Substitution effects on the absorption spectra of nitrophenolate isomers

Charge-transfer excitations highly depend on the electronic coupling between the donor and acceptor groups. Nitrophenolates are simple examples of charge-transfer systems where the degree of coupling differs between ortho, meta and para isomers. Here we report the absorption spectra of the isolated anions in vacuo to avoid the complications of solvent effects. Gas-phase action spectroscopy was done with two different setups, an electrostatic ion storage ring and an accelerator mass spectrometer. The results are interpreted on the basis of CC2 quantum chemical calculations. We identified absorption maxima at 393, 532, and 399 nm for the para, meta, and ortho isomer, respectively, with the charge-transfer transition into the lowest excited singlet state. In the meta isomer, this π-π* transition is strongly redshifted and its oscillator strength reduced, which is related to the pronounced charge-transfer character, as a consequence of the topology of the conjugated π-system. Each isomer's different charge distribution in the ground state leads to a very different solvent shift, which in acetonitrile is bathochromic for the para and ortho, but hypsochromic for the meta isomer.     

The phototransposition in acetonitrile and the photoaddition of 2,2, 2-trifluoroethanol to the six isomers of dimethylbenzonitrile

The six dimethylbenzonitriles can be divided into two independent triads in their photochemical reactivity. The first triad is comprised of the 2,3-dimethyl, 3,4-dimethyl, and 2,6-dimethyl isomers (11-2,3, 11-3,4, and 11-2,6, respectively); the second triad is comprised of the 2,4-dimethyl, 2,5-dimethyl, and 3,5-dimethyl isomers (11-2,4, 11-2,5, and 11-3,5, respectively). In acetonitrile, phototransposition converts the members of one triad to other members of the same triad, although only 11-3,4 was reactive enough to have significant conversion approaching a steady-state composition. Irradiation in 2,2,2-trifluoroethanol (TFE) resulted in the formation of addition products, 6-cyano-X,Y-dimethylbicyclo[3.1. 0]hex-3-en-2-yl 2,2,2,-trifluoroethyl ethers, but in significant yield only from 11-3,4 of the first triad and 11-2,4 of the second triad. The 11-3,4 isomer gave seven major regio- and stereoisomers; the 11-2,4 isomer gave three different regio- and stereoisomers. These addition products were all explained by formation of bicyclo[3. 1.0]hex-3-en-1-yl cations resulting from protonation by TFE at C6 followed by nucleophilic trapping by TFE. From these and previous results on aromatic nitriles, a consistent mechanistic picture is obtained where the critical carbon in determining the products of the phototransposition and photoaddition reactions is the cyano substituted one.     

The photochemical addition of 2,2,2-trifluoroethanol to methoxy-substituted stilbenes

The excited-state lifetime of the trans-stilbene chromophore in acetonitrile is prolonged by methoxy substituents in the meta positions. The long-lived singlet excited state of trans-3,5-dimethoxystilbene (trans-2d) is quenched upon the addition of 2,2,2-trifluoroethanol (TFE), and the Markovnikov ether is observed as the major product from steady-state irradiations. The results indicate that the reaction pathway proceeds through a carbocation intermediate.     

Photochemistry of the three carboxypyridines in water: a pH dependent reaction

The photochemistry of ortho, meta and para-carboxypyridines (pK(a)(1)= 1.0-2.1 and pK(a)(2)= 4.7-5.3) in aqueous medium was studied by laser-flash photolysis and product studies. At pH < pK(a)(1), hydroxylated compounds are produced with low quantum yields. Within the pH range 4-7, ortho and meta isomers undergo dimerization together with decarboxylation with a quantum yield showing a very sharp maximum around pK(a)(2)([small phi](max)= 0.09 and 0.01, respectively) while the para isomer is photostable. End-of-pulse transients assigned to triplet states were detected by laser-flash photolysis at pH < pK(a)(1) and pH > 4. Additionally, the carboxypyridinyl radicals were detected as secondary intermediates at pH < pK(a)(1) and 4 < pH < 7 and the OH-adduct radicals at pH < pK(a)(1). This is in favour of an electron transfer reaction between triplet and starting compound producing a charge transfer species. The radical anion would escape as carboxypyridinyl radical while the radical cation may add water at pH < pK(a)(1) yielding the OH-adduct radical or may undergo decarboxylation at pH > 4. The high quantum yield of phototransformation of the ortho isomer at pH > 4 is due to an easy decarboxylation process. A reaction scheme is proposed accounting for the dependences of [small phi] on both the pH and the carboxypyridines concentration. This study points out the distinct pattern of reactivity of carboxypyridines depending on the ionisation state of starting compounds and isomeric substitution.     

A comparative analysis of the UV/Vis absorption spectra of nitrobenzaldehydes

In a joint experimental and theoretical study, the UV/Vis absorption spectra of the three isomers (ortho, meta, para) of nitrobenzaldehyde (NBA) were analyzed. Absorption spectra are reported for NBA vapors, cyclohexane and acetonitrile solutions. All spectra are poor in vibronic structure and hardly affected in shape by the surroundings (vapor or solution). Moderate solvatochromic shifts of ～ -0.2 eV are measured. For all isomers vertical transition energies, oscillator strengths, and excited state dipole moments were computed using the MS-CASPT2/CASSCF and CC2 methods. Based on these calculations the experimental transitions were assigned. The spectra of all isomers are characterized by weak (ε(max) ≈ 100 M(-1) cm(-1)) transitions around 350 nm (3.6 eV), arising from nπ* absorptions starting from the lone pairs of the nitro and aldehyde moieties. The next band of intermediate intensity peaking around 300 nm (4.2 eV, ε(max) ≈ 1000 M(-1) cm(-1)) is dominated by ππ* excitations within the arene function. Finally, strong absorptions (ε(max) ≈ 10,000 M(-1) cm(-1)) were observed around 250 nm (5.0 eV) which we ascribe to ππ* excitations involving the nitro and benzene groups.     

Differentiation of isomeric substituted diaryl ethers by electron ionization and chemical ionization mass spectrometry

A series of isomeric substituted diaryl ethers, i.e., 2- or 4-NO2, 5- FC6H3OC6H4 (4-R), where R=H, COCH3, COOCH3, NO2, CHO, OCH3 etc., which comprise ortho and para isomers with respect to the position of the nitro group are studied under GC-EI-MS and CI-MS conditions. The EI mass spectra of ortho and para isomers show distinct fragment ions, where the [MOH]+ and [MOHO]+ ions specifically appeared in all spectra of the ortho isomers (I), whereas the para isomers (II) contain [MO]+ and [MNO]+ ions. The [MOHCO]+ and [MOHNO]+ ions in I, and [MNO2]+ ion in II are the other specific fragment ions observed but feasibility of these fragment ions are found to depend on the nature of the substituent (R). The substitution (R) effect is also clearly reflected in the formation of fragment ions due to sigma-cleavage process with or without hydrogen migration. Similar differences in the formation of specific fragment ions are also observed in ortho and para isomers of substituted aryl naphthyl ethers. The methane/CI of isomeric compounds resulted in the same set of fragment ions, but prominent differences are observed in the relative abundance of [MHNO]+, which is relatively higher in para isomers compared with corresponding ortho isomer.     

Activation of C-H bonds of arenes: selectivity and reactivity in bis(pyridyl) platinum(II) complexes

The reaction of [PtMe2(NN)] and B(C6F5)3/H2O in CF3CH2OH with arenes Ar-H gives [PtAr{HOB(C6F5)3}(LL)] if the bis(pyridyl) ligand NN forms a six-membered, but not five-membered, chelate ring; methyl-substituted arenes give selectivity for metalation of meta > para > ortho, but methoxy-substituted arenes give ortho > meta, para.     

Investigations in the benzazole and naphthazole series

1-Benzazolyl-3-methyl-5-(p- or o-)tolylformazans (benzazolyl residues: 1-benzylbenzimidazole, benzothiazole, benzoxazole) have been synthesized and their structures have been studied by IR and UV spectroscopy. It has been found that in solutions the formazans are present in the open form and do not contain a chelate ring. Formazans containing an o-tolyl residue do not differ in respect of the structure of the formazan chain from their p-tolyl isomers, but in solutions of the complexes with nickel and copper the influence of the ortho methyl group is shown in the higher coloration of the complex of the ortho isomer than of the para isomer.     

Unusual luminescence characteristics of aminobiphenyls

Analysis of fluorescence solvatochromic shifts of ortho, meta and para aminobiphenyls reveals that the change in dipolemoment of m-aminobiphenyl on excitation is more when compared to other isomers. This change is due to the resonance interaction of unsubstituted phenyl ring with -NH2 group at meta position in the excited singlet state. The fluorimetric titration curves of three aminobiphenyls are found to be different from each other. The stretched sigmoidal curves obtained for m-aminobiphenyl indicates that the rates of proton transfer in S1 state are comparable to the rates of fluorescence.     

The direct detection of an aryl azide excited state: an ultrafast study of the photochemistry of para- and ortho-biphenyl azide

Ultrafast laser flash photolysis (266 nm) of para- and ortho-biphenyl azide in acetonitrile produces azide excited states that have broad absorption bands centered at 480 nm. The para-biphenyl azide excited singlet state has a lifetime of 100 fs. The excited-state lifetime of the ortho-azide isomer is 450 +/- 150 fs. Decay of the azide excited states is accompanied by the formation of the corresponding known singlet nitrenes (para, lambdamax = 350 nm, ortho, lambdamax = 400 nm). Singlet para-biphenylnitrene is born with excess energy and undergoes vibrational cooling with a time constant of 11 ps to form the long-lived (tau approximately 9 ns) relaxed singlet nitrene. Singlet ortho-biphenylnitrene decays with a lifetime of 16 ps in acetonitrile at ambient temperature.     

Molecular conformations of aminophenylimidazoles exhibiting antiulcer activities

To experimentally clarify a possible stereostructure-activity relationship proposed for H2-receptor antagonists, three 5-aminophenylimidazoles (1, 2 and 3), in which respective amino groups are located on the ortho, meta and para positions of the benzene ring, were synthesized and examined for their conformational characteristics using X-ray diffraction and proton nuclear magnetic resonance (1H-NMR) methods, and for antiulcer activities on rats and H2-receptor antagonist activities in guinea pig. The ortho isomer 1, which preferentially formed an intramolecular N-H (amino)...N (imidazole) hydrogen bond, showed the highest antiulcer activity with half the efficacy of cimetidine. On the other hand, none of 1, 2 and 3 showed significant H2-receptor antagonist activity. Based on these results, the conformational characteristic for the exhibition of antiulcer activity has been discussed.     

Generation of regiospecific carbanions under electrospray ionisation conditions and their selectivity in ion-molecule reactions with CO2

Regiospecific formation of carbanions from a set of geometrical (cis and trans isomers) and five different sets of positional isomers (ortho, meta and para isomers) of aromatic carboxylic acids is reported under negative electrospray ionisation conditions by decarboxylation of the carboxylate anions. The structures of decarboxylated anions, [(M-H)-CO(2)](-), are studied by ion-molecule reactions with carbon dioxide in the collision cell of a triple quadrupole mass spectrometer. The [(M-H)-CO(2)](-) ions generated from the trans and meta/para isomers react with CO(2) to produce product ions corresponding to the addition of one CO(2), which confirms the survival of the [(M-H)-CO(2)](-) ions as carbanions. On the other hand, the [(M-H)-CO(2)](-) ions generated from cis and ortho isomers failed to react with CO(2) due to rapid isomerisation of the initially generated carbanion to a aromatic carboxylate/oxide anion, which is unreactive with CO(2), through a facile intramolecular proton transfer from the proton-containing substituent to the carbanion site. When the experiments were performed at high desolvation temperatures (300 degrees C), instead of 100 degrees C, the relative abundance of [(M-H)-CO(2)](-) ions and the corresponding CO(2) adduct in ion-molecule reaction experiments increased significantly due to minimisation of proton exchange. Quantum chemical calculations on some of the generated isomeric carbanions and their isomerised products due to proton transfer support the selective stability of carbanions.     

C-H vs C-C bond activation of acetonitrile and benzonitrile via oxidative addition: rhodium vs nickel and Cp* vs Tp' (Tp' = hydrotris(3,5-dimethylpyrazol-1-yl)borate, Cp* = η(5)-pentamethylcyclopentadienyl)

The photochemical reaction of (C(5)Me(5))Rh(PMe(3))H(2) (1) in neat acetonitrile leads to formation of the C-H activation product, (C(5)Me(5))Rh(PMe(3))(CH(2)CN)H (2). Thermolysis of this product in acetonitrile or benzene leads to thermal rearrangement to the C-C activation product, (C(5)Me(5))Rh(PMe(3))(CH(3))(CN) (4). Similar results were observed for the reaction of 1 with benzonitrile. The photolysis of 1 in neat benzonitrile results in C-H activation at the ortho, meta, and para positions. Thermolysis of the mixture in neat benzonitrile results in clean conversion to the C-C activation product, (C(5)Me(5))Rh(PMe(3))(C(6)H(5))(CN) (5). DFT calculations on the acetonitrile system show the barrier to C-H activation to be 4.3 kcal mol(-1) lower than the barrier to C-C activation. A high-energy intermediate was also located and found to connect the transition states leading to C-H and C-C activation. This intermediate has an agostic hydrogen interaction with the rhodium center. Reactions of acetonitrile and benzonitrile with the fragment [Tp'Rh(CNneopentyl)] show only C-H and no C-C activation. These reactions with rhodium are compared and contrasted to related reactions with [Ni(dippe)H](2), which show only C-CN bond cleavage.     

Spectral characteristics of ortho, meta and para dihydroxy benzenes in different solvents, pH and beta-cyclodextrin

Spectral characteristics of ortho, meta and para dihydroxy benzenes (DHB's) have been studied in different solvents, pH and beta-cyclodextrin. Solvent study shows that: (i) the interaction of OH group with the aromatic ring is less than that of amino group both in the ground and excited states, (ii) in absorption, the charge transfer interaction of OH group in para position is larger than ortho and meta positions. pH studies reveals that DHB's are more acidic than phenol. The higher pK(a) value of oDHB (monoanion-dianion) indicates that the formed monoanion is more stabilized by intramolecular hydrogen bonding. DHB's forms a 1:1 inclusion complex with beta-CD. In beta-CD medium, absorption spectra of DHB's mono and dianions shows unusual blue shifts, whereas in the excited state, the spectral characteristics of DHB's follow the same trend in both aqueous and beta-CD medium.     

Structural orientation in the condensations of 2.4-dimethyl-1,3-pentadiene

Summary It was shown for the first time that the diene condensation of 2,4-dimethyl-1,3-pentadiene with acrylonitrile results in the formation not only of the ortho isomer (III), but also of the meta isomer (IV) in 3–5% yield.     

Bulk polymerization of diallyl benzene-dicarboxylates I. Influence of temperature on allyl group reactivity

The bulk polymerization of the three isomeric diallyl benzene-dicarboxylates was carried out in the temperature range 80–285°C. The progress of the polymerization process was examined by determination of the conversion of allyl groups double bonds. The reactivity of these groups in the polymerization increases in the following order of isomers: ortho < para < meta at 80–230°C. At temperatures above 200°C the thermal polymerization with activation energies for ortho, meta and para isomers 32, 27, and 28 kcal/mol of allyl group, respectively, has been observed. With the increase of temperature from 80 to 230°C for each of the monomers the number of allyl groups consumed when forming one C? C chain (degree of chain polymerization) decreases, but at the same time the kinetic chain length increases several times. The results have been explained by the growing role of chain transfer reactions with simultaneous increase of an ability to reinitiation by occured radicals. The mechanisms of thermal polymerization have been proposed.     

Acid generation in the thermal decomposition of diaryliodonium salts

Diphenyliodonium tetrafluoroborate and diphenyliodonium hexafluorophosphate have been found to generate up to two equivalents of hydrogen fluoride per equivalent of the iodonium salt by pyrolysis at 239°C in the neat state and at 150°C in the presence of anisole or nitrobenzene. The formation of hydrogen fluoride is presumed to arise by dissociation of hydrogen tetrafluoroborate or hydrogen hexafluorophosphate initially formed, due to the high temperatures, thus giving rise also to the Lewis acids boron trifluoride and phosphorus pentafluoride, respectively. A detailed analysis of the volatile organic products of the decomposition of the diphenyliodonium salts was also carried out. Many products were identified in all of the cases studied. For example, the neat decomposition of diphenyliodonium tetrafluoroborate afforded benzene, fluorobenzene, iodobenzene, the three isomeric iodobiphenyls, biphenyl, three isomeric terphenyls, and one or more of the diiodobiphenyls, iodoterphenyls, and polyaromatics. Among the iodobiphenyls, the ortho and para isomers were found to predominate over the meta isomer. The terphenyl isomers did not exhibit this ortho, para selectivity. It was significant that decomposition of the diaryliodonium salts in anisole suspension did not afford methoxybiphenyls or iodomethoxybiphenyls. An interpretation of these results is presented. © 1996 John Wiley & Sons, Inc.     

Theoretical study of the decomposition reactions in substituted nitrobenzenes

The influence of substituent nature and position on the unimolecular decomposition of nitroaromatic compounds was investigated using the density functional theory at a PBE0/6-31+G(d,p) level. As the starting point, the two main reaction paths for the decomposition of nitrobenzene were analyzed: the direct carbon nitrogen dissociation (C6H5 + NO2) and a two step mechanism leading to the formation of phenoxyl and nitro radicals (C6H5O + NO). The dissociation energy of the former reaction was calculated to be 7.5 kcal/mol lower than the activation energy of the second reaction. Then the Gibbs free energies were computed for 15 nitrobenzene derivatives characterized by different substituents (nitro, methyl, amino, carboxylic acid, and hydroxyl) in the ortho, meta, and para positions. In meta position, no significant changes appeared in the reaction energy profiles whereas ortho and para substitutions led to significant deviations in energies on the decomposition mechanisms due to the resonance effect of the nitro group without changing the competition between these mechanisms. In the case of para and meta substitutions, the carbon-nitro bond dissociation energy has been directly related to the Hammett constant as an indicator of the electron donor-acceptor effect of substituents.     

Loss of benzene to generate an enolate anion by a site-specific double-hydrogen transfer during CID fragmentation of o-alkyl ethers of ortho-hydroxybenzoic acids

Collision-induced dissociation of anions derived from ortho-alkyloxybenzoic acids provides a facile way of producing gaseous enolate anions. The alkyloxyphenyl anion produced after an initial loss of CO(2) undergoes elimination of a benzene molecule by a double-hydrogen transfer mechanism, unique to the ortho isomer, to form an enolate anion. Deuterium labeling studies confirmed that the two hydrogen atoms transferred in the benzene loss originate from positions 1 and 2 of the alkyl chain. An initial transfer of a hydrogen atom from the C-1 position forms a phenyl anion and a carbonyl compound, both of which remain closely associated as an ion/neutral complex. The complex breaks either directly to give the phenyl anion by eliminating the neutral carbonyl compound, or to form an enolate anion by transferring a hydrogen atom from the C-2 position and eliminating a benzene molecule in the process. The pronounced primary kinetic isotope effect observed when a deuterium atom is transferred from the C-1 position, compared to the weak effect seen for the transfer from the C-2 position, indicates that the first transfer is the rate determining step. Quantum mechanical calculations showed that the neutral loss of benzene is a thermodynamically favorable process. Under the conditions used, only the spectra from ortho isomers showed peaks at m/z 77 for the phenyl anion and m/z 93 for the phenoxyl anion, in addition to that for the ortho-specific enolate anion. Under high collision energy, the ortho isomers also produce a peak at m/z 137 for an alkene loss. The spectra of meta and para compounds show a peak at m/z 92 for the distonic anion produced by the homolysis of the O--C bond. Moreover, a small peak at m/z 136 for a distonic anion originating from an alkyl radical loss allows the differentiation of para compounds from meta isomers. Copyright (c) 2008 John Wiley & Sons, Ltd.     

Noncatalytic kinetic study on site-selective H/D exchange reaction of phenol in sub- and supercritical water

The site-selective H/D exchange reaction of phenol in sub- and supercritical water is studied without added catalysts. In subcritical water in equilibrium with steam at 210-240 degrees C, the H/D exchange proceeds both at the ortho and para sites in the phenyl ring, with no exchange observed at the meta site. The pseudo-first-order rate constants are of the order of 10(-4) s(-1); 50% larger for the ortho than for the para site. In supercritical water, the exchange is observed also at the meta site with the rate constant in the range of 10(-6)-10(-4) s(-1). As the bulk density decreases, the exchange slows down and the site selectivity toward the ortho is enhanced. The enhancement is due to the phenol-water interaction preference at the atomic resolution. The site selectivity toward the ortho is further enhanced when the reaction is carried out in benzene/water solution. Using such selectivity control and the reversible nature of the hydrothermal deuteration/protonation process, it is feasible to synthesize phenyl compounds that are deuterated at any topological combination of ortho, meta, and para sites.