Distortion from planarity in arenes produced by internal rotation of one single hydroxyl hydrogen: The case of alternariol

Alternariol (AOH) and alternariol methyl ether (AME) are compounds toxic to farm animals and humans produced by ubiquitous fungi of the Alternaria genus. Information on chemical and physical properties of these compounds is rather scarce although their X-ray structures are known. AOH and AME are composed of three aromatic rings with three hydroxyl groups. However, the crystal structure of AOH is nonplanar whereas that of AME which differs only in one methyl group substituting one hydroxyl hydrogen is planar. By means of quantum calculations we find that the internal rotation of that single hydrogen affects the structure of AOH producing deviations from planarity near 5°. We also show that although quantum calculations predict energy differences about 5 kcal/mol between planar and perpendicular conformations of that hydrogen, its rotation has no noticeable effects on electron density properties that could indicate modifications of aromaticity features. Based upon these results, we suggest an explanation to the nonplanarity of the AOH crystal solely in terms of the spatial arrangement of molecules in the crystal interconnected through a network of hydrogen bonds involving rotatable hydroxyl hydrogens that produce distortions from planarity.     

Correlation between chain conformations and optical anisotropy of thin films of an organo-soluble polyimide

An organo-soluble polyimide was successfully synthesized by two step polycondensation accompanied with chemical imidization. Optical anisotropy of thin films was detected by a prism-coupler technique. The results showed that the optical anisotropic properties of thin films prepared from solutions in different solvents depend on the solution properties. It is concluded that the more expanded the chain conformation in solution, the larger the negative birefringence of thin films. © 1997 Elsevier Science Ltd.     

Molecular motion of isolated polybutadiene chains tethered on the fresh surface of polytetrafluoroethylene

The polybutadiene (PBD) chains tethered on the fresh surface of a polytetrafluoroethylene (PTFE) were produced by a block copolymerization of PTFE with 1,3-butadiene in vacuum at 77 K. The extremely low segmental density of the tethered chains was estimated by a spin labelling technique. The tethered chains can be regarded as ‘isolated polymer chains’. The PBD tethered chain has an unpaired electron at the chain end. We studied the molecular mobility of the PBD tethered chains by electron spin resonance (e.s.r.) spectroscopy, using the PBD chain end radical as a probe. The site exchange motion between two conformations at the chain end was observed in the temperature range 77–173 K and the rate of the exchange motion was determined by spectral simulations. The tethered chains are mobile even below 218 K which is a glass transition temperature of 1,4-PBD in the bulk. The high mobility of the PBD chains tethered on the PTFE surface is attributed to: the PBD chains have a very large space around the chains because of an extremely low segmental density on the PTFE surface. © 1997 Elsevier Science Ltd.     

Chain-length and tacticity effects on the conformational behavior of MMA-oligomer thin films on an Au (1 1 1) substrate

Chain-length and tacticity effects on the conformational behavior, including density distributions and conformations, of MMA-oligomer thin films deposited on an Au (1 1 1) substrate at room temperature were investigated using molecular dynamics (MD) simulations. Three kinds of MMA-oligomer thin film with the oligomers containing m repeat units with m = 5, 10, and 20, respectively, were selected to examine the chain-length effects. Three types of MMA-oligomer thin film (made of isotactic, syndiotactic, and atactic PMMA isomers, respectively) were utilized to inspect the tacticity effects. For ease of investigation, the thin film was divided into three regions (the contact, the bulk, and the surface regions) according to the through-thickness distribution of density of the thin film. For short-chain thin films (with m = 5 and 10), the density distribution had a remarkable peak in the contact region of the thin film. The density peak, however, was not evident for the long-chain thin film (with m = 20). Regarding tacticity, the isotactic thin films had a prominent peak density in the contact region as compared with the syndiotactic and atactic thin films. Moreover, the MMA oligomers were found to exhibit a flattened conformation parallel to the Au substrate in the contact region and a slightly flattened conformation in the surface region for both the short-chain and the long-chain thin films. The flattened conformation remained in the bulk region of the long-chain thin film. However, it was not present in the bulk region of the short-chain thin film, which had a randomly orientated conformation–a characteristic similar to bulk MMA. It was also observed that the tacticity of the MMA-oligomer thin films did not have obvious effects on the conformations of the thin films for both the short-chain and the long-chain cases.     

Solvent effects on conformation of a sex pheromone ofCydia (Laspeyresia pomonella L.; Lepidoptera: Tortricidae: Olethreutinae), codlemone (8,10-dodecadienol)

The conformations of codlemone (8,10-dodecadienol) and of a model analog have been studied by CNDO calculations. Stable conformations are found for the folded forms when the polarity of the solvent is taken into account.     

Enhanced chain order and dependence of the linear absorption of poly (p-phenylenevinylene) on the solution environment of its precursor polymer

A large change in the linear absorption spectrum of poly (p-phenylenevinylene) (PPV) occurs when it is prepared from freshly synthesized and undialysed poly(sulfonium chloride) salt. The red shift of the linear absorption maximum of the PPV is 30-50 nm compared to that obtained from dialysed prepolymer. Subsequent changes in the chain order, as monitored by the changes in the linear absorption spectrum of the converted PPV, have been investigated as a function of solvent, time and temperature. The use of water in the polysulfonium ion synthesis is critical for the attainment of high chain order. Water was also found to be better than methanol in preserving the chemical and conformational integrity of the precursor salts. The observed effects of water have been attributed to its high ionizing and solvating ability, and a lower nucleophilicity compared to methanol.     

Conformational characteristics of poly(N-vinyl pyrrolidone)

Conformational energies are calculated for poly(N-vinyl pyrrolidone) (PVP) chains as a function of stereosequence using semiempirical potential functions appropriate to peptides and n-alkanes. The planar pyrrolidone side groups are permitted to adopt both conformations which result in an eclipsed arrangement of the pyrrolidone NCH₂ or N(CO) and the C^aH^a bonds. Solvent interactions were considered in the manner used to treat other vinyl polymers bearing planar side groups. Dimensions and dipole moments were calculated using the RIS model developed for PVP from the conformational energies considering both the effects of stereosequence and temperature. Dipole moments were measured for three PVP samples with molecular weight ranging from 10 000 to 360 000. The dimensions and dipole moments calculated for atactic PVP chains agree with the dimensions reported in the literature and the dipole moments measured here.     

Large PAHs Derived from Benzanthrone and Naphthanthrone. A Semiempirical Study

In a systematic semiempirical study, AM1 and PM3 methods are employed to calculate minimum, transition state, and higher order saddle point conformations of planar and overcrowded C₃₄H₁₈ 4–15 and C₃₈H₁₈ 16–21 LPAHs, potential products of peri - peri reductive coupling of benzanthrone (2) and naphthanthrone (3), respectively. The most stable LPAHs in these series are planar C_2v-4 (C₃₄H₁₈) and twisted - folded C₂-21 (C₃₈H₁₈). Among overcrowded regions, two coves are consistently more destabilizing than one fjord, according to both semiempirical methods. The non-planar members of the C₃₄H₁₈ and C₃₈H₁₈ series adopt twisted - folded conformations as global minima and folded - twisted conformations as transition states. The barriers for enantiomerization of the LPAHs with a fjord, cove, and two coves are 18.5–24.0 (AM1) and 16.0–19.3 (PM3), 11.1–11.8 (AM1) and 7.2–7.5 (PM3), 12.0–12.4 (AM1) and 8.1–8.4kcal/mol (PM3), respectively. A comparison of calculated and experimentally determined geometries of LPAHs shows a good correlation between the two types of methods.     

Conformational change of a helical polymer molecule induced by periodic modulation of the internal coordinates

Conformational change of a helical polymer molecule induced by periodic modulation of the internal coordinates around constant values of a uniform helix was studied by numerical calculation. We paid attention to the ‘spatial resonance' found by Yamamoto et al., who carried out analytical calculation to a linear approximation (Yamamoto, M., Kasai, K. and Hikichi, K., J. Macromol. Sci. (Phys.), 1967, B1(2), 213). The spatial resonance means that when the wavelength of the modulation is equal to one turn of the helix, a finite amplitude of the modulation of the internal coordinates leads to divergence of the fluctuation in the external coordinates. We found that the molecular conformation in the spatial resonance induced by the modulation of the internal rotational angle or the bond angle is not a straight helix but is deformed into a ring-shaped helix, which we call ‘ringed-coil' conformation. On the other hand, the modulation of the bond length does not give rise to the spatial resonance. When both the internal rotational angle and the bond angle are modulated in a proper way the molecular conformation remains as a straight helix, but the atomic positions are displaced from those of the uniform helix. A few possible applications of the concept of the spatial resonance are proposed.     

Conformational change of a helical polymer molecule induced by periodic modulation of internal rotational angles

Conformation of a helical polymer molecule with periodic modulation of the internal rotational angle was numerically calculated. It was found that the modulation with wavelength equal to one turn of the helix leads to a ‘ringed-coil’ conformation. When the amplitude of the modulation was one degree the ring was composed of 956 atoms with a diameter 200 times the bond length. © 1997 Elsevier Science Ltd.