ABSTRACT Recent advances in on‐surface chemistry have enabled the synthesis and structural characterization of even‐numbered cyclo[n]carbons, traditionally classified as either doubly aromatic ( n = 4k + 2) or doubly antiaromatic ( n = 4k) based on their in‐plane and out‐of‐plane π‐electron circuits. However, recent studies have increasingly questioned this classification, suggesting instead that these molecules are more accurately described as non‐aromatic. In this work, we computationally examine the electron affinities and (anti)aromatic character of cyclo[n]carbons with n = 16–30 using energetic, structural, and electronic aromaticity descriptors. Adiabatic electron affinity (AEA) analysis reveals a high degree of uniformity across the series of both nominally aromatic and antiaromatic members. Aromatic stabilization energy (ASE) values, derived from homodesmotic and disproportionation reactions, indicate slight destabilization only for C 16 and C 20 , and low stabilization for the remaining systems. In particular, ASE is less than 2 kcal/mol for cyclo[n]carbons with n ≥ 24. This suggests that neither aromatic nor antiaromatic character significantly contributes to the thermodynamic stability of larger cyclocarbons. EDDB analysis further supports this conclusion, with only about 22%–27% of π‐electrons participating in delocalization. While delocalization is slightly greater in cyclo[n]carbons with n = 4k + 2, the difference diminishes with increasing size. Upon two‐electr

The cyclohexatriene contributors would be expected to show alternating bond lengths, the double bonds being shorter (1.34 Å) than the single bonds (1.54 Å). An alternative representation for benzene (circle within a hexagon) emphasizes the pi-electron delocalization in this molecule, and has the advantage of being a single diagram. In cases such as these, the electron delocalization described by resonance enhances the stability of the molecules, and compounds composed of such molecules often show exceptional stability and related properties. Evidence for the enhanced thermodynamic stability of benzene was obtained from measurements of the heat released when double bonds in a six-carbon ring are hydrogenated (hydrogen is added catalytically) to give cyclohexane as a common product. In the following diagram cyclohexane represents a low-energy reference point. Addition of hydrogen to cyclohexene produces cyclohexane and releases heat amounting to 28.6 kcal per mole.

This chapter delves into the foundational concepts of chemical structure and bonding, essential for understanding molecular interactions and properties. The chapter begins with hybridization, exploring how atomic orbitals combine to form hybrid orbitals, influencing molecular geometry and bonding properties. Bond length and bond angles are analyzed, providing insight into the spatial arrangements and distances between atoms in molecules, which are crucial for predicting molecular shape and reactivity. The concept of bond energy is introduced, highlighting the energy changes associated with bond formation and dissociation, essential for thermodynamic and kinetic considerations.The chapter then focuses on localized bonds and their impact on molecular stability and reactivity, contrasting them with delocalized electrons in various bonding scenarios. Van der Waals forces are examined as weak intermolecular forces that play significant roles in physical properties such as boiling and melting points. The chapter proceeds with an analysis of the inductive effect and the electromeric effect, both of which describe electron shifts within molecules under the influence of electronegativity and external fields, respectively.Resonance and hyperconjugation are covered as mechanisms for electron delocalization, contributing to molecular stability and influencing chemical reactivity. Hydrogen bonding, a critical intermolecular force, is discussed in terms of its formation, significance in bi

oscillator model of aromaticity, extracyclic resonance energy, aromatic fluctuation index, nuclear independent chemical shift, and para-delocalization index. In Azaborines are a unique class of aromatic boron and nitrogen containing heterocycles isoelectronic and isostructural to carbon-containing aromatic compounds such as benzene. These novel compounds possess unique electronic characteristics that provide them unprecedented and adaptable reactivity and photophysical properties. Their properties enable them to greatly alter molecular reactivity without Although 1,3- and 1,4-azaborines do not have a lone pair adjacent to an empty p orbital to facilitate this π interaction, they still retain their aromatic character. For 1,3-azaborines, the structure is planar and the bonds lengths sit between those of single and double bonds. Initial theoretical studies of 1,3-azaborine reported structural characteristics that were less aromatic than benzene but possessed significant delocalization of π electrons. The synthesis of this elusive azaborine isomer and subsequent crystal structure provided empirical evidence of electron delocalization in 1,3-azaborines as they displayed intra-ring bonds shorter than the expected single bond length, but longer than the expected length of the respective double bonds. Theoretical studies comparing the three isomers of azaborines found that the charge separation present in 1,3-azaborines resulting from the 1,3-arrangement of the heteroatoms enhances the delocalization of electrons in the ring. As a result, researchers have labeled 1,3-azaborines as the most aromatic of the three isomers as calculated by density functional theory calculated metrics such as harmonic oscillator model of aromaticity, extracyclic resonance energy, aromatic fluctuation index, nuclear independent chemical shift, and para-delocalization index. In 1,4-azaborines, ab initio calculations reported significant π electron delocalization around the ring, but that the conjugated π system was interrupted by the boron atom. Upon the synthesis of a 1,4-di-tert-butyl-1,4-azaborine by Braunschweig and coworkers in 2012, X-ray crystal structures revealed a planar structure similar to other isomers. Braunschweig and coworkers performed DFT calculations that yielded geometric optimizations in good agreement with their structural data. Their DFT investigation of the electronic structure of 1,4-di-tert-butyl-1,4-azaborine supported classifying the molecule as aromatic. However, this aromaticity has the caveat that the 1,4-arrangement of boron and nitrogen causes a directionality to the delocalization moving from nitrogen to boron. Although the three isomers of azaborines possess some similar characteristic, the relative…

As shown in the diagram, the positive charge is distributed over carbons #2 and #4 so it is at these sites that the nucleophilic component bonds. Note that resonance stabilization of the allyl cation is greater than comparable stabilization of 1,3-butadiene, because charge is delocalized in the former, but created and separated in the latter. An explanation for the temperature influence is shown in the following energy diagram for the addition of HBr to 1,3-butadiene. The initial step in which a proton bonds to carbon #1 is the rate determining step, as indicated by the large activation energy (light gray arrow). The second faster step is the product determining step, and there are two reaction paths (colored blue for 1,2-addition and magenta for 1,4-addition). The 1,2-addition has a smaller activation energy than 1,4-addition, but the 1,4-product is more stable than the 1,2-product. At low temperatures, the products are formed irreversibly and reflect the relative rates of the two competing reactions. This is termed kinetic control. At higher temperatures, equilibrium is established between the products, and the thermodynamically favored 1,4-product dominates.