Interplay of π-Electron Delocalization and Strain in [n](2,7)Pyrenophanes

Abstract

The geometries of a series of [n](2,7)pyrenophanes (n = 6-12) were optimized at the B3LYP/6-311G** DFT level. The X-ray crystal structures determined for the [9](2,7)- and [10](2,7)pyrenophanes agreed excellently with the computed structures. The degree of nonplanarity of the pyrene moiety depends on the number of CH2 groups in the aliphatic bridge and, as analyzed theoretically, influences the strain energy and the extent of pi-electron delocalization in the pyrene fragment. Various indices, e.g., the relative aromatic stabilization energies (DeltaASE), magnetic susceptibility exaltations (Lambda), nucleus-independent chemical shifts (NICS), and the harmonic oscillator model of aromaticity (HOMA) were used to quantify the change in aromatic character of the pyrene fragment. DeltaASE and relative Lambda values (with respect to planar pyrene) were evaluated by homodesmotic equations comparing the bent pyrene unit with its bent quinoid dimethylene-substituted analog. The bend angle, alpha, DeltaASE, and Lambda were linearly related. The aromaticity decreases smoothly and regularly over a wide range of bending, but the magnitude of the change is not large. The differences between planar pyrene (alpha = 0 degrees) and the most distorted pyrene unit (alpha = 39.7 degrees in [6](2,7)pyrenophane) are only 15.8 kcal/mol (DeltaASE) and 18.8 cgs-ppm (Lambda). Also, the geometry-based HOMA descriptor changes by only 0.07 unit. The local NICS descriptors of aromatic character also correlate very well with the global indices of aromaticity. In line with the known reactivity of pyrenophanes, the variations of NICS(1), a measure of pi-electron delocalization, were largest for the outer, biphenyl-type rings. The strain energies of the pyrene fragments were much larger and varied more than those evaluated for the bridge. Both strain energies were interrelated (correlation coefficient R = 0.979) and depend on the bend angle, alpha.

Quadrannulene: A Nonclassical Fullerene Fragment

“…For this reason, we discuss measurements from the geometry calculated at the B3LYP/6-311G** level of theory, which is known to provide reliable geometries for many aromatic compounds. [18] Nonetheless, the diffraction data agree with the calculated (DFT) geometry and hence prove the structure of TMS 4 -TBQ.…”

Substituent effects in mono- and disubstituted 1,3,5,7-cyclooctatetraene derivatives in natural and planar conformations

“…It should be mentioned that HOMA can be used for any p-electron systems or parts of the systems, and also for non-planar systems. [69][70][71][72][73][74] Results and discussion…”

Syntheses of the Smallest Carbon Nanohoops and the Emergence of Unique Physical Phenomena

“…47,71,72 The distortion found in the benzene rings of [5]CPP is trumped, however, by those in the single phenyl ring of several paracyclophanes 73−75 and the pyrene moieties of [9](2,7)pyrenophane and 1,7-dioxa[7]-(2,7)pyreneophanes. 76,77 In addition, several other cyclophanes have been predicted computationally to have highly distorted aromatic rings, though the crystal structures of these compounds have not been obtained. 78,79 The outstanding solubility of [5]CPP compared with other cycloparaphenylenes permitted extensive solution-state characterization, providing initial insight into the newest member of the [n]CPP family.…”