Least motion path

Ehrenson S 1974 Analysis of least motion paths for molecular deformations J. Am. Chem. Soc. 96 3778... 

Fig. 8. a) Non-least-motion approach of two methylenes. The sequence 1-2—3 shows the mutual orientation of the two fragments with decreasing distance, b) Energy variation along the least-motion path (i) and along the least-energy path (2). The abscissa scale is the C-C distance (A). The vertical bar is 1 eV on the ordinate energy scale. 

We first comment on the so-called least motion path (LMP), in which the two H atoms move away from the CO atoms, maintaining a Czv symmetry, as shown in Fig. 16.6. Earlier calculations of all sorts indicate that this path does not cross... 

Ring contraction by the non-least-motion path requires the intermediate formation of an acyclic species that is able to undergo structural changes that finally provide a new three-membered ring in which a formerly exo-cyclic atom has moved to a ring position. An explanation of this behavior is not always simple (discussed later) because unexpected products may... 

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... 

In the photolysis of methyl-(3,5,5-trialkyl-3,5-dihydro-[l,2,3]triazol-4-ylidene)-amine (34) in solution, aziridine imines are formed quantitatively, about 85% on the non-least-motion path and 15% on the least-motion path (90CB2195). The gas-phase pyrolysis of 34 affords only the product of the former path (93CB2683). 

As mentioned earlier, [5 —> 3 + 2] ring contraction of a five-membered ring compound with an exocyclic double bond can lead to a three-membered ring either on the least-motion or on the non-least-motion path. In the most general case, three different products are possible one is formed directly on the former and two can be formed indirectly on the latter path. We have investigated the relevant question of selectivity in some detail (92TH1). 

If it is a real intermediate and not too short-lived, it is likely that the thermodynamically most stable product will be formed, and this seems to be the usual case. Examples are found in the decompositions of 25-27, which follow the least-motion path, and 31-37, which follow the non-least-motion path. The most obvious exception is the triazolinone system 30 (Scheme 1). 

If a major contributor to least motion effects in. solution is the varying degrees of solvent reorganisation required for varying degrees of nuclear motion, then it is not clear what function of atomic displacements should be minimised to determine the least motion path. Following Hine (1966a), the square of the atomic displacement has been minimised, but if solvation forces are involved, this will be accurate only for small displacements. 

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. 

Basch investigated the photodecomposition of keten and carried out both SCF and MCSF calculations with an augmented DZ basis set. It was concluded that the first excited triplet state of keten can form CH2( Ba) and CHaCMi) relatively rapidly and the first excited singlet state of keten can give CH2( R2) easily in a near-least-motion path. However, the formation of CHj(. 4i) from the first excited singlet state of keten by a near-least-motion path seemed to be very improbable. 

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). 

Approximate non-least-motion paths have been determined by optimization of the geometrical parameters as functions of the C-C, C-Si or Si-Si distance at the extended Hiickel or at the ab initio SCF level. The results of the various studies " " can be summarized as follows ... 

Two singlet silylenes dimerize to give a ground-state disilene with a substantial barrier via the least-motion path and without a barrier via the non-least-motion path. On the other hand, two triplet silylenes dimerize to give adiabatically an excited state of disilene. 

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