Bonds that cannot break

It is worth noting that the force fields correspond to a fixed (and unchangeable during computation) system of chemical bonds. The chemical bonds are treated as springs, most often satisfying Hooke s law (harmonic), and therefore unbreakable. Similarfy, the bond angles are forced to satisfy Hooke s law. Such a force flexible MM field is known as flexible molecular mechanics. Tb decrease the number of variables, 

Molecular mechanics represents a method of finding a stable configuration of the nuclei by using a minimization of V R) with respect to the nueleai eoordinates (for a moleeule or a system of moleeules). 

A big problem in molecular mechanics is that the final geometry is very close to the starting one. We start from a boat (chair) conformation of cyclohexane and obtain a boat (chair) equilibrium geometry. The very essence of molecular mechanics however, is that when started from some, i.e. distorted boat (chair) conformation, we obtain the perfect, beautiful equilibrium boat (chair) conformation, which may be compared with experimental results. Molecular mechanics is extremely useful in conformational studies of systems with a small number of stable conformations, either because the molecule is small, rigid or its overall geometry is fixed. In such cases all or all reasonable , conformations can be investigated and those of lowest-energy can be compared with experimental results. 

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In contrast, aU the electrons in the compound on the right are localized. The lone-pair electrons on nitrogen caimot be shared with the adjacent carbon because carbon cannot form five bonds. The octet mle requires that second-row elements be surrounded by no more than eight electrons, so sp hybridized carbons cannot accept electrons. Because an sp hybridized carbon has a rr bond that can break, has a positive charge, or has an unpaired electron, it can accept electrons without violating the octet rule. 

Notice, as you study the following resonance contributors and practice drawing them, that electrons (tt electrons or lone pairs) are always moved toward an sp or sp atom. (Remember that an sp carbon is either a positively charged carbon or a double-bonded carbon and an sp carbon is generally a triple-bonded carbon Sections 1.8, 1.9, and 1.10). Electrons cannot be moved toward an sp carbon because an sp carbon has a complete octet and does not have a tt bond that can break, so it cannot accommodate any more electrons. 

The following species do not have delocalized electrons. Electrons cannot be moved toward an sp hybridized atom because an sp hybridized atom has a complete octet and it does not have a tt bond that can break, so it cannot accept any more electrons. 

The harmonic approximation is only valid for small deviations of the atoms from their equilibrium positions. The most obvious shortcoming of the harmonic potential is that the bond between two atoms cannot break. A physically more realistic potential is the Morse potential (Fig. 8.1) ... 

This is the case for the quadricyclane - to norbornadiene" reaction. Although the C2K reaction path provides an attractive interpretational tool for understanding the progress of this reaction, its highest point represents a conical intersection at which the two relevant states have the same energy at the same geometry. This point cannot be a transition state, so that lowering the symmetry in any direction leads to a stabilization. The result is an asynchronous reaction path in which one of the two cyclopropane bonds is broken first to form the biradical-like transition state la. The second bond can then break to form the norbornadiene radical cation 2. 

Not all fibers yield fibers on comminution. Fibrous varieties of quartz (Si02), for example, are formed from tightly bonded, aligned helical fibers that cannot be separated mechanically (Frondel, 1978). Fibrous calcite (CaCOa), when crushed or ground, breaks into equant grains of rhombic shape. The fragments reflect the cleavage characteristics of the mineral. 

On that basis, conformers A and E can break down by the loss of the axial alkoxy group. Conforrner can undergo a cleavage via the fission of the carbon-oxygen bond of the ring but conforrner cannot break down (no primary electronic effect). Conforrner must therefore be unreactive and this prediction was verified experimentally (38, 50) by studying the reactivity of the tricyclic orthoester 62 (54-56) which is a perfect rigid model for conforrner , as shown by X-ray analysis (57). Indeed, compound 6 was found completely stable under the mild acid conditions that are normally used for the hydrolysis of other cyclic orthoesters. Thus, conforrner is a remarkably unreactive conforrner which must be eliminated. 

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