Benzene ring deformations

Representative data for the doorway vibrations of ACN (C-C=N bend 379 cm-1), NM (NO2 rock 480 cm-1), and benzene (ring deformation 606 cm-1) are shown in Fig. 16. It would be preferable to pump the doorway vibration directly, but suitably powerful ultrashort pulse sources are not yet available in the needed range (here 16-26 pm). We have been able to understand the behavior of doorway vibrations by watching energy run in and out of these vibrations after C-H stretch pumping however, this indirect method of excitation complicates the problem somewhat. 

A. Domenicano, Structural Substituent Effects in Benzene Derivatives. In Accurate Molecular Structures, A. Domenicano and I. Hargittai, eds., Oxford University Press, Oxford, 1992, pp. 437-468 See, also, A. R. Campanelli, A. Domenicano, F. Ramondo, I. Hargittai, Group Electronegativities from Benzene Ring Deformations A Quantum Chemical Study. J. Phys. Chem. A, 2004, 108, 4940-1948. 

Campanelli, A. R. Arcadi, A. Domenicano, A. Ramondo, F. Hargittai, I. Molecular structure and benzene ring deformation of three ethynylbenzenes from gas-phase electron diffraction and quantum chanical calculations, J. Phys. Chem. A 2006,110, 2045-2052. 

The crystal of 2 OPr recrystallized from EtOH/H20 solution, and the mixed crystal of the same ethyl and propyl cinnamate derivatives (2 OEt and 2 OPr), on photoirradiation for 2h at room temperature with a 500 W super-high-pressure Hg lamp, afforded the highly strained tricyclic [2.2] paracyclophane (2 OEt-2 OPr-cyclo) crystal quantitatively (Maekawa et ai, 1991b). A crystal structure analysis was carried out of a single crystal of the complex of 2 OEt-2 OPr-cyclo with HFIP (recrystallization solvent) in a 1 2 molar ratio. Fig. 13 shows the molecular structure of 2 OEt-2 OPr-cyclo viewed along the phenylene planes. The short non-bonded distances and deformation of the benzene rings, as seen in Fig. 13, are common to those of [2.2] paracyclophanes, as previously reported (Hope et ai, 1972a,b). 

Arynes present structural features of some interest. They clearly cannot be acetylenic in the usual sense as this would require enormous deformation of the benzene ring in order to accommodate the 180° bond angle required by the sp1 hybridised carbons in an alkyne (p. 9). It seems more likely that the delocalised 7i orbitals of the aromatic system are left largely untouched (aromatic stability thereby being conserved), and that the two available electrons are accommodated in the original sp2 hybrid orbitals (101) ... 

The orbitals of the second row can be hypothetically obtained from those of the first row by deforming the orbital around the benzene ring in a clockwise direction. If the orbital is moved even further in that direction, one can pass from the second row to the third row, and eventually from the third row to the fourth row. In fact, this transition from the first row to the last row is a continuous process, and there exist infinitely many sets of localized orbitals of intermediate character, only two of which have been indicated in the second and the third rows.s7) Again the first column contains the superimposed fifth strongest contours for each set of molecular orbitals (in the case of the orbitals of the third row the fifth strongest contour happens to divide up into two disconnected parts). 

An early application was to the pathway for conformational isomerization of molecules Ar3Z, with three aromatic rings on the same centre (Mislow, 1976). Typically the system is pyramidal (tetrahedral overall where there is a fourth substituent on Z), and the rings are close enough in space that they cannot rotate independently about the Z-Ar bond. Triphenylphosphine oxide, to take a specific example, crystallizes in a propeller conformation [4 Z = P=OJ which is chiral, with all three benzene rings rotated in the same sense from the relevant C-P-O plane. A study (Bye et al., 1982) of deformations from this geometry for more than 1000 related structures in various environments allowed a detailed description of the pathway for... 

The magnitude of the above-mentioned shifts of the ortho absorptions by various substituents (ortho shift) lies between the relatively large shifts of the hydrogen atoms cis to the substituents in vinyl compounds and the smaller ortho shifts of the usual benzene derivatives. The ortho shifts in the [2.2]paracyclophane system are thus attributed to an increased double-bond character in the deformed benzene rings, where canonical structures such as 77 could possibly contribute to stabilization of the molecule. 

The deformation of the benzene ring in substituted benzenes is a sensitive indicator of substituent effects. Extensive experimental evidence accumulated over the past two decades, mainly from X-ray diffraction studies of solid state samples However, the first report of a ring distortion in a benzene derivative was done by Keidel and Bauer in their pioneering (1956) gas-phase electron diffraction study of the molecular structure of phenylsilane Recently a... 

The deformation of the benzene ring has also been determined in the electron diffraction study of o-phenylene-sulfite by Schultz et al. °. This seemingly com-... 

Ultraviolet spectra of aromatic systems are often used to probe strain-induced perturbations in the K-system. Out-of-plane deformations of the benzene ring shift the 260 nm band to the red and increase its intensity. Classical examples are [2.2]paracyclophane (286 nm) and Pascal s twisted benzenes. The for a given transition reveals changes in the energy of the filled/unfilled gap, whereas the extinction coefficient reveals the efficiency of the transition. 

The structures of the compounds were verified by H NMR spectroscopy. Two doublets of benzene ring protons (7 = 9 Hz) characteristic of mutual ott, ti-positions indicated angularly ring-fused structures. The shifts of NMR peaks and the features of the IR spectra (containing deformed out-of-plane vibrations at 800 cm ) were characteristic of 1,2,3,4-tetrasubstitution and so also confirmed the angular ring fusion. 

---