Composites with photosensitive 5,10,15,20-tetrakis(<Emphasis Type="Italic">N</Emphasis>-methylpyridinium-4-yl)porphyrin entrapped into silica gels

Photosensitive cationic 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin (TMPyP) was entrapped into microporous silica gels prepared by sol–gel method from tetrakis(2-hydroxyethoxy)silane (THES) and tetramethoxysilane (TMOS), resp., using different water and PEG 600 contents in the initial mixture. The absorption spectra of both composites showed that incorporation of TMPyP has led to a bathochomic shift (ca. 8 nm) of the Soret band and to a decrease in their molar absorption coefficient compared to TMPyP in solution. The TMPyP encapsulation kept the molecular state of the porphyrin in the free-base monomer form. In comparison to TMOS analogs, THES composites showed prolonged shape stability at least for 3 months, one-order higher rate of chemical substrate photooxidation and higher photobiocidal activity against E. coli.     

Structure of decaazonium Tricyclo[28.2.2.213.16]tetratriacontane Decatetrafluoroborate Decahydrate

A new inclusion complex, decaazonium tricyclo[28.2.2.2^13.16]tetratriacontane (L) decatetrafluoroborate decahydrate [(H₁₀L)(BF₄)₁₀10H₂O] (I), has been prepared and characterized by X-ray crystallography. Crystal data: a = 8.156(2), b = 29.125(6), c = 12.981(3) , = 96.76(3)°, space group P2₁/c, Z = 2, the final R = 0.0662 for 5376 reflections with I> 2 (I) (MoK radiation). The azamacrocycle of the complex is a 10-charged cation. The components of the supramolecular system in crystal are stabilized by the hydrogen bonds between the macrocycle cations, the water molecules, and the tetrafluoroborate anions. The water molecules make a solvation shell of the macrocycle forming an octahydrate. The tetrafluoroborate anions alternate with the water molecules stitched to the macrocycle, forming polymer chains in the directions parallel to the z axis of the crystal, and serve as bridges between the macrocycle cations related by the c plane. The set of hydrogen bonds including weak ones unite the components of the complex into a three-dimensional framework. All tetrafluoroborate anions in the structure are disordered.     

Synthesis,Structure, and Redox Chemistry of a Vitamin-B12s-Related Macrocyclic Complex of Cobalt(I)

(±)-[1-hydro-8H-HDP]cobalt(I) 1

1 Full name of 1: [2,2,3,3,7,7,8,8,12,12,13,13,17,17,18,18,-hexadecamethyl-2,3,7,8,12,13,17,18,-octahydro-1H,23H-10,20-diaza-porphinato]cobalt(I); full name of 2a : dibromo[2,2,3,3,7,7,8,8,12,12,13,13,17,17,18,18-hexadecamethyl-2,3,7,8,12,13,17,18-octahydro-1H,21H-10,20-diazaporphinato]cobalt(III).

2 For the nomenclature of [HDP]-complexes see addendum in [2].

is obtained by chemical or electrochemical four-electron reduction of (±)-dibromo- or (±)-dicano[1-hydroxy-8H-HDP]cobalt(III) 2a or 2b ⁴, respectively. The crystal nad molecular structure of 1 was determined by combination of X-ray analysis and MS, ¹H-, and ¹³C-NMR spectroscopy. Square-planar coordinated Co(I) lies closely to the best plane through the four N-atoms which form the first coordination sphere. Thermodynamic data for the coordination of axial bases with the cation of [1-hydroxy-8H-HDP]cobalt 2 in its different metal oxidation states were determined. The pathway of the overall four-electron reduction of 2a to 1 was elucidated: it involves a two-electron reduction of the central metal, a two-electron reduction of the macrocycle accompanied by elimination of the OH-group and final protonation at C(1). Evidence for an intramolecular electron transfer between the central metal and the macrocycle is presented.     

Crystal Structure of Triaqua(1,10-diaza-18-crown-6)chlorobarium Chloride

The crystal structure of triaqua(1,10-diaza-18-crown-6)chlorobarium chloride [Ba(DA18C6)Cl(H₂O)₃]⁺Cl^–(I) is studied by X-ray diffraction analysis: space group Pnma, a= 14.912, b= 13.590, c= 10.456 Å, Z= 4. Structure Iis solved by the direct method and refined by the full-matrix least-squares method in the anisotropic approximation: R= 0.079 for all 3194 measured independent reflections (CAD-4 diffractometer, MoK ). Crystal Iexists as complex guest–host cations and Cl^–anions connected via interionic hydrogen bonds. The cations and anions are located in the msymmetry plane. The Ba²⁺ion (coordination number 10) is in the cavity of the DA18C6 macrocycle and is coordinated by its six heteroatoms (2N + 4O) and also by the Cl^–anion and two O atoms of two water molecules from one side of the macrocycle and by the O atom of the third water molecule from another side. The DA18C6 ligand in Ihas the conformation of a crown with an approximate D _3d symmetry.     

A simple and sensitive resonance scattering spectral method for determination of hydroxyl radical in Fenton system using rhodamine S and its application to screening the antioxidant

Based on resonance scattering (RS) effect of rhodamine dye association particles, a new resonance scattering method for the determination of hydroxyl free radical from Fenton reaction was developed. In HCl-NaAc buffer solution, the OH of Fenton reaction oxidized the excess I⁻ to I₃⁻. The I₃⁻ combined, respectively, with rhodamine B (RhB), butyl rhodamine B (b-RhB), rhodamine 6G (RhG) and rhodamine S (RhS) to form association particles that exhibit stronger resonance scattering effect at 420 nm and 610 nm. However, the RS peak at about 610 nm was interfered with its synchronous fluorescence peak at 580 nm for RhB, 580 nm for b-RhB, 560 nm for RhG and 560 nm for RhS, respectively. The concentration of H₂O₂ in the range of 0.648-21.6 μmol/L, 0.423-13.0 μmol/L, 0.216-13.0 μmol/L and 0.092-13.0 μmol/L was linear to its resonance scattering intensity at 420 nm. Its detection limit was 0.15 μmol/L, 0.10 μmol/L, 0.092 μmol/L and 0.044 μmol/L, H₂O₂, respectively. This RhS RS method was applied to selection of the antioxidant, with satisfactory results.     

Cationic Porphyrin–Anionic Surfactant Mixtures for the Promotion of Self‐Organized 1:4 Ion Pairs in Water with Strong Aggregation Properties

We performed a systematic study on the spectroscopic and aggregation properties of stoichiometric mixtures (1:4) of the tetracationic meso‐tetrakis(4‐N‐methylpyridinium)porphyrin (H₂TMPyP) and three sodium alkylsulfate surfactants (tetradecyl, hexadecyl, and octadecylsulfate) in an aqueous solution. The objective was to build a supramolecular aggregate, which would favor the internalization of tetracationic porphyrins in cells without chemical modification of the structure of the porphyrin. We show that stoichiometric H₂TMPyP/alkylsulfate (1:4) mixtures lead to the formation of large hollow spherical aggregates (60–160 nm). The TEM images show that the membrane of these aggregates are composed of smaller aggregates, which are probably rod‐like micelles. These rod‐like micelles have a hydrophobic core composed of the alkyl chains of the alkylsulfate surfactant, whereas the charged surface corresponds to the tetracationic porphyrins.     

Formation of mono-, bi-, and polyradicals upon reduction of poly(arylenesulfophthalides) by metallic lithium

The reduction of poly(biphenylenesulfophthalide) (1), poly(fluorenylenesulfophthalide) (2), and poly(terphenylenesulfophthalide) (3) by metallic lithium in DMSO was studied using UV-visible and ESR spectroscopies. The reduction of compounds 1 and 2 affords blue diamagnetic color centers with absorption bands at 568 and 350 nm (shoulder) for 1 and at 576 and 360 nm (shoulder) for 2. The color centers were attributed to quinoid structures of the Chichibabin"s hydrocarbon type, being biradicals in the ground singlet state. The spectra of compounds 1 and 2 also exhibit weak absorption bands at 420 nm, which are assigned to monoradicals of the triarylmethyl type. The reduction of compound 3, for which the formation of quinoid structures is energetically unfavorable, leads to polyradicals of the triarylmethyl type with a high content (100%) of unpaired electrons in the main polymer chain. These radicals are characterized by absorption bands at 430 nm (allowed transition) and 638 nm (forbidden transition). The paramagnetic centers in all polymers under study give singlet lines with g = 2.0028 and H 10 Oe in the ESR spectra. The color centers and radicals of the triarylmethyl type observed for the poly(arylenesulfophthalides) under study are assumed to be formed upon the dissociative electron transfer from lithium to the sulfophthalide cycles of the polymeric molecules. The PM3 calculations show a high electron affinity of the sulfophthalide cycle and a higher propensity of the fluorenyl bridge to form quinoid structures than that of the biphenyl bridge.     

Electronic and vibrational circular dichroism spectroscopic study of non-covalent interactions of meso-5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin with (dG-dC)10 and (dA-dT)10

The non-covalent interactions of (dG-dC)₁₀ and (dA-dT)₁₀ with 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin (TMPyP) were studied using the combination of electronic circular dichroism (ECD), vibrational circular dichroism (VCD) spectroscopy, and UV–vis and IR absorption spectroscopy at different ratios of both components r = [oligonucleotide]/[TMPyP] = 2/1–10/1 where [oligonucleotide] and [TMPyP] are the amount concentrations of oligonucleotide per base-pair and TMPyP, respectively. It was shown that TMPyP with (dG-dC)₁₀ provided hemiintercalative binding mode for r = 4/1 that is manifested in vibrational spectra: The absorption band assigned to the C6O6 stretching vibration of guanine is shifted from 1683 to 1672 cm⁻¹, the corresponding VCD couplet from 1694(−)/1674(+) to 1684(−)/1663(+) cm⁻¹ and its intensity decreases. The absorption band assigned to the C2O2 stretching vibration of cytosine is shifted from 1652 to 1644 cm⁻¹ and its intensity increases. TMPyP with (dA-dT)₁₀ provided three binding modes: (i) external binding to the phosphate backbone, (ii) external minor groove binding for the ratios >6/1 and (iii) external major groove binging associated with the partial B- to Z-transition for the ratios <4/1. The major groove binding is manifested in VCD spectra by the intensity decrease of the bands 1655 and 1638 cm⁻¹ assigned to the thymine vibrations while the bands assigned to the adenine vibrations are unchanged. In the (dA-dT)₁₀–TMPyP complexes, the external binding to the phosphate backbone accompanied by self-stacking of porphyrins along the phosphate backbone chain is preferred at temperatures higher than 40 °C.     

Supramolecular design of a porphyrin-[60]fullerene photocurrent generation system on a DNA scaffold fabricated by a conjugate polymer film

DNA which binds monocationic [60]fullerene (1) and tetracationic porphyrin (TMPyP) was readily fabricated by electrochemical oxidative polymerization of 3.4-ethylenedioxythiophene (EDOT) and the resultant poly(EDOT) composite was deposited on an ITO electrode as a stable thin film. Spectral and CV analyses established that one 1 and one TMPyP are bound per 57 nucleobase units, that is, every three pitches of DNA. Photoirradiation of this 1/TMPyP/DNA-poly(EDOT) film generated a photocurrent in 3.8% quantum yield, which was much higher than those obtained from 1/DNA and TMPyP/DNA systems. One can conclude, therefore that the photoexcited energy of TMPyP is transferred to 1, which is collected by the electron-conducting poly(EDOT) film. The present paper shows that DNA is useful as a scaffold to arrange redox-active couples in a one-dimensional matrix.     

Crystal structure,fluorescence spectroscopy,and electrochemical property of two neodymium coordination polymers with phenoxy acids

Two nine-coordination coordination polymers of neodymium, [Nd₂(p-C₈H₄O₄)(o-C₈H₄O₄)₂ ? 4H₂O] _n (I), [Nd₂(C₁₀H₄O₈)(C₁₀H₂O₈) ? 2H₂O] _n (II), have been prepared by hydrothermal reaction of Nd(NO₃)₃ ? 6H₂O with terephthalic acid and phthalic acid, or benzenetetracarboxylic anhydride and determined by means of IR, UV, fluorescence, TG-DTA, cyclic voltammetry (CV) and X-ray single-crystal diffraction methods (CIF files CCDC nos. 1006206 (I), 979309 (II)). Yellow-green luminescence could been observed at 391 nm (λ_ex = 305 nm) for complex I and 370 nm (λ_ex = 331 nm) for the complex II. The emission of complexes I and II may be due to the π* → n transition, which may be assigned to the ligand-to-metal charge-transfer bands. Compared with complex II, the complex I exhibits a stronger fluorescence intensity for the different coordinated environment. Cyclic voltammetric measurement of the two compounds reveal that the compounds both have a couple of irreversible redox peak, indicating that the two polymers were both corresponded to the unusual Nd(III)/Nd(V).     

Mass spectrometric studies on effects of counter ions of TMPyP4 on binding to human telomeric DNA and RNA G-quadruplexes

A comparative study on human telomeric DNA G-quadruplex binding of meso-5,10,15,20-tetrakis(N-methyl-4-pyridyl)porphyrin (TMPyP4) between its two salt forms, i.e., tetratosylate and tetrachloride, was conducted by using ESI-TOF-MS, UV-melting measurement, and molecular modeling methods. Besides cation TMPyP4, the tosyl anion was found to bind to human telomeric DNA G-quadruplex with multiple binding stoichiometries from 1:1 to 3:1 observed in ESI-TOF-MS spectra, indicating that the stabilization activity of TMPyP4 tetratosylate on G-quadruplex is derived from a synergetic effect of both TMPyP4 cation and tosyl anion. A molecular modeling study suggests that a tosyl anion fills up the vacant space between TMPyP4 cation and DNA G-quadruplex and thus stabilizes the complex by 3.8 kcal/mol. Therefore, it is estimated that TMPyP4 tetratosylate’s activity might not reflect the real effect of TMPyP4 cation in some bioassays related to G-quadruplex stabilization. This was verified by the results of less binding affinity of TMPyP4 tetrachloride with DNA G-quadruplex obtained from ESI-TOF-MS measurement, and of 2.27 °C less thermal stabilization of TMPyP4 tetrachloride for DNA G-quadruplex, compared to its tetratosylate under the same conditions. Our study demonstrated the influence of counter ions of TMPyP4 on G-quadruplex binding, which sheds light on the proper usage of TMPyP4 salt in the chemical and biological research associated with G-quadruplex binding. Subsequently, the binding of TMPyP4 tetrachloride to human telomeric RNA G-quadruplexes was studied with ESI-TOF-MS technique. The binding constants of TMPyP4 with human telomeric G-quadruplexes indicated that TMPyP4 binds to human telomeric RNA G-quadruplex one order of magnitude stronger than DNA counterpart. This is a comprehensive mass spectrometric report on binding study of TMPyP4 with human telomeric DNA/RNA G-quadruplexes.

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An ESR and Optical Spectroscopic Study of the Mechanism of Photostimulated Reactions in Hydrogen Cyanide γ-Irradiated at 77 K

Changes in the electronic absorption spectra and ESR spectra in the course of photobleaching of radiolyzed solid HCN with light of different wavelengths (236–600 nm) were studied by ESR and optical spectroscopy. Two bands at 270 and 290 nm in the optical spectrum were attributed to the presence of H₂C=N and HC=NH radicals, respectively (the molar absorption coefficients are k ₂₇₀ 2.7 × 10²l mol^–1cm^–1and k ₂₉₀ 1.5 × 10²l mol^–1cm^–1, respectively). Structureless broad bands with maximums at 313 and 465 nm, which were detected after the exposure of a sample to light with 300 nm, can belong to the cyanide ions (CN^–) and H₂C=N⁺cations (the molar absorption coefficients of the ions are k _ion= (0.4–1.0) × 10²l mol^–1cm^–1). In the photobleaching of -irradiated HCN ( = 236–280 nm), H₂C=N⁺radicals were additionally formed by the photoinduced reaction of electron transfer from the CN^–anion to the H₂C=N⁺cation. The amount of these radicals generated in the course of photobleaching is several times greater than that of the same radicals formed in the radiolysis via hydrogen atom addition to the multiple bond of HCN molecules.     

Influence of draw ratio on morphological and structural changes in hot-drawing of UHMW polyethylene fibres as revealed by DSC

The melting behaviour of gel-spun, ultra-high molecular weight polyethylene fibres was investigated in an attempt to characterize their morphology after various stages of hot-drawing at 148 C. In this drawing process a shish-kebab morphology is transformed into a smooth fibrillar structure. It was concluded that this transition initially proceeds by pulling elastically inactive loops, originally present in the folded chain lamellae of the shish-kebabs, taut between entanglements. Thereafter a considerable amount of entanglements is removed by pulling molecular chain ends through them, until ca. 2.5 entangelements per molecule remain in the ultimately drawn fibres. The fibrils in the fully drawn fibres were found to be composed of chain-extended orthorhombic crystallites with an average length of 70 nm, which are interrupted by defect regions (containing trapped entanglements and chain ends) of about 4 nm in length. If free shrinkage of the fibre was allowed, this structure had an equilibrium melting temperature of 140.5 C. Upon constrained melting of the filaments, a solid-solid phase transition could be observed in the DSC thermograms (at a temperature of ca. 150 C), associated with a transition of the chain-extended orthorhombic blocks in the fibrils into a hexagonal phase. The heat effect associated with this solid-solid transition in perfectly crystalline polyethylene (H _o–h ) was estimated to be 205 kJ/kg, whereas for the heat of fusion of the hexagonal phase (H _h–m ) a value of 81 kJ/kg was assessed.     

Spectral properties and solution behavior of gold(III) coordination compounds with water-soluble porphyrins

Gold(III) coordination compounds with three water-soluble porphyrins―5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H₂TSPP^4–), 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin (H₂TMPyP⁴⁺), and 5,10,15,20-tetrakis(4-N,N,N-trimethylaminophenyl)porphyrin (H₂TTMAPP⁴⁺)―have been studied. Complex [Au(TTMAPP)]⁵⁺ has been prepared for the first time. The analysis of coordination-induced shifts of proton signals in NMR spectra and intensities of Q bands in absorption spectra indicates the high degree of bond covalence in the studied metal porphyrins and a partial transfer of electron density from porphyrin to gold ion. The cationic complexes [Au(TMPyP)]⁵⁺ and [Au(TTMAPP)]⁵⁺ in aqueous solutions has been found to exist in monomeric form, while anionic complex [Au(TSPP)]^3– undergoes dimerization upon growth of concentration and solution ionic strength. Equilibrium constant for dimerization has been calculated, the constant has been found to decrease when temperature rises. Thermodynamic parameters of dimerization process have been determined: ΔH° =–31.8 kJ/mol and ΔS° =–13.8 J/mol K.     

Spectrophotometric determination of vanadium and niobium: Xylenol orange as a chromogenic reagent

Summary The formation of red coloured chelates of vanadium(V) and niobium(V) with xylenol orange (DCAC) having _max at 490 nm (at p_H 5.0) and 530 nm (at p_H 5.5) have been reported. The colour formation has been applied to the spectrophotometric determination of metal ions. The colour has been found to be stable between p_H 3.5–6.5 and 2.0–7.0 respectively. The sensitivity of the reagent is 0.051g/cm² of vanadium and 0.093g/cm² of niobium. Maximum colour intensity has been found for Nb-DCAC chelate at p_H 5.5 whereas no appreciable change has been found for V-DCAC chelate between p_H 4.5 and 6.5.

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Zusammenfassung Über die Bildung rot gefärbter Chelate von Vanadin und Niob mit Xylenolorange wurde berichtet. Deren Absorptionsmaxima liegen bei 490 nm (p_H 5,0) bzw. bei 530 nm (p_H 5,5). Die Farbreaktion wurde für die Bestimmung der beiden Metalle verwendet. Die Farbe ist zwischen p_H 3,5 und 6,5 bzw. zwischen p_H 2,0 und 7,0 stabil. Die Empfindlichkeit für Vanadin beträgt 0,051g/cm², für Niob 0,093g/cm². Die Farbe des Niobchelates erreicht bei p_H 5,5 ein Maximum, während die des Vanadinchelates zwischen p_H 4,5 und 6,5 keine wesentlichen Änderungen erkennen läßt.

     

Synthesis of a New Thiophene/Quinoxaline CT‐Type Copolymer with High Solubility and Its Basic Optical Properties

A new π‐conjugated charge‐transfer‐type copolymer of electron‐donating thiophene and electron‐accepting quinoxaline was prepared by organometallic polycondensation. The polymer was soluble in organic solvents such as tetrahydrofuran, and showed a UV‐vis peak at long wavelengths of 598 nm in chloroform and 629 nm in the film. Its film exhibited a χ⁽³⁾ peak in the resonance region with a χ⁽³⁾ value comparable to that of regioregular head‐to‐tail poly(3‐hexylthiophene‐2,5‐diyl).

     

Nitroaniline Derivatives of 2-Oxo-1-naphthylideneamines—Molecular Self-Assembling <Emphasis Type="Italic">via</Emphasis> C—H⋯O Intermolecular Hydrogen Bonds and Stabilization of O—H⋯N and N—H⋯O Tautomers in Solution and Solid State

Three Schiff-bases of the Ar—CH—N—Ar type were synthesized from 2-hydroxy-1-naphthaldehyde and o- (1), m- (2), and p-nitroaniline (3) in order to investigate the shift of the keto-amine/enol-imine tautomeric equilibrium that arises by the introduction of the electron-withdrawal nitro group. The compounds have been investigated experimentally, in the solid state and in the solution, by IR, NMR, UV spectroscopy, and X-ray single crystal analysis, and theoretically by using density functional theory (DFT). The tautomeric equilibrium is strongly shifted towards the keto-amine form despite of the nitro group position at N-phenyl ring in the solid state of molecular structures of 1, 2, and 3 (especially indicated by C—O and C—N bond distances). The keto-amine tautomeric form of the compounds is characterized by a strong, resonance assisted hydrogen bond (RAHB) of the N—H O type (the N O distances are 2.530(8), 2.554(2) and 2.555(5) Å in 1, 2, and 3, respectively). The molecules are assembled in the crystalline state into a 3D HB network (1), centrosymmetrical dimers (2) or infinite chains (3) by the C—H O intermolecular hydrogen bonds. In contrast, UV/VIS spectra of the chloroform solution of the title compounds reveal predominance of the enol-imine form. The molecular geometries and relative stabilities of the compounds determined by using DFT find good agreement with the experimental data.     

Copper (II)-thiolate complexes with novel tripodal- and tetrapodal-like benzimidazoles

Summary The tripodal copper(II) thiolate complexes Cu(L1)(Cl)-2H₂O and Cu(L1)(mim)(Cl) [H(L1) = N-(2-mercaptoethyl)-N,N-bis (pyrid-2-ylmethyl)amine and mim = N-methylimidazole] have been isolated. Both contain monomeric copper(II) and display two ligand field bands and axial cryogenic e.p.r. spectra, suggesting a squarebased geometry. A copper(II) thiolate complex Cu(L2)(Cl)-H₂O [H(L2) = N-(2-mercaptoethyl)-N,N,N-tris(benzimidazol-2-ylmethyl)-1,2-ethanediamine] with a CuN₅S chromophore has been also isolated. It exhibits only one ligand field band and an axial cryogenic e.p.r. spectrum, consistent with a distorted tetragonal coordination geometry. All the thiolates display intense S^– Cu^II charge transfer bands in the u.v. region, suggesting equatorial thiolate coordination. All of the complexes exhibit irreversible electrochemical behaviour.     

BrNO—thermodynamic properties,the ultraviolet/vis spectrum,and the kinetics of its formation

The equilibrium has been studied between 275°and 363°K. Third-law calculations lead to ΔH°₂₉₈(1) = -11.50 ±0.17 kcal/mol, from which Absorption bands of BrNO in the ultraviolet with e (γ_max = 215 nm) = 1.84±0.17 × 10⁴ 1/mol·cm, and in the red with e (γ_max = 708 nm) = 7.7±1.9 1/mol·cm at 298°K have been investigated. The rate of formation of BrNO has also been measured between 275°and 363°K.     

Relative Stability ofcis andtrans conformers of 2- and 3-fluorostyrene: An ab initio SCF-MO study

Ab initio SCF-MO calculations, mainly at the 6-31G F^* level with 3-21G F^* fully optimized geometries, were performed for 2- and 3-fluorostyrene in different comformations.Structures and conformational preferences of these molecules are compared with available data and discussed. It was found that the 2-fluorostyrene molecule has acis-trans energy difference of ca. 2.4 kJ mol^–1, showing a small barrier to planarity of 0.26 kJ mol^–1 in thetrans form. While the reduced stability of thecis form is mainly ascribed to a repulsive F ... vinyl group interaction, the partial loss of resonance stabilization between the ring and the vinyl group in the trans form and the H₆ ... H_c repulsive interaction in this form are taken to explain the local maximum at=180. In the 3-fluorostyrene molecule, thecis form is favored over thetrans by ca. 0.5 kJ mol^–1. An appreciable asymmetry of charge distribution found for thetrans form is assumed to slightly overweigh the difference in nuclear repulsion energy favorable to this conformation, thus explaining the observed relative stability.