Aromaticity Versus strain

3 Aromaticity Versus Strain The Mills-Nixon effect discussed in the 

Cyranski et al. examined the series of pyrenophanes where the tethering chain has 6-12 carbon atoms. The nonplanar distortion is measured as the bend angle a, defined as the angle made between the outside ring plane and the horizon, or more simply, the angle C11-C2-C7. Relative ASE is computed using Reaction 

which cleverly avoids the complication of knowing (1) what is the ASE of pyrene itself and (2) what is the strain energy in these compounds. 

All of these trends are consistent with rednced aromaticity with increased out-of-plane distortion of the pyrene framework. What may be surprising is the relatively small loss of aromaticity in this sequence. Even though the bend angle is as large as almost 40, the loss of relative ASE is only 16 kcal mol , only about a quarter of the ASE of pyrene itself. Apparently, aromatic systems are fairly robust. 

---

The C NMR spectrum of cyclobutadiene-iron tricarbonyl consists of a singlet at 209.0 ppm for the carbonyl carbons and a doublet centered at 61.0 ppm, Jc-H = 191 Hz (193). The large C-H coupling constant suggests hybridization at carbon between sp and sp which is consistent with the strained cyclobutadiene ring. In a plot of C versus chemical shifts the cyclobutadiene resonances fall on the same line as resonances for aromatic molecules, their ions and their metal n complexes. 

This mechanical model is depicted in Fig. 21. The Eqs. (15) and (17) have been confirmed for aromatic polyamide fibers by a variety of experiments. Figure 22 shows the dynamic compliance versus the orientation parameter measured during extension of medium and high-modulus PpPTA fibers. It confirms the linear relation (15) and yields e = 240 G Nm and go = 2 G Nm . It has been shown that the tensile curves of the second and higher extensions of an aramid fiber are well described by Eq. (17) [143]. A relation between the strain and the dynamic... 

---