Puckering

However, most impurities and defects are Jalm-Teller unstable at high-symmetry sites or/and react covalently with the host crystal much more strongly than interstitial copper. The latter is obviously the case for substitutional impurities, but also for interstitials such as O (which sits at a relaxed, puckered bond-centred site in Si), H (which bridges a host atom-host atom bond in many semiconductors) or the self-interstitial (which often fonns more exotic stmctures such as the split-(l lO) configuration). Such point defects migrate by breaking and re-fonning bonds with their host, and phonons play an important role in such processes. 

Graphite reacts with alkali metals, for example potassium, to form compounds which are non-stoichiometric but which all have limiting compositions (for example K C) in these, the alkaU metal atoms are intercalated between the layers of carbon atoms. In the preparation of fluorine by electrolysis of a molten fluoride with graphite electrodes the solid compound (CF) polycarbon fluoride is formed, with fluorine on each carbon atom, causing puckering of the rings. 

FIGURE 3 11 Nonplanar ( puckered ) conformation of cydobutane The nonplanar con formation avoids the eclipsing of bonds on adjacent carbons that characterizes the planar con formation... 

Cyclobutane has less angle strain than cyclopropane and can reduce the torsional strain that goes with a planar geometry by adopting the nonplanar puckered confer matron shown m Figure 3 11... 

The puckered nature of this ligand has been neglected in the figure. 

Figure 6.42 (a) Cyclobutane, its ring-puckering vibration and dihedral angle, (b) Cyclopentene... 

Figure 6.43 (a) Ring-puckering potential function for cyclopentene. The reduced coordinate z is... 

C4H8 (cyclobutane) ring-puckering, 191 CgHg (benzene)... 

In the chapter on vibrational spectroscopy (Chapter 6) 1 have expanded the discussions of inversion, ring-puckering and torsional vibrations, including some model potential functions. These types of vibration are very important in the determination of molecular structure. 

The metallic taste (12,19,20) is not ascribed to any special taste buds or mouth area. Along with pungency (the hot taste of peppers), astringency (the puckering taste of alum), and cold taste (the cool effect of menthol), the metallic taste is called a common chemical sense (21). 

Dioxolane also pseudorotates essentially freely in the vapor phase. 2,2 -Bi-l,3-dioxolane (128) has been shown by X-ray crystallography to have a conformation midway between the half-chair and envelope forms. The related compound 2-oxo-l 3-dioxolane (129) shows a half-chair conformation. This result is confirmed by microwave spectroscopy and by NMR data. Analysis of the AA BB NMR spectra of the ring hydrogen atoms in some 1,3-dioxolane lerivatives is in agreement with a puckered ring. Some 2-alkoxy-l,3-dioxolanes (130) display anti and gauche forms about the exocyclic C(2)—O bond. 

Chemical shifts for some representatives of the less common class of A -pyrazolines are shown in Table 13. These cyclic enehydrazines are also probably puckered (69) however, no X-ray determination nor conformational study has been carried out to solve this problem. Compound (70) is representative of the rare N(l)-unsubstituted A -pyrazolines prepared by Burger et al. (79T389). 

Four-membered heterocycles are often puckered rather than planar (Table 4), As expected, exo- and e do-unsaturation tend to make these systems planar. 

Bond angles ( ) Pucker angle Ring inversion Section... 

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