Stabilities and Spin Distributions of Benzannulated Benzyl Radicals

Abstract

The effects have been studied of mono- and dibenzannulation of a benzyl radical with hybrid density functional theory (B3LYP) and quadratic configuration interaction theory (QCISD). Bond dissociation energies and enthalpies are reported that were determined at the common level QCISD/6-311G**//B3LYP/6-31G* for the benzylic C-H bonds of toluene 1H, the monobenzannulated polycyclic aromatic hydrocarbons (PAH) 1- and 2-methylnaphthalene 2H and 3H, the dibenzannulated PAHs 9-methylanthracene 4H and 9-methylphenanthrene 5H, and the model hydrocarbons 1-phenylpropene 6H and propene 7H. The conformational preferences and the symmetries of 1H-7H and of their corresponding radicals 1-7 have been determined. The analysis of the electron and spin density distributions of radicals 1-7 at the QCI level are reported, and these high-level data are discussed in comparison to results obtained with density functional theory and with an awareness of a general perception shaped by Hückel molecular orbital theory. The results show in a compelling fashion that electron and spin delocalization onto an annulated arene is not the decisive principle for stabilization of the benzyl radicals formed by homolysis of the methylated PAHs C10H7-CH3 and C14H9-CH3, and instead, the analysis of QCI spin density distributions suggests that spin delocalization onto annulated arenes is avoided as much as possible while spin polarization does occur to a significant extent.

Impact of Conjugation and Hyperconjugation on the Radical Stability of Allylic and Benzylic Systems: A Theoretical Study

“…This differential enhancement in the rotational energy barriers implies that the allyl radical is able to utilize the π-conjugation more effectively for radical delocalization, in turn increasing the rotational barrier by 4.6 kcal/mol. This result is similar to the findings of Sui et al, who investigated the spin distributions of benzannulated benzyl radicals . On the contrary, the benzyl radical is not able to utilize the extended conjugation effectively.…”

“…It is clear for the benzyl radical that utilization of the phenyl and allyl units is minimal, highlighting our original claim that benzyl carbons are much less likely to benefit from conjugation from an energetic or rotational barrier perspective. This perspective on radical delocalization is shared by Sui et al, who suggested that spin density distributions onto annulated arenes is avoided as much as possible . Interestingly, the degree of spin delocalization on the benzene ring in 2A is in quantitative agreement with the populations in 2B , further indication that benzyl radicals do not utilize the aromatic unit to a greater extent than with compound 2A .…”

“…This result is similar to the findings of Sui et al, who investigated the spin distributions of benzannulated benzyl radicals. 60 On the contrary, the benzyl radical is not able to utilize the extended conjugation effectively. The overall extent of electron delocalization and stability in the aforementioned systems is further discussed in the subsequent section with the aid of population analyses.…”

“…Thus, this study aims to comprehend the relative stability of allyl and benzyl radicals using a common precursor (Scheme ) and the impact of increased π-conjugation and hyperconjugation (Schemes –, vide infra). Previous work has shown that concepts important for allyl and benzyl radical stability, namely resonance, conjugation and hyperconjugation, can be investigated through approaches which compute electron occupations and spin densities such as natural bond orbitals (NBO) ,− or the quantum theory of atoms in molecules (QTAIM). , These calculations provide relevant knowledge to forward many fields where these resonantly stabilized radicals play a crucial role, and build a framework to facilitate a larger discussion of these radicals through insights gained by population analyses, which to our knowledge is not discussed in the scientific literature at this point.…”

Factors Influencing the Regioselectivity of the Oxidation of Asymmetric Secondary Amines with Singlet Oxygen

“…From what is known computationally of benzyl radicals,30 extension of the conjugated systems to naphthyl groups makes radical delocalization less efficient, and the calculated BDEs of 1‐naphthyl and 2‐naphthyl CH bonds are higher than those of benzylic CH bonds by about 1 kcal mol −1 . This is reflected in the poor selectivity observed (Table 1, entries 5 and 6) and led us to investigate the influence of BDE on oxidation selectivity.…”