Isomeric pyridylcarbenes and their Si and Ge heavier analogues at DFT: stabilization through π–p or n–p interaction

To realize the effects of the pyridyl group on the stability, multiplicity, and geometry of isomeric pyridylcarbenes and their heavier Si and Ge analogues, a theoretical study was performed at B3LYP/6-311++G(d,p)//B3LYP/6-31G(d). The behavior of nitrogen atom was totally different in each multiplicity (singlet and triplet), structural isomer (ortho-, meta-, and para-), and divalent center (C, Si, and Ge). All pyridylcarbenes have triplet ground states, while the stable silylene and germylene analogues are singlet. The pyridyl group stabilizes both singlet and triplet states divalent centers with more pronounced effects on the singlet states in the order: carbene>germylene>silylene. While all planar species benefit from common π–p conjugative interaction of the pyridyl ring, in the ortho-isomers of 2-pyridylsilylene and 2-pyridylgermylene there is another interaction, n–p, that leads to two stable non-planar conformers. This finding is confirmed by NBO charges, calculated UV–vis spectra, philicity indices (N and ω), and isodesmic reactions.     

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.     

Ortho effect in the mass spectrometric behaviour of N(pyrimidin-4-yl)aminobenzoic acids and their methyl esters

Ortho-. meta- and para-isomers of N-(pyrimidin-4-yl)aminobenzoic acid and their methyl esters were investigated by electron impact mass Spectrometry. Their fragmentation was found to be strongly dependent on the position of the substituent in the aminobenzoic moiety. Two different kinds of ortho effect were studied and confirmed with the aid of deuterium-labelled derivatives.     

Differentiation of Protonated Aromatic Regioisomers Related to Lignin by Reactions with Trimethylborate in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer

Several lignin model compounds were examined to test whether gas-phase ion–molecule reactions of trimethylborate (TMB) in a FTICR can be used to differentiate the ortho-, meta-, and para-isomers of protonated aromatic compounds, such as those formed during degradation of lignin. All three regioisomers could be differentiated for methoxyphenols and hydroxyphenols. However, only the differentiation of the ortho-isomer from the meta- and para-isomers was possible for hydroxyacetophenones and hydroxybenzoic acids. Consideration of the previously reported proton affinities at all basic sites in the isomeric hydroxyphenols, and the calculated proton affinities at all basic sites in the three methoxyphenol isomers, revealed that the proton affinities of the analytes relative to that of TMB play an important role in determining whether and how they react with TMB. The loss of two methanol molecules (instead of one) from the adducts formed with TMB either during ion–molecule reactions, or during sustained-off resonance irradiated collision-activated dissociation of the ion–molecule reaction products, revealed the presence of two functionalities in almost all the isomers. This finding supports earlier results suggesting that TMB can be used to count the functionalities in unknown oxygen-containing analytes.     

A semiempirical investigation of interelectronic exchange coupling in bisected poly(1,4-phenylene) polycation model systems

AM 1 SCF -MO -CI computations find the bisected biphenyl dication to have nearly degenerate triplet and singlet states, with the lowest-energy state being a quininoid singlet planar dication. The bisected perchlorobiphenyl dication favors a triplet ground state by a small amount (0.4-1.9 kcal/mol), in qualitative agreement with recent experimental findings and with theoretical expectations that such an orthogonal open-shell pi-system should exhibit ferromagnetic exchange coupling. The higher oligomeric bisected para-linked phenylenes polycations do not show an appreciable computational preference for a high-spin multiplicity ground state either with or without perchlorine substitution. Chlorine substitution para to the 1,1′-linkage may lend a unique stabilization to the biphenyl system, which is not available in higher oligomeric analogs of poly(1,4-phenylene)s. The small magnitude of ferromagnetic exchange in these systems suggests that small geometric or substituent effects may confound experimental efforts to design polymeric ferromagnetic materials by this strategy. © 1992 John Wiley & Sons, Inc.     

ortho‐, meta‐, and para‐Dihydroindenofluorene Derivatives as Host Materials for Phosphorescent OLEDs

This work reports the first structure–properties relationship study of ortho [2,1‐c]‐, meta [1,2‐a]‐, and para [1,2‐b]dihydroindenofluorenes, highlighting the influence of bridge rigidification on the electronic properties. This study has made it possible to devise an extended π‐conjugated molecule with both a high triplet state energy level and excellent thermal and morphological stability. As a proof of concept, dihydroindenofluorenes were used as the host in sky‐blue phosphorescent organic light‐emitting diodes (PhOLEDs) w ith high performance.     

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.     

Conformational Dependence of Triplet Energies in Rotationally Hindered N- and S-Heterocyclic Dimers: New Design and Measurement Rules for High Triplet Energy OLED Host Materials

A series of four heterocyclic dimers has been synthesized, with twisted geometries imposed across the central linking bond by ortho-alkoxy chains. These include two isomeric bicarbazoles, a bis(dibenzothiophene-S,S-dioxide) and a bis(thioxanthene-S,S-dioxide). Spectroscopic and electrochemical methods, supported by density functional theory, have given detailed insights into how para- vs. meta- vs. broken conjugation, and electron-rich vs. electron-poor heterocycles impact the HOMO–LUMO gap and singlet and triplet energies. Crucially for applications as OLED hosts, the triplet energy (E_T) of these molecules was found to vary significantly between dilute polymer films and neat films, related to conformational demands of the molecules in the solid state. One of the bicarbazole species shows a variation in E_T of 0.24 eV in the different media—sufficiently large to “make-or-break” an OLED device—with similar discrepancies found between neat films and frozen solution measurements of other previously reported OLED hosts. From consolidated optical and optoelectronic investigations of different host/dopant combinations, we identify that only the lower E_T values measured in neat films give a reliable indicator of host/guest compatibility. This work also provides new molecular design rules for obtaining very high E_T materials and controlling their HOMO and LUMO energies.     

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.     

Direct assignment of positional isomers by mass spectrometry: ortho,meta and para acyl and amidyl anilines and phenols and derivatives

A direct MS/MS method for the ortho, meta or para configuration assignment of any single molecule that forms reference ions upon ionization and dissociation is demonstrated. Gas-phase structure diagnostic ion–molecule reactions with acetonitrile are shown to distinguish the isomeric 2-, 3- and 4-hydroxybenzoyl cations and the 2- from the 3- and 4-aminobenzoyl cations. These reference ions, which display indistinguishable 15 eV collision-induced dissociation product ion mass spectra, react with acetonitrile to yield characteristic ratios of product ions, most particularly for the 2-isomers. The reactivity of the 2-benzoyl cations is the most characteristic since the ortho configuration allows for [4⁺ + 2] polar cycloaddition that yields relatively stable heterocycles in N-protonated forms. Distinction of the reference isomeric 2-, 3- and 4-hydroxy- and aminobenzoyl cations permits, therefore, partially or completely, direct ‘MS-only’ positional assignment of either ortho, meta or para configuration for any single molecule that forms such reference ions upon ionization and dissociation. This “class-universal” method for direct MS assignment of a single positional isomer should therefore be applicable to many members of the homologous series of isomeric ortho, meta and para acyl and amidyl anilines and phenols and derivatives. Such molecules dissociate, or are likely to dissociate, after or during ionization processes to form the reference and structurally diagnostic ortho, meta or para hydroxy- or aminobenzoyl cations. Copyright © 2004 John Wiley & Sons, Ltd.     

Urea as ammonia equivalent in aryl halides amination catalyzed by palladium complexes

Urea reaction with nonactivated aryl bromides and chlorides under catalysis with palladium complexes led to the formation in 65–95% yield of triarylamines from para-and meta-substituted aryl halides and of diarylamines from ortho-isomers.     

Use of two-parameter correlations for studying the radical reactivity of aromatic compounds

Two-parameter equation correlations are reported for radical reactions of aromatic compounds. In these correlations polar and resonance substituent constants identical with the substituent constants of aliphatic compounds were used. The equations correlate the rate constants for H abstraction reactions and for the addition of a variety of free radicals to the ortho-, meta-, and para-substituted aromatic compounds. Besides, they correlate parameters of the spectra for substituted aromatic radicals. The correlations show that the effects of para substituents on the reactions studied are nearly entirely resonance effects, whereas for the meta- and ortho-substituted compounds polar (inductive) effects become essential. Application of the two-parameter correlations permits also to determine the structure of transition states (σ or π-complex) in free-radical reactions.     

Mass spectra of some isopropyl benzene derivatives. A study of the ortho effect

The mass spectra of a series of ortho, meta and para substituted isopropyl benzene derivatives have been determined where the second substituent is ? COOH, ? C(O)NH₂, ? C(O)C₆H₅, ? C(Ph)(=NPh) or ? CH(Ph)(NHPh). Two bis-isopropylbenzophenones have also been studied. The spectra are characterized by prominent ortho effects which distinguish the ortho derivatives from the meta and para.     

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.     

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.     

The ortho‐Substituent Effect on the Ag‐Catalysed Decarboxylation of Benzoic Acids

A combined experimental and computational investigation on the Ag‐catalysed decarboxylation of benzoic acids is reported herein. The present study demonstrates that a substituent at the ortho position exerts dual effects in the decarboxylation event. On one hand, ortho‐substituted benzoic acids are inherently destabilised starting materials compared to their meta‐ and para‐substituted counterparts. On the other hand, the presence of an ortho‐electron‐withdrawing group results in an additional stabilisation of the transition state. The combination of both effects results in an overall reduction of the activation energy barrier associated with the decarboxylation event. Furthermore, the Fujita–Nishioka linear free energy relationship model indicates that steric bulk of the substituent can also exert a negative effect by destabilising the transition state of decarboxylation.     

Rational Design of Azothiophenes—Substitution Effects on the Switching Properties

A series of substituted azothiophenes was prepared and investigated toward their isomerization behavior. Compared to azobenzene (AB), the presented compounds showed red-shifted absorption and almost quantitative photoisomerization to their (Z) states. Furthermore, it was found that electron-withdrawing substitution on the phenyl moiety increases, while electron-donating substitution decreases the thermal half-lives of the (Z)-isomers due to higher or lower stabilization by a lone pair–π interaction. Additionally, computational analysis of the isomerization revealed that a pure singlet state transition state is unlikely in azothiophenes. A pathway via intersystem crossing to a triplet energy surface of lower energy than the singlet surface provided a better fit with experimental data of the (Z)→(E) isomerization. The insights gained in this study provide the necessary guidelines to design effective thiophenylazo-photoswitches for applications in photopharmacology, material sciences, or solar energy harvesting applications.     

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.     

Functionalization of Hexa‐peri‐hexabenzocoronenes: Investigation of the Substituent Effects on a Superbenzene

We have demonstrated that the iridium‐catalyzed direct borylation of hexa‐peri‐hexabenzocoronene (HBC) enables regioselective introduction of boryl groups to the para‐, ortho‐, and meta‐substituted HBCs in high yields. The boryl groups have been transformed into various functionalities such as hydroxy, cyano, ethynyl, and amino groups. We have elucidated that the substituents significantly influence the photophysical properties of HBCs to enhance fluorescence quantum yields. DFT calculations revealed that the origin of the substituent effect is the lift in degeneracy in the frontier orbitals by an interaction with electron‐donating and electron‐withdrawing substituents at the para‐ and ortho‐positions. The change in molecular orbitals results in an increase of the transition probability from the S₀→S₁ states. In addition, the two‐photon absorption cross‐section values of para‐substituted HBCs are significantly larger than those of ortho‐ and meta‐substituted HBCs.     

Electron-impact studies—LXX. Substituent effects in the negative-ion spectra of nitroaryl esters

The negative-ion mass spectra of a variety of substituted nitroaryl benzoates and aryl nitrobenzoates have been studied. Pronounced proximity effects are observed in the spectra of ortho-substituted compounds. The meta and para pairs fragment in a similar manner. Linear plots are observed for many cleavages using either the McLafferty or Harrison methods. Evidence is provided which suggests that the fragmentations of meta and para isomers may proceed by the same mechanistic pathway. The magnitude of ρ does not appear to provide precise information concerning the nature of the transition state for a particular reaction of a negative ion.