Extent of Cyclic π-Electron Delocalization Modification in Exocyclically Substituted Fulvenes

Abstract

Fulvene derivatives, 20 mono- and 14 di-exocyclically substituted, were optimized at the B3LYP/6-311+G level of theory. Aromaticity indices include aromatic stabilization energy (ASE), Schleyer's Nucleus Independent Chemical Shift (NICS), (3)He chemical shifts, anisotropy and exaltation of magnetic susceptibility, and the Harmonic Oscillator Model of Aromaticity (HOMA), which is a geometry-based descriptor. These indices were used to estimate the extent of a cyclic pi-electron delocalization due to the substituent effect. A dramatic variation of these indices was found, indicating great sensitivity of the pi-electron structure of the ring. Except for anisotropy of magnetic susceptibility, all other indices exhibited perfect equivalence.

How the Substituent Effect Influences π-Electron Delocalisation in the Ring of Reactants in the Reaction Defining the Hammett Substituent Constants σm and σp

“…The above results may be compared with analogous data for exocyclically substituted fulvene [15] and heptafulvene [16]. In both cases very good correlations with σ p (and σ p -for electron accepting substituents in the case of fulvene and σ p + for the electron donating substituents in the case of heptafulvene, respectively) were observed and the dispersion of the data measured by variance or interquartile ranges was around ten times greater than in the case of meta and para substituted benzoic acid derivatives and their anions.…”

Electronic properties and aromaticity of substituted diphenylfulvenes in the ground (S0) and excited (T1) states

“…Previously, the correlation of the number of π electrons in the monosubstituted fulvenes with σ p + Hammett constants and aromaticity index NICS were investigated [13], in addition to the correlations of a few magnetic indices of aromaticity with ASE and HOMA for disubstituted fulvenes [14]. However, the correlations had been studied only for the molecules in the ground electronic state.…”

“…Fulvene has been shown by spectroscopic methods to be a planar, non-aromatic molecule of C 2v symmetry with C-C bonds that alternate in length, consistent with localized single and double bonds [1,2]. The fulvene properties, as well as properties of its derivatives (fulvenes), have been the subject of numerous experimental [2][3][4][5] and theoretical investigations [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Further, the methods for generation of fulvenes and their rich cycloaddition chemistry have been recently reviewed [3].…”

“…Further, the methods for generation of fulvenes and their rich cycloaddition chemistry have been recently reviewed [3]. The properties of fulvenes studied by theoretical methods have included the aromaticity and electronic structures [5,[7][8][9][10][12][13][14][15][16][17][18][19]. It has been found that electron-donating substituents attached to the exocyclic carbon in fulvenes significantly increase their aromatic character [5,7,13,14].…”

Avoiding pitfalls of a theoretical approach: the harmonic oscillator measure of aromaticity index from quantum chemistry calculations

“…However, when the calculated quantities (e.g., bond lengths) are mixed with the experimental ones, the shortcomings of quantum chemical treatment are bound to resurface. Unfortunately, HOMA is routinely calculated in just such a way: the theoretically obtained bond lengths for a studied system are combined with the parameterization based on experimental geometries of the reference molecules [7,[11][12][13][14][15][16][17][18]. One may have justified suspicions that HOMA computed in such a way would undergo strong changes with a change of the basis set, computational method, or even the exchange-correlation functional of DFT (a great variety of which have been recently developed and presented for general use).…”