Lowest energy pathway

The overall picture of the many results which have been obtained with hetero-substituted cyclohexane rings is a very consistent one. Cyclohexane itself in its lowest energy conformation adopts the so-called chair conformation, as depicted in Figure 3 by the two outer formulae (a, b). These are contained in energy wells ca. 42 kJ moP deep. Another conformation, of low abundance in cyclohexane at normal temperatures, but which is important in some substituted derivatives, is the twist form (c, d). This is ca. 22 kJ moP less stable than the chair forms, and it lies on the lowest-energy pathway between them. 

In our laboratory we have utilized multiphoton infrared laser activation of metal ion-hydrocarbon adducts to probe the lowest energy pathways of complex reaction systems (6). Freiser and co-workers have utilized dispersed visible and uv radiation from conventional light sources to examine photochemical processes involving organometallic fragments... 

Absorption of infrared photons identification of lowest energy pathway, 35,36f... 

While the lowest-energy pathway involved, again, ligand dissociation (phosphine) followed by coordination of methane to form the... 

The kinetics and mechanism of pyrrole pyrolysis were investigated by ab initio quantum-chemical calculations. It was revealed that pyrrole undergoes tautomerization to form 2H- and 37/-pyrroles prior to any thermal decomposition. It has been shown that the major product, HCN, arises from a hydrogen migration in pyrrole to form a cyclic carbene with the NH bond intact. Ring scission of the carbene leads to an allenic imine of HCN and propyne which is the lowest energy pathway. The 277-pyrrole... 

Thus, following the lowest energy pathway between minima might require C3-03 bond rotation in 8. This cursory examination indicates that, while the location of energy minima may not be affected by non-anomeric hydroxyl conformations, the relative energies of these minima may indeed depend on these conformations. 

Experimentally based intuitive arguments have been presented to arrive at a regional rule for optical activity of d-d transitions of conformational isomers of octahedral metal complexes. Conformational preferences for chelate rings formed by 1,3-pn in its octahedral mono, bis, and tris metal complexes have been studied by calculation of the conformational energies. In all cases, the chair conformation was found to be the most stable. The lowest energy pathway for converting from one chair configuration into another has a barrier to activation of about 7 kcal mol Conformational types of metal-edta complexes have been studied. ... 

Note that there are many possible transition states (different coordinates may be singled out as the reaction coordinate). What this means is that merely finding a transition state does not guarantee that this is the transition state , meaning that it is at the top of the lowest-energy pathway that smoothly connects reactants and products. While it is possible to verify the smooth connection of reactants and products. 

The ratio of Type II fragmentation to Type II cyclization products may depend strongly on the excited state from which reaction occurs. The lowest-energy pathway for fragmentation requires continual orbital overlap between developing p bonds. Cyclobutanol formation, however, has less stringent orbital orientation requirements. When the configuration of the ketone is unfavorable... 

Calculations by Francisco examined decomposition reactions of ethylsilylene EtSiH242. Elimination to form SiH2 and ethylene is computed to be the lowest energy pathway, with... 

Most of other trajectories obtained in this study exhibited intermediate characters between trajectories (a) and (b). An important message was that the chemical reaction does not always proceed through the lowest energy pathway with optimal solvation. In conclusion, the simulations for the first time illustrated how the atoms in reacting molecules behave in solution at the molecular level, which was made possible by using full QM simulations with the recently developed FMO-MD methodology. 

In view of the fact that nature always chooses the lowest energy pathway, one may wonder why these reactions do not proceed via a more balanced transition state with more advanced delocalization, which would presumably lower its energy. This question has been discussed at considerable length in our 1992 review4 and hence only an abbreviated version is presented here. 

O or N protonation mechanism was the lowest energy pathway, the authors preferred the O protonation mechanism, via (78). Steric effects at N(l) and C(2) were found to have a significant effect on the rate of reaction. 

We turn to empirical force field calculations in order to choose between these two mechanisms. 44 Such calculations indicate that the two-ring flip mechanism is the lowest-energy pathway, and yield a barrier of 20 kcal/mol for the two-ring flip of 7. 44> The experimental free energy of activation for stereoisomerization derived from the temperature-dependent 1H-nmr spectrum, is AG 67 21.9 kcal/mol 43, in excellent agreement with the calculated value. This high barrier admits of the possibility that 7 is separable into its optical antipodes at moderately low temperatures. 

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