Mechanochemical dethreading of a cyclobis(paraquat-p-phenylene)-derived [2]rotaxane

The elongation of a model of the rotaxane (a) described by Stoll et al. (DOI: 10.1021/ol200801b) was simulated using the CoGEF method (DFT B3LYP/6-31G*, gas). The resulting energy/elongation curve (b), and the related structures (c), reveal that the mechanochemical dissociation of this rotaxane proceed via dethreading at relatively low force (Fmax = 0.96 nN) as the cyclobis(paraquat-p-phenylene) macrocycle can slip over the 2,6-diisopropylphenyl stopper. 

Constrained Geometries simulate External Force (CoGEF) calculations were performed on Spartan 14/20 following Beyer’s method (10.1063/1.481330). The distance between the anchor groups was constrained and increased in increments of 1 Å (0.25Å around Emax). The energy was minimized by molecular mechanics (MMFF) with Spartan 14 then DFT (B3LYP/6-31G*, gas phase) with Spartan 20 using the Energy Profile function implemented in Spartan. The relative energy of each intermediate was determined by setting the energy of the first structure to 0 kJ/mol. The Fmax value was determined from the slope of the final 40% of the energy/elongation curve (i.e. from 0.6 Emax to Emax).