To gain insight into the origin of NH4 + selectivity, attempts were made to grow single crystal of 2d ⊃ NH4 + for structural characterization but they all failed. We instead turned to DFT calculation to optimize both the structures and energies of 2a ⊃ NH4 + in different possible conformations of 2a. Based on these results, a conformation preference switch from 4u to trans-2u2d takes place upon NH4 + binding, which is >1.6 kcal/mol more stable than the next preferred conformers (4u), and significantly more preferred than 3u1d and cis-2u2d by >6 kcal/mol (Figure ). However, due to the small energy difference, we believe both 4u and trans-2u2d conformations are predominated in 2a ⊃ NH4 + . In the minimized structure of 2a ⊃ NH4 + (cis-2u2d), it showed that NH4 + sits in the center of the host cavity, completely surrounded by four naphthyl groups of 2a through multiple N-H⋯𝝿 interactions (avg. N-H⋯Npcentroid distance = 2.154 Å) (Figure ). This represents an unconventional binding mode in common ammonium ionophores, where polar-H-N interactions are often governed in most literature ammonium receptor examples.