Excited state aromatization assisted push-pull abilities of two unsaturated oxazolone derivatives

Molecular structures of two unsaturated oxazolone derivatives involving furan rings, which are decorated with p-tolyl (FurM) and 4-nitro phenyl (FurN), were investigated by X-ray crystallography and quantum chemical calculations. Their ground and excited states were examined by DFT and TD-DFT computations with the aid of topological electron density studies, NBO and Charge Decomposition Analysis. Both compounds have push-pull (D–π–A) framework using oxazolone ring as π-linker. Depending on the transitions from the ground to excited states, intramolecular charge transfer (ICT) in both compounds results in aromatization of oxazolones. Push-pull ability of FurN has more pronounced than that of FurM. The use of furan instead of almost fully aromatic benzenoid ring reduces HOMO?LUMO band gap due to relatively low aromaticity level of furan. Introduction of nitro substituent leads to a further reduction in HOMO?LUMO gap. In addition, electronic redistribution in the excited state results in aromatization of oxazolone moieties without elongation of carbonyl bonds.     

On the stability of methyl cations with aromatic substituents. Theoretical approach

The extreme stability of methyl cations surrounded by certain aromatic fragments was studied using density functional theory methods. The pK_R+, aromaticity, ionization potential and dipole moment of tris[6-(dimethylamino)-1-azulenyl]methyl [1] and 2,6,10-tris(dimethylamino)trioxatriangulenium [2], as well as the corresponding for crystal violet [3] and tert-butanol [4], these last as the reference precursors of stable carbenium, were calculated and the results were found to be in agreement with experimental results previously reported. Furthermore, the calculations suggest that the large stability is a direct consequence of inductive effects as well as the intrinsic aromaticity of the branches as measured by the HOMA (harmonic oscillator model of aromaticity) method.     

杂硼原子簇B6X-(X=N, P, As, Sb, Bi)稳定性和芳香性研究

利用Gaussian 98程序, 采用从头算和密度泛函理论方法, 对B6X-(X=N, P, As, Sb, Bi)杂硼原子簇进行了理论研究, 优化得到了其稳定平衡构型, 讨论了其振动光谱和稳定性等, 通过自然键轨道(NBO)、分子轨道(MO)和核独立化学位移(NICS)分析, 确定这些杂硼原子簇都有离域的π电子和σ电子成键轨道, 满足4n+2电子规则, 具有芳香性, 与纯B6- 或B62- 原子簇呈反芳香性不同.     

Local aromaticity study of heterocycles using n-center delocalization indices: the role of aromaticity on the relative stability of position isomers

A quantitative study on local aromaticity based on n-center electron delocalization indices, n being the number of atoms in the ring, is performed on a series of heterocycles containing N, O or S. The results indicate that the order of stability within a series of position isomers is not controlled by aromaticity but by other structural factors. Thus, for a certain series of monocycles position isomers (diazoles, triazoles, tetrazoles, diazines, triazines, and tetrazines) the most stable compound is the least aromatic one and vice versa. However, aromaticity controls the stability for series of isomers where these structural factors are similar. For the case of isocompounds, like isobenzopyrrole, isobenzofuran or isobenzothiophene, the large decrease in the aromaticity of the benzene ring with regard to their isomers makes them less stable.     

Natural resonance structures and aromaticity of the nucleobases

Natural resonance theory (NRT) and nucleus- independent chemical shift (NICS) analyses have been applied to the standard nucleobases adenine, guanine, cytosine, uracil, and thymine. The molecular electron densities were obtained from density functional theory calculations at the B3LYP level and ab initio calculations at the HF, MP2, and CCD levels. Compared with the dominance of the two Kekulé structures in benzene, the structural modifications in the forms of endocyclic heteroatoms and exocyclic substituents introduce various degrees of charge separation in nucleobases. As a result, the leading resonance structures for cytosine, uracil, and thymine are found to be covalent structures, but their weightings decrease to ~30% in the NRT expansion. For adenine and guanine, the covalent structures have weightings of ~20%, and the leading ionic resonance structures have weightings of as high as about 8%. Methods that include electron correlation effects, B3LYP, MP2, and CCD, give smaller weightings for the covalent structures than HF. However, MP2 and CCD results often include “strange” resonance structures with connections between unbonded vicinal atoms, making DFT at the B3LYP level the better choice for calculating these molecules’ electron density. The NICS at the ring center shows that the six-membered rings in cytosine, uracil, thymine, and guanine are nonaromatic with NICS within − 3 to − 1 ppm, while it is − 7.3 ppm for the six-membered ring in adenine. The NICS of the five-membered rings of adenine and guanine is around − 12 ppm, a slight decrease from the value of − 15.0 ppm for pyrrole.     

C36X(X=O,NH,S)各种可能异构体的芳香性研究

采用拓扑共振能方法对富勒烯C36X(X=O,NH,S)开环结构中的所有可能的异构体及阳离子和阴离子芳香性进行了理论研究. 计算结果表明,C36X的芳香性高于C36. C36X的阳离子因其共振能为负值而具有反芳香性. 反之,C36X的阴离子因共振能为正值而具有芳香性和较高的稳定性. C36的D6h和D2d异构体中杂原子X插入在5-5键时得到的化合物最稳定. 从理论上预测了C36X的负离子能形成稳定的金属富勒烯. 对C36X的阳离子和阴离子的芳香性进行了解释.     

Stabilization Studies on Divalent Five‐Membered Rings C4H4C,C4H4Si,C4H4Ge,C4H4Sn,and C4H4Pb

Energy differences, ΔX_S‐t (X = E, H and G) (ΔX_S‐t = X_(singlet)‐X_(triplet)) between singlet (s) and triplet (t) states are calculated at B3LYP/6‐311++G (3df,2p). The DFT calculations show that the triplet state of C₄H₄C is a ground state with planar conformer respect to its corresponding nonplanar singlet state. Both singlet and triplet states of C₄H₄M (M = Si, Ge, Sn and Pb) have a planar conformer with the singlet ground state. Four isodesmic reactions are presented for determining the stability energies, SE. NICS calculations are carried out for C₄H₄M to determine the aromatic character.     

Heavy analogues of the 6π-electron anionic ring systems: Cyclopentadienide ion and cyclobutadiene dianion

The heavy analogues of the anionic 6π-electron systems, lithium 1,2-disila-3-germacyclopentadienide 2⁻ · [Li⁺(thf)], 1,2-disila-3,4-digerma- and 1,2,3,4-tetrasilacyclobutadiene dianions 7²⁻ · 2[K⁺(thf)₂] and 8²⁻ · 2[K⁺(thf)₂], were synthesized by the reduction of the neutral precursors 1, 3 and 4, respectively. 2⁻ · [Li⁺(thf)], the heavy analogue of the cyclopentadienide ion, is an aromatic compound, whereas 7²⁻ · 2[K⁺(thf)₂] and 8²⁻ · 2[K⁺(thf)₂], the heavy analogues of the cyclobutadiene dianion, are both non-aromatic.     

The theoretical study of aromaticity in N-heteroatom compounds

In this article, we used nucleus-independent chemical shifts (NICS), aromatic stabilization energies (ASE), and magnetic susceptibility isotropic, calculated with (density functional theory) B3LYP levels at the 6-311+G (d, p) basis set, to evaluate the aromaticity of a set of 12 six-member planar π-electron aromatic systems: mono- and multi-substituted benzenes by nitrogen atoms. NICS were calculated at the center of the rings, 0.5 Å and 1 Å above the molecular plane, and in order to reflect the π-electron effects, we compare the values of NICS(0.5). The result statistically revealed significant correlations among the above three criteria. The comparison result was obtained as benzene > mono-nitrogen benzene > di-nitrogen benzene > tri-nitrogen benzene > tetra-nitrogen benzene > penta-nitrogen benzene.     

一种新型包含平面四配位碳二硼有机化合物的理论研究

采用B3LYP/6-311+G**方法, 研究了一种新型的包含平面四配位碳(ptC)二硼有机化合物C9B2H6的结构、稳定性和振动频率. 计算结果表明, C9B2H6结构的稳定性和两个硼原子的位置有很大关系, 硼原子起给予σ电子和接受π电子的作用. 在C9B2H6的15个异构体中, 最稳定的结构是具有C2v对称性的异构体(1,5), 在异构体(1,5)中, 两个硼原子位于同一个六元环中且与ptC相邻. 而且占据的π轨道说明异构体(1,5)具有10个π电子, 满足4n+2规则. 计算的核独立化学位移(NICS)值显示异构体(1,5)强的芳香性位于C9B2H6的两个三元环而不是两个六元环上.