Dominant conformers, interconversion

The structures and relative energies for the basic conformations of silacyclohexane 173 were calculated using HF, MP2, RI-DFT and MM3 methods (00ZAAC853). All predict the chair form to be the dominant conformation (boat and twist forms, also found, are less stable) the transition state for the chair-to-chair interconversion consists of a sofa-like half-chair conformation (cf. Scheme 57). The steric energy of the transition state is 6.57 kcal/mol higher than that of the chair conformation. 

The n-site Bloch-McConnell equations describe the evolution of nuclear spin magnetization in the laboratory or rotating frames of reference for molecules subject to chemical or conformational interconversions between n species with distinct NMR chemical shifts. Trott and Palmer used perturbation theory to approximate the largest eigenvalue of the Bloch-McConnell equations and obtain analytical expressions for the rotating-frame relaxation rate constant and for the laboratory frame resonance frequency and transverse relaxation rate constant. The perturbation treatment is valid whenever the population of one site is dominant. The new results are generally applicable to investigations of kinetic processes by NMR spectroscopy. 

Note that there are two conformers, one with the two H8 atoms on the same side of the coordination plane (syn isomer) and the other with one H8 atom on each side of the coordination plane (anti isomer). Refine and save both using the MOMEC97 force field. Section 17.14 describes how to enforce planarity of a coordination compound. Two dimensional NMR methods can be used to determine which isomer dominates - as long as interconversion of the isomers is not rapid on the NMR time scale. The data used here are hypothetical and we have assumed that one isomer dominates to the exclusion of the other and that there is no interconversion, i. e., the observed NMR spectrum is that of an isomerically pure compound. 

The pseudorotation concept was introduced to describe the continuous interconversion of puckered forms of the cyclopentane ring [104]. The same concept is applied to the furanose geometry where the Cl, 04, and C4 atoms lie in one plane and the C2 and C3 atoms lie above and below that plane. A statistical analysis of X-ray crystal structures of nucleosides and nucleotides has shown that North (N) 130 and South (S) 131 conformations are the most dominant forms, which has been the basis of the assumption of the two-state N — S pseudorotational equilibrium in solution (Fig. 2.44). 

---