Azaboracyclooctatetraenes Reveal that the Different Aspects of Triplet State Baird-Aromaticity are Nothing but Different

Abstract

The Baird-aromaticity of BN/CC cyclooctatetraene isosteres (azaboracyclooctatetraenes) in their lowest triplet states (T1) has been explored through computations of various aromaticity indices describing the different aspects of aromaticity (magnetic, electronic, energetic and geometric). While cyclooctatetraene (COT) is aromatic in its T1 state following Baird’s 4n rule, we now reveal that the degree of Baird-aromaticity of the BN isosteres varies extensively with aromaticity aspect considered. According to energetic and electronic indices, the thermodynamically most stable octagonal B4N4H8 isomer, having alternating B and N pattern (borazocine, B4N4COT-A), is only weakly aromatic or nonaromatic in T1, while the magnetic descriptors suggest it to have about two thirds the Baird-aromaticity of T1 state COT (3COT). The extent of Baird-aromaticity of intermediate BN/CC isosteres also varies markedly with aspect investigated. On the other hand, magnetic indices indicate that borazine (B3N3H6) in its singlet ground state (S0) is weakly aromatic. This opposite features in the magnetic aromaticity descriptors of, respectively, S0 state borazine (1borazine) and 3B4N4COT-A can be linked to differences in the symmetries of the orbitals involved in the virtual transitions from occupied to unoccupied orbitals which describe the response of a molecule in an external magnetic field. For azaboraCOTs, the magnetic aspect of T1 state Baird-aromaticity (response aromaticity) is not related to the electronic and energetic aspects (intrinsic aromaticity), findings that underline earlier observations on differences between the various aspects of the aromaticity phenomenon (or phenomena).

Aromaticity: Quo Vadis

“…The feature Cina refers to as ground state aromaticity is also found for the electronically excited state; the ground state is rather the electron configuration(s) of a particular state". In this sense, a possibility is to use the term "intrinsic aromaticity" instead of "ground state aromaticity" as proposed by Ottosson et al 59 However, the choice of the aromaticity tool is up to the users. If we rely on magnetic properties as a probe of aromaticity, we might list some molecules as aromatic, which might not be considered as aromatic if we chose intrinsic criteria of aromaticity, i.e., energetic, structural, and electronic criteria".…”

In the quest of Hückel–Hückel and Hückel–Baird double aromatic tropylium (tri)cation and anion derivatives

“…Except for X = F, the X–X lone pair repulsions are strong enough to force the molecule to pucker losing the potential σ‐aromatic character. Through the distortion of the planar geometry, the X–X distance increases from 0.05 for X = Cl to 0.15 Å for X = I. Puckering is not unexpected given that (i) the angle strain in seven‐membered rings (7‐MRs) can promote puckering as compared to the hexagonal 6‐MRs, which are ideal for sp 2 C atoms [ 45 ] ; (ii) the lowest out‐of‐plane (oop) frequency changes from 212 cm −1 in planar 1 C 7 H 7 + to 81 cm −1 in planar 1 C 7 F 7 + (see Table S2), showing that increasing the size of X in 1 C 7 X 7 + increases steric congestion that promotes puckering of the 7‐MR; and (iii) the highest occupied molecular orbital (HOMO) of D 7h C 7 X 7 + (X = Cl, Br, and I) contains seven in‐plane antibonding interactions in the ring of X substituents (see Figure 3). The same type of HOMO with maximal in‐plane antibonding interaction between the substituents was found by Fowler and Havenith in D 4h constrained C 8 I 8.…”

Excited‐state aromaticity and antiaromaticity special issue

“…The paper by Escayola et al [ 16 ] describes the double Hückel and Baird aromaticity of the triplet state of C 7 Br 7 +3 , which features an inner Hückel‐aromatic tropylium ring and an outer weak Baird‐aromatic Br 7 ring. Finally, Preethalayam et al [ 17 ] analyze the extent of Baird aromaticity in the T 1 states of BN/CC cyclooctatetraene isosteres, revealing large differences between the magnetic aspect of aromaticity (response aromaticity) and the electronic and energetic aspects (intrinsic aromaticity).…”