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.
Spectroscopic studies of the interaction between crown ethers and organic nitriles.
The interaction of 18-crown-6 (18C6), diaza-18-crown-6 (DA18C6), and dithia-18-crown-6 (DT18C6) with acetonitrile and malononitrile was investigated using infrared spectroscopy, 1H and 13C NMR spectroscopy, and molecular modeling. The interaction between 18C6 and the nitriles involves hydrogen bonding between the CH3/CH2 protons of the nitriles and the ether oxygens of the crown. In contrast, the interaction between the organic nitriles and DA18C6 involves the nitrogen of the nitrile group and the N-H groups of the crown. Due to the bulkiness of the sulfur atoms of DT18C6, no interaction was observed to occur between acetonitrile and the crown.
Temperature study of intra- and inter-molecular coupling and Fermi resonance constants in the Raman spectra of liquid water using Fourier deconvolution
The structure of the Raman OH stretching band of water has been investigated from 4 to 90 °C using a Fourier deconvolution technique. The overlapped components in the isotropic and anisotropic spectra have been resolved. The information on the positions of the components in the deconvoluted spectra was used for calculation of the constants for intra- and inter-molecular coupling and Fermi resonance and for an estimation of the component frequencies according to the model presented. For the first time, the value of the asymmetric Fermi resonance constant and the temperature dependences of all coupling constants have been obtained. Their temperature behaviour is in good agreement with the data in the literature concerning the different phases of water. It was established that the intramolecular coupling constant increases and the intermolecular coupling constant decreases as the temperature increases. The Fermi resonance constant for symmetric components tends to decrease with increasing temperature. The decrease in the symmetric Fermi resonance interaction with increasing temperature is due to the increase in the distance between the unperturbed fundamental and overtone band positions. The asymmetric Fermi resonance constant tends to increase with increasing temperature and this determines the strengthening of the asymmetric Fermi resonance interaction, which exhibits a limiting value at high temperatures.