Structural and vibrational study of the Ci(1) conformation of 18-crown-6.
Optimized geometry, vibrational frequencies and IR absorption intensities were calculated for the Ci(1) conformation of 18c6. Optimized geometry was compared with the experimental geometry and that of the Ci(2) conformation of 18c6. The Ci(1) conformation is more planer than the Ci(2) conformation of 18c6, which provides an explanation that free 18c6 in the crystal phase has a Ci(1) conformation. Vibrational frequencies were scaled using an unvaried one-scale-factor scaling of 0.928. Calculated vibrational frequencies were compared with the experimental vibrational frequencies. The root-mean-square deviation of the difference between the calculated and experimental frequencies was only 15 cm(-1). This excellent agreement between the calculated and experimental frequencies is an indication of the proper assignment of the fundamental vibrational frequencies of the Ci(1) conformation of 18c6.
Complexation of Syndiotactic Polystyrene with Crown Ethers: 12-Crown-4, 15-Crown-5 and 18-Crown-6
Solid-state complexation of syndiotactic polystyrene (sPS) with three crown ether compounds, 1,4,7,10-tetraoxacyclododecane (12-crown-4, 12c4), 1, 4, 7, 10, 13-pentaoxacyclopentadecane (15-crown-5, 15c5), and 1, 4, 7, 10, 13, 16-hexaoxacyclooctadecane (18-crown-6, 18c6) took place when a film of sPS/chloroform clathrate was subjected to a guest exchange procedure assisted by a plasticizing agent. The new guest molecules were incorporated into the crystalline region of the sPS film, without causing a large conformational change of the host sPS helices. X-ray diffraction and thermogravimetric measurements showed that sPS forms co-crystalline structures with 12c4 and 15c5 presented four guest molecules per unit cell, whereas little information about the lattice dimension was obtained for sPS/18c6 because no X-ray reflections appeared in the low-angle region. IR spectroscopic and X-ray diffraction investigation suggested that the (010) uniplanar orientation of the starting sPS/chloroform clathrate was preserved during the course of guest exchange. The comparison of IR spectra between a crown ether in the liquid and crystalline states and its corresponding sPS complex showed that the crown ether incorporated into the host sPS lattice was in a conformational state similar to its liquid state, which was a mixture of two or more different conformers. It was suggested from IR and Raman spectral data that sPS/12c4 complex contained a high percentage of the Ci type conformer found in crystalline 12c4. It was inferred that the larger the molecular ring, the higher its flexibility, which leads to the lower conformational correlation of crown ether between the crystalline state and the sPS complex.
Macrocyclic receptor chemistry: Experimental and theoretical studies on molecular recognition
Abstract Synthetic macrocyclic receptors containing an intramolecular cavity (“crypt”) display remarkable binding properties towards a variety of charged substrates (spherical cations and anions, small molecules, functionalized chains, complexes, etc ...) and provide elements for an analysis of the factors involved in the “lock and key” substrate ⊂ receptor complementarity within the “cryptate” inclusion complex formed. Empirical (molecular mechanics) calculations on a typical macrocycle (18-crown-6) and its alkali complexes account for several aspects of spherical recognition (flexibility of the ligand, weak hydrophilicity of the complexed cation, binding specificity, and the “macrocyclic effect”) and give insight into conformational dependent energies of the ligand, a factor not otherwise available. The relative energies of various conformations of a macrobicyclic cryptand have also been investigated. Finally, the discrimination of L D peptides by the enzyme α-chymotrypsin in the course of the enzyme catalyzed hydrolysis, is calculated to occur at the transition state rather than at the initial non-covalent binding step.