Facts and artifacts about aromatic stability estimation

The stability of a set of 105 five-membered π-electron systems (involving aromatic, non-aromatic and anti-aromatic species) was evaluated using six isodesmic reactions of which two belong to the subclass of homodesmotic reactions, which are based on cyclic and acyclic reference systems. We demonstrate that the ‘Resonance Energies’ derived from isodesmotic schemes have obvious flaws and do not correct or cancel other contributions to the energy, such as the changes of hybridization, homoconjugation of heterosubstituted cyclopentadienes, conjugative interactions of CC or CX (X=N or P) with a π or pseudo π orbital at Y (Y=O, S, NH, PH), strain, etc. as effectively as possible. Likewise, ‘aromatic stabilization energies (ASE)’ derived from homodesmotic schemes based on the acyclic reference compounds do not give satisfactory results. We strongly recommend that only cyclic reference compounds should be used for ASE and other aromaticity evaluations. The analysis is based on ab initio optimized geometries at B3LYP/6-311+G^∗∗.     

Dications of 3-phenylindenylidenefluorenes: evaluation of antiaromaticity of indenyl and fluorenyl cations by magnetic measures

Dications of p-substituted 3-phenylindenylidenefluorenes were prepared to examine the response of the resulting indenyl and fluorenyl cationic systems to magnetic measures of antiaromaticity. All measures, (1)H NMR shifts, nucleus independent chemical shifts (NICS(1)(zz)), and magnetic susceptibility exaltation, Lambda, supported the antiaromaticity of the dications 3a-f2+. The 1H NMR shifts and NICS(1)(zz) showed that the indenyl ring system was less antiaromatic than the fluorenyl ring system, contrary to the antiaromaticity of indenyl monocations compared to fluorenyl monocations. The presence of a phenyl substituent in the 3-position was able to stabilize the indenylidene cation through resonance, decreasing its antiaromaticity, but even in the absence of the 3-phenyl substituent, the indenyl system of indenylidenefluorene dications was less antiaromatic than the fluorenyl system. The decreased antiaromaticity of the 3-phenylindenylidenefluorene dications over the unsubstituted indenylidenefluorene dication was supported by (anti)aromatic (de)stabilization energy calculations, ASE.     

On the aromatic stabilization of corannulene and coronene

The application of set of homodesmotic reactions allowed us to estimate the aromatic stabilization energy (ASE) of corannulene and coronene. Appropriate reactions have been applied to balance syn/anti mismatches in di-, tetra- and hexamethylene substituted derivatives. Based on many different polycyclic reference structures that compensate the effect of strain in the corannulene moiety the value of ASE comes to 44.5 kcal mol(-1). Planar corannulene is more stabilized by cyclic π-electron delocalization by ca. 10.7 kcal mol(-1), as compared with a bowl-shaped system. A similar approach for coronene leads to an ASE equal to 58.4 kcal mol(-1).     

Studies of 9-fluorenyl carbocations. intramolecular hydride migration in a substituted 9-fluorenyl carbocation

The substituted fluorenyl cation, 9-(diphenylmethyl)fluoren-9-yl cation (4), is formed under stable ion conditions (low temperature/strong acid) from its corresponding alcohol 3. This ion is transformed to a substituted diphenyl methyl cation 8 at ambient temperature via an apparent 1,2-hydrogen shift. Irradiation of 9-(diphenylmethyl)fluoren-9-ol in methanol gives products derived from the corresponding cation along with radical-derived products from C-C and C-O homolysis processes. The laser flash photolysis of this alcohol gave a transient corresponding to cation 4. All of the photoproducts are derived from cation 4 or radical pathways. High level MO calculations point to a high barrier (23.8 kcal x mol(-1)) for the 1,2-hydride shift. This barrier is the consequence of the minimum energy conformation of this fluorenyl cation which is less than ideal for the periplanar geometry necessary for this process.     

Sigma-pi energy separation in homodesmotic reactions.

A well-established quantity for specifying the aromaticity or antiaromaticity of cyclic conjugated molecules is the so-called aromatic stabilization energy (ASE), which can be derived-either experimentally or theoretically-from appropriate homodesmotic reactions. To gain further insight into the origin of aromaticity, several schemes have been devised to partition ASE into nuclear and electronic as well as sigma and pi contributions, some of which have resulted in contradictory statements about the driving force of aromatic stabilization. Currently, these contradictions have not been resolved and have resulted in a confusing distinction between two different types of aromaticity: extrinsic and intrinsic aromaticity. By investigating different homodesmotic reactions we show that, in contrast to ASE itself, the individual contributions that enter the ASE can strongly depend on the type of reaction. Caution is therefore advised if conclusions or physical interpretations are derived from the individual components. The contradictions result from the fact that some reactions suffer from an imbalance in the number of interaction terms at the two sides of the reaction equation. The concept of isointeractional reactions is introduced and results in the elimination of the imbalance. For these reactions, the contradictions disappear and the distinction between intrinsic and extrinsic aromaticity becomes unnecessary. As far as the sigma-pi partitioning is concerned, several schemes proposed in the literature are compared. Contradictory results are obtained depending on the partitioning scheme and reaction used. In this context, it is demonstrated that for the partitioning of the electron-electron interaction, the scheme introduced by Jug and K?ster is the one that is most theoretically grounded.     

Homodesmotic reaction for fullerenes

The ASE (aromatic stabilization energy) for C₆₀ and C₇₀ have been evaluated by a new homodesmotic reaction and the results support the early established fact that C₇₀ is more stable than C₆₀.     

Dications of fluorenylidenes. The relationship between redox potentials and antiaromaticity for meta- and para-substituted diphenylmethylidenefluorenes

[reaction: see text] Electrochemical oxidation of meta-substituted diphenylmethylidenefluorenes (3a-g) results in the formation of fluorenylidene dications that are shown to be antiaromatic through calculation of the nucleus independent chemical shift (NICS) for the 5- and 6-membered rings of the fluorenyl system. There is a strong linear correlation between the redox potential for the dication and both the calculated NICS and sigma(m). Redox potentials for formation of dications of analogously substituted tetraphenylethylenes shows that, with the exception of the p-methyl derivative, the redox potentials for these dications are less positive than for formation of the dications of 3a-g and for dications of p-substituted diphenylmethylidenefluorenes, 2a-g. The greater instability of dications of 2a-g and 3a-g compared to the reference system implies their antiaromaticity, which is supported by the positive NICS values. The redox potentials for formation of the dications of meta-substituted diphenylmethylidenes (3a-g) are more positive than for the formation of dications of para-substituted diphenylmethylidenes (2a-g), indicating their greater thermodynamic instability. The NICS values for dications of 3a-g are more antiaromatic than for dications of 2a-g, which is consistent with their greater instability of the dications of 3a-g. Although the substituted diphenylmethyl systems are not able to interact with the fluorenyl system through resonance because of their geometry, they are able to moderate the antiaromaticity of the fluorenyl cationic system. Two models have been suggested for this interaction, sigma to p donation and the ability of the charge on the substituted ring system to affect delocalization. Examination of bond lengths shows very limited variation, which argues against sigma to p donation in these systems. A strong correlation between NICS and sigma constants suggests that factors that affect the magnitude of the charge on the benzylic (alpha) carbon of the diphenylmethyl cation affect the antiaromaticity of the fluorenyl cation. Calculated atomic charges on carbons 1-8 and 10-13 show an increase in positive charge, and therefore greater delocalization of charge in the fluorenyl system, with increasing electronegativity of the substituent. The change in the amount of positive charge correlated strongly with NICS, supporting the model in which the amount of delocalization of charge is related to the antiaromaticity of the species. Thus, both aromatic and antiaromatic species are characterized by extensive delocalization of electron density.     

Dianion and dication of tetrabenzo[5.7]fulvalene. Greater antiaromaticity than aromaticity in comparable systems

The dianion, 5(2-), and dication, 5(2+), of tetrabenzo[5.7]fulvalene represent an aromaticity/antiaromaticity continuum in which the fluorenyl system changes from aromatic in 5(2-) to antiaromatic in 5(2+). Conversely, the antiaromatic dibenzotropylium system of 5(2-) becomes an aromatic system in 5(2+), allowing an examination of aromaticity/antiaromaticity within the same carbon framework. Dianion 5(2-) was prepared and characterized by (1)H NMR spectroscopy. The fluorenyl system of 5(2-) showed the downfield shifts expected for an aromatic system, while the dibenzotropylium system showed the paratropic shifts expected for an antiaromatic system. The conclusions from (1)H NMR spectroscopy were supported by NICS(1) zz calculations for each system. Comparison of the (1)H NMR spectrum and NICS(1) zz of 5(2-) with those of 5(2+) supported the assignments of aromaticity/antiaromaticity for each system. Aromaticity/antiaromaticity were further examined through comparison of the degree of bond length alternation, which showed that the bond length alternation was slightly greater for the antiaromatic ring systems than for the aromatic systems. However, when structures of 5(2-) and 5(2+) with no bond length alternation were examined, there was a dramatic increase in the degree of antiaromaticity for the antiaromatic ring systems as evaluated through NICS. This result suggests that a decrease in bond length alternation results in an increase in antiaromaticity as well as an increase in aromaticity. The magnitude of the antiaromaticity of the fluorenyl system in 5(2+) was greater than the magnitude of the aromaticity in the fluorenyl system of 5(2-), with similar effects shown by the analogous tropylium systems. This is consistent with the behavior of the antiaromatic dication of tetrabenzo[5.5]fulvalene, compared to that of its aromatic dianion, and also with the behavior of the cyclopentadienyl cation/anion and tropylium cation/anion.     

Resonance revisited: A consideration of the calculation of cyclic conjugation energies

A homomolecular differential bond separation reaction may be defined as the difference between the conventional bond separation reactions involving the unsaturated system and its saturated counterpart. Such a reaction is homomolecular in that the basic molecular structures involved are the same on both sides of the reaction. The type of homodesmotic reaction that also conserves structure in this way may be termed a homomolecular homodesmotic reaction. Both types of homomolecular reactions are readily related to hydrogenation reactions and, more importantly, to each other. Δ B(n), the energy of the homomolecular differential bond separation reaction involving a system with n double bonds, and H(n), the corresponding homomolecular homodesmotic reaction, are related by: where h(1) and h(e) are the hydrogenation energies of the system's monoene and of ethylene, respectively. Both types of reactions yield measures of cyclic conjugation energies that for certain classifications of molecules are simply related to each other. Consideration of extra conjugation in the monoenes allows a ready interpretation of those cases in which a simple classification is not obtained. Ab initio calculations illustrating these effects have been carried out on a variety of molecules including many five- and six-membered ring systems using second order Møller-Plesset and density functional approaches. © 1997 by John Wiley & Sons, Inc.     

The Fluorenyl Cation

The fluorenyl cation is a textbook example for a 4π antiaromatic cation. However, contrasting results have been published on how the annelated benzene rings compensate the destabilizing effect of the 4π antiaromatic five‐membered ring in its core. Whereas previous attempts to synthesize this cation in superacidic media resulted in undefined polymeric material only, we herein report that it can be generated and isolated in amorphous water ice at temperatures below 30 K by photolysis of diazofluorene. Under these conditions, the fluorenylidene is protonated by water to give the fluorenyl cation, which could be characterized spectroscopically. Its absorption in the visible‐light range matches that previously obtained by ultrafast absorption spectroscopy, and furthermore, its IR spectrum could be recorded. The IR bands in amorphous ice very nicely match predictions from DFT and DFT/MM calculations, suggesting the absence of strong interactions between the cation and surrounding water molecules.     

The Fluorenyl Cation

The fluorenyl cation is a textbook example for a 4π antiaromatic cation. However, contrasting results have been published on how the annelated benzene rings compensate the destabilizing effect of the 4π antiaromatic five‐membered ring in its core. Whereas previous attempts to synthesize this cation in superacidic media resulted in undefined polymeric material only, we herein report that it can be generated and isolated in amorphous water ice at temperatures below 30 K by photolysis of diazofluorene. Under these conditions, the fluorenylidene is protonated by water to give the fluorenyl cation, which could be characterized spectroscopically. Its absorption in the visible‐light range matches that previously obtained by ultrafast absorption spectroscopy, and furthermore, its IR spectrum could be recorded. The IR bands in amorphous ice very nicely match predictions from DFT and DFT/MM calculations, suggesting the absence of strong interactions between the cation and surrounding water molecules.     

Stability and aromaticity of the cyclopenta-fused pyrene congeners

The aromaticity of all possible cyclopenta-fused pyrene congeners has been investigated at various levels of theory. On the basis of the calculated resonance energies and magnetic properties (delta(1)H data, magnetic susceptibility anisotropies, and NICS values), the overall aromaticity of these compounds is found to decrease gradually with increasing number of externally fused five-membered rings. The relatively small differences (<5 kcal/mol) in thermodynamic stability of the isomeric dicyclopentapyrenes (E(tot): dicyclopenta[cd,fg]- > dicyclopenta[cd,jk]- > dicyclopenta[cd,mn]pyrene), which differs from the aromaticity order based on the magnetic criteria (dicyclopenta[cd,mn]- > dicyclopenta[cd,fg]- > dicyclopenta[cd,jk]pyrene), is shown by model calculations to be dominated by sigma-strain imposed on the pyrene skeleton by sequential cyclopenta-fusion. This is supported by the computed homodesmotic reaction energies and aromatic stabilization energy (ASE(isom)) from isodesmic aromatic-nonaromatic isomerization, and by the model calculations on "distorted" cyclopenta[cd]pyrenes. The elusive tetracyclopenta[cd,fg,jk,mn]pyrene is computed to be bowl-shaped; its corresponding planar geometry is the transition state for bowl-bowl interconversion.     

A family of fluorenyl dendrons for porphyrin dendrimers synthesis

A series of dendrons bearing various number of fluorenyl donor groups have been synthesized. First, the reference compound 2-(bromomethyl)-9H-fluorene (8) with one fluorenyl unit, then dendron 10, with two fluorenyl arms, and finally new generation dendrons, 11 and 12, bearing four peripheral fluorenyl arms were synthesized and characterized. A series of different generations of porphyrin dendrimers, obtained from these dendrons are also presented. Preliminary results on higher generation dendrimers are reported as well. Under mild basic conditions, surprisingly, a new compound 1 incorporating a fluorenyl unit in the cycle and three pendant fluorenyl arms was obtained by an intramolecular reaction of brominated tetrapod dendron 12.     

Photochemical pinacol rearrangements of unsymmetrical diols

The photochemical pinacol reaction of a series of nonsymmetrical 9-fluorenyl-substituted vic-diols was investigated and compared with their acid-catalyzed thermal reaction. Unlike the thermal reaction, the radiation-induced processes involve only fluorenyl cations, as is reflected in differences of product distribution between the two reactions. From the product studies, substituent migratory aptitudes are reversed in the photochemical process, suggesting that kinetic control takes place under neutral conditions unlike the acid-catalyzed thermal reactions. The presence of fluorenyl cation intermediates and their lifetimes were established by laser flash spectroscopy studies.     

The reaction of protonated dimethyl ether with dimethyl ether: temperature and isotope effects on the methyl cation transfer reaction forming trimethyloxonium cation and methanol.

Fourier transform ion cyclotron resonance mass spectrometry has been used to study the temperature and deuterium isotope effects on the methyl cation transfer reaction between protonated dimethyl ether and dimethyl ether to produce trimethyloxonium cation and methanol. From the temperature dependence of this bimolecular reaction it was possible to obtain thermodynamic information concerning the energy barrier for methyl cation transfer for the first time. From the slope of an Arrhenius plot, a value for DeltaH(++) of -1.1 +/- 1.2 kJ mol(-1) was obtained, while from the intercept a value for DeltaS(++) of -116 +/- 15 J K(-1) mol(-1) was derived. This yields a DeltaG(++)(298) value of 33.7 +/- 2.1 kJ mol(-1). All thermodynamic values were in good agreement with ab initio calculations. Rate constant ratios for the unimolecular dissociation forming trimethyloxonium cation and the dissociation re-forming reactants were extracted from the apparent bimolecular rate constant. Attempts at modeling the temperature dependence and isotope effects of the unimolecular dissociation forming trimethyloxonium cation were also made.     

Silaaromaticity in polycyclic systems: a computational study

Density functional theory (B3LYP) calculations were performed to examine the effect of Si substitution on the aromaticity of some polycyclic hydrocarbons using geometric criterion (HOMA), isodesmic isomerization reactions, homodesmotic equations, NICS values, chemical hardness, and out-of-plane distortive tendencies. The HOMA values are lower and the NICS values are higher in the Si-substituted rings compared to those in the hydrocarbon counterpart, whereas the homodesmotic equations predict little loss of aromaticity upon Si replacement in polycylic systems. The chemical hardness values decrease and the out-of-plane distortive tendency increases upon silicon substitution. The relative energies of the positional isomers and the causative factors are analyzed. The high reactivity of some silaaromatics toward dimerization is explained based on local softness indices.     

Solvent-dependent C-OH homolysis and heterolysis in electronically excited 9-fluorenol: the life and solvation time of the 9-fluorenyl cation in water

The primary pathways of the photodecomposition of 9-fluorenol (FOH) were studied in polar and nonpolar solvents by use of laser flash-photolysis with a resolution time of 10 ps. In solvents of high polarity, that is, in 1,1.1,3,3,3-hexafluoroisopropanol (HFIP), 2,2,2-trifluoroethanol (TFE), formamide or water, the fluorenyl cation, F+, forms by heterolytic C-O bond cleavage. In H2O, the initial (10 ps) spectrum of F+ has lambdamax at <460 nm. This absorption red-shifts with T = 25 ps to the "classical" spectrum with lambdamax = 510-515 nm. This process is assigned to the solvation of the initial "naked" cation, or rather, the contact ion pair. The lifetime of the solvated fluorenyl cation in H2O (or D2O) and TFE was measured to be tau 20 ps and 1 ns, respectively. In solvents of lower polarity such as alkanes, ethers and alcohols, the long-lived (tau 1/2 1 micros) fluorenyl radical, F., (lambdamax = 500 nm) forms through homolytic C-O cleavage. In addition to the radical and the cation, the vibrationally relaxed excited singlet state of FOH is seen with its absorption at approximately 640 nm; its lifetime is strongly dependent on the solvent, from 10 ps for formamide to 1.7 ns for cyclohexane. The rate constant for singlet decay increases exponentially with the polarity of the solvent (as expressed by the Dimroth-Reichardt ET value) or with the Gutmann solvent acceptor number. The relaxation of S1 to S0 is accompanied by homolytic C9-O bond cleavage (except in HFIP, TFE, and water, where S1 is not seen).     

Isolation of an Antiaromatic 9-Hydroxy Fluorenyl Cation

Fluorenyl cations are textbook examples of 4π electron antiaromatic five-membered ring systems. So far, they were reported only as short-lived intermediates generated under superacidic conditions or by flash photolysis. Attempts to prepare a m-terphenyl acylium cation by fluoride abstraction from a benzoyl fluoride gave rise to an isolable 9-hydroxy fluorenyl cation that formed by an intramolecular electrophilic attack at a flanking mesityl group prior to a 1,2-methyl shift and proton transfer to oxygen.     

Ab initio and density functional theory studies of the structure,gas-phase acidity and aromaticity of tetraselenosquaric acid

Results of ab initio self-consistent-field (SCF) and density functional theory (DFT) calculations of the gas-phase structure,acidity (free energy of deprotonation,G0) and aro-maticity of tetraselenosquaric acid (3,4-diselenyl-3-cy-dobutene-1,2-diselenone,H2C4Se4) are reported.The global minimum found on the potential energy surface of tetraselenosquaric acid presents a planar conformation.The ZZ iso-mer was found to have the lowest energy among the three planar conformers and the ZZ and ZE isomers are very dose in energy.The optimized geometric parameters exhibit a bond length equalization relative to reference compounds,cyclobu-tanediselenone,and cydobutenediselenol.The computed aromatic stabilization energy (ASE) by homodesmotic reaction is -77.4 (MP2(fu)/6 - 311 G //RHF/6 - 311 G) and - 54.8 kJ/mol (B3LYP/6 - 311 G //B3LYP/6 -311 G).The aromaticity of tetraselenosquaric add is indicated by the calculated diamagnetic susceptibility exaltation (A) - 19.13 (CSGT(IGAEM) - RHF/6 - 311 G// RHF/6-