Steric isotope effects gauged by the bowl-inversion barrier in selectively deuterated pentaarylcorannulenes

Abstract

Motivated by a greater bowl depth and barrier to bowl inversion in sym-1,3,5,7,9-pentamanisylcorannulene compared to corannulene, an experimental plan is developed to measure the effective hydrogen/deuterium steric kinetic isotope effect (KIE). Symmetry arguments are used to design orthogonal isotope labeling patterns so that the barrier for the CD3 compound can be measured in the presence of the CH3 compound. This scheme eliminates the differential uncertainty in the temperature measurement by allowing both barriers to be measure in the same sample, which in turn reduces the error in determining the differential barrier. Ab initio computations corroborate the structure and isotope effect found experimentally. The predicted and determined steric KIE at 250 K is 1.08 (modified QUIVER at M06-2X/cc-pVDZ) and 1.22 +/- 0.06 (VT-NMR), respectively. The results stem from differences in zero-point energy of the CH and CD motions; however, the phenomenology makes the CD3 group appear effectively "stickier" than CH3. The more the C-H...X interaction steepens the well, the "stickier" C-D should appear to be relative to C-H--an important consideration for molecular recognition and one supported by stronger binding constants for deuterated substrates.