Planar state

Fig. 4 Ground-state and excited-state energies of the TICT complexes thioflavin T (a) and 9-(dicyanovinyl)-julolidine (DCVJ) (b) as a function of the intramolecular rotation angle (data from Stsiapura et al. [13] and Allen et al. [14]). In both cases, energy levels were determined by quantum mechanical simulations. For thioflavin T, the energy difference between Si and S0 corresponds to approximately 400 nm in the planar state and 470 nm in the twisted state. In the case of DCVJ, the energy differences correspond to 310 and 960 nm, respectively. The DCVJ energy levels reflect a rotation around the vinyl double bond...

The average shape of the radical will therefore be a compromise between strain due to hydrogen-hydrogen repulsion in the completely planar state and torsional strain of the partial double bonds in the skew state. Such a compromise should result in a propeller form in which the blades are slightly feathered out of the plane. It is possible that the radical exists in two isomeric forms, one corresponding to a symmetrical propeller and the other to a propeller in which one of the blades has been tilted the wrong way. Although such isomerism has been ob-... 

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. 

The transition state to C—N rotation is less polar than the ground state, and therefore barriers to this rotation are increased by increased solvent polarity (20,83). For similar reasons, the barriers to passage through the planar state in Case 2 push-pull ethylenes increase moderately with increasing solvent polarity (143). 

Fig. 8 Free energy profiles derived from the lattice model. The folded side of the barrier slopes to the folded state very gently. Numerous small barriers not shown) connect planar states with different n. A representative planar-planar lattice conformer is shown...

However, the bonds around the nitrogen retain some pyramidal character (Fig. 2-5, bottom). Even more important is the fact that there is flexibility. As a result, the torsion angle co may vary over a range of 15° or even more from that in the planar state.80 81/81a... 

Figure 12-20 Representation of (a) achiral and (b) chiral conformations of frans-cycloalkenes, using frans-cyclooctene as a specific example. For frans-cyclooctene, the achiral state is highly strained because of interference between the inside alkenic hydrogen and the CH2 groups on the other side of the ring. Consequently the mirror-image forms are quite stable. With frans-cyclononene, the planar state is much less strained and, as a result, the optical isomers are much less stable. With frans-cyclodecene, it has not been possible to isolate mirror-image forms because the two forms corresponding to (b) are interconverted through achiral planar conformations corresponding to (a) about 1016 times faster than with frans-cyclooctene.

With ammonia, inversion of this type occurs about 4 x ]010 times per second at room temperature, which corresponds to the planar state being less stable than the pyramidal state by about 6 kcal mole-1. With aliphatic tertiary amines, the inversion rate is more on the order of 103 to 105 times per second. Such rates of inversion are much too great to permit resolution of an amine into its enantiomers by presently available techniques. 

TABLE 3. Free-energy barriers to rotation through the planar state (kcalmol 1), C1—C2 bond lengths (pm) and twist angles (deg) for compounds 8-10... 

Nonsteric contributions may affect the barriers (1) the electronic effect of the substituent which modifies the conjugation in the planar state (2) the eventual interaction between ortho substituent and the 7i-cloud of the second ring in the perpendicular state (3) dipole-dipole interactions in the planar state (4) hydrogen bonding or complexation in the planar transition state. 

Van der Waals potential functions for non-bonded interactions display an attractive and a repulsive region 93>. Attractive interactions are small, too small to lead to a detectable effect on nitrogen inversion barriers in the present state of data accuracy. The repulsive portion of the curve is however very steep. Thus the presence of bulky substituents leads to appreciable nonbonded repulsions which are stronger in the pyramidal than in the planar state, where repulsions are partially relieved by the opening of the angle 0. As a consequence the pyramidal state is destabilized with respect to the planar TS and the inversion barrier is expected to decrease. 

In the reactions shown in Eqs. 7(a) and 7(b), we tried to symbolize this dynamic behavior in a simplified manner. Certainly, the rotation motion around the Ar-O-OH axis generates a diversity of momentary electron distributions and thus very mixed conformer states. This diversity can hardly be introduced into an all-comprising chemical model. Hence, for simplification we define and further consider only two borderline states the planar state stands for the donor molecules with low angle of twist, whereas the perpendieular state includes the molecules with higher deformation angle. 

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