Nonplanar Form Research Articles

Trans-Difluorosilicon(IV) Complexes of Tetra-p-tolylporphyrin and Tetrakis(p-(trifluoromethyl)phenyl)porphyrin: Crystal Structures and Unprecedented Reactivity in Hexacoordinate Difluorosilanes(1).

Single-crystal X-ray structures have been determined for the difluoro(porphyrinato)silicon(IV) complexes trans-(Por)SiF(2) (Por = the dianions of tetra-p-tolylporphyrin (TTP) and tetrakis(p-(trifluoromethyl)phenyl)porphyrin). Crystallographic data for (TTP)SiF(2).Et(2)O: monoclinic, space group C2/c, a = 30.228(4) Å, b = 9.913(4) Å, c = 15.474(5) Å, alpha = gamma = 90 degrees, beta = 114.58(3) degrees, V = 4217(2) Å(3), Z = 4, R1 = 0.0588. Crystallographic data for (TTFP)SiF(2): monoclinic, space group C2/c, a = 31.557(2) Å, b = 9.546(1) Å, c = 15.941(1) Å, alpha = gamma = 90 degrees, beta = 115.83(1) degrees, V = 4322.4(8) Å(3), Z = 4, R1 = 0.0489. In both structures, the silicon lies in a slightly distorted octahedral geometry (average distances: Si-F 1.642 Å and Si-N 1.919 Å) with the fluorides in a trans configuration, and the porphyrin is in a ruf nonplanar form. The trans-(Por)SiF(2) structures were compared to the structures of related hexacoordinate cis-difluorosilanes and other group 14 metalloporphyrins. (TTP)SiF(2) readily reacts with excess MeMgBr or LiPh to give (TTP)SiMe(2) or (TTP)SiPh(2), respectively, in contrast to related hexacoordinate cis-difluorosilanes which do not react with strong nucleophiles. The enhanced reactivity of (TTP)SiF(2) may be a combination of a trans-effect, even though the structural parameters for cis and trans Si-F bonds and Si-N bonds are essentially the same, and a single-electron transfer process involving the porphyrin ligand.

Photomodulation of the hydrophilic properties of acrylic polymers containing side-chain azobenzene chromophores

Partially hydrophilic polymers have been prepared by free-radical copolymerization of trans-4-acryloyloxyazobenzene with N-tert-butylacrylamide and N-isopropylacrylamide. A turbidimetric method has been adapted to investigate the dependence of the polymer hydrophilic–hydrophobic balance on chemical composition and extent of photoisomerization of side-chain azobenzene chromophores. Irradiation at 366 nm of polymer suspension in THF–water induces the isomerization of azobenzene groups from the planar apolar trans form to the nonplanar polar cis form. Correspondingly, the solution turbidity is appreciably modified. Experimental results are discussed in terms of polymer structural parameters. Suspension stability and the possibility of carrying out photochemical cycles have been also investigated. Keywords: photochromic polymers, photoresponsive polymer, photomodulation of polymer properties, azobenzene-containing polymers, acryloyloxyazobenzene, N-tert-butylacrylamide.

Reaction of benzotriazole ultraviolet light absorbers with free radicals

The photolysis of benzotriazole ultraviolet light absorber additives (Bz-UVAs) in acrylic/melamine clearcoats suggests that Bz-UVAs are destroyed by clearcoat-driven photooxidation chemistry. Tinuvin 900 is converted to parent benzotriazole and phenol ring oxidation products. Bz-UVAs are not destroyed by photolysis in acrylic/melamine clearcoats in the absence of oxygen. Solution photolysis experiments reveal that Bz-UVAs are not attacked by free radicals generated by thermal decomposition of azoisobutyronitrile (AIBN) or cumene hydroperoxide (CHP) in ethanol or cyclohexane. Bz-UVAs are also not attacked by free radicals generated by photolytic decompostion of AIBN or CHP in cyclohexane. Bz-UVAs, however, are destroyed when AIBN or CHP are photolytically decomposed in ethanol. Photolysis without oxygen leads to Bz-UVA phenol ring solvent and initiator adducts. Photolysis with oxygen leads to cleavage to form free benzotriazole and to destruction of the phenol ring. It is suggested that an excited state of the non-planar Bz-UVA form may be susceptible to free radical attack. Detailed liquid chromatography tandem mass spectrometry (LC/MS/MS) product analysis results are reported in a companion paper.

Structural effects on the thermochromic properties of polythiophene derivatives

Temperature dependent optical absorption measurements have revealed important thermochromic effects in poly(3-dodecylthiophene) and poly[(3-octyloxy)-4-methylthiophene] between 25 and 150 °C which are related to a conformation transition of the conjugated polymer from a co-planar structure to a non-planar form. This conformational transition is caused by an increase of the repulsive intrachain steric interactions upon heating which then force the thiophene backbone to adopt a non-planar conformation. On the other hand, as observed with poly(3-octyloxythiophene), if the thermally induced disorder in the side chains does not lead to large steric interactions, the polymer can keep a highly conjugated structure even at high temperatures whereas, if the steric interactions are too strong (e.g. as in poly(3,3′-dihexyl-2,2′-bithiohene), no co-planar conformation can take place.

State-selectivity of excited-state intramolecular proton transfer in a ‘‘double’’ benzoxazole: Jet spectroscopy and semiempirical calculations

The fluorescence excitation spectra of free 2,5-bis(2′-benzoxazolyl)-hydroquinone (BBXHQ) cooled in a supersonic jet are examined. By using double-resonance saturation spectroscopy, we show that adjacent vibronic transitions, leading to either blue or red fluorescence, belong to a single molecular species in the electronic ground state. The molecular geometries of the enol and keto forms which are relevant for intramolecular transfer of a single hydrogen atom in the S1 state are obtained by the MNDO/H method. The translocation distance is calculated to be 0.43 Å. The complexity of the vibronic spectrum, near the electronic origin for the S0(1Ag) → S1(1Bu) transition of the enol form at 4178.1 Å, is attributed to vibronic coupling with the nonplanar excited keto form due to H tunneling.

Electrical conductivity of iodine-doped poly( para-phenylene vinylene) model compounds blended with polystyrene

We synthesized 15 trimeric poly( para-phenylene vinylene) (PPV) model compounds containing various substituents in the phenyl rings and characterized the structure by 1H and 13C NMR. The trans,trans-isomers are almost planar, while those containing methoxy groups in the central ring exhibit the ortho-methoxy- anti conformation. For cis,cis-isomers a non-planar form is proposed in which the three phenyl rings are perpendicular to the plane of the olefinic links. The solubilities were found to increase by (i) CH 3 and OCH 3 groups in the central ring, (ii) heavy OCH 3 substitution in the terminal rings and (iii) cis-configurations of the ethylenic bonds. The more soluble compounds (> 2 g/100 ml CHCl 3) were blended with polystyrene and iodine to give black electrically conducting films. Their conductivity ( 0.1-1 Ω −1 cm −1 ), which varies with the ratio polystyrene:model compound:iodine, is very stable at room temperature.

Solid state cross polarization/magic angle spinning 13C NMR investigation of alkoxy-substituted poly( p-phenylene vinylene) homo- and copolymers

The comparison of solid state and solution 13C NMR spectra of the model compound trans, trans-2,5-dimethyl-1,4-bis(2-(3,4,5-trimethoxyphenyl)ethenyl) benzene showed that essentially the same conformation exists in both states. The molecule is almost planar exhibiting an ortho-methyl- syn form. For the cis, cis-isomer a non-planar form is proposed in which the three phenyl rings are perpendicular to the plane containing the double bonds. The information from the model compounds was used to analyse the cross polarization/magic angle spinning (CP-MAS) 13C NMR spectra of a series of 2,5-di-alkoxy-substituted homo- and copolymers of poly( para-phenylene vinylene) (PPV). For the all- trans polymers we propose the ortho-alkoxy- anti conformation and for the cis-containing polymers we propose the conformation in which the phenyl rings are perpendicular to the olefinic plane. The first atom of the alkoxy chain is always in the phenyl plane. Estimates of the degree of polymerization gave 9< n<18 for these compounds synthesized via Wittig reactions. The value agrees well with results of other methods. Measurements of relaxation times ( T 1p(H), T 2(H), T CH, T 2(C)) were performed on a selection of the compounds. The T 1p(H) values vary linearly with the length of alkoxy chain. The molecular dynamics of alkoxy-substituted PPVs is dominated by a segmental mobility of these chains, with minimal interference from other molecular motions. Methyl rotation is more rapid in long and unbranched chains than in short, or branched ones. A rationalization of specific densities observed in these polymers can be given in terms of the observed molecular mobility.

The case of glycine continued: some contradictory SCF results

The energies of five conformations of glycine were determined by ab initio HF/SCF gradient geometry optimization using 13 different basis sets. The conformations considered are the stretched form ( I), a heavy-atom framework planar ( II) and non-planar ( III) cyclic form, and a planar ( IV) and non-planar ( V) bifurcated structure. Form I is the global energy minimum in all calculations, but contradictory results are obtained for the stabilities of the other forms. Of the cyclic forms, II is a local energy minimum at the 4-31G, 6-31G, 6-31+G, and (10s,5p/4s) levels, but a saddle point in calculations with both smaller and larger basis sets (including 6-31G ∗∗, 6-31 + G ∗∗, and 6-31 + + G ∗∗). Similarly contradictory results are obtained for IV. There is considerable scatter in calculated energy differences and in the optimized values of the non-planar N-C-CO torsions. Form I and the planar cyclic form II were previously found in the microwave spectrum of glycine. The significance of the new calculations for interpretations of the microwave data is discussed. It is concluded that assumption of slight non-planarity of the cyclic form, with a minimum at III and a low lying transition state at II is not necessarily in conflict with the available microwave data. In addition it is possible to conclude that a third conformer, IV or V, hitherto not observed, may exist in glycine vapors in amounts which are similar to those of the observed conformer II.

Die Struktur von cyclischem C6S6 und C6O6

Nicht aromatisch, sondern chinoid ist das energetisch günstigste Isomer des in der Gasphase nachgewiesenen C6S6. Die Energiedifferenz zwischen A und B wurde durch ab‐initio‐Rechnungen zu 10.9 kcal mol−1 bestimmt. Nahezu energiegleich mit B ist dagegen das Hexathioketon C, das in einer Sesselkonformation vorliegen sollte.magnified image

Deformation and stability of technologically imperfect cylindrical shells in a nonuniform stress state

The authors examined the problem of deformation and stability of thin-walled shells on the basis of technological mechanics and the multifactorial theoretical-experimental approach. The combined effect of the nonplanar form of the end of a smooth cylindrical shell on its subcritical behavior and the process of stability loss is examined. The investigations are carried out by the method of holographic interferometry and multifactorial experiment; the experimental results are used to construct the numerical solution.

Effects of nonplanar outboard wing forms on a wing

It is possible for a constant span to obtain better aerodynamic performance from a wing with a nonplanar outboard wing form than from a wing with a planar outboard form, despite the added drag from the increased wetted area. Furthermore, the semispan rolling-moment characteristics indicate the lower wing-root bending moment for some nonplanar configurations. These conclusions are based on an experimental and computational investigation of the aerodynamic characteristics of planar and nonplanar outboard wing forms. Seven different configurations - planar rectangular, nonplanar rising arc, nonplanar drooping arc, planar sheared, sheared with dihedral, sheared with anhedral, and planar elliptical - were investigated for two different spans. Flow-visualization photographs indicate that there are three vortex systems associated with the sheared forms. The lower induced drag coefficients of nonplanar wings are believed to accrue from the movement of vorticity away from the center-of-span line, resulting, in some instances, in induced efficiencies higher than that of a planar elliptical wing. Flow surveys indicate that the effective span, as determined by the location of the tip vortex, might not be a sufficient yardstick of the induced performance of a nonplanar wing.

Damage development during rupture of heterogeneous brittle materials: a numerical study

Abstract We have used a finite element method with an iterative algorithm to simulate rupture development in complex brittle materials such as rocks. A simple set of continuous distributions of properties has been used, and only tensile failure of elements allowed. Wider strength distribution leads to more damage and less abrupt mechanical breakdown characteristics. Variations in strength are found to have a greater effect on the geometry of breakdown than variations in elastic properties. The material behaviour simulated here gives a basic explanation for the extent to which natural fractures in the earth, such as joints and dykes, have a non-planar form.

Transient optical absorption spectroscopy of the photochemical spiropyran-merocyanine conversion

The photochemical ring-opening reaction of 1′,3′,3′-trimethylspiro-[2H-1-benzopyran-2,2′-indoline] has been followed by transient absorption spectroscopy, covering the range 370–650 nm with a temporal resolution of 0.42 ps. During the pump pulse, induced transient absorption is observed only at wavelengths shorter than 500 nm, with a maximum near 440 nm. The absorption at 440 nm decays within ≈5 ps, while a broad absorption band appears at 550 nm in the region characteristic of merocyanine absorption. The risetime of the absorption at 550 nm is approximately 1.3 ps. The reaction is described as a barrierless photoisomerisation on the excited singlet potential energy surface towards a nonplanar cisoid form, followed by internal conversion and rapid attainment of planarity.

Ab initiocalculations on SO2-and the NaSO2charge transfer complex

UHF-SCF calculations of SO2 - and the ion pair NaSO2 were performed using the 3–21 G* and 6–31 G* basis sets. Calculations indicate that NaSO2 has, at the equilibrium point, planar C 2v symmetry structure. A non-planar form was found as a stationary point on the energy surface and represents a saddle point for the Na migration. Vibrational frequencies of SO2 - and of the planar complex were computed and compared with the experimental ones.

Time-resolved resonance raman studies of the photochromic reaction of 6-nitro-1′,3′,3′-trimethylspiro[2 H-1-benzopyran-2,2′-indoline

The time-resolved resonance Raman spectra of 6-nitro-1′,3′,3′-trimethylspiro[2 H-1-benzopyran-2,2′-indoline] reveal that at least five transient species are involved in the photochromic reaction of this compound. One of the transient species is very short lived and its rapidly converted into other transient species. This short-lived species is assigned to a structure having the trans—cis—cis (TCC) configuration with respect to the three C-C partial double bonds of the merocyanine-like skeleton (non-planar cis-cisoid form). Comparison of the transient resonance Raman spectra of this compound with those of 1′,3′,3′-trimethylspiro[2 H-1-benzopyran-2,2′-indoline] led to the assignment of the remaining four transient species to the TTC, CTC, TTT and CTT configurations. The effects of the nitro substituent on the relative stabilities of the configurations of the transient species are also discussed.

Photophysics of a sulfonated 2‐hydroxyphenylbenzotriazole UV absorber in solution and in polymer substrates

AbstractThe total absorption spectrum of the UV absorber sodium 2‐(2′‐hydroxy‐5′‐methylphenyl) benzotriazole sulfonate (TINS) has been resolved into contributions by two ground‐state forms in acetonitrile‐water solvent mixtures and hydrogen bonding polymer and protein substrates. These two ground‐state species can be assigned to (i) a planar intramolecularly hydrogen bonded form which can undergo the efficient nonradiative energy dissipation process of excited‐state intramolecular proton transfer (ESIPT), and (ii) a fluorescent nonplanar species in which ESIPT is effectively prevented. The mole fractions of each contributing conformer in the polymeric substrates have been estimated. In all the polymer systems studied, a significant proportion of the TINS molecules exist in the nonplanar form and thus may not contribute to photostabilization of the substrate. The proportion of nonplanar molecules varied from 11 to 86% in synthetic polymers and 20 to 41% in protein substrates.

Tautomerism of pyrazoline-5-thione and triazoline-5-thione studied by variable temperature photoelectron spectroscopy

Photoelectron spectra at variable temperatures demonstrate that the rate of thermal isomerization of pyrazoline-5-thione (1a) into methylthiopyrazole(s) in the gaseous phase is metal catalysed. Similar isomerizations of triazolethione (2a and 3a) are at best only slightly so.Furthermore, photoelectron spectra and MNDO calculations show that gaseous methylthiopyrazoles (1b and 1c) and methylthiotriazoles (2b and 2c) exist predominantly in planar form for the b isomer and in nonplanar form for the c isomer. The ΔH0 for the rotameric equilibria is 2.7–4.2 kJ•mol−1.

Molecular Force Fields of s-trans-1,3-Butadiene and the Second Stable Conformer

Abstract Normal-coordinate analyses of s-trans-1,3-butadiene and its various deuterated and 13C-substituted analogs were performed using the group-coordinate force field, and a set of force constants was determined. The infrared spectra of the second stable conformer of 1,3-butadiene-d0, -1,1,4,4-d4, and -d6 were analyzed on the basis of normalcoordinate calculations of the planar s-cis form and a nonplanar gauche form. Such an analysis suggests that the second stable conformer takes a gauche form rather than the s-cis. To increase the number of reliable data to be used in this study, the infrared and Raman spectra of the d0, 1,1,4,4-d4, and d6 species were observed in low-temperature Ar matrices. The infrared spectra of the second stable conformer were also observed using a combination of the high-temperature nozzle and low-temperature matrix techniques.

Diffusion of molecules on biological membranes of nonplanar form. A theoretical study

Lateral mobility of molecules on cell membranes has been recently studied by fluorescence photobleaching recovery (FPR) techniques. The interpretation of these results in terms of diffusion along the membranes is based on the assumption that the surface is planar, although biological membranes may have blebs and microvilli. To study the effect of nonplanarity on the diffusion rate, the diffusion equation along curved surfaces was derived and was solved numerically for a "wavy" surface of the form A cos kx cos ky. Calculations show that for k = 10 pi micrometer-1 and a bleached spot of 1 micrometer in diameter, the time dependence of the intensity of fluorescence in the bleached spot depends on A at A less than 0.5 micrometer, while at higher values of A (a and 2 micrometer) the dependence is weak. If one calculates diffusion coefficients from FPR measurements and assumes that the membrane is planar, the resulting diffusion coefficient is not less than about half of the real one. Because of the tortuous shape of the spot boundary, increasing the microvilli length from 0.5 micrometer to 1 or 2 micrometer does not change the diffusion rates. These considerations are valid for times when the diffusion is dominated by molecules that were initially located close to the spot boundary.

ChemInform Abstract: CONFORMATION ANALYSIS. 6. THE MOLECULAR STRUCTURE OF GASEOUS TETRABROMOFORMALDAZINE AS DETERMINED BY ELECTRON DIFFRACTION

The structure of gaseous tetrabromoformaldazine (BrZCNNCBrz) has been investigated by electron diffraction at a nozzle-tip temperature of 112 'C. In contrast to formaldazine itself which exists in two conformations, the tetrabromo com- pound was found only in a single, nonplanar form. The data were fit well by a model of C2 symmetry with planar NCBr2 groups, a torsion angle CNNC equal to 72.1 ' (5.2) relative to 0' for the planar anti form, and a torsion around the double bond NNCBr equal to 16.6' (3.9) taken in such a sense as to increase the separation between an unshared nitrogen electron pair and the bromine atom syn to it in the adjacent NCBrz group. Other geometrical parameter values with uncertainties estimated at 20 are r(C-Br) = 1.877 8, (0.005), r(N-N) = 1.381 8, (0.018), r(C=N) = 1.266 8, (0.008), LCNN = 114.5' (l.6), and LNCBr = 126.2' (1 .O) and 1 18.2' (I. 1) where the last two angles correspond to a rocking displacement of the CBrz group away from the nitrogen atom of the adjacent NCBrz group. The conformation is discussed in comparison with similar mole- cules. Our recent investigation of the structure of gaseous for- maldazine' (2,3-diaza-1,3-butadiene) revealed that the mol- ecule existed as a mixture of two rotational conformers, a more stable anti (4 = 0) form and a less stable gauche (4 = 120O) form. Replacement of the hydrogen atoms in formaldazine with much larger atoms such as bromine would be expected to create larger steric interactions between the terminal (bromine) atoms and the lone-pair electrons on the nitrogen atoms in a possible anti conformer, and between bromine atoms on opposite ends of the molecule in a gauche or syn conformer. Such interactions would clearly play an important role in de- termining both the nature and the relative amounts of con- formers of the molecule. No structure information has so far been reported for tetrabromoformaldazine and we decided to study it in connection with our program of work on the struc- tures and compositions of conjugated systems.