An Experimental Thermochemical and Theoretical Study of Triquinacene: Definitive Disproof of Its Neutral Homoaromaticity

Exploring the antioxidant activity of thiaflavan compounds: a quantum chemical study

Conceptual DFT tools (HAT, SPLET, SET-PT, aromaticity index,…) have been used to explore the antioxidant activity of thiaflavan compounds, and predict which derivate should be the best one.

Phospha- and arsa-bridged cyclononatetraenides: novel zwitterionic 10π aromatic hemispheres

To probe the limits of the internal charge compensation approach for the generation of 10π aromatic hemispheres derived from hetero-acepentalenide zwitterions, the phospha- and arsa-bridged cyclononatetraenides were studied computationally by DFT at the B3LYP/6-311++G(d,p) level, and compared with the known aza-analog.

Decasilahexahydrotriquinacene and Decasilaisotwistane: σ Conjugation on a Bowl Surface

The first bowl-shaped oligosilane, hexadecamethyldecasilahexahydrotriquinacene (1), and a related oligosilane, hexadecamethyldecasilaisotwistane (2), were synthesized, and their structures and properties were studied. The results revealed importance of σ conjugation on a bowl surface: the HOMOs of 1 are σ orbitals delocalized on the bowl surface, whereas the LUMO is a pseudo π* orbital on the convex and concave sides of the bowl surface.

Efficient synthesis of aza-triquinacene derivatives via cycloaddition of 2,6-diazasemibullvalenes with nitroso compounds

The reaction between 2,6-diazasemibullvalenes and nitroso compounds was investigated. Aza-triquinacene derivatives of interesting structural and synthetic chemistry were generated highly selectively in good to excellent isolated yields. This reaction, which was rarely found between common aziridine derivatives and nitroso compounds, could be attributed to the rigid polycyclic ring system and the substitution patterns of 2,6-diazasemibullvalenes. Δ(1) -Bipyrroline derivatives were formed in excellent yields when these aza-triquinacene derivatives were treated with SmI2 .

A theoretical study of the intramolecular proton transfer and calculation of the nucleus independent chemical shift in juglone and some of its derivatives

In the present study, first, the intramolecular proton transfer (IPT) process of juglone and its derivatives were theoretically investigated in the gas phase and the effect of electron-withdrawing and electronreleasing substituents in different positions of the phenyl and benzoquinone rings of juglone on the IPT process was studied in which the geometries, energies and thermodynamic functions of the compounds were obtained using DFT calculations at the B3LYP/6-31+G(2d,p) level. Next, the influence of IPT on changing the aromaticity of the phenyl and benzoquinone rings was investigated. To determine the aromaticity of the rings, nuclear independent chemical shift (NICS) values were calculated for the ground state and transition state structures (GS1,TS,GS2) using the continues set of gauge transformations (CSGT) procedure at the B3LYP/6-311+G(2d,p) level.

Imidazolium-based ionic liquids. 1-methyl imidazolium nitrate: thermochemical measurements and ab initio calculations

In this work data of the molar enthalpies of formation of the ionic liquid 1-methylimidazolium nitrate [H-MIM][NO3] was measured by means of combustion calorimetry. The molar enthalpy of fusion of [H-MIM][NO3] was measured using DSC. Experiments to vaporize the ionic liquid into vacuum or nitrogen stream in order to obtain vaporization enthalpy have been performed. Ab initio calculations of the enthalpy of formation in the gaseous phase have been performed for the ionic species using the G3MP2 theory. The combination of traditional combustion calorimertry with modern high-level ab initio calculations allow the determination of the molar enthalpy of vaporization of the ionic liquid under study. The ab initio calculations indicate that [H-MIM][NO3] is most probably separated into the neutral species methyl-imidazole and HNO3 in the gaseous phase at conditions of the vaporization experiments.

A theoretical study of pericyclic rearrangements catalyzed by lithium

The role of lithium cation in the isomerization from diademane to triquinacene and in the Claisen reaction from phenyl allyl ether to 6-allyl-2,4-cyclohexadienone was analyzed. The nature of the interaction of the lithium ion with the reacting molecules in the transition state was studied using supermolecule and perturbational methods. The aromaticity of the transition state in presence of lithium was compared with that for the same reaction in absence of catalyst, employing tools such as nucleus-independent chemical shift and anisotropy of the induced current density. Our results support that the catalytic effect is caused principally by a more favorable electrostatic interaction of lithium cation with the transition states of both reactions.

The Gaseous Enthalpy of Formation of the Ionic Liquid 1-Butyl-3-methylimidazolium Dicyanamide from Combustion Calorimetry, Vapor Pressure Measurements, and Ab Initio Calculations

Ionic liquids are attracting growing interest as alternatives to conventional molecular solvents. Experimental values of vapor pressure, enthalpy of vaporization, and enthalpy of formation of ionic liquids are the key thermodynamic quantities, which are required for the validation and development of the molecular modeling and ab initio methods toward this new class of solvents. In this work, the molar enthalpy of formation of the liquid 1-butyl-3-methylimidazolium dicyanamide, 206.2 +/- 2.5 kJ.mol-1, was measured by means of combustion calorimetry. The molar enthalpy of vaporization of 1-butyl-3-methylimidazolium dicyanamide, 157.2 +/- 1.1 kJ.mol-1, was obtained from the temperature dependence of the vapor pressure measured using the transpiration method. The latter method has been checked with measurements of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, where data are available from the effusion technique. The first experimental determination of the gaseous enthalpy of formation of the ionic liquid 1-butyl-3-methylimidazolium dicyanamide, 363.4 +/- 2.7 kJ.mol-1, from thermochemical measurements (combustion and transpiration) is presented. Ab initio calculations of the enthalpy of formation in the gaseous phase have been performed for 1-butyl-3-methylimidazolium dicyanamide using the G3MP2 theory. Excellent agreement with experimental results has been observed. The method developed opens a new way to obtain thermodynamic properties of ionic liquids which have not been available so far.

Which NICS aromaticity index for planar pi rings is best?

Five increasingly sophisticated aromaticity indexes, based on nucleus-independent chemical shifts (NICS), were evaluated against a uniform set of aromatic stabilization energies (ASE) for 75 mono- and polyheterocyclic five-membered rings. While acceptable statistical correlations were given by all of the NICS methods, the most fundamentally grounded index, NICS(0)pizz (based on the pi contribution to the out-of-plane zz tensor component), performed best statistically (cc=0.980) and in practice. The easily computable NICS(1)zz index is a useful alternative (cc=0.968).

Heats of Formation of [2.2]Paracyclophane-1-ene and [2.2]Paracyclophane-1,9-diene − An Experimental Study

The enthalpies of formation [Delta(g)] of tricyclo[8.2.2.2(4,7)]hexadeca-1(13),2,4(16),5,7(15),10(14),11-heptaene (2, 1,2-dehydro[2.2]paracyclophane or [2.2]paracyclophane-1-ene) and tricyclo[8.2.2.2(4,7)]hexadeca-1(13),2,4(16),5,7(15),8,10(14),11-octaene (3, 1,2,9,10-dehydro[2.2]paracyclophane or [2.2]paracyclophane-1,9-diene) have been determined by measuring their heats of combustion in a microcalorimeter and their heats of sublimation by the transpiration method. Values of the strain energies (SE) [SE(2) = 34.7 kcal mol(-)(1), SE(3) = 42.0 kcal mol(-)(1)] have been derived from the gas-phase heats of formation and are compared with those from MM3 and PM3 calculations and with the corresponding value SE(1) = 30.1 kcal mol(-)(1) for the parent tricyclo[8.2.2.2(4,7)]hexadeca-1(13),4(16),5,7(15),10(14),11-hexaene (1, [2.2]paracyclophane). The higher strain energies of 2 and 3 (by 4.6 and 11.9 kcal mol(-)(1)) are in accord with the well-known increased reactivities of their aromatic rings as a consequence of their increased bending. As revealed by an X-ray crystal structure analysis, the bending in the monoene 2 corresponds to that of 1 and 3 at one of two bridging corners.

Stability and three-dimensional aromaticity of closo-NB(n-1)H n azaboranes, n = 5-12

Computations on all the possible positional isomers of the closo-azaboranes NB(n)()(-)(1)H(n)() (n = 5-12) reveal substantial differences in the relative energies. Data at the B3LYP/6-311+G level of density functional theory (DFT) agree well with expectations based on the topological charge stabilization, with the qualitative connectivity preferences of Williams, and with the Jemmis-Schleyer six interstitial electron rules. The energetic relationship involving each of the most stable positional isomers, 1-NB(4)H(5), NB(5)H(6), 2-NB(6)H(7), 1-NB(7)H(8), 4-NB(8)H(9), 1-NB(9)H(10), 2-NB(10)H(11), NB(11)H(12), was based on the energies (DeltaH) of the model reaction: NBH(2) + (n-1)BH(increment) --> NB(n)()H(n)()(+1) (n = 4-11). This evaluation shows that the stabilities of closo-azaboranes NB(n)()(-)(1)H(n)() (n = 5-12) increase with increasing cluster size from 5 to 12 vertexes. The "three-dimensional aromaticity" of these closo-azaboranes NB(n)()(-)(1)H(n)() (n = 5-12) is demonstrated by their the nucleus-independent chemical shifts (NICS) and their magnetic susceptibilities (chi), which match one another well. However, there is no direct relationship between these magnetic properties and the relative stabilities of the positional isomers of each cluster. As expected, other energy contributions such as topological charge stabilization and connectivity can be equally important.

A study into the possible homoaromatic nature of some related carbene and cationic intermediates with the potential for transannular interaction

A comparison of the aromatic nature of the cations and carbenes centered at the 8-position of tricyclo[3.2.1.0(2,4)]octane, the 9-position of pentacyclo[4.3.0.0(2,4).0(3,8).0(5,7)]nonane, the 6-position of tricyclo[3.1.1.0(2,4)]heptane, and the 4-position of tetracyclo[3.3.0.0(2,8).0(3,6)]octane has been undertaken. Hybrid density functional theory calculations have been used to examine the geometric, energetic, and magnetic characteristics of each cation, singlet carbene, and triplet carbene. The results shed light on the flexibility of the polycycles to achieve stable intermediates and on the breadth of homoaromaticity.

Aromaticity and antiaromaticity in fulvenes, ketocyclopolyenes, fulvenones, and diazocyclopolyenes

The structures, energies, natural charges, and magnetic properties of 3-, 5-, 7-, and 9-membered cyclic polyenes 1-4, respectively, with exocyclic methylene, keto, ketenyl, and diazo substituents (a-d, respectively) were computed at the B3LYP/6-311G+ **//B3LYP/6-311+G** level to elucidate their aromatic and antiaromatic properties. The corresponding conjugated cyclic cations le and 3e were also studied. The criteria used are isomerization energies (ISE), magnetic susceptibility exaltations (lambda), aromatic stabilization energies (ASE), nucleus independent chemical shifts (NICS), and bond length alternation (deltaR). Planar C2v structures were found to be the lowest energy minima with the exceptions of diazocyclopropene (1d), cycloheptafulvenone (3c), diazocycloheptatriene (3d), and all of the cyclononatetraene derivatives (4). The fulvenes (1a-4a) have modest aromatic or antiaromatic character, and are used as standards for comparison. By these criteria the ketenylidene and diazo cyclopropenes and cycloheptatrienes 1,3-c,d and oxo cyclopentadiene and cyclononatetraene 2,4b are antiaromatic, while the 5- and 9-ring ketenyl and diazo compounds and 3- and 7-ring ketones are aromatic. The degree of aromatic/antiaromatic character decreases with ring size. The consistent agreement with Hückel rule predictions for all the criteria shows their utility for the evaluation of the elusive properties of aromaticity and antiaromaticity.

Resurrection of neutral tris-homoaromaticity

Neutral in-plane tris-homoaromaticity is evaluated in tris(bismethano)benzene (15) and modifications of this parent structure in which the pi-orbitals might interact in the plane established by the unsaturated carbon atoms (in-plane conjugation). On the basis of magnetic susceptibility exaltation, nucleus-independent shift (NICS), and aromatic stabilization energy (ASE, evaluated via homodesmotic and isodesmic equations using B3LYP/6-311+G + ZPVE energies, as well as by MM3 and MM4 force field computations), we identified triene 17, a triply bridged analogue of 15, as the system where homoaromaticity is most effective. The NICS(total) in the center of 17 is -30.1 ppm and the diatropic pi-contribution is -18.0 ppm. This structure possesses more than one-third of the aromatic stabilization of benzene and is the best candidate for neutral tris-homoaromaticity ever proposed. The previously described tris-(bismethano)-benzene (15) also shows homoaromaticity but to a smaller extent compared to 17. Structure 18, which is closely related to 17, also is significantly homoaromatic but, as evaluated by MM3, strain partially counteracts the stabilizing effects from homoconjugation. Such a counteracting increase in strain largely cancels or even overwhelms the stabilization from homoconjugation in all other species considered in this study.

Dissected Nucleus-Independent Chemical Shift Analysis of π-Aromaticity and Antiaromaticity

Analysis of the basic π-aromatic (benzene) and antiaromatic (cyclobutadiene) systems by dissected nucleus-independent chemical shifts (NICS) shows the contrasting diatropic and paratropic effects, but also reveals subtleties and unexpected details.

Structures and Energies of 10-Azatriquinacene and Its Hydrogenation Products: A Computational Study

Enthalpies of formation and hydrogenation, vertical ionization potentials, and proton affinities of 10-azatriquinacene and its hydrogenation products have been computed at the B3LYP/6-311+G level of density functional theory. Like the parent triquinacene, 10-azatriquinacene is not homoaromatic on the basis of the energetic, geometric, and magnetic criteria. The hyperfine coupling constants of the nitrogen-centered cation radicals are also calculated.