How Far Does Substituents’ Impact Reach? The Transmission of Electronic Substituent Effects along the Polyphenylene Chain and the Evaluation of Through-Bond and Through-Space Contributions

Substituent Effect Study for 13Cβ SCS in a Meta-Styrene Series. Reynolds Dual Substituent Parameter Model Investigation

This article investigates the transmission of the meta-substituent, X, effect to the carbon β (C_β) of 17 different styrenes (X-C₆H₄-CH═CYZ) using the literature ¹³C substituent chemical shift (SCS) of C_β. The ¹³C SCSs of C_β were modeled by Reynolds substituent field σ_F and resonance σ_R constants. The model is considered a dual substituent parameter model, SCS = ρ_Fσ_F + ρ_Rσ_R. The coefficients of determination of the modeled ¹³C SCS for the 17 series fall between 0.9878 and 0.9983. The π-polarization concept was used to rationalize the field and resonance effects felt at C_β. The ρ_F values of the meta-series were all lower than the para-series ρ_F values. The substituent field effect transmits to C_β from C_ipso. The substituent resonance effect transmits from C_ipso to C_β initially through the π-bond to C_ortho and C_para, next to the side chain, and then the generated charge will induce the vinyl group via π-polarization. The ρ_F felt at C_β was almost double the ρ_R when compared with the short distance between C_meta and C_β, as opposed to C_ortho and C_para. Stabilization of the π-polarization of the vinyl group will increase, with the field and resonance effects felt at C_β and vice versa.

Substituent effects in the so-called cationπ interaction of benzene and its boron-nitrogen doped analogues: overlooked role of σ-skeleton

Despite massive efforts to pinpoint the substituent effects in the so-called cationπ systems, no consensus has been yet reached on how substituents exercise their effects in the interaction of the aromatic molecule with the metal ion. The π-polarization (the Hunter model) and the direct local effect (the Wheeler-Houk model) are two lines of thought applied to this problem, but the justification of both approaches is based on insufficiently proven assumptions and approximations. In order to shed more light on this issue we propose a new approach which enables us to gauge directly the energetic trends resulting from the interaction of the ring with the cation. In our method we add one more partitioning level to the interacting quantum atoms (IQA) scheme and decompose the IQA interaction energies into contributions resulting from σ and π electron densities of the aromatic ring. The new approach, which is named partitioned-IQA, abbreviated as p-IQA, has been applied to complexes of derivatives of benzene or azaborine interacting with a sodium cation. The p-IQA approach reveals that in these systems both σ and π electronic moieties are polarized. Interestingly, for the majority of cases the σ-polarization outweighs the π one, contrary to the Hunter model. However, the Wheeler-Houk model is not precise, either, since the σ-polarization shows some degree of non-locality. In addition, the substituents are found to have a negligible influence on the ring orbital-overlapping capability, i.e. the covalency. Therefore, the substituent effect in the cationπ interaction is a nonlocal classical effect, indicating that neither Hunter model nor Wheeler-Houk model is able to fully describe all the aspects of the substituent effects. The p-IQA conclusions for the considered systems have been compared with the results from the functional-group SAPT (F-SAPT) method. We believe that the presented partitioning in the IQA framework will provide a deeper insight into the substituent effects in the cationπ interactions, which is beyond the σ-π atomic charge population separation.

Substituent control of the ultrafast twisted intramolecular charge transfer rate in dimethylaminochalcone derivatives

The rate of TICT relaxation in dimethylaminochalcone derivatives is shown to be controlled by acceptor strength. Variation in the charge pulling capacity of the acceptor modifies the torsional barrier along the TICT coordinate in the S₁ state, resulting in a tunable TICT relaxation rate.

The Distant Double Bond Determines the Fate of the Carboxylic Group in the Dissociative Photoionization of Oleic Acid

The valence threshold photoionization of oleic acid has been studied using synchrotron VUV radiation and imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. An oleic acid aerosol beam was impacted on a copper thermodesorber, heated to 130 °C, to evaporate the particles quantitatively. Upon threshold photoionization, oleic acid produces the intact parent ion first, followed by dehydration at higher energies. Starting at ca. 10 eV, a large number of fragment ions slowly rise suggesting several fragmentation coordinates with quasi-degenerate activation energies. However, water loss is the dominant low-energy dissociation channel, and it is shown to be closely related to the unsaturated carbon chain. In the lowest-barrier process, one of the four allylic hydrogen atoms is transferred to the carboxyl group to form the leaving water molecule and a cyclic ketone fragment ion. A statistical model to analyze the breakdown diagram and measured rate constants yields a 0 K appearance energy of 9.77 eV, which can be compared with the density functional theory result of 9.19 eV. Alternative H-transfer steps yielding a terminal C=O group are ruled out based on energetics and kinetics arguments. Some of the previous photoionization mass spectrometric studies also reported 2 amu and 26 amu loss fragment ions, corresponding to hydrogen and acetylene loss. We could not identify such peaks in the mass spectrum of oleic acid.