Variation of the stereoparameters for description geometry of calix[4]arenes - more suitable solution for "flat systems"
Abstract
This article deals with a variation of previously introduced parameters α, β, and δ which were used to describing the all possible conformations of these compounds, cone, partial cone, 1,2- and 1,3-alternate conformers of methylene- and heteroatom-bridged calix[4] arenes. Usefulness of these parameters α, β, δ have been already demonstrated, but, it seems, to be more suitable to do any variations them for flat systems.
The background for the parameters α, β, δ lies in the representation of the scale of the angles of the calix[4] arene rings towards the reference plane. The original scale 0 degrees - 360 degrees for these angles depicts very well the differences between the calix[4] arene conformations but has two significant disadvantages.
The first one is the discontinuity of the scale. The angles α(i) range approximately from 0 degrees to 110 degrees and from 250 degrees to 360 degrees because of the sterical hindrance that would occur should the calixarene phenyl ring enter the cavity.
The second disadvantage is the discontinuity at the point α(i) = 0 degrees, it means the case of flat systems. These flat systems are usually the 'transitions states' between the calixarene conformations and cannot be observed using parameters α, β, δ.
To eliminate the difficulties in describing the 'transition states' between the calix[4] arene conformations caused by the discontinuities of the original scale, the new scale should be introduced. New parameters α', β', δ' were introduced by subtracting 360 degrees for each 'negative' ring present in the structure (one for partial cone conformers, two for 1,2- and 1,3-alternate conformers); after the parameters α, β, δ are calculated.