Flow birefringence and conformational characteristics of molecules of para-and meta-isomers of polyoxyphenyl-benzoxazoleterephthalamides

Flow birefringence (FB) has been used to study sulphuric solutions of two homologous series of polyoxyphenylbenzoxazoleterephthalamides (POPhBT) differing in the position (para- or meta-) of phenyl ring in the chain. In the framework of the FB method alone by using the theory of flow birefringence for kinetically rigid wormlike chains, it was possible to determine quantitatively the optical anisotropy of the monomer unit Δa = (330 ± 30) 10^?25 cm^?3 and the length of the Kuhn segment $\text{A = (330 ± 30)}\text{A}\text{?}$ and $\text{A = (115 ± 20)}\text{A}\text{?}$ for para- and meta-isomers, respectively. Analysis of possible mechanisms of flexibility in the chains of both polymers gives theoretical values of the rigidity parameter A in good agreement with experimental values of A. confirming the validity of the molecular models used.     

Bonding information in tungsten(VI) compounds from directly bonded fluorine and fluorophenoxy subtituents

The series of compounds (FC₆H₄O)_nWF_6-n, where n = 1-6 and F is meta or para to oxygen, has been prepared and all fluorine nmr chemical shifts determined. The W-$\text{F}$, para-$\text{F}$, and meta-$\text{F}$ resonances all shift upfield as a function of n with approximate relative sensitivities of 1, 1/20, and 1/30, respectively. All chemical shifts are also found to be sensitive to molecular stereochemistry, with subtituents trans to oxygen shifted to higher field than those trans to fluorine. ¹⁹F data is also reported for the complete series (C₆H₅O)_nWF_6-n     

Mechanisms of reactions of halogenated compounds: Part 7. Effects of fluorine and other groups as substituents on nucleophilic aromatic substitution

The effect of fluorine as a substituent group on nucleophilic aromatic substitution is discussed, where a fluorine atom located ortho to the point of substitution may be of variable activating influence, whereas fluorine located para is slightly deactivating and meta is activating. A rationale of these effects is presented and evidence to support polar influences by ortho fluorine is advanced. The influence of CN, CF₃, CF₂H and CFH₂ is also established by comparison of appropriate measured rate constants and compared with the activation effects of ring nitrogen.     

Disentangling Excimer Emission from Chiral Induction in Nanoscale Helical Silica Scaffolds Bearing Achiral Chromophores

The synthesis and characterization of diketopyrrolopyrroles and perylenemonoimidodiesters linked to a substituted benzoic acid in the ortho, meta, and para positions, are reported. Grafting of these dyes on the surface of chiral silica nanohelices is used to probe how the morphology of the platform at the mesoscopic level affects the induction of chiroptical properties onto achiral molecular chromophores. The grafted structures are weakly (diketopyrrolopyrroles) or strongly (perylenemonoimidodiesters) emissive, exhibiting both locally-excited state emission and a broad, structureless emission assigned to excimers. The dissymmetry factors obtained using circular dichroism highlight optimized supramolecular organization between the chromophores for enhancing the chiroptical properties of the system. In the ortho- derivatives, poor organization due to steric hindrance is reflected in a low density of chromophores on walls of the silica-nanostructures (<0.1 vs. >0.3 and up to 0.6 molecules/nm² for the ortho and meta or para derivatives, respectively) and lower g_abs values than in the other derivatives (g_abs<2×10⁻⁵ vs 6×10⁻⁵ for the ortho and para derivatives, respectively). The para derivatives presented a better organization and increased values of g_abs. All grafted chromophores evidence varying degrees of excimer emission which was not found to directly correlate to their grafting density.     

The effect of ortho substituents in SRNl reactions. Some synthetic applications

Several $\text{ortho}$-substituted iodobenzenes were allowed to react, via the S_RNl mechanism, with mixtures of diethyl phosphite ion and pinacolone enolate ion. From product composition data, the relative reactivities of the two nucleophiles were reckoned. They represent relative reactivities in attachment to aryl radical intermediates. Most of the $\text{ortho}$ effects are slight, but there is evidence that large $\text{ortho}$ substituents hinder attachment of the phosphite more than the enolate nucleophile. The methoxy and diethoxyphosphoryl substituents, when $\text{para}$ to the site of substitution, respectively favour and disfavour the phosphite with respect to the enolate ion. S_RNl reactions of $\text{ortho}$-substituted halobenzenes are, for some substituents, followed by other reactions which afford products of synthetic interest.     

Origin of the Regioselectivity in the Gas‐Phase Aniline+CH3+ Electrophilic Aromatic Substitution

Nonadiabatic ab initio molecular dynamics simulations are carried out to monitor the attack of CH₃⁺ on aniline in the gas phase to form the corresponding σ complexes. The reaction is ultrafast and is governed by a single electron transfer within 30 fs, which involves two sequential conical intersections and finally produces a radical pair. Positive‐charge allocation in the aromatic compound is found to govern the substitution pattern in ortho, meta, or para position. Although the major products in the first step of the electrophilic aromatic substitution are the ortho and para σ complexes, initially 26 % of the simulated trajectories also form meta complexes, which then undergo H shifts, mainly to the para position.     

Carbon-13 NMR of quinone methides

The ¹³C NMR spectra of four ortho- and seven para-quinone methides were assigned using chemical shift and long-range carbon-proton coupling information. The carbonyl shifts are compared with those in ortho- and para-benzoquinones. The chemical shifts of the carbonyls of the p-quinone methides are observed at δ 186.2–186.4 for the three ortho-di-tert-butyl-substituted compounds and at δ 180.7–181.5 for the four ortho-oxy-substituted compounds. In the three o-quinone methides with meta, para-dioxy substituents, the carbonyl signals are at δ 184.2–185.4. The carbonyl signal of the one o-quinone methide with no oxygen substitution is shifted downfield to δ 200.9, apparently as a result of hydrogen bonding to the nearby hydroxyl.     

Chemical shifts for tritons in the ortho,meta and para positions of toluene as determined by tritium NMR spectroscopy

Chemical shifts for tritons in ortho, meta and para positions in toluene have been determined using a 64 MHz tritium NMR spectrometer. The order of shift is meta>para>ortho, whereas the calculated shift order for protons is meta>ortho>para.     

Concerning the chloromethylation of alkylbenzenes and polystyrenes by chloromethyl methyl ether

Chloromethyl methyl ether-stannic chloride treatments of toluene, ethylbenzene, isopropylbenzene and $\text{tert}$-butyl benzene afforded respectively, 26.5, 16.8, 5.5 and 2.5% $\text{ortho}$-products. No $\text{meta}$-products could be detected (less than 0.5%). Similar chloroalkylation of linear polystyrenes followed by conversion of the chloromethyl groups to hydroxylmethyl, yielded products possessing 5.0 ± .3% $\text{ortho}$-substitution.     

Theoretical and experimental interpretations of phenol oxidation by the hydroxyl radical

Phenol oxidation by OH radicals produced by the Fenton reaction was studied and the oxidation process was monitored by the UV–visible, ¹³C NMR and LC techniques. The results show that benzoquinone is formed. In the NMR and LC experiments, since the peaks corresponding to isomers ortho and para- benzoquinones are unresolved, DFT was used to determine the branching ratios of the isomers formation that coincides with their ΔG values (ortho > para > meta): 72% for ortho, 23% for para and 5.0% for meta. Furthermore, the energy profile of the OH attack at ortho is quite similar to that at the para position while the meta position attack is less favored by 2.0 kcal/mol.     

13C chemical shifts of phenol derivatives. A correlation with electronic structure

The ¹³C chemical shifts of twenty chloro- and nitro-substituted phenols have been measured in d-chloroform. The influence of substituents on the screening at C-1, on the total charge at this position and on the dipole moments is reported and analysed. The results of this study suggest that variations of the chemical shifts at C-1 for meta-methyl-, meta-chloro-, meta-nitro-, ortho- and para-methyl-, ortho- and para-chlorosubstituents mainly reflect charge density changes. The influence of ortho-nitro groups on the screening of C-1 is more complex.     

An “Ortho Effect” in Electrophilic Aromatic Nitrations: Theoretical Analysis and Experimental Validation

Usually, a π‐donor substituent acts as an ortho/para directing group in an electrophilic aromatic substitution reaction, and a π‐acceptor substituent acts as a meta directing group. Interestingly, when a π‐acceptor substituent is meta to a π‐donor substituent, certain electrophilic aromatic nitration occurs ortho to the acceptor substituent rather than para. The “ortho effect”, highlighted in various text books, has been tentatively analyzed here based on ab initio calculations. The reliability of the calculations was verified by the corresponding experimental data, including a new‐designed electrophilic aromatic nitration that also gave reasonable product distributions.     

Degradation thermique des trois isomeres Ortho,meta, para du polymethoxymethylstyrene

A kinetic study of the overall thermal decomposition of the three isomers (ortho, meta and para) of polymethoxymethylstyrene has been carried out. The reaction order is one over a wide interval of mass loss, for the meta and para polymers; there is no order for the first one. The corresponding activation energy has been calculated as 32.5 kcal mol⁻¹ for the meta polymer and 31 kcal mol⁻¹ for the para polymer.The volatile compounds, identified by gas chromatography coupled with mass spectrometry, are chiefly monomers. The thermal decomposition reaction is probably a depolymerization reaction coupled with crosslinking for the meta and para isomers.     

Variational RRKM theory calculation of thermal rate constant for carbon—hydrogen bond fission reaction of nitro benzene

The present work provides quantitative results for the rate of unimolecular carbon-hydrogen bond fission reaction of benzene and nitro benzene at elevated temperatures up to 2000 K. The potential energy surface for each C-H (in the ortho, meta, and para sites) bond fission reaction of nitro benzene was investigated by ab initio calculations. The geometry and vibrational frequencies of the species involved in this process were optimized at the MP2 level of theory, using the cc-pvdz basis set. Since C-H bond fission channel is barrier less reaction, we have used variational RRKM theory to predict rate constants. By means of calculated rate constant at the different temperatures, the activation energy and exponential factor were determined. The Arrhenius expression for C-H bond fission reaction of nitro benzene on the ortho, meta and para sites are k(T) = 2.1 × 10¹⁷exp(?56575.98/T), k(T) = 2.1 × 10¹⁷exp(?57587.45/T), and k(T) = 3.3 × 10¹⁶exp(?57594.79/T) respectively. The Arrhenius expression for C-H bond fission reaction of benzene is k(T) = 2 × 10¹⁸exp(?59343.48.18/T). The effect of NO₂ group, location of hydrogen atoms on the substituted benzene ring, reaction degeneracy, benzene ring resonance and tunneling effect on the rate expression have been discussed.     

Atomemissionsspektroskopie mit laseranregung an graphit unter erhöhtem Druck—I. Plasmaeigenschaften

The paper describes the vaporization of graphite, the plasma form, the plasma properties and the interaction of laser radiation with the plasma for when the beam of a free running laser is focused on graphite in air at atmospheric pressure and in He, O₂, or Ar at pressures in a range from (1–40) × 10⁵ Pa.The power densities of the focused laser radiation (focal distances of condenser lens 150 to 50 mm) reach 10⁷$\text{W}\text{cm}{}_2$. The extent of vaporization depends on the pulse energy (under 40 × 10⁵ Pa not unambiguously, however) and the ambient pressure.The plasma is observed with normal, framing (to 10⁷ frames per second), and streak photography (to 25 $\text{mm}\text{μs}$). The plasma form depends, among other things, on the ambient gas, the power density of the laser pulse on the sample, and the ambient pressure. Increasing the pressure in the range from 25 × 10⁵ to 40 × 10⁵ Pa modifies the plasma form from a “plume” to a “fungus” with toroidal head. The average velocity of the plasma particles in air is about 3 × 10⁵$\text{cm}\text{s}$, which is 10 times higher than the velocity of the luminous plasma front. Under increased pressure a strong interaction between the laser radiation and the plasma takes place resulting in absorption by inverse bremsstrahlung (>50% in 40 × 10⁵ Pa He). In this way the luminosity of the plasma is enhanced by a factor of 100 (delay to the start of the laser pulse about 50 μs). This absorption of the laser beam by inverse bremsstrahlung becomes the dominating mechanism of excitation of the plasma and causes a strong temperature gradient from the point of absorption to the fringe.     

Absolute configuration assignment of ortho,meta, or para isomers by mass spectrometry

A general method based solely on mass spectrometric techniques for the absolute configuration assignment of ortho, meta, or para isomers of acyl nitrobenzenes and derivatives is described. Instead of comparing the mass spectra of the three intact molecules of each positional isomer and investigating each one of the many sets of positional isomers, the method generalizes the effort by performing structural analysis on configurationally diagnostic fragment ions that are common for a given class of compounds. These ions must therefore retain the positional information of the parent molecules and be unequivocally distinguished. Nitrobenzoyl cations are common and stable fragment ions of most acyl nitrobenzenes and derivatives retaining the respective ortho, meta, or para configuration of the precursor molecules. The different NO₂ and CO⁺ ring alignments profoundly influence their collision-induced dissociation and bimolecular reactivity, and the isomeric 2-, 3-, and 4-nitrobenzoyl cations are found to be unequivocally distinguished using both approaches. Absolute ortho, meta, or para positional assignment by tandem MS of every isomeric molecule of the acyl nitrobenzene class and derivatives forming detectable amounts of any of those diagnostic nitrobenzoyl cations is, therefore, possible. The ability to perform absolute (non-comparative) configuration assignment using such diagnostic ions is exemplified for a single test molecule of (2R)-(−)-2-methylglycidyl 4-nitrobenzoate. The general application of this absolute MS-only method for other classes of positional isomers is discussed.