Perlin effect reverse

Ab initio calculations at the MP2 level of theory of a collection of substituted 1,3-dioxanes, 1,3-oxathianes, and 1,3-dithianes have been employed to study both the position of the conformational equilibria and the validity of the Perlin effect <2005T7349>. The 7h,c coupling constant proved to be a valuable tool in conformational analysis both twist conformers, in addition to the chair and alternative chair forms, could be readily identified simply by comparing experimental Vh,c coupling constants to the corresponding calculated values in the particular forms. In addition, the Perlin and reversed-Perlin effects of the C2-H fragments, c in 1,3-dioxanes,... 

The analysis of H and C data, particularly H chemical shifts and C-H coupling constants, is indeed often used to determine C-H bond orientation. In cyclohexane, / C-H coupling constants of equatorial C-H bonds ( /c-Heq) greater than those of axial bonds ( /c-Hax) (the Perlin effect). This effect also operates in 1,3-dioxanes for the C-2 position protons <73ACS2676>. In contrast, the C-5 position protons display a reverse Perlin effect when the conformation is locked by a 2-t-butyl substituent <94JCS(P2)ii5i>. Such a reverse Perlin effect has been observed for all ring protons in... 

Figure 8,8 Reverse Perlin effect (equatorial C-H bond elongation) stems from the Plough effect in 1,3-dioxane, 1,3-dithiane and 1,3-oxathiane but the W-effect is greater in azacyclohexanes.

Reverse Perlin effect In contrast to cyclohexane, axial protons at the p-carbons of heterosubstituted cyclohexanes may have coupling constants larger than those of the equatorial protons (i.e. the reverse Perlin effect). In 1,3-dioxanes and 1,3-dithianes, equatorial C-H bonds at the five-position have been shown to display this behavior (Figure 12.42). 

The term reverse can be misleading for these systems, because the normal and the reverse PerUn effects share the same structural origin - the shorter C-H bond has a greater C-H coupling constant. In this sense, the reverse Perlin effect is perfectly normal ... 

The discovery of the reverse Perlin effect led to subsequent refinement of the general picture for homoano-meric effects. For example, the earlier suggestion that the W-arrangement of interacting orbitals (pseudoe-quatorial lone pair is responsible for the reverse Perlin effect in 1,3-dioxanes has been tested using... 

Figure 12.43 (a) Reverse Perlin effect originates from homoanomeric effects in saturated heterocycles,... 

An effect similar to the well-known reverse Perlin effect has been observed by Silla et al. on the couplings of a- and p-o-glucopyranosyl fluoride tetracetate, both in nonpolar and polar solution VcFaxOf -229.6 Hz and VcFeq of —220.7 Hz have been found in cyclohexane solution. This has been called by the authors reverse fluorine Perlin-like effect , and shown to be ruled by dipolar interactions rather than by hyperconjugation. The reverse fluorine Perlin-like effect does not have a general relationship with the anomeric effect, and it can be useful to determine the structure and stereochemistiy of organofluorine compounds. 

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