Anomeric-Schleyer hyperconjugative interaction as a convenient avenue for aromaticity enhancement of phospholes

Due to the insufficient interaction of the phosphorus lone pair with the butadiene moiety, the aromaticity of the phosphole ring is lower than that of its counterpart's pyrrole, furan, and thiophene. Considering the high importance of phosphole core in organic chemistry, increasing its stability through reinforcement its aromaticity can be very valuable. In the present work, the aromaticity of the phosphole on the anomeric carbon in both the axial and equatorial conformers of the unsaturated six-membered heterocycles, using structural, electronic, energetic, and magnetic indices were investigated by the DFT-B3LYP/6-311++G(d,p) computational method. Electron pumping through anomeric and then Schleyer hyperconjugative interaction increase the aromaticity of the phosphole ring in axial conformer of compounds 1–11 . Based on various types of aromaticity indices, the results showed that the phosphole ring in the axial position has far higher aromaticity than the equatorial position. The phosphole ring containing cyano groups shows an efficient anomeric effect and, as a result, higher aromaticity. Excellent correlations were observed between aromaticity indices with different backgrounds.     

Substituent effects on the energetics and aromaticity of aminomethylbenzoic acids

The standard (p0 = 0.1 MPa) molar enthalpies of combustion of six aminomethylbenzoic acids were measured at T = 298.15 K by static bomb calorimetry. With these values, the standard molar enthalpies of formation in the crystalline state were obtained. Combining these results with the standard molar enthalpies of sublimation, the standard molar enthalpies of formation in the gaseous phase were derived. For the 10 possible isomers, the obtained experimental results were compared to and correlated with the relative stability obtained by ab initio calculations at the B3LYP/6-311++G(d,p) level of theory. Seeking a better understanding of the aromatic behavior and energetics of aminomethylbenzoic acids in the gas phase, calculations of NICS values, HOMA indices, and dihedral angles between the aromatic carbon and the amino group, Phi(Ar-NHH), were also performed computationally. The significant differences observed in the energetics, as well as in the NICS values, HOMA indices, and Phi(Ar-NHH) dihedral angles for these 10 isomers suggest a strong dependency on the identity and relative position of the three substituents on the benzene ring. This study points out a marked tendency for a decrease of the ring aromaticity, accompanied by an increase in the respective system stability, as the conjugation between the substituents becomes more extensive.     

The applicability of three-dimensional aromaticity in BiSn(n)- Zintl analogues

Three-dimensional aromaticity is shown to play a role in the stability of deltahedral Zintl clusters and here we examine the connection between aromaticity and stability. In order to gain further insight, we have studied Zintl analogs comprised of bismuth doped tin clusters with photoelectron spectroscopy and theoretical methods. To assign aromaticity, we examine the ring currents induced around the cage by using the nucleus independent chemical shift. In the current study, BiSn(4)(-) is a stable cluster and fits aromatic criteria, while BiSn(5)(-) is found to fit antiaromatic criteria and has reduced stability. The more stable clusters exhibit an aromatic character which originates from weakly interacting s-states and bonding orbitals parallel to the surface of the cluster, while nonbonding lone pairs perpendicular to the surface of the cluster account for antiaromaticity and reduced stability. The effect of three-dimensional aromaticity on the electronic structure does not result in degeneracies, so the resulting variations in stability are smaller than those seen in conventional aromaticity.     

Theoretical study of stability, structures, and aromaticity of multiply N-confused porphyrins.

The total electronic energy and nucleus-independent chemical shift (NICS) of 95 isomers of N-confused porphyrin (NCP: normal porphyrin (N(0)CP), singly N-confused porphyrin (N(1)CP), doubly N-confused porphyrin (N(2)CP), triply N-confused porphyrin (N(3)CP), and fully N-confused porphyrin (N(4)CP)) have been calculated by the density functional theory (DFT) method. The stability of NCP decreased by increasing the number of confused pyrrole rings. Namely, the relative energies of the most stable isomers in each confusion level increased in a stepwise manner approximately by +18 kcal/mol: 0 (N(0)CP1), +17.147 (N(1)CP2), +37.461 (N(2)CPb3), +54.031 (N(3)CPd6), and +65.636 kcal/mol (N(4)CPc8). In this order, the mean plane deviation of these isomers increased from 0.000 to 0.123, 0.170, 0.215, and 0.251 A, respectively. The unusual tautomeric forms of pyrrole ring with an sp(3)-carbon were found in the stable forms of N(3)CP and N(4)CP. The NICS values at the mean position of the 24 core atoms were nearly the same for the most aromatic isomers regardless of the confusion level: -15.1280 (N(0)CP1), -13.8493 (N(1)CP2), -13.7267 (N(2)CPd1), -11.7723 (N(3)CPb5), and -13.6224 ppm (N(4)CPa6). The positive correlation between aromaticity and stability was inferred from the plots of NICS and the relative energy of NCP for N(0)CP, N(1)CP, and trans-N(2)CP. On the other hand, the correlation was negative for cis-N(2)CP, N(3)CP, and N(4)CP isomers.     

运用密度泛函活性理论的信息论方法研究苯并富烯衍生物的芳香性

文献中已有越来越多的芳香性体系被发现,同时也有越来越多的芳香性指标被提出来,但是如何解释芳香化合物稳定性的起源以及理解芳香性的本质仍然是当今理论化学中一个悬而未决的难题。运用我们新近提出的密度泛函活性理论信息论方法,不久前我们曾对一系列富烯衍生物进行了系统研究并得到了一个全新的认识。本文进一步探讨苯并富烯衍生物的芳香性行为,目的在于考察一个或多个苯环与富烯连接之后其芳香性发生变化的情况。运用香农熵,费舍尔信息,Ghosh-Berkowitz-Parr熵,Onicescu信息能,信息增益,以及相对Rényi熵六个信息量,和四种芳香指标,ASE,HOMA,FLU和NICS,我们系统地研究了信息量和芳香性指标在单、双、三苯并富烯衍生物中的相关性。我们发现,不管是否有苯环与富烯相连,芳香指标和信息量的交叉相关性都是一样的。这表明,虽然苯环本身具有芳香性,但苯环与富烯相连并不能改变富烯的芳香性与反芳香性本质。苯并富烯衍生物与富烯衍生物的芳香性和反芳香性一致。苯并富烯衍生物的芳香性和反芳香性完全取决于富烯本身的芳香性和反芳香性。这些结果为认识和理解复杂体系芳香性和反芳香性起源和本质将提供有益的启示。     

Substituent effects and aromaticity of six-membered heterocycles

Structure, aromaticity, relative stability, and conformational flexibility of nitro and amino substituted monoheterocyclic analogous of benzene were studied by ab initio quantum-chemical method at MP2/aug-cc-pvDZ level of theory. Amino derivatives were found to be slightly less aromatic than nitro derivatives. Strong push–pull interactions were found in α- and γ-aminochalcogenopyrylium cations and, in less extent, in α- and γ-aminopyrydines. These molecules are less aromatic but more stable, as compare to their β-isomers. All heterocycles with 3rd and 4th row heteroatoms reveal C–NO₂ bond elongation accompanied by C–Heteroatom bond shortening in β-nitro isomers, and strong inequality of two endocyclic C–Heteroatom bonds in α-amino isomers.     

Effect of the H-bonding on aromaticity of purine tautomers

Four tautomers of purine (1-H, 3-H, 7-H, and 9-H) and their equilibrium H-bonded complexes with F(-) and HF for acidic and basic centers, respectively, were optimized by means of the B3LYP/6-311++G(d,p) level of theory. Purine tautomer stability increases in the following series: 1-H < 3-H < 7-H < 9-H, consistent with increasing aromaticity. Furthermore, the presence of a hydrogen bond with HF does not change this order. For neutral H-bonded complexes, the strongest and the weakest intermolecular interactions occur (-14.12 and -10.49 kcal/mol) for less stable purine tautomers when the proton acceptor is located in the five- and six-membered rings, respectively. For 9-H and 7-H tautomers the order is reversed. The H-bond energy for the imidazole complex with HF amounts to -14.03 kcal/mol; hence, in the latter case, the fusion of imidazole to pyrimidine decreases its basicity. The ionic H-bonds of N(-)···HF type are stronger by ~10 kcal/mol than the neutral N···HF intermolecular interactions. The hydrogen bond N(-)···HF energies in pyrrole and imidazole are -32.28 and -30.03 kcal/mol, respectively, and are substantially stronger than those observed in purine complexes. The aromaticity of each individual ring and of the whole molecule for all tautomers in ionic complexes is very similar to that observed for the anion of purine. This is not the case for neutral complexes and purine as a reference. The N···HF bonds perturb much more the π-electron structure of five-membered rings than that of the six-membered ones. The H-bonding complexes for 7-H and 9-H tautomers are characterized by higher aromaticity and a much lower range of HOMA variability.     

Substituent effect on the aromaticity of the silolide anion

The effect of the substituents on the planarity and aromaticity of the silolide anion was studied computationally. It was revealed that π-electron acceptor groups (e.g., silyl or trimethylsilyl) at the α position of the ring reduce substantially the inversion barrier about the central silicon increasing the aromaticity according to isomeric stabilization energy (ISE) and NICS values. In the planar and highly aromatic silolide anions, the mesomeric structures with the largest weight exhibit a Si=C double bond and a negative charge which is located at the α or β carbons based on NRT calculations. 2,5-disilylsilacyclopentadienides coordinated by naked Li⁺ have planar minima, however, further coordination by THF molecules (as in a solution) reduces somewhat the flattening of the silicon pyramid. NMR calculations were carried out to understand the connection between the ²⁹Si chemical shift and the aromaticity of the ring. It was revealed that the commonly accepted charge transfer–chemical shift relationship is strongly influenced by the substituents and the counter cation. THF complexation of the Li counter cation has a small influence on the NMR shift.     

2-酰基-1,3-环二酮类化合物互变异构的理论计算

采用MP2/6-31+G**//B3LYP/6-31+G**方法,系统地研究了2-酰基-1,3-环二酮类化合物的互变异构性质,探讨了环的饱和性、环的大小、环上杂原子以及溶剂效应对各互变异构体相对稳定性的影响。结果表明:(1)未烯醇化的异构体由于无法形成分子内氢键而具有较高的能量;(2)环上杂原子的推电子效应提高了邻位羰基氧上的负电荷,从而有利于相应环外烯醇式异构体的稳定;(3)对化合物3-酰基-1H-吡啶-2,4-二酮,3-酰基吡喃-2,4-二酮,2-酰基环戊-4-烯-1,3-二酮和3-酰基吡咯-2,4-二酮,由互变异构形成的环的芳香性在各异构体的相对稳定性中扮演着决定性的作用。     

The influence of aza-substitution on azole aromaticity

A homogeneous set of values for the aromaticity indices ASE (Aromatic Stabilisation Energy), HOMA (Harmonic Oscillator Model of Aromaticity) and NICS(1) (Nucleus-Independent Chemical Shift) for azoles has been investigated using multiple linear regression analysis. Statistically-significant relationships were found between the aromaticity indices and the number of nitrogen atoms at positions 2/5 and 3/4 of the ring. Aza-derivatives of pyrrole, furan and thiophene all gave similar relationships. For all three indices aza-substitution at positions 2 and/or 5 increases aromaticity. However, aza-substitution at positions 3 and/or 4 decreases classical aromaticity (ASE and HOMA) but increases magnetic aromaticity (NICS(1)). These indices appear to be measuring different properties of the azoles. The influence of aza-substitution on these different aspects of aromaticity is tentatively rationalized in terms of either bond length equalization or uniformity of π electron distribution.     

Cyclic carbon cluster dianions and their aromaticity

Cyclic carbon cluster dianions (CC(2))(2-)(n)(n = 3-6) are investigated by ab initio methods with regard to their geometric properties, electronic stability, and aromaticity. The unique wheel-like structures of these dianions consist of a n-membered carbon ring, where a C(2) unit is attached to each carbon atom. All investigated dianions represent stable gas-phase dianions. While the smallest member of this family (CC(2))(2-)(3) is clearly aromatic, the aromatic character decreases rapidly with increasing ring size. The geometries and the aromaticity of the cyclic clusters (CC(2))(2-)(n)(n = 3-6) can be nicely explained using resonance structure arguments.     

Local aromaticity of the six-membered rings in pyracylene. A difficult case for the NICS indicator of aromaticity

In this work, we have analyzed the local aromaticity of the six-membered rings (6-MRs) of planar and pyramidalized pyracylene species through the structurally based harmonic oscillator model of aromaticity (HOMA), the electronically based para-delocalization index (PDI), and the magnetic-based nucleus independent chemical shift (NICS) measurements, as well as with maps of ring current density. According to ring currents and PDI and HOMA indicators of aromaticity, there is a small reduction of local aromaticity in the 6-MRs of pyracylene with a bending of the molecule. In the case of NICS, the results depend on whether the NICS value is calculated at the center of the ring (NICS(0)) or at 1 A above (NICS(1)(out)) or below (NICS(1)(in)) the ring plane. While NICS(1)(out) values also indicate a slight decrease of aromaticity with bending, NICS(0) and NICS(1)(in) wrongly point out a large increase of aromaticity upon distortion. We have demonstrated that the NICS(0) reduction in the 6-MRs of pyracylene upon bending is due to (a) a strong reduction of the paratropic currents in 5-MRs and (b) the fact that, due to the distortion, the paratropic currents point their effects in other directions.     

Ring-current aromaticity in open-shell systems

The ab initio ipsocentric approach to calculation and mapping of induced orbital current density is extended to open-shell π systems. ROHF/6-31G^** calculations document the (magnetic) aromaticity of planar 4nπ triplet states of 4π/8π annulenes and isobenzofulvene, and quintet excited-state azulene. An orbital model for currents rationalises the generalised form of Baird’s rules, by which 4n + 2 annulenes are aromatic (antiaromatic) in states of even (odd) total spin (vice versa for 4nπ annulenes). In contrast to geometric criteria, ring-currents predict antiaromaticity for the doublet naphthalene radical anion.     

Y vs. cyclic delocalization in small ring dications and dianions: The dominance of charge repulsion over Huckel aromaticity

Coloumbic repulsion in four membered ring dications and dianions is more important than Hückel aromaticity and leads to a preference for Y-delocalize isomers with more favorable π charge distributions.     

Local aromaticity of [n]acenes, [n]phenacenes, and [n]helicenes (n = 1-9)

The local aromaticity of the six-membered rings in three series of benzenoid compounds, namely, the [n]acenes, [n]phenacenes, and [n]helicenes for n = 1-9, has been assessed by means of three probes of local aromaticity based on structural, magnetic, and electron delocalization properties. For [n]acenes our analysis shows that the more reactive inner rings are more aromatic than the outer rings. For [n]phenacenes, all indicators of aromaticity show that the external rings are the most aromatic. From the external to the central ring, the local aromaticity varies in a damped alternate way. The trends for the [n]helicene series are the same as those found for [n]phenacenes. Despite the departure from planarity in [n]helicenes, only a very slight loss of aromaticity is detected in [n]helicenes as compared to the corresponding [n]phenacenes. Finally, because of magnetic couplings between superimposed six-membered rings in the higher members of the [n]helicenes series, we have demonstrated that the NICS indicator of aromaticity artificially increases the local aromaticity of their most external rings.     

Magnetic properties and aromaticity of o-, m-, and p-benzyne

The relative aromaticities of the three singlet benzyne isomers, 1,2-, 1,3-, and 1,4-didehydrobenzenes have been evaluated with a series of aromaticity indicators, including magnetic susceptibility anisotropies and exaltations, nucleus-independent chemical shifts (NICS), and aromatic stabilization energies (all evaluated at the DFT level), as well as valence-bond Pauling resonance energies. Most of the criteria point to the o-benzyne相似文献   

Density functional theoretical investigation of the aromatic nature of BN substituted benzene and four ring polyaromatic hydrocarbons

We have studied the topological and local aromaticity of BN-substituted benzene, pyrene, chrysene, triphenylene and tetracene molecules. The nucleus-independent chemical shielding (NICS), harmonic oscillator model of aromaticity (HOMA), para-delocalization index (PDI) and aromatic fluctuation index (FLU) have been calculated to quantify aromaticity in terms of magnetic and structural criteria. We find that charge separations due to the introduction of heteroatoms largely affect both the local and topological aromaticity of these molecules. Our studies show that the presence of any kind of heteroatom in the ring not only reduces the local delocalization in the six membered ring, but also affects strongly the topological aromaticity. In fact, the relative orders of the topological and local aromaticity depend strongly on the position of the heteroatoms in the structure. In general, more ring shared BN containing molecules are less aromatic than the less ring shared BN molecules. In addition our results provide evidence that the structural stability of the molecule is dominated by the σ bond rather than the π bond.