The tt Scale

Table 10.13 shows that Z and T(30) values are generally in the same order as Y values. Other scales, the tt scale,4"0 the tt scale,4111 and the Py scale,402 are also based on spectral data.403... 

In the absence of Tween 80 both the para- and meta-TFMS tt values are evenly distributed along the uppermost tt axes. When Tween 80 is added, the tt scales for both the para and meta derivatives are severely compressed, as shown in the lower portion of each scale. 

This procedure was applied to numerous solvatochromic scales [Z, Xr. (30)], using 4-nitroanisole as the solute incapable of H bonding. As in the development of the TT scale, the a scale is averaged over several processes. A (3 scale of HBA ability (basicity) was established in a similar manner. Table 8-17 lists some it, a, and (3 values. ... 

Catalan (1995) has developed a set of polarity parameters known as the solvent bipolarity-polarizability (SPP) scale. Like the tt scales, the SPP parameters are based on the abihty of the solvent to shift the positions of absorption bands in a test molecule used as a probe. The effect, known as solvatochromism, utilizes 2-(N,Af-dimethylamino)-7-nitrofluorene (DMANF) by measuring the shift in the absorption spectrum as a series of solvents is used. The value of SPP for each solvent is calculated from the relationship... 

We have recently developed (33) a generalized solvent polarity scale 7T which for aliphatic, aprotic, monofunctional solvents (henceforth called select solvents ) correlates quite well with all the other most widely used empirical polarity scales based on a variety of chemical and physical properties (34). It is significant that both the tt scale and the other solvent polarity scales show excellent correlations with the molecular dipole moments of the select solvents. 

Solvent Effects on ir -Type Properties Where hydrogen bonding effects are excluded, solvent effects on 7r -type properties are simply related to the TT scale by equations of the form. 

Factor (b) is a well-known phenomenon (80). Recently Skulski and Kan-abus (81) presented experimental results hinting at the formation of such complexes between benzene and p-nitroanisole, one of the indicators used to establish the tt scale. Since the work of Winstein and Feldman (82) has established the existance of C.T. complexes between aromatic hydrocarbons and tropylium cation, the possibility of weak nucleophilic attack by aromatics on incipient carbonium ions seems reasonable. 

This is not the case with the tt scale, which exhibits not only more disparate polarizability and dipolarity coefficients, but also slight contamination with acidity and basicity effects as revealed by the following equation ... 

Hence these probes respond to solvent polarity qualitatively, and almost quantitatively, in the same way. It is now possible to set up a scale of solvent sensitivity, based on 1, in which the value of v(l), 3j, for cyclohexane is set equal to zero while that for dimethyl sulfoxide (DMSO) is set equal to 1.00, and values for all other solvents lying, by hypothesis, between these hmits. Parallel behavior is found for 3. The resulting values constitute the tt scale. 

Figure 3. Low-energy vibronic spectrum in a. 11 electronic state of a linear triatomic molecule, computed for various values of the Renner parameter e and spin-orbit constant Aso (in cm ). The spectrum shown in the center of figure (e = —0.17, A o = —37cm ) corresponds to the A TT state of NCN [28,29]. The zero on the energy scale represents the minimum of the potential energy surface. Solid lines A = 0 vibronic levels dashed lines K = levels dash-dotted lines K = 1 levels dotted lines = 3 levels. Spin-vibronic levels are denoted by the value of the corresponding quantum number P P = Af - - E note that E is in this case spin quantum number),...

Sulfur dioxide is an example of a simple Lewis base that carries two sets of inequivalent n-pairs, one set on each O atom. The n-pair model (in which the tt bonding pairs are not drawn and are ignored here) is shown in Fig. 10. The geometries of S02 HF [126,127], S02 HC1 [28,126] and S02- C1F [70] have all been determined from investigations of their rotational spectra. Each molecule is planar and belongs to the Cs point group. Scale drawings for S02 HC1 and S02- C1F are displayed in Fig. 10. 

Case 1. (90°) 8> s (0°) (Fig. 1). This is a normal push-pull system, and a donor group A has different environments in the energy minima near 0° and near 180°, provided that the acceptor groups X and Y are different. The energy required to pass across the tt barrier can be measured by monitoring the band shape of the A resonance when the preexponential lifetimes are intermediate on the NMR time scale. 

Turning now to direct theoretical evaluations, we consider Ar as the displacement (on the C=C axis) of the centroid of the tt orbital with respect to the center 1. Of course, such a displacement can differ from zero only if some hybridization is allowed, which, in the case of a tt orbital, must consist in admixture of the suitable dTT orbital. The hybrid in question was determined [222] from 4-3IG calculations with d polarization functions for carbon and optimization of all scale factors, followed by a calculation of in situ valence orbitals, and of their characteristics, according to Del Re and Barbier [143]. The inward shifts (on the C=C axis) of the tt orbital centroids are close to 0.03 A (Table 11.1). 

This expression contains all characteristic structures (([-function, In 2, 7T and a rational number) which one usually encounters in the results of the loop calculations. Let us emphasize that the relative scale of these subleading terms is rather large, of order tt, which is just what one should expect for the constants accompanying the large logarithm. 

Recent studies by Schuster and collaborators67,54, based on nanosecond laser flash techniques, revealed important conclusions, including (a). The enone excited state responsible for the photocycloaddition is the jt-Tt which possesses different polarization than the n-7T state, considered in rationalizing the effect of the oriented jr-complex. (b) Direct measurement of the reactivity scale of alkenes measured by nanosecond flash photolysis provided different results from those obtained with no consideration of the diradical fragmentation to starting materials. 

The side group characteristics needed to move the Tt up or down the temperature scale have been mentioned earlier. The data shown in Table 3.1 illustrate these principles for a number of different side groups. 

Figure 2.20. Right part The polariton dispersion at a few tens of reciprocal centimeters below the bottom of the excitonic band, vs the wave vector, or the refractive index n = ck/w (notice the logarithmic scale). The arrows indicate transitions with creation of one acoustical phonon, with linear dispersion in k (with a sound velocity of 2000 m/s). For the transitions T, Tt, T3 the final momentum is negligible compared to the initial momentum, and the unidimensional picture suffices. For the transitions between T3 and the point A, the direction of the final wave vectors should be taken into account. Left part The density of states m( ) (2.141) of the polaritons in the same energy region. This diagram explains why the transitions T, will be much slower than the transitions around T3 and the point A. The very rapid increase of m( ) at a few reciprocal centimeters below E0 shows the effect of the thermal barrier.

In contrast to the extraordinary stability of TT dimer, C-containing CPDs are prone to deamination, converting C to U or 5-methyl-C to T in the dimer. The half-life for deamination of TC and CT dimers appears to be several hours. It is very likely that CPD deamination is biologically important, as it occurs well within the time scale of cell division cycle of -24 hr. Thus, some unrepaired CPDs may have... 

---