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.     

Muonium atom addition to pyridine and to the pyridinium cation

Muonium atoms are found to add mainly to the N-atom and to the ortho and meta positions in pyridine in almost equal amounts, giving pyridinyl and 2-azacyclohexadienyl radicals and 3-azacyclohexadienyl radicals, respectively (no addition was observed at the para site); on protonation, as in a sample of PyH⁺ BF₄⁻, the reactivity follows the positive charge density, being enhanced for addition to the N(H⁺) position, reduced for the ortho/meta positions, and now conferring a significant yield of the para isomer. These results for genuine reactions of hydrogen atoms are discussed in view of studies of radical formation in these materials, made using EPR spectroscopy, when they are exposed to γ-rays in cryogenic matrices, for which mechanisms involving charge-neutralization of initial radical ions are proposed. Copyright © 2001 John Wiley & Sons, Ltd.     

Thiomethylation of aromatic amines: efficient method for the synthesis of heterocyclic compounds

Primary aromatic amines were thiomethylated by formaldehyde and hydrogen sulfide. N-Substituted 1,3-thiazetidines, 4,5-dihydro-1,3,5-dithiazines, 3,4,5,6-tetrahydro-2H-1,3,5-thiadiazines, and 4,5-dihydro-1,3,5-oxathiazines were prepared for the first time starting from meta- and para-toluidines, meta-, para-, and ortho-anisidines, and para-xylidine. Amines characterized by higher mobility of hydrogen atoms produced previously unknown four-membered thiazetidines, whereas amines characterized by lower mobility of hydrogen atoms gave six-membered thiadiazines. The sorption properties with respect to silver were studied for the compounds, which were prepared from p-toluidine and p-anisidine.     

Reversal of order of chemical shifts. Differential solvent effects in the hydrogen shifts of toluene,m-xylene and o-xylene as determined by 320 MHz tritium NMR spectroscopy

Chemical shifts for the aromatic tritons of toluene decrease in the order meta>para>ortho for pure toluene, but in the order meta>ortho>para for dilute solutions of toluene in carbon tetrachloride, chloroform, cyclohexane and dimethyl sulphoxide; m-xylene shifts are 5>4,6>2 in the pure state and 5>2>4,6 in carbon tetrachloride and dimethyl sulphoxide; o-xylene shifts are 4,5>3,6 in the pure state and reversed in carbon tetrachloride.     

Analytical MO study of the singlet–triplet coupling in xylylenes

The singlet–triplet energy difference in para-, meta-, and ortho-xylylenes is studied as the interaction of two radical centers through the benzene ring. An SCF perturbative procedure adapted to open-shell systems leads to two benzyl-like nonbonding molecular orbitals (NBMOS ) and to benzene-like occupied and vacant MOS whatever the xylylene isomer. The superposition of these NBMOS in para-, meta-, and ortho-positions and their interaction with the benzene-like MOS lead, at the configuration interaction level, to the following results: The exchange energy (which favors the triplet state) and the charge transfer energy (which favors the singlet state) are important only in the meta-xylylene; the dynamic (or double) spin polarization favors the triplet in meta and the singlet in para and ortho-isomers; the super-exchange energy (which favors the singlet) is important only in para- and ortho-isomers. The above results are independent of the chosen geometry.     

Synthesis and characterisation of polymethacrylates containing para-, meta- and ortho-monosubstituted azobenzene moieties in the side chain

Three sets of novel side-chain liquid crystalline polymers with monosubstituted azobenzene moieties in the side-chain have been studied. These are poly(p-(4′-methoxy-4-oxyhexyloxy azobenzene) benzyl methacrylate) (PPHABM), poly(m-(4′-methoxy-4-oxyhexyloxy azobenzene) benzyl methacrylate) (PMHABM) and poly(o-(4′-methoxy-4-oxyhexyloxy azobenzene) benzyl methacrylate) (POHABM). The chemical structure of the monomers was confirmed by ¹H NMR, ¹³C NMR spectroscopy and elemental analysis. The structural characterisation of the polymers was performed by ¹H NMR spectroscopy and gel permeation chromatography, and their phase behaviour and liquid crystalline properties were studied using differential scanning calorimetry, polarised optical microscopy and wide-angle X-ray diffraction. The results show that the transitional behaviour of side-chain liquid crystalline polymers containing monosubstituted azobenzene moieties depends strongly on the position of the substituent on the azobenzene moiety; for example, the ortho-monosubstituted polymers do not form liquid crystalline phases, but all the para- and meta-monosubstituted polymers exhibit a smectic A phase. Furthermore, the glass transition temperature (T_g ) of the polymers decreases in the order, para > meta > ortho. For the PPHABM and PMHABM polymers the isotropic temperature (T_i ) and liquid crystalline range (ΔT, from T_g to T_i ) are found to be in the order, para > meta, although it is surprising that the associated enthalpy changes in these polymers is the opposite order, meta > para.     

The influence of the macroring structure on the enthalpies of solution of tetrapyridylporphyrin derivatives in water

The enthalpies of solution of 5,10,15,20-tetra(N-methylpyrid-4,3,2-yl)porphyrin tetraiodide were determined calorimetrically at 298.15 K. Changes in the position of the heteroatom in the pyridyl fragment of the substituted porphyrin (meta- and ortho- positions) substantially decreased the endothermic effect of solution compared with the para- isomer. This was related to changes in the energy of the molecular crystal lattice, which decreased in the series para- > meta- > ortho-isomer.     

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.     

The fragmentation of ortho-methoxyphenyl-2-propanone oximes and related compounds–further examples of an ortho-effect

An [M ? 31]⁺ ion was a prominent fragment in the mass spectra of three ortho-methoxy-phenyl-2-propanone oximes and is shown to be due to the expulsion of a methoxyl radical from the molecular ion as a result of an ortho-effect. In contrast, an [M ? 31]⁺ ion was absent from the spectra of a structurally related ketone and a hydroxylamine, and was not observed in the spectra of meta- and para-methoxyphenyl-2-propanone oximes.     

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.     

Concerning the chemical shifts of the ring protons of toluene as determined by 1H NMR spectroscopy of deuterated material

Chemical shifts of the ring protons are determined by ¹H NMR of 99% deuterated toluene. At low temperatures the deuterons relax rapidly, yielding sharp ¹H peaks for the ortho, meta and para protons. These are compared with shifts obtained by other methods.     

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.     

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.     

A ‘meta effect’ in the fragmentation reactions of ionised alkyl phenols and alkyl anisoles

The competition between benzylic cleavage (simple bond fission [SBF]) and retro‐ene rearrangement (RER) from ionised ortho, meta and para RC₆H₄OH and RC₆H₄OCH₃ (R = n‐C₃H₇, n‐C₄H₉, n‐C₅H₁₁, n‐C₇H₁₅, n‐C₉H₁₉, n‐C₁₅H₃₁) is examined. It is observed that the SBF/RER ratio is significantly influenced by the position of the substituent on the aromatic ring. As a rule, phenols and anisoles substituted by an alkyl group in meta position lead to more abundant methylene‐2,4‐cyclohexadiene cations (RER fragmentation) than their ortho and para homologues. This ‘meta effect’ is explained on the basis of energetic and kinetic of the two reaction channels. Quantum chemistry computations have been used to provide estimate of the thermochemistry associated with these two fragmentation routes. G3B3 calculation shows that a hydroxy or a methoxy group in the meta position destabilises the SBF and stabilises the RER product ions. Modelling of the SBF/RER intensities ratio has been performed assuming two single reaction rates for both fragmentation processes and computing them within the statistical RRKM formalism in the case of ortho, meta and para butyl phenols. It is clearly demonstrated that, combining thermochemistry and kinetics, the inequality (SBF/RER)_meta < (SBF/RER)_ortho < (SBF/RER)_para holds for the butyl phenols series. It is expected that the ‘meta effect’ described in this study enables unequivocal identification of meta isomers from ortho and para isomers not only of alkyl phenols and alkyl anisoles but also in other alkyl benzene series. Copyright © 2012 John Wiley & Sons, Ltd.     

Chemical ionization mass spectra of mononitroarenes

The H₂ and CH₄ chemical ionization mass spectra of a selection of substituted nitrobenzenes have been determined. It is shown that reduction of the nitro group to the amine is favoured by high source temperatures and the presence of water in the ion source. The H₂ chemical ionization mass spectra are much more useful for distinguishing between isomeric compounds than the CH₄ CI mass spectra because of the more extensive fragmentation. For ortho substituents bearing a labile hydrogen abundant [MH ? H₂O]⁺ fragments are observed. When the substituent is electron-releasing both ortho and para substituted nitrobenzenes show abundant [MH? OH]⁺ fragment ions while meta substituted compounds show abundant loss of NO and NO₂ from [MH]⁺. The latter fragmentation is interpreted in terms of protonation para to the substituent or ortho to the vitro function, while the first two fragmentation routes arise from protonation at the nitro group. When the substituent is electron-attracting the chemical ionization mass spectra of isomers are very similar except for the H₂O loss reaction for ortho compounds.     

Relative Stabilities and Molecular Structures of Isomeric Dihalobenzenes. A DFT Study

The molecular structures, total energies, and other computational data of benzene, and its monoand dihalogenated derivatives (halogen = F, Cl, Br) have been studied by DFT calculations. The main aim of the study was to estimate the relative stabilities (energies) of the ortho, meta, and para isomers of the six series of dihalobenzenes investigated. The computational data show that the ortho isomers always have the highest, and the meta isomers usually, but not always, the lowest total energies. Thus, 1,2-difluorobenzene is ca. 16.6 kJ mol^–1, and 1,4-difluorobenzene 2.5 kJ mol^–1 less stable than 1,3-difluorobenzene. Among the other isomeric dihalobenzenes, the differences in stability are less pronounced. For the dibromo-, dichloro-, and bromochlorobenzenes, the para compounds are calculated to be slightly (0.2–0.4 kJ mol^–1) more stable than their meta isomers. In addition to the thermochemical aspect of the study, the computational molecular structures of the halobenzenes are compared with available experimental data and discussed in terms of the substituent-induced deformation of the ideal geometry of the benzene ring. The computational electric dipole moments, especially for the fluorine-containing compounds, compare favorably with the respective experimental (gas-phase) values.     

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.     

ESR studies of trapped free radicals and polymer deposition in xylene plasmas

Rates of polymer deposition and radical trapping in xylene plasmas produced in a Siemens tube with 60-Hz current decreased linearly with increasing flow rate but increased linearly with power and decreased in the order ortho > meta > para at a constant flow rate and pressure. These results indicate that radical recombination and disproportionation are unimportant and that the polymerization rate equation is first order in free radical concentration.     

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.     

Efficient double bond migration of allylbenzenes catalyzed by Pd(OAc)2-HFIP system with unique substituent effect

A novel catalyst system of Pd(OAc)₂-HFIP induces double-bond migration of allylbenzenes under mild conditions with low catalytic loading to afford 1-propenylbenzenes. The reaction shows a unique substituent effect that is highly dependent on the distance of substituents from the allylic moiety. Thus, the reactivity of substrates bearing a methyl group is ordered in para > meta > ortho, whereas it is entirely reversed as ortho > meta > para for methoxy and chloro substituents.