Principle of least structural change

Even more striking is the exclusive formation of 3-methoxybenzenamine in the animation of 2-chloromethoxybenzene. Notice that this result is a violation of the principle of least structural change (Section 1-1H) ... 

These considerations gave rise to the principle of least structure change by Muller (1856) ... 

The idea that unsymmetrical molecules will orient at an interface is now so well accepted that it hardly needs to be argued, but it is of interest to outline some of the history of the concept. Hardy [74] and Harkins [75] devoted a good deal of attention to the idea of force fields around molecules, more or less intense depending on the polarity and specific details of the structure. Orientation was treated in terms of a principle of least abrupt change in force fields, that is, that molecules should be oriented at an interface so as to provide the most gradual transition from one phase to the other. If we read interaction energy instead of force field, the principle could be reworded on the very reasonable basis that molecules will be oriented so that their mutual interaction energy will be a maximum. 

The possibility that reacting species prefer to react along those paths in which they undergo the least modification has always been intuitively attractive. At one time or another, so-called principles of minimum structural change or deformation, configurational change, and minimum atomic and electronic motion have been invoked (Wheland, 1960 Hine, 1966). To account for Michael s rule of favored anti 1,2-addition, Pfeiffer formulated acetylenes as tram-heat structures in 1904 Frankland (1912) suggested that anti elimination is favored by an inherent tendency to centric symmetry. The more conscious applications of PLM by Muller after 1886, are probably misapplications of the principle, since they were usually concerned with complex pyrolytic reactions above 1000° (Muller and Peytral, 1924). 

In order to specify the secondary or tertiary nature of these other non-primary products, complementary experiments can be carried out which study the reactions of the primary products systematically, either alone or in the presence of the reactants or of other primary products. The principle of least change of structure could also be used, which states that the structural changes which enable a species to change into another must be as small as possible. 

The above reactions are otherwise reasonably compatible with the principle of least change of structure as they correspond to likely schemes... 

Similarily, Yoshida and Ranby (23) found a well-resolved 24-line spectrum for the growing vinyl acetate chain in dilute aqueous solution five nonresolved lines may be seen for the vinyl acetate popcorn system. Least structured is the styrene spectrum, with three nonresolved lines. In principle, the same features appear in the substituted styrenes represented in Figures 13 and 14, and the vinylpyridines (Figure 15). Only the form of the shoulders changes. With better resolution, these spectra should consist of at least six lines. 

As can be seen, this index attains its maximal value of unity for two identical structures (9 = 9,

monotonously decreases. The use of this index for the formulation of the least motion principle arises from the following simple idea. Let us assume that we are on a reaction path at point characterised by the wave function P(, q> ) and we are looking for such an infinitesimally close structure (9, (p) for which the transformation (9,

requires minimal change in electronic configuration. This condition is equivalent to a search of the direction along which the derivative of K at the point = 9 and q> = q> attains its minimum. This directional derivative can be mathematically described as (27),... 

This type of scission of the molecule complies with the general principle of the least change in structure. 

Principles for the interpretation of photoelectron spectra of silicon compounds (Sections II, IV and V) are as follows. Although the rather instantaneous ionization within 10 15 s does remove all difficulties concerning M 0 structure changes and specific M 0 state dynamics, still electronic relaxation is possible and especially the differences in the correlation of n electrons in M and n-1 in M 0 should not be neglected. Therefore, all PE spectroscopic information available on individual M 0 states (Section IV.B) should predominantly be used for their assignment. Last but not least, orbital models derived from simplified calculations should only be trusted, if their essentials can be substantiated by comparison of equivalent states within series of chemically related molecules9 (Section IV.A). 

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