Abstract
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•Transition state barriers for hydroquinone rotation are stabilized by van der Waals interactions and hydrogen-bonding.•The regular pillar-type structure is destroyed by substitution at hydroquinone units.•Rotational barriers decrease for alkyl substitution up to n-propyl due to stabilization by dispersion interactions.
Pillar[5]arenes, a type of novel macrocycles containing di-substituted hydroquinone units linked by methylene bridges in para-positions, have attracted extensive attention in supramolecular chemistry as interesting candidates to be used in the preparation of host-guest complexes. Functionalization by means of rim substitution and sustaining an ordered substituent arrangement on both sides of the rim is important for the development of new pillararene-based materials. In order to achieve this, the rim inversion process of rotating the hydroquinone units through the pillar[5]arenes has to be controlled. In this context we have studied the effect of different types of hydroquinone substituents on the rotational energy profile using density functional theory combined with the hybrid M06-2X functional. The influence of polar (CH2F, CH2Cl, CH2OH, CH2SH, CH2NH2) and nonpolar alkyl (CH3, CH2CH3, CH2CH2CH3, CH(CH3)2 and CH2CH2CH2CH3) substituents on the on the energy barriers of the rotation mechanism, and different local minima was investigated. The stabilization of the intermediate structures by non-covalent van der Waals and interactions and also by hydrogen bonds constitute a major factor affecting barrier heights. In case of polar substituents, the largest barriers were found for CH2OH and CH3 substitutions and the lowest ones for CH2SH and CH2NH2. For the alkyl series, the barrier decreased significantly up to propyl due to increasing stabilizing dispersion interactions while it increased again for n-butyl since the chain did not fit in well the cavity to rotate through.