Bonds twisted

The 6-acetoxy-substituted 1,4-diothiocin 13 exhibits no aromatic character and is not planar with the C7 — C8 bond twisted at an angle of 11 as revealed by X-ray crystallographic analysis.9 1,4-Dithiocin 13 on warming above 50 C forms acetoxybenzene. 

E. Stretched cr-bond , Twisted jr-bond and Carbene Minima... 

This equation has been used by Sundstrom and coworkers [151] and adapted to the analysis of femtosecond spectral evolution as monitored by the bond-twisting events in barrierless isomerization in solution. The theoretical derivation of Aberg et al. establishes a link between the Smoluchowski equation with a sink and the Schrodinger equation of a solute coupled to a thermal bath. The reader is referred to this important work for further theoretical details and a thorough description of the experimental set up. It is sufficient to say here that the classical link is established via the Hamilton-Jacobi equation formalism. By using the standard ansatz Xn(X,t)= A(X,i)cxp(S(X,t)/i1l), where S(X,t) is the action of the dynamical system, and neglecting terms in once this... 

Aberg, U., Akesson, E., Alvarez, J.-L., Fedchenia, I. and Sundstrom, V. Femtosecond spectral evolution monitoring the bond- twisting event in barrierless isomerization in solution, Chem.Phys., 183 (1994), 269-288... 

The formation of a TICT state is often invoked even if no dual fluorescence is observed. For donor-acceptor stilbenes (PCT-2 and PCT-3), the proposed kinetic scheme contains three states the planar state E reached upon excitation can lead to state P (non-fluorescent) by double-bond twist, and to TICT state A by singlebond twist, the latter being responsible for most of the emission. 

Aromatic amines are not the only type of molecule to undergo a spontaneous twist in the excited state, but DMABN is the best-studied example. Other such compounds are stilbene-type molecules, where the double bond twists in the excited state. The electronic structure for both cases, twisting double bonds and twisting single or partly double bonds of 7r-donor linked to 7r-acceptor (TICT molecules), can be related to each other and characterized in the framework of quantum-chemical treatment as described in Section III. 

However, it is to be expected that the rotation of the dimethylamino groups out of the plane should diminish their donor capacity. This is supported by the observation that the C=C barriers are lower in acceptor-substituted 1-dimethylamino-1-methyl-thioethenes (ketene N,5-acetals, 26) than in the l,l-bis(dimethylamino)ethenes (24) with the same acceptor combination (Table 6), in spite of the fact that dimethylamino groups in general are much better donors than methylthio groups33. However, the situation is not quite simple, since in a crystallographic study the ketene A S-acetal 26a was found to have the dimethylamino group twisted 25° out of the plane with a C1=C2 bond twist of ca 20°67. 

A, single crystal X-ray structure of which shows a linear backbone with the en ligands bound to the central Ru.573 The central Ru—N bonds are eclipsed with respect to the Ru—N (equatorial) bonds of both end groups, which are themselves eclipsed relative to one another.573 Ruthenium red is also identified as the product when [Ru(NH3)sN2]2+ is oxidized in air on Y-type zeolite.342... 

Crystallographic structure analysis reveals that the rhenium atom in 27 is an extremely distorted four-legged piano stool, with the Re—C bond twisting below the plane. The Re—C single bond is somewhat long, at 2.29 A, as is the Re—O bond of the metallacycle (2.075 A), though other bond lengths are normal (Re-CO 1.876,1.935 A C-C 1.525 A, C-O 1.298 A, C=0 1.191 A). 

The data in Table 1 refer to the nonradiative decay rate k , in DCS and DCM and are indicative of the reaction to the photochemical funnel through double-bond twisting. They reveal that k, is highly polarity dependent, slowest in strongly and fastest in weakly polar solvents (negative solvatokinetic effect). In view of the above, we recognize this as signifying that the funnel state P is of less polar nature than the precursor state E. ... 

---