Antiaromatic Molecules Research Articles

Benzocyclobutadienyl anion: formation and energetics of an antiaromatic molecule

Benzocyclobutadienyl diazirine (2) was synthesized and reacted with hydroxide ion in a Fourier transform mass spectrometer to afford the conjugate base of benzocyclobutadiene (1a). Authentication of the ion structure was carried out by a derivatization experiment (i.e., la was converted to benzocyclobutenone enolate, which has previously been studied), and its reactivity was explored. Thermochemical data for benzocyclobutadiene (1) were obtained (deltaH(o)acid (1) = 386 +/- 3 kcal mol(-1), EA(1r) = 1.8 +/- 0.1 eV, and C-H BDE (1) = 114 +/- 4 kcal mol(-1)), compared to MP2 and B3LYP calculations, and contrasted to a series of model compounds. Cyclobutadienyl radical appears to be quite different from benzocyclobutadienyl radical (1r) and worth further exploration.

Raison d'être of apparently antiaromatic 1,2-dithiin derivatives in nature

Although living organisms tend to avoid synthesizing antiaromatic molecules because of their thermodynamic and kinetic instability, some composite plants synthesize apparently antiaromatic 1,2-dithiin derivatives. Molecular orbital calculations at the HF/6-31G** level revealed that the antiaromaticity of the 1,2-dithiin ring is markedly reduced by assuming a non-planar molecular structure. It was found that both 1,2- and 1,4-dithiin rings are essentially nonaromatic with the Dewar resonance energies being close to zero. This must be the primary reason why 1,2-dithiins occur in nature.

Theoretical studies of novel aromatic molecules and transition states

Abstract

Derivatives of Molecular Valence as a Measure of Aromaticity

Derivatives of the molecular valence have been calculated ab initio within the new non-finite-difference approach elaborated on earlier for the global hardness and the Fukui function indices. The group of 10 five-membered-ring molecules C4H4X has been chosen for a test, using the exaltation of the total magnetic susceptibility (Λ) as a reference measure of their aromaticity. An excellent correlation has been found between the molecular valence derivatives in the nucleophilic regime and the exaltation Λ, for both the aromatic and antiaromatic molecules. Calculation of the valence derivatives provides an absolute measure of aromatic character that is not directly dependent on the size of the molecule and does not require adopting any standard reference molecule.

Biomimetic Reactions Catalyzed by Cyclodextrins and Their Derivatives.

Cyclodextrins are extremely attractive components of artificial enzymes and other biomimetic materials. They are readily available, they bind hydrophobic substrates into their cavities in water solution, and they have two rims of hydroxyl groups (Figure 1) that can either react with substrates themselves or be used to attach other catalytic and functional groups. Of course, they have disadvantages. For one, unless they are extensively modified their complexes with substrates can be rather flexible and, perhaps, with unpredictable preferred geometry. They are also unstable to strong acid. Thus for some purposes such synthetic cavity species as calixerenes1 or synthetic macrocycles2-4 may have advantages. However, one of the chief advantages of cyclodextrins is highly attractivesthey are readily available, so it is possible to avoid the synthesis of a binding group and go directly to studies of what can be achieved with their use. Afterward, the lessons learned may be applied to other systems with advantage. This review will cover all the literature on reactions in which cyclodextrins bind substrates and then either catalyze their reactions or mimic a step in an enzymatic catalytic sequence. However, it will not describe work in which cyclodextrins simply change the course of a reaction without playing an obvious catalytic role involving substrate binding. For example, there are systems in which the main function of the cyclodextrin seems to be to complex a metal ion and keep it in solution.5-11 There are other studies in which binding into a cyclodextrin simply alters the selectivity of attack by an external reagent in some way12-24 or causes solubilization to facilitate phase transfer catalysis.12,25,26 Presumably such other areas are described elsewhere in this volume. While much work on artificial enzymes using cyclodextrins has been done in the author’s laboratory, and will be described, every effort is made to describe all the relevant work in the field. Several reviews of this subject already exist and should be consulted for further information.2,27-70 The readily Ronald Breslow, born in 1931 in Rahway, NJ, completed his B.A. in chemistry in1952, his M.A. in medical science in 1953, and his Ph.D. in chemistry in 1955 with R. B. Woodward, all at Harvard University. After a postdoctoral year with Alexander Todd in Cambridge, he came to Columbia University where he is now University Professor and Professor of Chemistry. His work on enzyme models, on novel conjugated aromatic and antiaromatic molecules, on electrochemical and hydrophobic methods in mechanistic chemistry, and on anticancer cytodifferentiating agents has been recognized by a number of awards, including the U.S. National Medal of Science. In 1996, he served as President of the American Chemical Society.

Structure and Bonding of Ordered Organic Monolayers of 1,3,5,7-Cyclooctatetraene on the Si(001) Surface: Surface Cycloaddition Chemistry of an Antiaromatic Molecule

Cycloaddition reactions have been utilized widely in organic chemistry and are now emerging as a method for attachment of organic molecules to silicon(001) surfaces in a controlled way. Scanning tunneling microscopy (STM), Fourier transform infrared spectroscopy (FTIR), and ab initio molecular orbital calculations have been used to investigate the structure and bonding of 1,3,5,7-cyclooctatetraene (COT), an antiaromatic molecule, on the Si(001)-2 × 1 surface. STM images show that the COT molecules spontaneously order on Si(001) at room temperature. The individual molecules are separated by 7.7 Å, equal to twice the separation between SiSi dimers on the underlying reconstructed Si(001)-2 × 1 surface. FTIR measurements in conjunction with STM experiments show that the COT molecules bond to Si(001) via the direct interaction of two CC units with two adjacent SiSi dimers within a single dimer row. Ab initio molecular orbital calculations show that the lowest-energy geometry bonding of two CC groups with two SiSi dimers is a di-σ configuration via a double [2 + 2] cycloaddition reaction. The result of this bonding is a monolayer organic film that terminates in an ordered array of alkene groups exposed to the vacuum.

Isodesmic and homodesmotic stabilization energies of [ n]annulenes and their relevance to aromaticity and antiaromaticity: is absolute antiaromaticity possible?

Isodesmic stabilization energies (ISEs) for conjugated molecules, which are determined by a balance of the stabilizing π-electron non-cyclic delocalization and either stabilizing aromatic or destabilizing antiaromatic π-electron cyclic delocalization, do not yield a reasonable estimation of aromaticity or antiaromaticity. Homodesmotic stabilization energies (HSEs) should be used for this purpose. The ISEs for [ n]annulenes are larger by the ( n 2 ) ISE( s-trans-1,3-butadiene) value than the corresponding HSEs. The experimental value of the ISE( s-trans-1,3-butadiene) is 59.4 ± 4.1 kJ mol −1 at 298 K. While cyclooctatetraene has only the relative antiaromatic character, cyclobutadiene features both relative and absolute antiaromaticity. Since the HSE and ISE of cyclobutadiene, corrected for its strain energy, are still negative (−174.0 and −55.2 kJ mol −1, respectively, at the MP4SDTQ/6-31G (d,p)//MP2/6-31G(d,p) + ZPE(HF/6-31G(d)) level), π-antiaromaticity rather than σ-strain is at the origin of the remarkable instability of cyclobutadiene. At the MP4SDTQ/6-31G(d,p)//MP2/6-31G(d,p) + ZPE(HF/6-31G(d)) level, the antiaromatic destabilization of cyclobutadiene corrected for the ring-strain and normalized per π-electron is −43.5 kJ mol −1, a value that is larger (in absolute value) than the antiaromatic destabilization of planar D 4 h cyclooctatetraene (−14.9 kJ mol −1 per π-electron) and the aromatic stabilization of benzene (17.7 kJ mol −1 per π-electron). In contrast to cyclobutadiene, planar D 4 h cyclooctatetraene has a positive ISE (118.8 kJ mol −1) indicating that the stabilizing π-electron non-cyclic delocalization overbalances the destabilizing antiaromatic π-electron cyclic delocalization. Nevertheless, planar cyclooctatetraene is assigned to the class of antiaromatic molecules since its HSE is −118.8 kJ mol −1 at the MP4SDTQ/6-31G(d,p)//MP2/6-31G(d,p) + ZPE(HF/6-31G(d)) level.

Complete account of the π-electron correlation in calculating ring currents in conjugated aromatic and antiaromatic systems

A semiempirical theory of ring π currents is developed, which accurately takes into account the electron correlation using the full configuration interaction (FCI) method. An elementary model is proposed, in which the calculation of the π-electron correlation is reduced to the model of generalized alternative Huckel systems. The FCI and SCF (self-consistent field) methods are used to calculate the diamagnetic susceptibilities and ring currents for some typical π systems. It is shown that, as a rule, the SCF method allows the qualitatively true description of the magnetic properties of condensed systems, although for antiaromatic molecules it often yields underestimated values of paramagnetic currents. The results of the FCI calculations suggest that electronic excitations can induce transitions from π paramagnetism to π diamagnetism (e.g., in fulvenes) and back (in quinones).

A theoretical study on the molecular structure of biphenylene in its first excited singlet and triplet states: quantum chemical calculations on the structural changes of an antiaromatic molecule upon excitation

The structural changes accompanying the electronic excitation of the antiaromatic biphenylene molecule were studied by semiempirical quantum chemical calculations, using the AM1 hamiltonian with configuration interaction (S 1, S 2, T 1, T 2, X̃ + states). The largest bond length differences of the molecular states investigated were observed within the central four-membered ring. Potential curves of the CC bonds which connect the two benzene rings have been calculated for all molecular states investigated. For the S 0 and S 1 states, potential hypersurfaces, using the two different bond lengths of the central four-membered “cyclobutadiene” subsystem as variables, were presented. Furthermore, the geometry of 1,3-cyclobutadiene in different states (S 0, S 1, T 1, X̃ + states) was investigated with the AM1-CI hamiltonian and compared with recent 6-31G ∗ MC-SCF ab initio results.

Phenanthrene dianion is not planar

Physical organic chemistry has produced a lot of arguments in favour of the flexibility of the skeleton of the phenanthrene dianion, 1 2-. Circumstantial evidence for the non-planarity of dianions derived from phenanthrene substituted at the 'bay' position has been published.'S2 It has also been demonstrated that 1 2is a highly paratropic species and is hence regarded as a so-called antiaromatic molecule.3i4 It was suggested that in order to minimize paratropicity the molecular skeleton may undergo a deformation so that the efficiency of the 71conjugation is reduced and antiaromaticity is less pronounced. At the same time semiempirical calculations predict that 12is planar, and only a sophisticated treatment including the counter-ion enabled the non-planarity of 12-/2 Li' to be achieved computationally.2*6 We find that ab initio calculations in minimal basis set (STO3G) as well as semiempirical methods (MIND0/3, MNDO, AMl) describe the ground state of 12as a planar structure. However, when the basis set for ab initio calculations was extended to STO 3-2 1 G and further to STO 4-3 1 G, the planar structure of 12proved to be not a minimum, but a transition state (vim = 5 3 cm-' on HF/3-21G level) between two nonplanar minima with a dihedral angle C(4)<(4a)-C(5a)-C(5) =

The ground state of antiaromatic 1,3,5,7-tetra-tert-butyl-s-indacene

The electronic and vibrational properties of the antiaromatic molecule 1,3,5,7-tetra-tert-butyl-s-indacene (TTBI) in the ground state are investigated in this paper. MO ab initio calculations have been performed on TTBI and the parent molecule, s-indacene (I). The results show that the C-C bonds of s-indacene alternate in length and the corresponding C 2h structure is more stable than the structure without bond alternation (D 2h symmetry), regardless of the basis set used. Bond length alternation is decreased by alkyl substitution in the 1, 3, 5, and 7 positions

Neutralization-reionization mass spectrometry

The use of neutralization-reionization mass spectrometry for polyatomic species has grown rapidly in the last 3 years. Mass-selected positive or negative ions are charge exchanged to form fast neutrals of chosen internal energies. Energy can be added to the neutrals by collision or photoexcitation, the neutral products reionized to positive or negative ions, and mass-selected product ions structurally characterized by further collisional dissociation and mass analysis. The method has found broad applicability for ion structure characterization and analysis; the lower isomerization tendency of neutrals, vs. that of the corresponding ions, has been particularly useful for hydrocarbon ion characterization. Of probably the broadest interest is the method's ability to prepare unusual, unstable neutral species from stable ions and to characterize the unimolecular chemistry of these neutrals. Detailed energy surfaces have been reported for carbenes, ylides, diradicals, clusters, organometallics, and hypervalent, sterically strained, and anti-aromatic molecules.

Inadequacy of one-electron approach for investigating antiaromatic molecules and their isomerization reactions

Inadequacy of one-electron approach for investigating antiaromatic molecules and their isomerization reactions

Two-photon spectroscopy of antiaromatic molecules: the case of biphenylene

The two-photon spectrum and polarization ratio of biphenylene have been measured at room temperature in the spectral range 225–370 nm, combining fluorescence-excitation and thermal-lensing techniques. The first excited Ag state 2Ag, has been observed around 265 nm and the second excited B1g, 2B1g, has its onset at ≈ 225 nm. Similarities and differences with alternant aromatics are pointed out. MO calculations of two-photon intensities support the interpretation of the experimental data.

Chemical Evolution, Biosynthesis, and Aromaticity

Abstract Naturally occurring heterocycles such as imidazole and thiophene do not conform to the rule of topological charge stabilization. Abiotic synthesis and biosynthesis of such heteroconjugated molecules can be rationalized in terms of aromatic stabilization. Antiaromatic molecules are formed neither in primitive-Earth-simulation experiments nor in living systems.

Radiationless processes in antiaromatic molecules: The photophysics of s-indacene studied by ultrashort light pulses

1,3,5,7-tetra-tert-butyl-indacene (TTBI), a non-alternant antiaromatic hydrocarbon, is investigated in solution by steady-state and time-resolved spectroscopy. A weak symmetry-forbidden S 0→S 1 absorption is observed in the wavelength range from 600 to 1100 nm. Intense S 0→S 2 and S 0→S n bands occur around 500 nm and in the ultraviolet. The S 2 level shows fluorescence with a quantum yield of 9×10 −5. Internal conversion directly to the ground state represents the main radiationless deactivation process of the S 1 and S 2 states resulting in short lifetimes of 18 and 2.5 ps, respectively. The radiationless S 2→S 0 decay channel is more effective than S 2→S 1 internal conversion as supported by CNDO/SDCI calculations of the molecular geometry in the S 0, S 1 and S 2 states.

4n PI Electron Triplet Species and Their Potential Use in Organic Ferromagnets

Abstract Antiaromatic cyclic conjugated molecules with 4n pi electrons and three-fold symmetry can have triplet ground states, but they may also distort to lower the singlet state below the triplet. Such distortion can be spontaneous, so the species are never true triplet ground state species, or distortion may be induced by a disymmetric environment. For instance, in crystalline solids an otherwise stable triplet species may adopt a singlet ground state because the environment within the crystal does not have the required three-fold symmetry. There are several theories suggesting that ferromagnetic organic materials might result if a ground state triplet species is one component of the material. We have been able to prepare a number of stable triplet species relevant to this area. The requirements for a triplet ground state, and for ferromagnetic interactions, are discussed.

9-Aminofluorenium Cations: A New Group of anti-Aromatic Molecules

Quantum-chemical and spectral studies on the protonated form of fluorenone-anil show that this compound has anti-aromatic character. Its real structure should be considered as: 9-amino derivative of the anti-aromatic fluorenium cation.

Structure and valence isomerization of antiaromatic molecules XI. Theoretical investigation of the valence isomerizations of cyclobutadiyne

Structure and valence isomerization of antiaromatic molecules XI. Theoretical investigation of the valence isomerizations of cyclobutadiyne

Structure and valence isomerizations of antiaromatic molecules. XIII. Relative stability of cyclobutadiene and its aza-substituted derivatives

Structure and valence isomerizations of antiaromatic molecules. XIII. Relative stability of cyclobutadiene and its aza-substituted derivatives