Behavior of Trapped\nMolecules in Lantern-Like Carcerand\nSuperphanes

Superphanes are a group of organic molecules from the\ncyclophane\nfamily. They are characterized by the presence of two parallel benzene\nrings joined together by six bridges. If these bridges are sufficiently\nlong, the superphane cavity can be large enough to trap small molecules\nor ions. Using ab initio (time scale of 80 ps) and classical (up to\n200 ns) molecular dynamics (MD) methods, we study the behavior of\nfive fundamental molecules (M = H<sub>2</sub>O, NH<sub>3</sub>, HF,\nHCN, MeOH) encapsulated inside\nthe experimentally reported lantern-like superphane and its two derivatives\nfeaturing slightly modified side bridges. The main focus is studying\nthe dynamics of hydrogen bonds between the trapped M molecule and\nthe imino nitrogen atoms of the side chains of the host superphane.\nThe length of the N···H hydrogen bond increases in\nthe following order: HF < HCN < H<sub>2</sub>O < MeOH <\nNH<sub>3</sub>. The mobility of the trapped molecule and its preferred\nposition inside the superphane cage depend not only on the type of\nthis molecule but also largely on the in/out conformational arrangement\nof the imino nitrogens in the side chains of the superphane. Their\ninward-pointing positions allow the formation of strong N···H\nhydrogen bonds. For this reason, these nitrogens are the preferred\nsites of interaction. The mobility of the molecules and their residence\ntimes on each side of the superphane have been explained by referring\nto the symmetry and conformation of the given superphane cage. All\nforce field MD simulations have shown that the encapsulated molecule\nremained inside the superphane cage for 200 ns without any escape\nevent to the outside. Moreover, our simulations based on some endohedral\ncomplexes in the water box also showed no exchange event. Thus, the\nsuperphanes we study are true carcerand molecules. We attribute this\nproperty to the hydrophobic side chains and their pinwheel arrangement,\nwhich makes the side walls of the studied superphanes fairly impenetrable\nto small molecules.