What Do Magnetic Shieldings Tell Us About Bonding, Aromaticity and Antiaromaticity in Mono-, Bi- and Tricyclic Molecules?

Some chemical concepts such as aromaticity, antiaromaticity, and chemical bonding have been evaluated for some molecules based on isotropic magnetic shielding calculations. This includes utilising points as local shielding probes which have been used as a single point or multiple points aligned in one-, two-, or three-dimensional grids. Each of the grids was placed at a specific location around/at the molecular space of the studied molecules. Nuclear and off-nucleus NMR shielding calculations were performed at different levels of theory using different quantum chemical methods of HF, MP2, and CASSCF with a variety of basis sets. The calculations were performed on some organic and inorganic mono-, bi-, and tricyclic molecules in their ground and, in some cases, low-lying excited states. These molecules are borazine, borazanaphthalene, deltate, squarate, croconate, rhodizonate, disulfur dinitride, naphthalene, anthracene and phenanthrene. Based on analysing and scanning the changes in the magnetic shielding data of values, 1D curves, 2D contour maps and 3D isosurfaces, the targeted molecular features for the above molecules have been obtained. The chemical bonding, aromaticity, and antiaromaticity of the molecules are assessed based on the above evaluations. The results show that both borazine and the borazanaphthalene are moderately aromatic. The oxocarbon dianions vary from aromatic deltate, moderately aromatic squarate to antiaromatic croconate and rhodizonate. Also, the vertical excitation of the moderately aromatic ground state disulfur dinitride leads to strongly antiaromatic S1 and moderate antiaromatic T1 states. Naphthalene shows obvious magnetic variations among its different electronic states. In terms of decreasing aromaticity, the naphthalene states follow this order: S2 (strongly aromatic) > S0 (aromatic) > T1 (antiaromatic) > S1 (strongly antiaromatic). Both anthracene and phenanthrene display a strong magnetic behaviour. The central ring of anthracene is more magnetically shielded than the two terminal rings, whereas a contrast shielding profile is found in phenanthrene rings. For all the above molecules, the magnetic shieldings around bonds help in understanding the overall magnetic behaviour and the aromaticity level

Reaction pathways and transition states of the C-C and C-H bond cleavage in the aromatic pyrene molecule - A Density Functional study

The activation and reaction energies of the C-C 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 C-C bond cleavage reactions include H atom migration, in many cases, leading to the formation of CH2 groups and H-C≡C- acetylenic fragments. The activation energy values of the C-C 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 C-C 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

Magnetic Shielding, Aromaticity, Antiaromaticity and Bonding in the Low-Lying Electronic States of S2N2

Aromaticity, antiaromaticity and chemical bonding in the ground (S₀), first singlet excited (S₁) and lowest triplet (T₁) electronic states of disulfur dinitride, S₂N₂, are investigated by analysing isotropic magnetic shielding, σiso(r), in the space surrounding the molecule for each electronic state. The σiso(r) values are calculated using state-optimized CASSCF/cc-pVTZ wavefunctions with 22 electrons in 16 orbitals constructed from gauge-including atomic orbitals (GIAOs). The S₁ and T₁ electronic states are 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 are 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 are CCSDT-GIAO/cc-pVTZ for S₀ and CASSCF(22,16)-GIAO/aug-cc-pVQZ for S₁ and T₁. The results strongly suggest that the S₀ electronic ground state of S₂N₂ is aromatic, but less so than is the electronic ground state of benzene, S₁ is profoundly antiaromatic, to an extent that removes any bonding interactions that would keep the atoms together, and 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