Aromaticity, antiaromaticity and chemical bonding in the ground (S0), first singlet excited (S1) and lowest triplet (T1) electronic states of disulfur dinitride, S2N2, were investigated by analysing the isotropic magnetic shielding, σiso(r), in the space surrounding the molecule for each electronic state. The σiso(r) values were calculated by state‐optimized CASSCF/cc‐pVTZ wave functions with 22 electrons in 16 orbitals constructed from gauge‐including atomic orbitals (GIAOs). The S1 and T1 electronic states were confirmed as 11Au and 13B3u, respectively, through linear response CC3/aug‐cc‐pVTZ calculations of the vertical excitation energies for eight singlet (S1–S8) and eight triplet (T1–T8) electronic states. The aromaticities of S0, S1 and T1 were also assessed using additional magnetic criteria including nucleus‐independent chemical shifts (NICS) and magnetic susceptibilities calculated at several levels of theory, the highest of which were CCSDT‐GIAO/cc‐pVTZ for S0 and CASSCF(22,16)‐GIAO/aug‐cc‐pVQZ for S1 and T1. The results strongly suggest that: 1) the S0 electronic ground state of S2N2 is aromatic but less so than the electronic ground state of benzene; 2) S1 is profoundly antiaromatic, to an extent that removes any bonding interactions that would keep the atoms together; and 3) T1 is also antiaromatic, but its antiaromaticity is more moderate and similar to that observed in the electronic ground state of square cyclobutadiene. S2N2 is the first example of an inorganic ring for which theory predicts substantial changes in aromaticity upon vertical transition from the ground state to the first singlet excited or lowest triplet electronic states.

Off ‐nucleus isotropic magnetic shielding ( σ iso (r)) and multi‐points nucleus independent chemical shift (NICS(0‐2 Å)) index were utilized to find the impacts of the isomerization of gas‐phase furfuraldehyde (FD) on bonding and aromaticity of FD. Multidimensional (1D to 3D) grids of ghost atoms (bqs) were used as local magnetic probes to evaluate σ iso (r) through gauge‐including atomic orbitals (GIAO) at density functional theory (DFT) and B3LYP functional/6‐311+G(d,p) basis set level of theory. 1D σ iso (r) responses along each bond of FD were examined. Also, a σ iso (r) 2D‐scan was performed to obtain σ iso (r) behavior at vertical heights of 0–1 Å above the FD plane in its cis , transition state (TS) and trans forms. New techniques for evaluating 2D σ iso (r) cross‐sections are also included. Our findings showed that bonds in cyclic and acyclic parts of FD are dissimilar. Unlike the C−O bond of furanyl, the C=O bond of the formyl group is magnetically different. C−C and C−H bonds in furanyl are found similar to those in aromatic rings. A unique σ iso (r) trend was observed for the C 2 −C 6 bond during FD isomerization. Based on NICS(0‐2 Å) values, the degree of aromaticity follows the order of cis FD< trans FD

Aromaticity reversals between the electronic ground (S 0 ) and low-lying singlet (S 1 , S 2 ) and triplet (T 1 , T 2 , T 3 ) states of naphthalene and anthracene are investigated by calculating the respective off-nucleus isotropic magnetic shielding distributions using complete-active-space self-consistent field (CASSCF) wavefunctions involving gauge-including atomic orbitals (GIAOs). The shielding distributions around the aromatic S 0 , antiaromatic S 1 ( 1 L b ), and aromatic S 2 ( 1 L a ) states in naphthalene are found to resemble the outcomes of fusing together the respective S 0 , S 1 , and S 2 shielding distributions of two benzene rings. In anthracene, 1 L a is lower in energy than 1 L b , and as a result, the S 1 state becomes aromatic, and the S 2 state becomes antiaromatic; the corresponding shielding distributions are found to resemble extensions by one ring of those around the S 2 and S 1 states in naphthalene. The lowest antiaromatic singlet state of either molecule is found to be significantly more antiaromatic than the respective T 1 state, which shows that it would be incorrect to assume that the similarity between the (anti)aromaticities of the S 1 and T 1 states in benzene, cyclobutadiene, and cyclooctatetraene would be maintained in polycyclic aromatic hydrocarbons.

The activation and reaction energies of the CC and C-H bonds cleavage in pyrene molecule are calculated applying the Density Functional Theory and 6-311G Gaussian basis. Different values for the energies result for the different bonds, depending on the location of the bond and the structure of the corresponding transition states. The CC bond cleavage reactions include H atom migration, in many cases, leading to the formation of CH2 groups and H-C≡Cacetylenic fragments. The activation energy values of the CC reactions are greater than 190.00 kcal/mol for all bonds, those for the C-H bonds are greater than 160.00 kcal/mol. The reaction energy values for the CC bonds range between 56.497 to 191.503 kcal/mol. As for the C-H cleavage reactions the activation energies range from 163.535 to 165.116 kcal/mol, the reaction energies are nearly constant, 117.500kcal/mol. The geometries of the transition states and reaction products are discussed too.

The reaction paths of the C-C and C-H bond cleavage in the anthracene and phenanthrene aromatic molecules are studied by applying the ab-initio DFT method. It is found that the C-C bond cleavage proceeds via a singlet aromatic transition state, compelled through a disrotatoric ring opening reaction. A suprafacial H atom shift follows the transition state, leading to the formation of a methylene -CH2 and an acetylenic or allenic moiety

This work revealed the spherical aromaticity of some inorganic E4 cages and their protonated E4H+ ions (E=N, P, As, Sb, and Bi). For this purpose, we employed several evaluations like (0D‐1D) nucleus independent chemical shift (NICS), multidimensional (2D‐3D) off‐nucleus isotropic shielding σiso(r), and natural bond orbital (NBO) analysis. The magnetic calculations involved gauge‐including atomic orbitals (GIAO) with two density functionals B3LYP and WB97XD, and basis sets of Jorge‐ATZP, 6‐311+G(d,p), and Lanl2DZp. The Jorge‐ATZP basis set showed the best consistency. Our findings disclosed non‐classical aromatic characters in the above molecules, which decreased from N to Bi cages. Also, the results showed more aromaticity in E4 than E4H+. The NBO analysis attributed the aromaticity in the above molecules to the residual density of the overlapping σ‐bonding orbitals. So, the aromaticity in these molecules is unlike the classical aromaticity that is associated with electron delocalization. Scanning 1D σiso(r) variation along E−E bonds indicated a lowering in the shielding trend from N to Bi cages. The 3D results showed a similar decrease in the relative volumetric diffusion of the magnetic activity, whereas the volumetric ratio of V1ppm/V2ppm is almost constant for all the E4 cages.

Herein, we report designing a new Δ (delta‐shaped) proton sponge base of 4,12‐dihydrogen‐4,8,12‐triazatriangulene (compound 1) and calculating its proton affinity (PA), aromatic stabilization, natural bond orbital (NBO), electron density ρ(r), Laplacian of electron density ∇2ρ(r), (2D‐3D) multidimensional off‐nucleus magnetic shielding (σzz(r) and σiso(r)), and scanning nucleus‐independent chemical shift (NICSzz and NICS). Density functional theory (DFT) at B3LYP/6‐311+G(d,p), ωB97XD/6‐311+G(d,p), and PW91/def2TZVP were used to compute the magnetic shielding variables. In addition, relevant bases like pyridine, quinoline, and acridine were also studied and compared. The protonation of compound 1 yields a highly symmetric carbocation of three Hückel benzenic rings. Comparing our findings of the studied molecules showed that compound 1 precedes others in PA, aromatic isomerization stabilization energy, and basicity. Therefore, the basicity may be enhanced when a conjugate acid gains higher aromatic features than its unprotonated base. Both multidimensional σzz(r) and σiso(r) off‐nucleus magnetic shieldings outperformed electron‐based techniques and can visually monitor changes in aromaticity that occur by protonation. The B3LYP/6‐311+G(d,p), ωB97XD/6‐311+G(d,p), and PW91/def2TZVP levels showed no significant differences in detailing isochemical shielding surfaces.

Is S2N2 aromatic? According to off‐nucleus shielding calculations reported here, S2N2 is the first example of an inorganic ring showing substantial changes in aromaticity upon transition from the aromatic ground state to either the strongly antiaromatic and unstable first singlet excited, or the antiaromatic lowest triplet electronic states. More information can be found in the Full Paper by P. B. Karadakov et al. on page 16803.