Conformation twisted

Yang JS, Hwang CY, Chen MY (2007) Bimodal fluorescence signaling based on control of the excited-state conformational twisting and the ground-state protonation processes. Tetrahedron Lett 48 3097-3102... 

The less stable puckered conformation of cyclohexane, with both parts puckered upward. The most stable boat is actually the twist boat (or simply twist) conformation. Twisting minimizes torsional strain and steric strain, (p. 113)... 

The smallest conceivable conjugated monocyclic diene is 1,3-cyclobutadiene. Several complexes involving cyclobutadiene are known. The compound itself is unstable and has not been studied by structural methods. It will therefore not be included in the present discussion. 1,3-Cyclobutadiene has, however, been isolated in argon matrices, and it has been established that the molecule has symmetry. For the tetra-tert-butyl derivative an envelope conformation (twist angle 7°) was found by X-ray methods, however the distances in the ring were obviously too similar for an antiaromatic system [1.464(3) and 1.483(3) A]. A redetermination at even lower temperatures gave more reasonable results (1.441 and 1.527 A) and a further analysis of the anisotropic parameters revealed that some residual disorder is still responsible for some equilibration and distances of 1.34 and 1.60 A were assumed to be the correct ones. ... 

An interesting difference in ring conformation, twist and chair respectively, is displayed by the cis- and trans-isomers of the dithiodihydrazidodimetaphosphoric esters (64), although bond-length and angle data are closely similar for the two isomers.New cyclic phosphorus(v)-hydrazine species (65) in which the phosphorus is in five-fold co-ordination have been prepared from a chlorophos-... 

The direct mapping of the electronic structure of the conjugated polymers is possible in photoelectron spectroscopy of thin films. The valence band, which is available from ultraviolet photoelectron spectroscopy, clearly shows shifts as well as a reduction of photoelectron peaks coming from the frontier k orbitals [31—33]. This is consistent with predictions from calculations using a Valence Effective Hamiltonian using a model of conformational twists separating every dimer on the chain. 

For the classical vibrational analysis ( which does not yet consider -dependent modes) we know that the conformational dependence of the IR and Raman spectra (frequencies and intensities) of polyaromatic molecules linked by "single" C-C bonds should be very small for small deviation out of planarity. As fully discussed in other simple cases [ 142,143 ] in principle the lowering of the symmetry produced by the conformational twisting should give IR /Raman activity originally silent for the fully coplanar higher symmetry system. Slight frequency shifts of a few torsionally dependent normal modes can also be predicted and calculated. 

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