyl and longer branches, χ_c does not exceed 20% of the total concentration of defects. In addition, after cold drawing, PE with low amounts (? < 1%) of butyl or longer chain branching, χ_c turns out to be zero; i.e., during deformation single molecular chain rearrangements leading to a chain segregation of defects into the amorphous phase must occur.     

Assigning the major isomers of fullerene C88 by theoretical 13C NMR spectra

The IPR isomers of fullerene C₈₈ have been studied using density functional theory. Structures of all C₈₈ isomers with non-zero HOMO–LUMO gaps were optimized at the B3LYP/STO-3G level. Those isomers having energies lower than 25 kcal/mol were subjected to geometry optimization using the 6-31G basis set. Isomer 17 has the lowest energy, followed by 7 and 33. All three isomers have large HOMO–LUMO gaps. ¹³C NMR chemical shifts were obtained employing the GIAO method. The comparison between predicted and measured NMR spectra strongly supports the observed C₈₈-1(C_s) as isomer 17, and isomers C₈₈-2(C₂) and C₈₈-3(C₂) as 7 and 33, respectively.     

含绿色荧光蛋白发色团双自由基分子的非线性光学性质

采用密度泛函理论(DFT)方法研究了系列含绿色荧光蛋白发色团双自由基分子光学异构体的几何结构、极化率(αs)和第一超极化率(βtot).结果表明,引入电子给受体取代基使分子的极化率增大,而对第一超极化率有不同影响.对于光照前的反式结构,引入电子受体βtot值增加,且βtot值随取代基吸电子能力的增强而增大;引入电子给体βtot值降低,且βtot值随取代基给电子能力的增强而减小.当分子变成相应的顺式结构时,其βtot值变化趋势与反式结构的结果正好相反.光异构化前后分子的βtot值变化不同,引入电子受体使顺式结构的βtot值比反式结构的小,其中―NO2使顺式结构的βtot值减小为反式结构的1/6;引入电子给体使反式结构的βtot值比顺式结构的小,其中―NH2使反式结构的βtot值减小为顺式结构的1/6.从而,光异构化起到调节非线性光学(NLO)响应的作用.