Double Aromaticity in Monocyclic Carbon, Boron, and Borocarbon Rings Based on Magnetic Criteria

Abstract

The double-aromatic character of selected monocyclic carbon, boron, and borocarbon rings is demonstrated by refined nucleus-independent chemical shift (NICS) analyses involving the contributions of individual canonical MOs and their out-of-plane NICS tensor component (CMO-NICS(zz)). The double aromaticity considered results from two mutually orthogonal Hückel p AO frameworks in a single molecule. The familiar pi orbitals are augmented by the in-plane delocalization of electrons occupying sets of radial (rad) p orbitals. Such double aromaticity is present in B(3) (-), C(6)H(3) (+), C(6) (4+), C(4)B(4) (4+), C(6), C(5)B(2), C(4)B(4), C(2)B(8), B(10) (2-), B(12), C(10), C(9)B(2), C(8)B(4), C(7)B(6), C(6)B(8), and C(14). Monocyclic C(8) and C(12) are doubly antiaromatic, as both the orthogonal pi and radial Hückel sets are paratropic. Planar C(7) and C(9) monocycles have mixed aromatic (pi) and antiaromatic (radial) systems.

Chemical Reasoning Based on an Invariance Property: Bond and Lone Pair Pictures in Quantum Structural Formulas

“…Bonding character in the σ-system of the cation is increased with increased bond angle at the phenyl carbon consistent with Walsh's rules. A similar increase in bond angle is found in the double aromatic [39] C 6 H 3 + cation. The VIF treatment of the σ-system for this cation is shown in Figure 26.…”

Diagnosis of the σ-, π- and (σ+π)-Aromaticity by the Shape of the NICSzz-Scan Curves and Symmetry-Based Selection Rules

“…In effect, this is mirrored on the shapes of the NICS zz -scan curves reported herein, which once again correctly predict the orbital-type of aromaticity in the isomeric benzyne molecules. [16]. It was claimed that the c-C 6 cluster is also σ-antiaromatic due to cyclic delocalization of its four lone-pair electrons.…”

“…Hofmann and Berndt [90] showed that the the NICS(1) values of the double aromatic isoelectronic [c-C 6 H 3 ] + and c-C 5 BH 3 molecules are significantly larger than that of the benzene molecule due to the contribution of an additional aromatic system, namely the σ-aromatic component. Akin to c-C 6 (D 3h ) the c-C 10 (D 5h ) cluster was also proved to be a double (σ + π)-aromatic molecule with enhanced aromaticity due to an additional diatropic subsystem (the diatropic in-plane radial system) [16]. This is consistent with the estimated excitation energies of the σ-and π-type transitions (Table 10), which indicate that the σ-aromatic component should be larger than the π-aromatic one and is mirrored on the shape of the NICS zz -scan curve shown in Figure 9b.…”

“…This is consistent with the estimated excitation energies of the σ-and π-type transitions (Table 10), which indicate that the σ-aromatic component should be larger than the π-aromatic one and is mirrored on the shape of the NICS zz -scan curve shown in Figure 9b. Schleyer et al [16] showed that the c-C 7 (C 2v ) cluster has mixed aromatic (π) and antiaromatic (radial) systems. According to our calculations for the c-C 7 (C 2v ) cluster the lowest σ-and π-T x,y -allowed transitions have much lower excitation energies than the σ-R z -allowed ones (Table 10) and therefore it is expected to be a double(σ + π)-aromatic molecule involving stronger π-than σ-aromatic component, in line with the shape of the NICS zz -scan curve (Figure 9b).…”

“…It is typically computed at ring centers, at points above, and even on grids [14] in and around the molecule. The NICS criterion continues to be enhanced and refined, with the out-ofplane zz-components of the shielding tensor elements NICS zz (0) and NICS zz (1) proposed to be a superior aromaticity descriptor [15,16]. Furthermore, the CMO-NICS criterion [17][18][19] of aromaticity dissects the total shieldings calculated at ring or cage centers, into individual contributions from each canonical molecular orbital (CMO).…”