Electronic states of cyclophanes with small bridges

Behavior of Trapped Molecules in Lantern-Like Carcerand Superphanes

Superphanes are a group of organic molecules from the cyclophane family. They are characterized by the presence of two parallel benzene rings joined together by six bridges. If these bridges are sufficiently long, the superphane cavity can be large enough to trap small molecules or ions. Using ab initio (time scale of 80 ps) and classical (up to 200 ns) molecular dynamics (MD) methods, we study the behavior of five fundamental molecules (M = H2O, NH3, HF, HCN, MeOH) encapsulated inside the experimentally reported lantern-like superphane and its two derivatives featuring slightly modified side bridges. The main focus is studying the dynamics of hydrogen bonds between the trapped M molecule and the imino nitrogen atoms of the side chains of the host superphane. The length of the N···H hydrogen bond increases in the following order: HF < HCN < H2O < MeOH < NH3. The mobility of the trapped molecule and its preferred position inside the superphane cage depend not only on the type of this molecule but also largely on the in/out conformational arrangement of the imino nitrogens in the side chains of the superphane. Their inward-pointing positions allow the formation of strong N···H hydrogen bonds. For this reason, these nitrogens are the preferred sites of interaction. The mobility of the molecules and their residence times on each side of the superphane have been explained by referring to the symmetry and conformation of the given superphane cage. All force field MD simulations have shown that the encapsulated molecule remained inside the superphane cage for 200 ns without any escape event to the outside. Moreover, our simulations based on some endohedral complexes in the water box also showed no exchange event. Thus, the superphanes we study are true carcerand molecules. We attribute this property to the hydrophobic side chains and their pinwheel arrangement, which makes the side walls of the studied superphanes fairly impenetrable to small molecules.

Circular Dichroisms of Mono- and Dibromo[2.2]paracyclophanes: A Combined Experimental and Theoretical Study

Circular dichroisms (CDs) of planar chiral 4-bromo[2.2]paracyclophane (1) and three isomeric dibromo[2.2]paracyclophanes (p-2, m'-2, and o'-2) were investigated experimentally and theoretically. They all exhibited strong multisignate Cotton effects (CEs) at the ¹L_b, ¹L_a, and ¹B transitions of the component (bromo)benzene chromophore and were comparable to each other. For all of the cyclophanes examined, the enantiomer that eluted earlier from a chiral high-performance liquid chromatography column (Chiralcel IA or IB) exhibited negative and positive CEs at the ¹L_b and ¹L_a bands, respectively, which were followed by a more complicated pattern of CDs at the higher-energy bands. These CD features were well reproduced by quantum chemical calculations, allowing us to unambiguously assign the absolute configurations of the first-eluted enantiomers as R _p in all of the cases examined. Interestingly, the CDs of 1 and 2, although largely comparable in shape, were still sensitive to the number and pattern of bromine substitution, showing closer resemblance between m'-2 and o'-2 and between p-2 and 1. The theoretical calculations also reproduced successfully these spectral resemblance between them. The anisotropy (g) factors for the ¹L_b bands of these cyclophanes were considerably large (∼10^-2), whereas those for the ¹L_a band were conventional in the order of 10^-3. In addition, a weak CE was observed in the low-energy region at around 320 nm, which turned out to originate from the interplanar interaction and is hence assigned to the "cyclophane band". The experimental g factors of this band were fairly large in the order of 10^-2, but the computation turned out to be quite challenging and were less well reproduced theoretically, ascribable to the forbidden nature of the transition.

Structure and NMR spectra of cyclophanes with unsaturated bridges (cyclophenes)

The calculated structures of several known and hypothetical cyclophanes with ethylene bridges (cyclophenes) are reported together with experimental and calculated values of their NMR parameters. Of the exchange-correlation functionals and basis sets used in this work, only the ωB97X-D/6-311++G(2d,2p) and ωB97X-D/cc-pVQZ yielded values of the C(sp3)-C(sp3) bond length close to the experimental data, although significant differences still remain. As far as the NMR parameters are concerned, except for close-lying signals, chemical shifts and coupling constants calculated at the ωB97X-D/cc-pVQZ level reproduce in most cases the experimental trends. Contrary to the calculations of geometries, an agreement between the values of the NMR parameters obtained at ωB97X-D/cc-pVQZ level and the experimental ones is the poorest compared with that of the ωB97X-D/6-311++G(2d,2p) one. Taking into account that the results of the different calculations show the same qualitative trends in most cases, we believe that they correctly describe the structure and properties of the hypothetical molecules studied here.