Conformational behavior, interconversion

The above energy profiles suggest significantly different conformational behavior in 7-10 both in terms of preferred geometries and modes of conformational interconversion in the gas phase. Inherent in the above analysis is the assumption that the 3-21G calculations provide a reliable picture of gas phase behavior. Of course, this assumption will require validation through calculations with more sophisticated basis sets, especially those employing d-orbitals (e.g., 6-31G ). 

The conformational landscapes of two ILs build from [NTf2] and from /V-propyl-and /V-butyI -A -methylpyrroIidinium were investigated from Raman spectroscopy, MD, and ab initio techniques [135], For [NTf2] the three-dimensional PES and the corresponding MD simulations confirmed the existence of two stable isomers (see also [37, 36]). The anion was calculated to be flexible, albeit hindered, but capable of interconversion between conformers in the liquid state, a result also confirmed by the Raman data. In the case of the ALV-dialkylpyrrolidinium cations, the PES showed a much more limited conformational behavior of the pyrrolidinium ring [135],... 

Saddle points, just like passes in a mountain area, provide the easiest way to pass from one valley to another. On the PES. saddle points therefore represent transition states providing the lowest energy barriers for conformational interconversions. The goal of conformational analysis (see Conformational Analysis 2 Conformational Analysis 3 and Conformational Anafysis Homotopy) is to find the set of interrelated energy minima and saddle points revealing the network of conformational interconversions (NCI). The NCI describes the dynamics of concerted atomic motion characteristic of the conformational behavior of a molecule. Of course, the interesting conformations are those of sufficiently low free energy to be populated at reasonable temperatures. 

Construct models of cw-decalin (cw-bicyclo[4.4.0]decane) and trans-Ascdlm.. Observe how it is possible to convert one conformation of cw-decalin in which both rings are in chair conformations to another all-chair conformation. This interconversion is not possible in the case of the trans-Ascdlm isomer. Suggest a reason for the difference in the behavior of the cis and trans isomers. Hint. What would happen to carbon atoms 7 and 10 of trans- scdlm if the other ring (indicated by carbon atoms numbered 1-6) is converted to the alternative chair conformation. Is the situation the same for cw-decalin ... 

In addition, all representatives are highly fluorescent in the solid state, but essentially nonfluorescent in solution at room temperature. Upon cooling the solutions of benzo[b][l,4]diazepines 51 cryo-fluorescence is observed, which can be attributed to a freezing of the ring interconversion and aggregation. This thermo responsive behavior of fluorophores as a consequence of restricted conformational... 

---