Description of Aromaticity with the Help of Vibrational Spectroscopy: Anthracene and Phenanthrene

Abstract

A new approach is presented to determine π-delocalization and the degree of aromaticity utilizing measured vibrational frequencies. For this purpose, a perturbation approach is used to derive vibrational force constants from experimental frequencies and calculated normal mode vectors. The latter are used to determine the local counterparts of the vibrational modes. Next, relative bond strength orders (RBSO) are obtained from the local stretching force constants, which provide reliable descriptors of CC and CH bond strengths. Finally, the RBSO values for CC bonds are used to establish a modified harmonic oscillator model and an aromatic delocalization index AI, which is split into a bond weakening (strengthening) and bond alternation part. In this way, benzene, naphthalene, anthracene, and phenanthrene are described with the help of vibrational spectroscopy as aromatic systems with a slight tendency of peripheral π-delocalization. The 6.8 kcal/mol larger stability of phenanthrene relative to anthracene predominantly (84%) results from its higher resonance energy, which is a direct consequence of the topology of ring annelation. Previous attempts to explain the higher stability of phenanthrene via a maximum electron density path between the bay H atoms are misleading in view of the properties of the electron density distribution in the bay region.

Tetrachlorinated Polycyclic Aromatic Dicarboximides: New Electron‐Poor π‐Scaffolds and NIR Emitters by Palladium‐Catalyzed Annulation Reaction

“…This observation is in accordance with our previously proposed mechanistic rationale for the annulation reaction of naphthalene dicarboximides with different PAH derivatives, where we assume that the direct C−H arylation, which follows the Suzuki–Miyaura cross‐coupling in this cascade reaction, proceeds through a Heck‐type mechanism and thus prefers C−C bonds with a pronounced olefinic character . Accordingly, the difference in the yields of 3 b for the regioisomers 1 b and 1 b ′ are reflected by the aromaticity/bond order in anthracene as in regioisomer 1 b the C−H arylation takes place at 1‐position with higher bond order, while in 1 b ′ at 9‐position having a lower bond order …”

C2 in a Box: Determining Its Intrinsic Bond Strength for the X1Σg+ Ground State

“…A molecule changes its electronic structure and thereby its intrinsic bond strength under the impact of increasing space confinement. The changes in bonding can be quantitatively assessed via the local stretching force constant . Confining the space in either z direction (σ direction: along the molecular axis) or x and y direction (π direction: perpendicular to the molecular axis, Figure a) leads to two basically different effects.…”

“…Confining the space in either z direction (σ direction: along the molecular axis) or x and y direction (π direction: perpendicular to the molecular axis, Figure a) leads to two basically different effects. 1) Any compression of the electron density within the bonding region causes the bond to strengthen as the bond becomes shorter and a larger bond stretching force constant results . 2) Apart from density compression, there can be a charge transfer from the He atoms to the confined molecule where a low‐lying unoccupied orbital(s) can accept negative charge.…”

11,11-Dimethyl-1,6-methano[10]annulene—An Annulene with an Ultralong CC Bond or a Fluxional Molecule?

“…The calculated BSOs provide a measure for the degree of π-delocalization in aromatic molecules, as was recently shown by Kalescky and co-workers. 86 These authors derived an AI (aromaticity index) based on benzene (AI = 1.0) and "Kekule benzene" (1,3,5-cyclohexatriene: AI = 0) where in the latter case 1,3-butadiene was used to define a suitable reference geometry. A fully aromatic system is predicted to have all conjugated CC bonds equal to the benzene CC bond length, whereas deviations from this value indicate the degree of bond alternation and bond strengthening (weakening).…”