Conformational isomerism

About: Conformational isomerism is a research topic. Over the lifetime, 11563 publications have been published within this topic receiving 199312 citations.

Kinetics of helix-handedness inversion: folding and unfolding in aromatic amide oligomers.

Abstract: A series of helically folded oligoamides of 8-amino-2-quinoline carboxylic acid possessing 6, 7, 8, 9, 10 or 16 units are prepared following convergent synthetic schemes. The right-handed (P) and the left-handed (M) helical conformers of these oligomers undergo an exchange slow enough to allow their chromatographic separation on a chiral stationary phase. Thus, the M conformer is isolated for each of these oligomers and its slow racemization in hexane/CHCl(3) solutions is monitored at various temperatures using chiral HPLC. The kinetics of racemization at different temperatures in hexane/CHCl(3) (75:25 vol/vol) are fitted to a first order kinetic model to yield the kinetic constant and the Gibbs energy of activation for oligomers having 6, 7, 8, 9, 10 or 16 quinoline units. This energy gives the first quantitative measure of the exceptional stability of the helical conformers of an aromatic amide foldamer with respect to its partly unfolded conformations that occur between an M helix and a P helix. The trend of the Gibbs energy as a function of oligomer length suggests that helix-handedness inversion does not require a complete unfolding of a helical strand and may instead occur through the propagation of a local unfolding separating two segments of opposite handedness.

Mixed-valence ruthenium complexes rotating through a conformational Robin-Day continuum.

Abstract: The conformational energy landscape and the associated electronic structure and spectroscopic properties (UV/Vis/near-infrared (NIR) and IR) of three formally d5/d6 mixed-valence diruthenium complex cations, [{Ru(dppe)Cp*}2(μ-C≡CC6H4C≡C)]+, [1]+, [trans-{RuCl(dppe)2}2(μ-C≡CC6H4C≡C)]+, [2]+, and the Creutz–Taube ion, [{Ru(NH3)5}2(μ-pz)]5+, [3]5+ (Cp=cyclopentadienyl; dppe=1,2-bis(diphenylphosphino)ethane; pz=pyrazine), have been studied using a nonstandard hybrid density functional BLYP35 with 35 % exact exchange and continuum solvent models. For the closely related monocations [1]+ and [2]+, the calculations indicated that the lowest-energy conformers exhibited delocalized electronic structures (or class III mixed-valence character). However, these minima alone explained neither the presence of shoulder(s) in the NIR absorption envelope nor the presence of features in the observed vibrational spectra characteristic of both delocalized and valence-trapped electronic structures. A series of computational models have been used to demonstrate that the mutual conformation of the metal fragments—and even more importantly the orientation of the bridging ligand relative to those metal centers—influences the electronic coupling sufficiently to afford valence-trapped conformations, which are of sufficiently low energy to be thermally populated. Areas in the conformational phase space with variable degrees of symmetry breaking of structures and spin-density distributions are shown to be responsible for the characteristic spectroscopic features of these two complexes. The Creutz–Taube ion [3]5+ also exhibits low-lying valence-trapped conformational areas, but the electronic transitions that characterize these conformations with valence-localized electronic structures have low intensities and do not influence the observed spectroscopic characteristics to any notable extent.