p‐Cresyl 2,3‐an­hydro‐5‐deoxy‐5‐phthal­imido‐1‐thio‐α‐d‐lyxo­furan­oside

The crystal structure of the title compound, C₂₀H₁₇NO₄S, (I), was determined in order to compare the solution and solid‐state conformations. The mol­ecule was synthesized as a building block for incorporation into oligosaccharides comprised of conformationally restricted furan­ose residues. The furan­ose ring adopts an envelope conformation with the ring O atom displaced above the plane (an ^OE conformation). The pseudorotational phase angle (P) is 88.6° and the puckering amplitude (τ_m) is 31.5°. The C₂—C₁—S—C(Ph) torsion angle is ?163.2 (2)°, which places the aglycone in the exo‐anomeric effect preferred position. The C1—S—C14 bond angle is 99.02 (13)° and the plane of the cresyl moiety is oriented nearly parallel to the four in‐plane atoms of the furan­ose ring envelope. The orientation about the C4—C5 bond is gauche–gauche [Bock & Duus (1994). J. Carbohydr. Chem. 13 , 513–543].     

Conformational Flexibility of the Methoxyphenyl Group Studied by Statistical Analysis of Crystal Structure Data

In a large sample of observed methoxyphenyl groups, the twist angle τ about the MeO-C_Ph bond measuring internal rotation of the MeO group shows a continuous distribution with maxima at (0°) (coplanar conformation) and (～90°) (perpendicular conformation). The preferred conformation of methoxyphenyl depends on the nature of the ortho--substituents: In general, it is coplanar in the case of one or two ortho-hydrogens, and perpendicular in the case of two substituents. The internal rotation of the MeO group is accompained by systematic variations in bond angles and bond distances: 1 if MeO is twisted out of plane, the bond angle CH₃? O? C_Ph decreases from 117.7°, until it reaches a minimum of 114.9° at τ = ±90°. The O? C? C angle which is syn to CH₃ for τ = 0° decreases from 124.6° to a minimum of 115.4° at τ = ±180°. These angles changes keep the nonbonded distance CH₃ …? ortho substituent maximal during internal rotation of MeO and tend to minimize the corresponding strain energy. (2) In the perpendicular conformation, the O-atom is ～ 0. 06 Å displaced from the Ph plane, O and CH₃ and being on opposite sides of this plane. In addition, small but systematic increases of bond lengths MeO? C_Ph and CH₃? O are observed. These variations indicate a decrease in conjugation with increasing twist angle. Their interdependence during twisting and the magnitudes of the changes are close values obtained by ab initio calculations.     

Ab initio studies of structural features not easily amenable to experiment. 25. Conformational analysis of methyl propanoate and comparison with the methyl ester of glycine

The molecular geometries of three conformations of methyl propanoate (MEP) (C? C? C?O torsions of 0°, 120°, and 180°) and the potential-energy surfaces of MEP (C? C? C?O torsions) and of the methyl ester of glycine (MEG) (N? C? C?O torsions) have been determined by ab initio gradient calculations at the 4-21G level. MEP has conformational energy minima at 0° and 120° of the C? C? C?O torsion, while the 60–90° range and 180° are energy maxima. For MEG there are two minima (at 0° and 180°) and one barrier to N? C? C?O rotation in the 60–90° range. The N? C? C?O barrier height is about twice as high (4 kcal/mol) as the C? C? C?O barrier. The 180° N? C? C?O minimum is characteristically wide and flat allowing for considerable flexibility of the N? C? C?O torsion in the 150–210° range. This flexibility could be of potential importance for polypeptide systems, since the N? C? C?O angles of helical forms are usually found in this region. The molecular structures of the methyl ester group CH₃OC(?O)CHRR′ in several systems are compared and found to be rather constant when R ? H and R′ ? H, CH₃, CH₃CH₂; or when R ? NH₂ and R′ ? H, CH₃, or CH(CH₃)₂.     

Ruthenium-Catalyzed Cross-Metathesis of Trisubstituted Vinylsilanes with Light Alkenes

In the cross-metathesis reaction of tri(methyl, ethoxy)vinylsilanes with propene and/or 1-butene catalyzed by RuCl₂(PPh₃)₃ activated in benzene at 115–130 °C, a series of l-alkenylsilanes of general formula CH₃(CH₂)_mCH = CHSiMe_3−n(OEt)_n, where m=0, 1, and n=0–3 (1-silyl-1-alkenes), as well as of formula CH₂=C(Me)SiMe_3−n(OEt)_n, where n=1, 2 (2-silyl-1-alkenes), were obtained. Additional products determined were allysilanes of general formula CH₂=CHCH₂SiMe_3−n(OEt)_n and CH₃CH= CHCH₂SiMe_3−n(OEt)_n, where n=1–3. © 1997 John Wiley & Sons, Ltd.     

Bond dissociation energies and radical heats of formation in CH3Cl,CH2Cl2, CH3Br,CH2Br2, CH2FCl,and CHFCl2

An analysis of thermochemical and kinetic data on the bromination of the halomethanes CH_4–nX_n (X = F, Cl, Br; n = 1–3), the two chlorofluoromethanes, CH₂FCl and CHFCl₂, and CH₄, shows that the recently reported heats of formation of the radicals CH₂Cl, CHCl₂, CHBr₂, and CFCl₂, and the C? H bond dissociation energies in the matching halomethanes are not compatible with the activation energies for the corresponding reverse reactions. From the observed trends in CH₄ and the other halomethanes, the following revised ΔH°_f,298 (R) values have been derived: ΔH°_f(CH₂Cl) = 29.1 ± 1.0, ΔH°_f(CHCl₂) = 23.5 ± 1.2, ΔH_f(CH₂Br) = 40.4 ± 1.0, ΔH°_f(CHBr₂) = 45.0 ± 2.2, and ΔH°_f(CFCl₂) = ?21.3 ± 2.4 kcal mol^?1. The previously unavailable radical heat of formation, ΔH°_f(CHFCl) = ?14.5 ± 2.4 kcal mol^?1 has also been deduced. These values are used with the heats of formation of the parent compounds from the literature to evaluate C? H and C? X bond dissociation energies in CH₃Cl, CH₂Cl₂, CH₃Br, CH₂Br₂, CH₂FCl, and CHFCl₂.     

Synthesis of comb‐type polycarbosilanes via nucleophilic substitution reactions on the main‐chain silicon atoms

A series of comb‐type polycarbosilanes of the type [Si(CH₃)(OR)CH₂]_n {where R = (CH₂)_mR′, R′ = ? O‐p‐biphenyl? X [X = H (m = 3, 6, 8, or 11) or CN (m = 11)], and R′ = (CF₂)₇CF₃ (m = 4)} were prepared from poly(chloromethylsilylenemethylene) by reactions with the respective hydroxy‐terminated side chains in the presence of triethylamine. The product side‐chain polymers were typically greater than 90% substituted and, for R′ = ? O‐p‐biphenyl? X derivatives, they exhibited phase transitions between 27 and 150 °C involving both crystalline and liquid‐crystalline phases. The introduction of the polar p‐CN substituent to the biphenyl mesogen resulted in a substantial increase in both the isotropization temperature and the liquid‐crystalline phase range with respect to the corresponding unsubstituted biphenyl derivative. For R = (CH₂)₁₁? O‐biphenyl side chains, an analogous side‐chain liquid‐crystalline (SCLC) polysiloxane derivative of the type [Si(CH₃)(O(CH₂)₁₁? O‐biphenyl)O]_n was prepared by means of a catalytic dehydrogenation reaction. In contrast to the polycarbosilane bearing the same side chain, this polymer did not exhibit any liquid‐crystalline phases but melted directly from a crystalline phase to an isotropic liquid at 94 °C. Similar behavior was observed for the polycarbosilane with a fluorocarbon chain, for which a single transition from a crystalline phase to an isotropic liquid was observed at ?0.7 °C. The molecular structures of these polymers were characterized by means of gel permeation chromatography and high‐resolution NMR studies, and the crystalline and liquid‐crystalline phases of the SCLC polymers were identified by differential scanning calorimetry, polarized optical microscopy, and X‐ray diffraction. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 984–997, 2003     

New interpretation of progression bands observed in infrared spectra of nylon‐m/n

A series of progression bands observed in the infrared spectra of nylon‐m/n and their model compounds have been interpreted in a new manner on the basis of simply coupled oscillator models of zigzag alkyl chains. Nylon‐m/n possesses the methylene sequences of (CH₂)_m and (CH₂)_n?2, and so the effective models of m and n ? 2 coupled oscillators, respectively, had previously been assumed for the methylene rocking–twisting mode, for example. However, the spectral patterns of progression bands predicted by this previously proposed model have been found to be inconsistent with those observed for many kinds of nylon samples with various m and n values. It is rather reasonable to assume that the effective numbers of oscillators should be m ? 2 and n ? 4 for the methylene rocking, twisting, and wagging modes of the (CH₂)_m and (CH₂)_n?2 sequences, respectively. In other words, the infrared progression bands observed for methylene local modes of nylon‐m/n may be interpreted reasonably with the data of n‐alkane molecules with the chemical formulae CH₃(CH₂)_m?2CH₃ and CH₃(CH₂)_n?4CH₃. For the C? C stretching modes, the equivalent n‐alkanes are CH₃(CH₂)_m?1CH₃ and CH₃(CH₂)_n?3CH₃, respectively. In the simply coupled oscillator model, the vibrational mode of one methylene group is represented by an oscillator, for example. Our new concept is to isolate the terminal oscillator adjacent to the amide group from the other oscillators in the inner parts of the methylene zigzag sequence. This corresponds to a physical situation in which the methylene group adjacent to the amide group shows a different vibrational behavior of larger amplitude than those of the inner methylene sequence, as supported by broad‐line NMR data and molecular dynamics calculations reported in the literature. Another possibility is a difference in the electron structure of the methylene unit adjacent to the amide group from that of the inner methylene sequence, resulting in a difference in the force constant and giving a vibrational decoupling between these two types of methylene units. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1294–1307, 2003     

Crystal structure of poly-p-xylylene. I. The α form

The molecular conformation and the crystal structure of α-form poly-p-xylylene has been determined by x-ray diffraction. The polymer has a monoclinic unit cell with a = 5.92, b = 10.64, c (fiber axis) = 6.55 Å, and β = 134.7°. Two chains pass through the unit cell, and the space groups is C2/m. The packing fraction is 0.705. One monomer unit makes up the fiber identity period and the internal rotation angles are 0° and 90° for the ? CH₂? CH₂? and ? CH₂? ?? bonds, respectively. All benzene rings are in parallel orientation, perpendicular to the ac plane.     

1‐(β‐d‐Erythrofuranosyl)adenosine

The title compound, also known as β‐erythroadenosine, C₉H₁₁N₅O₃, (I), a derivative of β‐adenosine, (II), that lacks the C5′ exocyclic hydroxymethyl (–CH₂OH) substituent, crystallizes from hot ethanol with two independent molecules having different conformations, denoted (IA) and (IB). In (IA), the furanose conformation is ^OT₁–E₁ (C1′‐exo, east), with pseudorotational parameters P and τ_m of 114.4 and 42°, respectively. In contrast, the P and τ_m values are 170.1 and 46°, respectively, in (IB), consistent with a ²E–²T₃ (C2′‐endo, south) conformation. The N‐glycoside conformation is syn (+sc) in (IA) and anti (−ac) in (IB). The crystal structure, determined to a resolution of 2.0 Å, of a cocrystal of (I) bound to the enzyme 5′‐fluorodeoxyadenosine synthase from Streptomyces cattleya shows the furanose ring in a near‐ideal ^OE (east) conformation (P = 90° and τ_m = 42°) and the base in an anti (−ac) conformation.     

Homoleptic Octahedral Methylamine Complexes of Manganese(II): Solvothermal Syntheses and Crystal Structures of [Mn(NH2CH3)6]Cl2 and the Pentaselenide [Mn(NH2CH3)6]Se5

[Mn(NH₂CH₃)₆]Cl₂ ( 1 ) and [Mn(NH₂CH₃)₆]Se₅ ( 2 ) were prepared by solvothermal reactions in liquid methylamine from MnCl₂ at 150 °C for 1 and from a mixture of MnCl₂, Rb₂Se and selenium at 120 °C for 2 . Both 1 and 2 were obtained in high yields as colorless and dark‐red crystals and represent the first homoleptic methylamine complexes with coordination number six. Compound 1 crystallizes rhombohedral (R$\bar{3}$ , Z = 3) and is built of only slightly distorted octahedral [Mn(NH₂CH₃)₆]²⁺ cations and Cl^– anions. Compound 2 crystallizes orthorhombic (Pnna, Z = 4) and is built of octahedral [Mn(NH₂CH₃)₆]²⁺ cations showing a strong angular distortion and of Se₅^2– anions in the form of chains in transoid conformation. DFT calculations reveal an almost undistorted ground state structure for [Mn(NH₂CH₃)₆]²⁺ with N–Mn–N angular distortions of 1° from orthogonality, close to the structure found for the complex in 1 . The calculated energy necessary for a distortion as found in the structure of 2 is rather low and amounts to 26 kJmol^–1 which is in the range typical for hydrogen bonds. The N–Mn–N angular distortions of the complex cation in 2 , observed in the range of 10°, is caused by cation‐anion interactions in the crystal structure by N–H ···· Se hydrogen bonds.     

Intramolecular excited state complexes in trichromophoric model compounds for polyesters derived from 2,6‐naphthalenedicarboxylic acid and aliphatic glycols: experiment,rotational isomeric state model,and molecular dynamics

Steady‐state fluorescence was used to measure the ratio of emission intensities, denoted I_D/I_M, for excited state complexes and excited monomers of five trichromophoric compounds, 2‐naphthyl‐COO‐(CH₂)_m‐OOC‐2,6‐dinaphthyl‐COO‐(CH₂)_m‐OOC‐2‐naphthyl, m = 2–6. The linear aliphatic alcohols H(CH₂)_nOH, n = 1–7, as well as mixtures of ethylene glycol and methanol, were used to change the viscosity of the medium, η. The values of I_D/I_M depend on η and m. A Rotational Isomeric State model and Molecular Dynamics simulations were used for interpretation of the experimental results. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 253–266, 1999     

Silicon‐modified surfactants and wetting: III. The spreading behaviour of equimolar mixtures of nonionic trisiloxane surfactants on a low‐energy solid surface

The spreading behaviour of binary and ternary equimolar mixtures of siloxane surfactants of general formula [(CH₃)₃SiO]₂CH₃Si(CH₂)₃ (OCH₂CH₂) _nOCH₃, n = 3–9, has been investigated. The mixtures show a pronounced temperature dependence on the initial spreading rate. Mixtures imitating the average oligoethylene glycol chain length n = 5 are the fastest spreaders at 15 °C. At 23 °C and 40 °C these mixtures spread fastest sucking n = 6 and n = 8, respectively. For a given average chain length an increasing length difference between the components of the binary mixtures reduces the initial spreading rate. Nevertheless, substantial differences between the phase transition temperature T_c from the lamellar phase (L_α) into the two‐phase state (2Φ) and the actual spreading temperature are tolerated. A clear relation between phase transition temperature T_c and initial spreading rate does not exist. Copyright © 1999 John Wiley & Sons, Ltd.     

Disproportionation reactions between alkyl and fluoroalkyl radicals. VII. Difluoromethyl with perfluoro-n-propyl and methyl radicals

Mixtures of 1,1,3,3-tetrafluoroacetone and perfluorodi-n-propyl ketone have been photolyzed together over the temperature range 50° to 200°C, and the disproportionation/combination ratio for n-C₃F₇ and CF₂H radicals has been determined to be Δ(n-C₃F₇, CF₂H) = 0.072 ± 0.003. A reevaluation of existing data on CH₃ and CF₂H radicals leads to a value of Δ(CH₃, CF₂H) = 0.35. The large variations in Δ for the reactions of alkyl and perfluoroalkyl radicals with CF₂H radicals are discussed. © John Wiley & Sons, Inc.     

Dielectric properties of oligomers. VII. Low-molecular-weight propylene glycols

The dielectric properties of low-molecular-weight propylene glycols HO? [CH(CH₃)CH₂O]_n? H (n = 3, 4, 5) were investigated to clarify the effect of chain length on the dielectric properties. The measurement of dielectric constant and dielectric loss was carried out over the frequency range 30 Hz to 30 MHz at temperatures of ?20 to ?65°C. The static dielectric constants of these glycols are between 10 and 30, slightly smaller than values for the corresponding ethylene glycols of the same degree of polymerization. All of the Cole–Cole arcs, even that for pentapropylene glycol, can be represented by the empirical Davidson–Cole equation. The dielectric properties of homologous propylene glycols are compared with those for the ethylene glycols and are discussed in terms of the effects of chain length and intermolecular hydrogen bonds.     

Effect of lateral bromo substituent on the phase behavior of four-ring azo/ester/azo liquid crystalline materials

ABSTRACT

In order to study the influence of lateral Br substitution on mesophase behaviour, five homologous series of 4-substituted phenylazo phenyl 4?-(3?-bromo-4?-alkoxyphenylazo) benzoates (In_a–e) have been synthesised. Within each homologous series, the alkoxy group varies from 6 to 16 carbons, while other terminal group substituents, X, are CH₃O, CH₃, H, Br and NO₂ groups; the mesophase behaviour of these series is compared with previously prepared laterally neat analogues, 4-substituted phenylazo phenyl 4?-(4?-alkoxyphenylazo) benzoates (IIn_a–e) and laterally methyl analogues, 4-substituted phenylazo phenyl 4?-(3?-methyl-4?-alkoxyphenylazo) benzoates (IIIn_a–e). Similar to lateral methyl analogues, the present series, lateral Br substitution showed that, independent of the polarity of the substituent X or the alkoxy-chain length, the nematic phase is predominant with relatively high stability and broad temperature ranges. The mesophase stability varies between 204.0°C and 335.0°C for the nematic phase and 169.6°C and 281.0°C for the SmA phase. Their total mesophase temperature ranges vary between 87.2°C and 201.4°C. All compounds were found to be thermally stable within the mesophase temperature range, except the lower homologue of the nitro and Br substituted derivatives. The obtained results are discussed in terms of molecular polarisability.     

Methanol[1‐(methoxymethanimidoyl)‐2‐(pyridin‐2‐ylmethyl)guanidine]bis(perchlorato)copper(II)

The title complex, [Cu(ClO₄)₂(C₉H₁₃N₅O)(CH₃OH)], was synthesized from a methanolysis reaction of N‐(methylpyridin‐2‐yl)cyanoguanidine (L³) and copper(II) perchlorate hexahydrate in a 1:1 molar ratio. The Cu^II ion is six‐coordinated by an N₃O₃ donor set which confers a highly distorted and asymmetric octahedral geometry. Three N‐donor atoms from the chelating 1‐(methoxymethanimidoyl)‐2‐(pyridin‐2‐ylmethyl)guanidine (L^3m) ligand and one O atom from the methanol molecule define the equatorial plane, with two perchlorate O atoms in the apical sites, one of which has a long Cu—O bond of 2.9074 (19) Å. The dihedral angle between the five‐ and six‐membered chelate rings is 8.21 (8)°. Two molecules are associated into a dimeric unit by intermolecular N—H...O(perchlorate) hydrogen bonds. Additionally, the weakly coordinated perchlorate anions also link adjacent [Cu(ClO₄)₂(L^3m)(CH₃OH)] dimers by hydrogen‐bonding interactions, resulting in a two‐dimensional layer in the (100) plane. Further C—H...O hydrogen bonds link the two‐dimensional layers along [100] to generate a three‐dimensional network.     

Synthesis and mesomorphism of the liquid crystal based on diosgenyl end-capped polycarbonate

ABSTRACT

To enhance the activity of the hydroxyl group of diosgenin, the diosgenyl derivative Dios–(CH₂)₆–OH with an alkyl spacer and end primary hydroxyl group was synthesised via the chain extension reaction between diosgenin and 1,6-hexanediol. Then the liquid crystal (LC) compounds Dios–(CH₂)₆–(trimethylene carbonate [TMC])_n with different TMC content were obtained via the ring-opening polymerisation of TMC with Dios–(CH₂)₆–OH as initiator and mesogenic unit, importantly, there was no catalyst added. The chemical structures of the Dios–(CH₂)₆–OH and Dios–(CH₂)₆–(TMC)_n were characterised by Fourier transform infrared (FT-IR) spectra and ¹H NMR. The mesomorphism was characterised by polarising optical microscopy and X-ray diffraction measurement, and the phase transition was investigated by differential scanning calorimetry. The results showed that Dios–(CH₂)₆–(TMC)_n displayed an enantiotropic smectic phase, and TMC content played an important role in the LC properties of Dios–(CH₂)₆–(TMC)_n. The higher the TMC content, the lower the phase transition temperature of Dios–(CH₂)₆–(TMC)_n.     

Reversible cyclomerization and long range hydrogen abstraction by electron impact of 7-coumarinyl diesters ROOC(CH2)nCOOR

The mass spectra of 4-methyl-7-coumarinyl and 7-coumarinyl diestes ROOC(CH₂)_nCOOR (n = 2-12) have ben studied by appearance potential measurements, deuterium labelling and by comparison with suitable reference compounds such as the mised diestes ROOC(CH₂)_nCOOR′ (R=4-methyl-7-coumarinyl and R′ = methyl and phenyl) and 3.4-dihydro-4-methyl-coumarinyl diestes. Observations on the fragment ions of m/e 324, produced from the 7-coumrinyl diestes and their photocyclomers, by elimination of the central bridge as O?C?CH? (CH₂)_n–2? CH?C?O, demonstrate the existence and reversible formation of cyclomeric molecular ions. A stable bound system between the coumrin end groups is formed only at high internal energies by expulsion of a hydrogen atom, followed by elimination of the central bridge from the [M? H]⁺ ion. It is also shown that the lifetime of the open form molecular ions decreases remarkably for chain lengths with n larger than 6.     

Dilute solution properties of poly(α-methylstyrene–ethylene) “regular” sequence copolymers

“Regular” sequence copolymers having the structure {[? CH₂? C(CH₃)(C₆H₅)? ]_m(CH₂? CH₂)_n}_p with relatively small values of m and n were prepared by means of “living” polymerization techniques. The intrinsic viscosities of fractions of these copolymers were obtained in various solvents including a theta solvent. The molecular weights of these fractions were determined by the Archibald ultracentrifugal method. The results show that the intrinsic viscosity–molecular weight relations of the regular sequence copolymers are affected not only by the average composition of the copolymer, but also by the sequence length in the copolymer molecule. It is suggested that the effective conformation of a chain element in the copolymer is not always the same as that in the homopolymer.     

Reactions of vinyl ethers with 2‐cyano‐2‐propyl and benzoyloxy radicals

tert‐Butyl, cyclohexyl, n‐propyl, and n‐dodecyl vinyl ethers have been used as comonomers with styrene and methyl methacrylate using ¹³C‐enriched samples of azobis(isobutyronitrile) and benzoyl peroxide as initiators at 60°C. Examination by ¹³C‐NMR spectroscopy of either (¹³CH₃)₂C(CN) or Ph¹³COO end‐groups in the products has shown that the vinyl ethers have low reactivities toward the 2‐cyano‐2‐propyl radical but high reactivities toward the benzoyloxy radical. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 771–777, 1999