Naphthalenes twisted

On the other hand, reactions in which the return to So occurs from a "non-spectroscopic minimum (Fig. 3, path g) are probably the most common kind. The return is virtually always non-radiativef). This may be the very first minimum in Si (Ti) reached, e.g., the twisted triplet ethylene, or the molecule may have already landed in and again escaped out of a series of minima (Fig. 3, sequence c, e). For instance, triplet excitation of trans-stilbene 70,81-83) gives a relatively long-lived trans-stilbene triplet corresponding to a first spectroscopic minimum in Ti. This is followed by escape to the non-spectroscopic , short-lived phantom twisted stilbene triplet, corresponding to a second and last minimum in Ti. This escape is responsible for the still relatively short lifetime of the planar nn triplet compared to nn triplet of, say, naphthalene. A jump to nearby So and return to So minima at cis- and trans-stilbene geometries complete the photochemical process ). 

In 1,8-bis(dimethylamino)naphthalene steric and lone pair interactions are minimised in a structure (Einspahr et al., 1973) having the amino groups twisted and displaced above and below the plane of the naphthalene ring, as in [53] in which the almost planar naphthalene ring is shown as a dashed line. In l,8-bis(dimethylamino)-2,7-dimethoxynaphthalene, steric interactions involving the methoxy substituents bring about a further distortion as in [54] (Woolf, 1980). 

Fig. 8 Correlation between the twist angle, t, and the pyramidalization at nitrogen, yN, for anilines (O) and naphthalenes (+), enamines ( ) and amides (A). Data points for amides and thioamides are not shown. Reprinted with permission from Gilli et ul. (1986). Copyright 1986 American Chemical Society.

Sometimes transformation of aromatic componnds into ion-radicals leads to stereochemically unusual forms. Octamethylnaphthalene is a nonplanar molecule twisted around the bond that is common for the two six-membered rings. The nitrosonium oxidation results in the formation of the cation-radical with the centrosymmetric flatten chairlike geometry (Rosokha and Kochi 2006). According to the authors, such a skeletal transformation improves the overall planarity of octamethylnaphthalene. For example, the mean deviation of the carbon atoms in the naphthalene core for the flatten chairlike cation-radical (0.007 nm) is less than half of the corresponding value for the neutral twisted parent (0.016 nm). Within this flatten carcass of the anion-radical, the spin density can be delocalized more effectively. 

The antiaromatic 127t-electron benzodithiadiazine (47 R = H), an inorganic naphthalene analogue, is obtained as a volatile deep blue solid by the reaction of PhNSNSiMes with SCI2, followed by an intramolecular condensation of HCl. The tetrafluoro derivative (47 R = F) is prepared by treatment of CeFsSNSNSiMcs with CsF in acetonitrile. In contrast to (47 R = H), which has an essentially planar structure in the solid state, the dithiadiazine ring in the tetrafluoro derivative is somewhat twisted. In the gas phase, electron diffraction studies show that (47) (R = F) is planar, whereas (47) (R = H) is... 

When interpreting the solution and solid-state ENDOR spectra of radical anions of oligophenylenevinylenes it is not perfectly clear to what extent the observed localization of the charge in a particular subunit can be attributed to counterion induced polarization or to structural changes such as a twisted conformation. It is therefore important to include rigid and planar structures with extended 7t-systems for which the ries of peri-fused naphthalenes 44 appears as a suitable example [120, 121]. Such two-dimensional ladder-type... 

CD Spectra of Naphthalene-Diene Derivatives with Twisted n-Electron System... 

Circular Dichroic Power of a Twisted Naphthalene-Diene System... 

In the case of (8aS)-(+)-l,8a-dihydroazulene 10,15 the composition of the apparent CD and UV bands was rather simple, because each of the apparent bands was composed of a single electronic transition. The case of chiral troponoid spiro compounds (15aS)-(-)-14 and (18a5)-(-)-15 was also simple because of their Ci symmetrical structures. 16 On the other hand, the Ji-electron chromophores of the twisted naphthalene-diene systems 22-26 are complex and have no symmetric character.18 Therefore, to clarify the applicability of the 7i-electron SCF-CI-DV MO method to such complicated systems, it is important to analyze the composition of the apparent CD and UV bands theoretically obtained. As illustrated in Figure... 

These simplified three-dimensional drawings of the enantiomers show that the two naphthalene rings are twisted almost perpendicular to each other, and the large —P(Ph)2 substituents prevent interconversion of these mirror images. 

---