Barriers to planarization

Figure 6.41 Potential energy curves and vibrational energy levels for an inversion vibration when the barrier to planarity is (a) infinite, (b) moderately low and (c) zero...

Molecules with an inversion vibration which is qualitatively similar to that of NH3 are formamide (NH2CHO) and aniline (C6H5NH2) in which the vibration involves primarily the hydrogen atoms of the NH2 group. Both molecules are non-planar, having a pyramidal configuration about the nitrogen atom, with barriers to planarity of 370 cm (4.43 kJ moP ) and 547 cm (6.55 kJ mon ), respectively. 

Another important factor affecting the electronic properties is the steric barrier to planarity along the polymer chain. Since polyheterocycles and polyarylenes must adopt a planar geometry in the ionized state to form quinoid-like segments, steric factors that limit the ability of the polymer to adopt geometries which are planar with respect to adjacent rings have a detrimental effect on the electronic properties (181). 

Not much information is available for the five-membered ring pyrrolidine (4). MM2 calculations predicted that the 2-half-chair form is preferred over a host of other conformers by an average of 0.3 kealmol-1 with a 4.37 kealmol-1 barrier to planarity. The equatorial hydrogen is calculated to be favored by AE = 0.20 kealmol-1 over the axial one. 

There is a qualitative change in the dependence of the Au2 = 1 transition moments on the inversion state at the top of the barrier to planarity, which is about 1800 cm [4,46-50]. Below the inversion barrier, the matrix elements ((i 2 l) lMj k2 ) (V2 — 1) ) are approximately equal. Above the barrier... 

The planarity of the tricoordinate phosphorus and thus the aromaticity of 4 can be influenced by substituents. It turns out from ab initio calculations that 7T-acceptor groups either at phosphorus or at the neighboring carbon have a planarizing effect. The barrier to planarity decreases to 1.56 kcal/mol (MP2/ 6-31G(d)) as a result of —BH2 substitution at phosphorus. The bond length alternation in the planar form of this substituted derivative increases in the... 

Planarization of the P-substituted azaphospholes results in structures with similar aromaticity to the planar phosphole. Replacement of the CH units by N in these rings has some effect on the barrier to planarization. The MP2/6-31C(d) barrier of 277-1,3,2-diazaphosphole was 17.6, while that of 377-1,5,3-diazaphosphole was 13.5 kcal/mol (phosphole 17.2 kcal/mol at the same level). ... 

Analysis of the far IR-spectra of 3,4-dihydro-2//- pyran (13) (72JCP(57)2572> and 5,6-dihydro-2/f- pyran (14) (81JST(71)97> indicates that for both molecules the most stable conformation is a half-chair form. The barrier to planarity is greater for the former compound. These preferred structures are in accord with the half-chair conformation established for cyclohexene and its derivatives. The conformational mobility of cyclohexene is greater than that of the 3,4-dihydropyran. The increased stabilization of the pyran has been attributed to delocalization of the v- electrons of the alkenic carbon atoms and the oxygen lone-pairs (69TL4713). 

Since, in the excited state, the fluorine atoms may be above or below the plane of the benzene ring the potential function for vn is W-shaped, like that in Figure 6.41(b). Fitting the observed vibrational energy levels to the potential function in Equation (6.93) gives the height of the barrier to planarity as 78 cm 1. 

The total barrier to planarity of thietane is 274 cm" (784 cal/mole) so that at room temperature only about 25% of the molecules occupy vibrational levels above the barrier. The barrier is greater than that in oxetane but less than that in cyclobutane. Ring-strain favors planarity whereas eclipsing of methylene protons favors torsion and nonplanarity, a-methylthietan possesses equatorial and axial con-formers observable at room temperature with an inversion barrier of 341 cm (975 cal/mole). 3,3-dimethylthietane has a barrier of 300cm (857 cal/mole). °... 

The H nmr spectrum of thietane 1,1-dioxide in a nematic solvent indicates the ring is nearly planar with a very low barrier to planarity, a result that agrees with variable temperature investigations at 300 and 100MHz. An early nmr study compared the chemical shifts of thietane, thietane 1,1-dioxide, oxetane, and cyclobutane. The chemical shift (6) for the a-protons is 4.09 and for the /3-protons, 2.14. The geminal coupling constant for the a-protons is — 14.0 Hz and for the /3-protons, —12.6 Hz. Other coupling constants are /cis 10-3 Hz, Jtmns ... 

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