Glycine in a basket: protonated complexes of 1,1,n,n-tetramethyl[n](2,11)teropyrenophane (n = 7, 8, 9) with glycine in the gas-phase

Abstract

Protonated complexes composed of a basket-like host molecule 1,1,n,n-tetramethyl[n](2,11)teropyrenophanes (TMnTP) (n = 7, 8, 9) and glycine as a guest were studied in the gas phase by experimental and computational methods. Blackbody infrared radiative dissociation (BIRD) experiments of [(TMnTP)(Gly)]H⁺ not only provided the observed Arrhenius parameters (activation energies, E^obs_a, and frequency factors, A) but also suggested the existence of two populations of isomeric complexes of [(TMnTP)(Gly)]H⁺, termed fast dissociating (FD) and slow dissociating (SD), due to their relative BIRD rate constants. Master equation modeling was conducted to obtain the threshold dissociation energies E₀ of the host–guest complexes. The relative stabilities of the most stable of the n = 7, 8, or 9 [(TMnTP)(Gly)]H⁺ complexes followed the trend SD-[(TM7TP)(Gly)]H⁺ > SD-[(TM8TP)(Gly)]H⁺ > SD-[(TM9TP)(Gly)]H⁺ by both BIRD and energy resolved sustained off-resonance irradiation collision-induced dissociation experiments (ER-SORI-CID). Computed structures and energies of [(TMnTP)(Gly)]H⁺ were obtained using B3LYP-D3/6-31+G(d,p) and for all TMnTP molecules, the lowest-energy structures were ones where protonated glycine was within the cavity of the TMnTP, despite the TMnTP molecules having a proton affinity 100 kJ mol⁻¹ higher than glycine. An independent gradient model based on the Hirshfeld partition (IGMH) and natural energy decomposition analysis (NEDA) were applied to visualize and reveal the nature of interactions between hosts and guest. The NEDA analysis suggested that the polarization (POL) component which described interactions between induced multipoles contributed the most to the [(TMnTP)(Gly)]H⁺ (n = 7, 8, 9) complexes.