A DFT study of isomeric conjugated,cross-conjugated and semi-conjugated six-membered heterocyclic mesomeric betaines

The structures and electronic properties of seven isomeric six-membered heterocyclic mesomeric betaines (HMBs) and four fully-covalent isomers have been calculated using B3LYP/6-311++G(p,d) methodology. The aromaticity indexes Bird I₆, HOMA, NICS(1) and aromatic stabilisation energy (ASE) have been calculated and compared. In agreement with an earlier study employing connectivity-matrix analysis, the previously unrecognised and little known semi-conjugated HMBs (Class 3) have calculated properties that are quite distinct from the isomeric conjugated HMBs (Class 1) and cross-conjugated HMBs (Class 2). In particular semi-conjugated systems have a higher degree of both classical and magnetic aromaticity than the isomers. In agreement with the calculated vertical ionisation potentials and electron affinities, the six-membered semi-conjugated species have high energy HOMOs and low energy LUMOs and these observations are consistent with the limited experimental data available. The results support the view that semi-conjugated rings should be recognised as a discrete class of dipolar heterocycle.     

The Adverse Effect of Benzannelation on the Aromaticity of Oxocinyl Anion: A Combined Experimental and Theoretical Study

A synthesis of 2H‐1‐benzoxocin from readily available compounds was accomplished. The potentially ‘aromatic’ π‐excessive systems 2H‐1‐benzoxocinyl and 6H‐dibenz[b,f]oxocinyl anions were generated from their corresponding conjugate acid precursors 7c and 8 , respectively. It was found that 2H‐1‐benzoxocinide 3d lacks the type of π‐frame stability associated with the parent 2H‐oxocinide 1d and that the dibenzo analog 5b is more unstable than 3d . Both 3d and 5b undergo rapid structural reorganization to form their corresponding stable isomeric anions. We were able to characterize the proton‐quenched products of these anions as the ring‐opened structures 15 and 18 , respectively. ¹H‐NMR and an ab initio calculation at the 6‐31g* level indicated that, unlike the ‘aromatic’ parent 2H‐oxocinide 1d and the aza analog 3c, 3d incorporates a non‐planar oxocinyl ring in which the negative charge is primarily localized on the pentadienyl moiety of the ring, but also partial delocalization of π‐electron density onto the benzene ring occurs.     

The calculated enthalpies of the nine pyrazole anions, cations, and radicals: a comparison with experiment

Enthalpies of 12 pyrazole species including neutral, anions, cations, and radicals have been calculated at the G3B3 level. The main conclusions are: (i) there are ten equilibria between species of which six have been measured experimentally and the agreement is excellent; (ii) two structures, cyclic and chain, have been found for the pyrazolium-radical 8 that are able to explain the electrochemistry of pyrazolium salts; (iii) the aromaticity, calculated as the NICS indexes, is related to the unexpected stability of the pyrazole anion 3.     

Tautomerism in bis(oxazolines): an experimental and theoretical study of proton transfer in 1,1-bis(4,4-dimethyl-1,3-oxazolin-2-yl)-ethane

The serendipitous isolation of an unusual protonated bis(oxazoline) prompted us to discuss the role of H⁺ on the kinetics and thermodynamics of the equilibrium between its ketimine and enamine forms. X-ray diffraction analysis revealed that the protonated bis(oxazoline) is in the Z-enamine form, the unipositive charge is counterbalanced by [(FeCl₃)₂O]²⁻ and [FeCl₄]⁻ anions. DFT calculation at the BP86/TZVP level showed that relative stability of enamine tautomer versus ketimine tautomer increases with the protonation of the nitrogen atom of the oxazolidine ring. At the same time, the barrier energy of tautomerism decreases.     

密度泛函理论研究吲哚并咔唑同分异构体结构,芳香性和光谱性质

在B3LPY/6-31G(d, p)基组水平上,利用密度泛函理论(DFT)优化了吲哚并咔唑五种同分异构体的几何和电子结构.基于这五种同分异构体的几何结构下,其吸收和发射光谱的研究使用相同的基组水平并采用极化连续介质模型(PCM)下用含时密度泛函理论(TD-DFT)计算.由于三种近似线性分子(吲哚并[2,3-a]咔唑、吲哚并[2,3-b]咔唑和吲哚并[3,2-b]咔唑)的电荷转移跃迁的振荡强度较大,这些异构体的发射光谱存在明显差异;基于吲哚并[2,3-c]咔唑和吲哚并[3,2-a]咔唑的构型特征,这两种同分异构体的发射谱具有高能量.比较计算结果表明,吲哚并[2,3-b]咔唑在这些分子中的振荡强度最大.这是因为当吲哚并咔唑的五种同分异构体的结构从基态变为激发态时,这些分子的分子轨道(MO)能级不同.由计算结果还得出,这五个分子适用于P型传输材料,并且每个分子的三个苯环均具有共轭效应.     

Aromaticity or non aromaticity in germanazenes? Theoretical studies on germanazenes, the N-cyano analogs and their carbodiimide isomers

Theoretical studies of germanazene rings [(Ge_II-NR)_2,3; R = H, Me, CN, Ph] have been performed at the DFT/B3LYP level. The fully optimized geometrical structures display four or six-membered planar rings of alternating germanium and nitrogen, in good agreement with the available X-ray experimental data. The hypothetical molecule (GeN-H)₂ presents only a small distortion from planarity. Although the planar conformation could indicate some degree of delocalization, the stabilization energy - estimated using the concept of homodesmotic reactions - indicates very little or no aromatic character in these molecules. The easy experimental formation of these germanazenes can be explained by di- (or tri-)merisation of the transient monomeric germylene-imine GeNR in its triplet state. When R = CN, in conformity with the experimental results, the most stable species is the isomeric carbodiimide form (GeNCN)_n, a result which is easily explained by the maximum spin density on the terminal nitrogen in the calculated monomer.     

Aromaticity of anthranil and its isomers, 1,2-benzisoxazole and benzoxazole

Direct computation of the π-current density, that is, the ‘ring current’, of anthranil (1) and its isomers 1,2-benzisoxazole (2) and benzoxazole (3) reveals different patterns of current flow: isomers 2 and 3 sustain strong benzene-like currents in the six-membered and bifurcated flow in the five-membered ring, whereas, in keeping with its lower thermodynamic stability, 1 has only a perimeter circulation without separate monocycle currents. Although the ring current criterion does not offer a sharp distinction between 2 and 3, their difference in thermodynamic stability is identical to that between isoxazole (4) and oxazole (5) suggesting an aromaticity order 1 < 2 ≈ 3.     

Examination of the mechanism of the intramolecular amination of N-(3-bromopyridin-2-yl)azaheteroarylamines and N-(2-chloropyridin-3-yl)azaheteroarylamines: a Pd-catalyzed amination and/or a base-assisted nucleophilic aromatic substitution?

The intramolecular amination of N-(3-bromopyridin-2-yl)azaheteroarylamines and N-(2-chloropyridin-3-yl)azaheteroarylamines was investigated. In this way we unraveled the mechanism of the ring closure reaction in the auto-tandem amination (inter- and intramolecular Pd-catalyzed amination) of 2,3-dibromopyridine with amino(benzo)(di)azines and 2-chloro-3-iodopyridine with amino(benzo)(di)azines, respectively. Depending on the substrate a Pd-catalyzed amination, a base-assisted nucleophilic aromatic substitution or a combination of both is occurring. An explanation based on the aromaticity of the amidine, supported by theoretical calculations, is provided. In addition we gained evidence that the intramolecular metal-catalyzed amination of N-(3-bromopyridin-2-yl)azaheteroarylamines and N-(2-chloropyridin-3-yl)azaheteroarylamines indeed involves a nitrogen atom that is not substituted with a hydrogen atom.     

Aromaticity of Rees-type hydrocarbons—a DFT computational study

The structure and aromatic properties of Rees hydrocarbons 7bH-cyclopent[cd]indene and its benzo-annelated derivative 1a and 2a, respectively, are examined by the B3LYP/6−31+G(d) calculations employing HOMA criterion of Krygowski and coworkers. It is shown that 1a possesses strong π-electron delocalization over the perimeter of the CC bonds, thus forming a quasi-[10]annulene pattern. Its aromatic character is determined to be 83%. In contrast, 2a is less convenient model system for [14]annulene. The reason behind is that the perimeter network of the potentially aromatic 14π-electrons is supplemented by two additional more local aromatic patterns involving 10π and 6π electrons. Consequently, the π-electron delocalization over the molecular rim is incomplete being thus diminished. The aromatic character of the peripheral bonds in both 1a ⁻ and 2a ⁻ anions formed upon deprotonation of the central C–H bond is decreased, since the role of the smaller rings in forming aromatic subsystems is increasing. Finally, polycyano substitution of 1a and 2a decreases aromaticity due to the price paid for the resonance effect taking place between the carbocyclic π-network and the double bonds of the CN groups. The resonance effect is particularly strong in anions derived by heterolytic cleavage of the C–H bond emanating from the central sp ³ carbon atom. Dedicated to Professor T. M. Krygowski for his outstanding scientific achievements on the occasion of his 70th birthday.     

The ring current in cyclopropane

Mapping of induced current density using the coupled Hartree-Fock “ipsocentric” (CTOCD-DZ/ 6-31G**//RHF/6-31G**) method shows a ring current arising in the σ framework of cyclopropane that is intense, diatropic and annular, and hence is consistent with the numerous literature attributions of σ-aromaticity to this molecule. Localised orbital analysis shows that this current can be attributed to the set of three carbon–carbon bonds. In a similar analysis, the four carbon–carbon bonds of (planar constrained) cyclobutane are found to give rise to a central paratropic current. The maps are consistent with the view that cyclopropane is strongly σ-aromatic, cyclobutane is weakly σ-anti-aromatic, and the larger cycloalkane systems are essentially σ-non-aromatic. Contribution to the Fernando Bernardi Memorial Issue.