Herein we propose a general mechanism for the racemization of [n]helicenes up to n = 24. It is a concerted process for n = 4-7, but a multi-step mechanism is followed for n≥ 8, involving 2n- 14 intermediates. The changes in the barriers are a delicate consequence of the steric hindrance and the π-interactions.
In this work, we analyze the interactions of alkali metal cations with [6]- and [14]helicene and the cation mobility of therein. We found that the distortion of the carbon skeleton is the reason that some of the structures which are local minima for the smallest cations are not energetically stable for K , Rb , and Cs . Also, the most favorable complexes are those where the cation is interacting with two rings forming a metallocene-like structure, except for the largest cation Cs , where the distortion provoked by the size of the cation destabilizes the complex. As far as mobility is concerned, the smallest cations, particularly Na , are the ones that can move most efficiently. In [6]helicene, the mobility is limited by the capture of the cation forming the metallocene-like structure. In larger helicenes, the energy barriers for the cation to move are similar both inside and outside the helix. However, complexes with the cation between two layers are more energetically favored so that the movement will be preferred in that region. The bonding analysis reveals that interactions with no less than 50 % of orbital contribution are taking place for the series of E -[6]helicene. Particularly, the complexes of Li show remarkable orbital character (72.5-81.6 %).
The possible existence of HSO in aqueous sulfuric acid is analyzed in detail. For bare HSO, the computed free energy barrier for the exergonic transformation of HSO into the HSOHO complex is only 3.8 kcal mol. The presence of water or sulfuric acid catalyzes the dehydration to such an extent that it becomes almost a barrierless process. In the gas phase, dehydration of HSO is an autocatalytic reaction as the water molecule produced by the decomposition of one HSO molecule induces further dissociation. Thus, in solution, the surrounding water molecules make the para-sulfuric acid a very vulnerable species to exist. The simulated Raman spectra also corroborate the absence of HSO in solution.
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