Accurate calculation of aromaticity of benzene and antiaromaticity of cyclobutadiene: new homodesmotic reactions

Newly designed homodesmotic reactions based on radical systems predict an absolute aromaticity of 29.13 kcal/mol for benzene and an absolute antiaromaticity of 40.28 kcal/mol for cyclobutadiene at the MP4(SDQ)/6-31G(d,p) level.     

The antiaromaticity of cyclobutadiene

The antiaromaticity of cyclobutadiene systems is explored by means of theoretical studies on quinone electrochemical oxidation reactions.     

Multiple aromaticity and antiaromaticity in silicon clusters.

A series of silicon clusters containing four atoms but with different charge states (Si4(2+), Si4, Si4(2-), and NaSi4-) were studied by photoelectron spectroscopy and ab initio calculations. Structure evolution and chemical bonding in this series were interpreted in terms of aromaticity and antiaromaticity, which allowed the prediction of how structures of the four-atom silicon clusters change upon addition or removal of two electrons. It is shown that Si4(2+) is square-planar, analogous to the recently discovered aromatic Al4(2-) cluster. Upon of two electrons, neutral Si4 becomes sigma-antiaromatic and exhibits a rhombus distortion. Adding two more electrons to Si4 leads to two energetically close structures of Si4(2-): either a double antiaromatic parallelogram structure or an aromatic system with a butterfly distortion. Because of the electronic instability of doubly charged Si4(2-), a stabilizing cation (Na+) was used to produce Si4(2-) in the gas phase in the form of Na+[Si4(2-)], which was characterized experimentally by photoelectron spectroscopy. Multiple antiaromaticity in the parallelogram Na+[Si4(2-)] species is highly unusual.     

Ground- and excited-state reactivity of iron porphyrinogens

The iron(II) porphyrinogen dication, [LDeltaDeltaFeII]2+, is a multielectron oxidant featuring the metal center in its reduced state and the ligand as the redox reservoir. Oxidations break the ligand's redox-active C-C bonds. Extremely short-lived excited states are consistent with extensive structural reorganization that accompanies charge-transfer excitation of the porphyrinogen.     

Ground- and excited-state intramolecular proton transfer in 3,5-dibromosalicylic acid

The existence of both ground- and excited-state proton transfer equilibrium between the enolic and keto tautomers of 3,5-dibromosalicylic acid has been investigated in aqueous solution using electronic absorption and luminescence emission spectroscopies, together with ab initio and semiempirical MO/CI calculations. The compound provides an example of a room temperature phosphorescence triplet-state emission from a phototautomer (ketonic) form. The interference from radiationless paths, which may in principle avoid the observation of the phosphorescence signal, was overcome by using a protective micellar medium and the presence of an external heavy atom.     

Theoretical notions of aromaticity and antiaromaticity: phenalenyl ions versus fluorenyl ions

The dications 6, 7, and 8 and dianions 9, 10, and 11 of the bistricyclic aromatic enes bifluorenylidene (1), 1,1'-biphenalenylidene (2), and 9-(9H-fluoren-9-ylidene)-1H-phenalene (4), as well as monocations 12a and 13a and monoanions 14a and 15a of phenalene (3) and fluorene (5), were subjected to a systematic DFT and ab initio study. B3LYP and MP2 methods were employed to estimate the relative aromaticity/antiaromaticity of these ions, using energetic, magnetic, and structural criteria. The couplings of monoions 12a-15a to give the respective diions 6-11 result in a similar destabilization in both the fluorene and phenalene series. The interactions between the C13H8 units in diions 6-11 are weak and are not expected to result in a significant loss of aromaticity/gain of antiaromaticity, as compared with the respective monoions. The antiaromaticity of bifluorenylidene dication (6), relative to that of two fluorenyl cations (12a), is only slightly enhanced as compared with the aromaticity of biphenalenylidene dication ((E)-7)) relative to that of two phenalenyl cations (13a). In particular, the homodesmotic reaction 6 + 2.13a = (E)-7 + 2.12a is only slightly exothermic, DeltaE(Tot) = -6.0 kJ/mol. The energetic effect of the analogous reaction involving anions 9 + 2.15a = (E)-10 + 2.14a is even smaller, DeltaE(Tot) = -3.4 kJ/mol. Bifluorenylidene dianion (9) and 1,1'-biphenalenylidene dianion ((E)-10) are aromatic, but the employed criteria disagree about their relative aromaticity. The electronic and structural properties of heteromerous dication 8 and dianion 11 lie between those of the homomerous diions. Thus, dications 6-8 and dianions 9-11 form a continuum of aromaticity/antiaromaticity.     

Ground- and excited-state interaction between aqua-uranyl(VI) and nitrate

Photoexcited (UO₂NO₃⁺)_aq is shown to have different structure and thermodynamic formation functions from ground-state (UO₂NO₃⁺)_aq and different photophysical and photochemical parameters from UO²⁺_2(aq). Results are discussed with relation to the electron-transfer nature of the lowest excited state of UO²⁺₂.     

Ground- and excited-state properties of inorganic solids from full-potential density-functional calculations

The development in theoretical condensed-matter science based on density-functional theory (DFT) has reached a level where it is possible, from “parameter-free” quantum mechanical calculations to obtain total energies, forces, vibrational frequencies, magnetic moments, mechanical and optical properties and so forth. The calculation of such properties are important in the analyses of experimental data and they can be predicted with a precision that is sufficient for comparison with experiments. It is almost impossible to do justice to all developments achieved by DFT because of its rapid growth. Hence, it has here been focused on a few advances, primarily from our laboratory. Unusual bonding behaviors in complex materials are conveniently explored using the combination of charge density, charge transfer, and electron-localization function along with crystal-orbital Hamilton-population analyses. It is indicated that the elastic properties of materials can reliably be predicted from DFT calculations if one takes into account the structural relaxations along with gradient corrections in the calculations. Experimental techniques have their limitations in studies of the structural stability and pressure-induced structural transitions in hydride materials whereas the present theoretical approach can be applied to reliably predict properties under extreme pressures. From the spin-polarized, relativistic full-potential calculations one can study novel materials such as ruthenates, quasi-one-dimensional oxides, and spin-, charge-, and orbital-ordering in magnetic perovskite-like oxides. The importance of orbital-polarization correction to the DFT to predict the magnetic anisotropy in transition-metal compounds and magnetic moments in lanthanides and actinides are emphasized. Apart from the full-potential treatment, proper magnetic ordering as well as structural distortions have to be taken into account to predict correctly the insulating behavior of transition-metal oxides. The computational variants LDA and GGA fail to predict insulating behavior of Mott insulators whereas electronic structures can be described correctly when correlation effects are taken into account through LDA+U or similar approaches to explain their electronic structures correctly. Excited-state properties such as linear optical properties, magneto-optical properties, XANES, XPS, UPS, BIS, and Raman spectra can be obtained from accurate DFT calculations.     

Ground- and excited-state electronic structure of an iron-containing molecular spin photoswitch

The electronic structure of the cation of [Fe(ptz)(6)](BF(4))(2), a prototype of a class of complexes that display light-induced excited-state spin trapping (LIESST), has been investigated by time-independent and time-dependent density-functional theories. The density of states of the singlet ground state reveals that the highest occupied orbitals are metal centered and give rise to a low spin configuration Fe(2+)(3d(xy) ( upward arrow downward arrow)3d(xz) ( upward arrow downward arrow)3d(yz) ( upward arrow downward arrow)) in agreement with experiment. Upon excitation with light in the 2.3-3.3 eV range, metal-centered spin-allowed but parity-forbidden ligand field (LF) antibonding states are populated which, in conjunction with electron-phonon coupling, explain the experimental absorption intensities. The computed excitation energies are in excellent agreement with experiment. Contrary to simpler models we show that the LF absorption bands, which are important for LIESST, do not originate in transitions from the ground to a single excited state but from transitions to manifolds of nearly degenerate excited singlets. Consistent with crystallography, population of the LF states promotes a drastic dilation of the ligand cage surrounding the iron.     

Ground- and excited-state prototropic equilibria of some hydroxylated benzo(a)pyrenes

Fluorescence excitation and emission spectroscopy are used to determine the ground and excited-state acid dissociation constants for 3-OH- and 9-OH-benzo(a)pyrene as well as the 4,5-, 7,8-, and 9,10-dihydrodiol derivatives. These constants and spectral shifts arising from substituent placement are used to show that in the vicinity of the hydroxyl substituents there is a greater charge localization for 9-OH-benzo(a)pyrene than for 3-OH-benzo(a)pyrene in both ground and lowest excited singlet states, and that for 9,10-dihydrodiol there is greater charge localization than for 4,5- and 7,8-dihydrodiol in the ground state. These results are briefly correlated with the carcinogenic potential of benzo(a)pyrenes.     

An energetic measure of aromaticity and antiaromaticity based on the Pauling-Wheland resonance energies

Various criteria based on geometric, energetic, magnetic, and electronic properties are employed to delineate aromatic and antiaromatic systems. The recently proposed block-localized wave function (BLW) method evaluates the original Pauling-Wheland adiabatic resonance energy (ARE), defined as the energy difference between the real conjugated system and the corresponding virtual most stable resonance structure. The BLW-derived ARE of benzene is 57.5 kcal mol(-1) with the 6-311+G** basis set. Kistiakowsky's historical experimental evaluation of the stabilization energy of benzene (36 kcal mol(-1)), based on heats of hydrogenation, seriously underestimates this quantity due to the neglect of the partially counterbalancing hyperconjugative stabilization of cyclohexene, employed as the reference olefin (three times) in Kistiakowsky's evaluation. Based instead on the bond-separation-energy reaction involving ethene, which has no hyperconjugation, as well as methane and ethane, the experimental resonance energy of benzene is found to be 65.0 kcal mol(-1). We derived the "extra cyclic resonance energy" (ECRE) to characterize and measure the extra stabilization (aromaticity) of conjugated rings. ECRE is the difference between the AREs of a fully cyclically conjugated compound and an appropriate model with corresponding, but interrupted (acyclic) conjugation. Based on 1,3,5-hexatriene, which also has three double bonds, the ECRE of benzene is 36.7 kcal mol(-1), whereas based on 1,3,5,7-octatetraene, which has three diene conjugations, the ECRE of benzene is 25.7 kcal mol(-1). Computations on a series of aromatic, nonaromatic, and antiaromatic five-membered rings validate the BLW-computed resonance energies (ARE). ECRE data on the five-membered rings (derived from comparisons with acyclic models) correlate well with nucleus-independent chemical shift (NICS) and other quantitative aromaticity criteria. The ARE of cyclobutadiene is almost the same as butadiene but is 10.5 kcal mol(-1) less than 1,3,5-hexatriene, which also has two diene conjugations. The instability and high reactivity of cyclobutadiene thus mainly result from the sigma-frame strain and the pi-pi Pauli repulsion.     

Ground- and excited-state proton transfer reaction of 3-hydroxyflavone in dimyristoylphosphatidylcholine liposome membrane

3-Hydroxyflavone (3HF), a molecule that exhibits excited-state intramolecular proton transfer, has been studied for its fluorescence characteristics in dimyristoylphosphatidylcholine (DMPC) liposome membrane. 3HF partitions to the lipid bilayer membrane with a reasonably large partition coefficient. On excitation at 417 nm, a weak emission from the ground-state anion species was observed at 483 nm, whereas excitation at absorption maxima (345 nm) gives the usual intense fluorescence of the phototautomeric emission at 530 nm. In this article, we report the observation of a ground-state proton transfer reaction of 3HF in DMPC liposome membrane.     

The aromaticity/antiaromaticity continuum. 1. Comparison of the aromaticity of the dianion and the antiaromaticity of the dication of tetrabenzo[5.5]fulvalene via magnetic measures

The aromaticity of the dianion (2) and the antiaromaticity of the dication (3) of tetrabenzo[5.5]fulvalene have been evaluated through magnetic criteria, (1)H NMR shifts, nucleus-independent chemical shifts, NICS, and magnetic susceptibility exaltation, Lambda. The sum of the NICS values, using the GIAO (gauge-independent atomic orbital) method, for 2 is -35.2; that of 3 is +38.2, indicating the aromaticity of 2 and the antiaromaticity of 3. Calculation of magnetic susceptibility exaltation using the CSGT (continuous set of gauge transformations) method gives a similar result, with Lambda of -81.8 ppm cgs for 2 and 95.8 ppm cgs for 3. The general validity of these values is supported by excellent agreement between the NMR shifts calculated by the GIAO and CSGT methods with experimental shifts. Comparison of 1H NMR shifts with those of model compounds allows evaluation of the magnitude of the diatropic shift in 2 and paratropic shift in 3 and supports their assignment as aromatic/antiaromatic, respectively. The agreement between calculated and experimental 1H NMR shifts is excellent for 3 in the absence of counterions but much better for 2 when counterions are included. Inclusion of counterions in the evaluation of diatropic shift for 2 gave a smaller shift than in the absence of counterions, suggesting a decreased aromaticity. When counterions were included in the calculation of Lambda, the value was also decreased, suggesting a decreased aromaticity. This observation has important consequences in the use of experimental data for the evaluation of aromaticity, and presumably antiaromaticity, of anions since, in most cases, there will be close interaction with counterions.     

Ground- and excited-state dynamic control of an anion receptor by hydrostatic pressure

Stimulus-responsive supramolecular architectures have become an attractive alternative to conventional ones for many applications in sensing, drug-delivery and switchable memory systems. Herein, we used an anion receptor (H: host) as a hydrostatic-pressure-manipulatable fluorescence foldamer and halide anions as chiral (binaphthylammonium) and achiral (tetrabutylammonium) ion pairs (SS or RR·X and TBA·X; X = Cl, Br), and then investigated their (chir)optical properties and molecular recognition behavior under hydrostatic pressures. The conformational changes and optical properties of H in various organic solvents were revealed by UV/vis absorption and fluorescence spectra and fluorescence lifetimes upon hydrostatic pressurization. The anion-recognition abilities of H upon interactions with SS or RR·X and TBA·X at different pressure ranges were determined by hydrostatic-pressure spectroscopy to quantitatively afford the binding constant (K_anion) and apparent reaction volume changes . The results obtained indicate that hydrostatic pressure as well as solvation plays significant roles in the dynamic control of the present supramolecular system in the ground and excited states. This work will provide a new guideline for further developing hydrostatic-pressure-responsive foldamers and supramolecular materials.

Hydrostatic pressure can control interactions of chiral countercations with helical receptors containing anions, causing remarkable chiroptical changes.     

Nucleus-independent chemical shifts (NICS): distance dependence and revised criteria for aromaticity and antiaromaticity

Nucleus-independent chemical shifts (NICS) have been used extensively for the identification of aromaticity properties of molecules, ions, intermediates, and transition states since their introduction in 1996 by Schleyer et al. Initially, probes (bq's) were placed at the centers of systems (NICS(0)) and later, 1A above the molecular planes (NICS(1)). However, contradicting assignments of aromaticity by NICS and other methods were found for some systems. In this article, an alternative NICS-based method is introduced. The method is based on scanning NICS values over a distance and separating them into in-plane and out-of plane contributions. The shapes of the plots of the chemical shifts and their components as a function of the distance of the NICS probe (bq) from the molecular plane give a clear indication of diamagnetic and paramagnetic ring currents. This method is applied to several (4n + 2)- and 4n pi-electron systems (molecules and ions) in the singlet and triplet electronic states, including some of the problematic systems mentioned above. It is also shown that relative aromaticities of rings in polycyclic systems (local aromaticities) cannot be estimated by comparing NICS or NICS-scan values.     

Stabilization of polyheteroatomic aza and phospha analogs of cyclobutadiene and benzene

A method is proposed for the stabilization of polyheteroatomic rings consisting only of nitrogen or phosphorus atoms by introduction of acceptors of the unshared electron pair of the heteroatom, particularly oxygen atoms, with the formation of oxide forms. The positions of introduction of the acceptor groups into four- and six-membered rings were determined. The conclusions drawn were confirmed by calculations by the MNDO method of tetrazete 1,3-dioxide and 1,3-ylid and hexaphosphorine 1,3,5-trioxide and 1,3,5-ylid.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 250–255, February, 1988.     

Ground- and excited-state electronic structure of an emissive pyrazine-bridged ruthenium(II) dinuclear complex

The synthesis, characterization, and electrochemical, photophysical, and photochemical properties of the binuclear compounds [(Ru(H8-bpy)2)2((Metr)2Pz)](PF6)2 (1) and [(Ru(D8-bpy)2)2((Metr)2Pz)](PF6)2 (2), where bpy is 2,2'-bipyridine and H2(Metr)2Pz is the planar ligand 2,5-bis(5'-methyl-4'H-[1,2,4]triaz-3'-yl)pyrazine, are reported. Electrochemical and spectro-electrochemical investigations indicate that the ground-state interaction between each metal center is predominantly electrostatic and in the mixed-valence form only a low level of ground-state delocalization is present. Resonance Raman, transient, and time-resolved spectroscopies enable a detailed assignment to be made of the excited-state photophysical properties of the complexes. Deuteriation is employed to both facilitate spectroscopic characterization and investigate the nature of the lowest excited states.     

Ground- and excited-state double proton transfer in lumichrome/acetic acid system: theoretical and experimental approach

Experimental time-resolved spectral and photon counting kinetic results confirm formation of an isoalloxazinic excited state via excited-state double proton transfer (ESDPT) catalyzed by a carboxylic acid molecule that forms a hydrogen-bond complex with the parent alloxazine molecule. This isoalloxazinic tautomer manifests itself as a distinct long-lived emissive species formed only in such alloxazine derivatives that were not substituted at the N1 nitrogen atom, being a product of the excited-state reaction occurring from the alloxazinic excited state. Theoretical calculations support the idea that the ESDPT occurs by the concerted mechanism. The calculated activation barrier in the excited state is much lower than the same barrier in the ground state and even disappears for the HOMO-1 to LUMO excitation, which explains the fact that the reaction takes place in the excited-state only. The reaction rate estimated from the emission kinetics is ca. 1.4 x 10(8) dm3 mol(-1) s(-1) in ethanolic solutions of lumichrome with added acetic acid.