Consequences of Curvature on Induced Magnetic Field: The Case of Helicenes

Abstract

Helicenes consist of several fused rings twisted around an axis, forming a cylindrical helix, with π‐delocalized electrons in the non‐planar rings. Induced magnetic fields dissecting the orbital contributions of [6]‐, [7]‐, and [14]helicene are discussed. Computations show a deshielding cone produced by the π‐electrons along the helical axis. Unexpectedly, the response of the core electrons produces a shielding cone, which is cumulative and sensitive to the curvature of the systems owing to the overlap of the other ring responses. A warning is provided regarding the evaluation of the delocalization in curved systems in which the x‐ and y‐components of the induced magnetic field become relevant.

Magnetically Induced Ring-Current Strengths of Planar and Nonplanar Molecules: New Insights from the Pseudo-π Model

“…Since helicenes consist of several rings forming a nonplanar structure with conjugated bonds and delocalized π-electrons, evaluating their aromatic character based on studies of magnetic properties has led to different conclusions. [56][57][58][59][60] In the case of the B ind analysis, the π-contribution to B ind z has a deshielded zone along the helical axis that grows with the number of rings in the helicene, giving rise to anomalous shieldings in the individual rings. 56 This behavior is reproduced by our pπ B ind z computations.…”

“…[56][57][58][59][60] In the case of the B ind analysis, the π-contribution to B ind z has a deshielded zone along the helical axis that grows with the number of rings in the helicene, giving rise to anomalous shieldings in the individual rings. 56 This behavior is reproduced by our pπ B ind z computations. This deshielding cone is a direct consequence of a diatropic pπ J ind flowing along the periphery and back into the helical skeleton (see Figure 8).…”

Visualizing electron delocalization in contorted polycyclic aromatic hydrocarbons

“…16,29 NICS has routinely been computed as single points 1 Å above the middle of each ring. 36,37 Richer pictures of NICS in molecules can be obtained by mapping NICS using 1D, [38][39][40][41] 2D, 37,[42][43][44][45][46][47][48] and 3D grids [49][50][51][52][53][54][55][56][57] of Bq. The latter approach can produce 3D maps of the magnetic properties of molecules as isovalue surfaces.…”

Aromaticity of ortho and meta 8‐Cycloparaphenylene and Their Dications: Induced Magnetic Field Analysis with Localized and Delocalized Orbitals in Strained Nanohoops

“…While the long-range shielding effect is dominated by contributions of π currents, at close range σ and core electrons have substantial contributions, whereas in curved systems such contributions may extent globally. [34] Previous theoretical studies on CPP aromaticity utilized the total electronic structure to calculate the magnetic response. [10,12,18] In order to have a direct representation of the magnetic field induced by the radial/perpendicular π currents which are relevant to aromaticity, only the π contributions have to be considered.…”

“…When a π aromatic ring is exposed to a uniform magnetic field oriented perpendicular to the ring plane, the mobile π electrons induce closed circuits of diatropic ring currents, which in turn induce a secondary magnetic field, opposing (shielding) the external field inside the ring forming a long‐range shielding cone, and reinforcing (deshielding) the external field outside the ring. While the long‐range shielding effect is dominated by contributions of π currents, at close range σ and core electrons have substantial contributions, whereas in curved systems such contributions may extent globally [34] . Previous theoretical studies on CPP aromaticity utilized the total electronic structure to calculate the magnetic response [10,12,18] .…”