Least motion reaction path

In order to assess the relative reactivity of different diacetylene monomers the packing parameters can be analysed in terms of a reaction along a least motion reaction path, i.e. the molecules simultaneously rotate and translate along the stacking direction In this model maximal reactivity is expected for d 5 A and 45°. 

The addition of H2Si to ethylene was studied at the MP2/6-31G //3-21G level298. The C2v path is symmetry forbidden282 and the reaction proceeds with no barrier via a non-least-motion Cs path. The overall exothermicity of the reaction is 47.9 kcal mol"1 at the above level of theory298. 

Consider an analogous least-motion reaction for the reductive elimination process in CH2R2 that yields CH2 and R—R. The MOs displayed in 19.74 and 19.75 are analogs of 19.70 and 19.71, respectively. They have been labeled according to the C2V symmetry of the molecule. A least-motion path then conserves C2V symmetry. The 0 level, 19.74, again becomes the C—C cr level, 19.76, of the... 

In some cases orbital symmetry rules can label the least-motion approach of two reacting fragments as forbidden. Semi-empirical MO calculations, such as EHT ones, can then be used to pick out the minimum-energy path, as outlined in the foregoing section. Another example is given by the reactions 17) ... 

For 5-alkylidenetetrazolines (33) three reaction paths have to be considered. They afford either N-methyl ketene imines and methyl azide, or, in addition to molecular nitrogen, a 3-alkylidenediaziridine (least-motion path) or a methyliminoaziridine (non-least-motion path). As was shown by... 

Basch in 1970 carried out one of the first MCSCF calculations on a polyatomic system216 in an investigation of the least-motion, coplanar approach of two methylenes to give C2H4. For two closed shell, singlet state methylenes, the reaction path is purely repulsive, and the reaction occurs for two appropriately oriented bent triplet methylenes. 

The situation with respect to the photobehavior of 7-chlorocoumarin is interesting (Fig. 11). There are two reaction pathways in this crystal one that favors the formation of the vyn-IIII isomer arising from reaction between the transla-tionally related molecules with a center-to-center distance of 4.54 A and another that would yield the anti-HT dimer, corresponding to the reaction between the centrosymmetrically related molecules, the center-to-center distance being 4.12 A. Experiment clearly shows that only the yy -HH dimer is obtained—not the one that would correspond to the path of least motion. This is supported by the results of the lattice energy calculations. The implication is that the shape of the free volume is anisotropic, with the larger volume or extension in the direction of the translational periodicity of 4.54 A. 

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),... 

Let us discuss now the most important conclusions that can be deduced from these figures. First, the most important conclusion concerns the comparison of the values of functional L along the optimal allowed and forbidden reaction paths. As can be seen, the value for the allowed conrotatory cyclisation is lower in absolute value than in the forbidden one. This confirms the intuitive expectation of the least motion principle that the extent of electron reorganisation should be smaller in allowed reactions than in the forbidden ones. On the basis of this primary test of reliability of the proposed model it is, in the next step, possible to start with the analysis and the classification of the reaction mechanisms for both individual reactions. Especially interesting in this connection is again the thermally forbidden disrotatory cyclisation. The reason for this... 

The so-called least-motion path has been popular for many years. In the original formulation, the elementary reactions that involve the least change in the atomic and electronic configurations are favored (160,161). This hypothesis has found numerous applications in organic and inorganic chemistry (162-167). However, it is necessary to admit that there exist rather numerous exceptions, primarily due to the fact that the non-least-motion pathway is symmetry allowed while the least-motion path is symmetry forbidden (168,169). Dimerization of singlet carbenes is a typical example. 

Both H2C and H2Si have the same frontier orbitals, denoted by a and 7c. Two possible reaction paths can be considered for the dimerization reaction a least-motion path where both molecules approach each other in a plane and a non-least-motion path where at large distances the fragments approach each other in perpendicular planes (Figure 46). 

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