An Ab Initio Investigation of the Structure and Alkaline Earth Divalent Cation Selectivity of 18-Crown-6

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.

Hydration to benzo-15-crown-5, benzo-18-crown-6 and the benzo-18-crown-6-potassium ion complex in the low-polar organic solvents

The coextraction of water with benzo-15-crown-5 (B1SC5), benzo-18-crown-6 (B18C6) and the B18C6-K+ complex into seven low-polar solvents, i.e., carbon tetrachloride (CTC), chloroform (CF), dichloromethane (DCM), 1,2-dichloroethane (1,2-DCE), benzene (BZ), chlorobenzene (CB) and o-dichlorobenzene (o-DCB), has been investigated. The mean hydration number, nH2O, of these solutes in the water-saturated organic solvents was determined. There is a trend that the nH2O values for any solutes increase with increasing the water concentration in the solvents. Those of B18C6 and B15C5 converge at almost 0.8 for B18C6 and 0.4 - 0.5 for B15C5 in the solvents with the relatively high water concentration, i.e., CF, 1,2-DCE, DCM, and nitorobenzene (NB). The nH2O value of B15C5 is about one-half of that of B18C6 for a given organic solvent. The dominant species of the B18C6-K+ complex in these solvents is non-hydrated. From these results, the hydration equilibrium constants, KH2O, in the organic solvents were estimated.

Theoretical study of the structure and vibrational spectra of the Ci(2) conformation of 18-crown-6

Optimized geometries of 18-crown-6 (18ce6) were calculated at the HF/6-31G* and B3LYP/6-31G* levels of theory for the D3d, Ci(1) and Ci(2) conformations. At the B3LYP level, the Ci(2) optimized geometry was higher in energy by 23.3 and 18.8 kcal mol(-1) than the Ci(1) and D3d optimized geometries, respectively. Harmonic force field, vibrational frequencies and IR absorption intensities were calculated at the corresponding optimized geometry at the B3LYP level for the Ci(2) conformation. Scaled Ci(2) frequencies were compared with the experimental frequencies of free 18ce6, Ci(1) conformation, and 18ce6-urea complex, Ci(2) conformation. This comparison showed possible misassignments in the fundamental vibrational frequencies of 18ce6.

Recognition of cationic species with synthetic receptors

In the past year progress in the study of cationic species has been made, particularly in our understanding of the factors which control the selective recognition of biologically important cations such as ammonium, alkali and alkaline earth metal ions, and of metal ions used in biomedicine such as lanthanides and iron(III). Based on this knowledge, several new hosts with improved transport, photophysical and biological properties have been designed.