Effect of the weak intermolecular C-H···O=C hydrogen bonding, solvent polarity and concentration on the frequency of the C=O stretch mode of 2-thiophenecarboxaldehyde.

The vibrational frequency shift in the C=O stretch mode of 2-thiophenecarboxaldehyde (2TC) in the condensed phase is still not fully understood. In this paper, the vibrational spectra of 2TC were investigated using the FT-Raman, FT-IR and resonance Raman spectroscopies in conjunction with the density functional theory calculation. The pure compound (in the neat liquid) exhibits three vibrational bands 1658, 1672 and 1687 cm-1 in the νC=O spectral region. It differs from the band pair 1672 and 1682 cm-1 for 2-cyclohexene-1-one (CHO) and the single band 1700 cm-1 for benzaldehyde. The relative intensities of observed bands vary with the polarity of aprotic solvents and the compound's concentration. In a diluted solution, the strongest band in the resonance Raman spectra of 2TC appears the C=O stretch mode at 1690 cm-1 in cyclohexane and 1674/1675 cm-1 in acetonitrile. The imparting factors that shift the C=O stretch mode frequency in the neat liquid and solvents with different polarities were examined. The spectral sources of the vibrational bands at 1658 and 1672 cm-1 in the neat liquid and a dilute solution were determined, and the resonance Raman spectra were assigned. It is concluded that tetramers and monomer are the major sources of the bands at 1658 and 1672 cm-1 in the neat liquid, respectively, while the monomer is the main source of the bands at 1674/1675 cm-1 in acetonitrile and the band at 1690 cm-1 in cyclohexane with a dilute concentration. The band's source at 1662/1663 cm-1 in acetonitrile (a dilute concentration) can be either the dimers or 2TC-CH3CN clusters. The C=O bond's electronic charge density is the main factor that shifts the vibrational frequency of the C=O stretch mode of 2TC monomer when an aprotic solvent is used. The larger the polarity of an aprotic solvent, the more negative the electronic charge-density of the C=O bond for the monomer, the lower the frequency of the C=O stretch mode.

Surface Surgery of the Nickel-Rich Cathode Material LiNi0.815Co0.15Al0.035O2: Toward a Complete and Ordered Surface Layered Structure and Better Electrochemical Properties.

A complete and ordered layered structure on the surface of LiNi0.815Co0.15Al0.035O2 (NCA) has been achieved via a facile surface-oxidation method with Na2S2O8. The field-emission transmission electron microscopy images clearly show that preoxidation of the hydroxide precursor can eliminate the crystal defects and convert Ni(OH)2 into layered ß-NiOOH, which leads to a highly ordered crystalline NCA, with its (006) planes perpendicular to the surface in the sintering process. X-ray photoelectron spectroscopy and Raman shift results demonstrate that the contents of Ni2+ and Co2+ ions are reduced with preoxidization on the surface of the hydroxide precursor. The level of Li+/Ni2+ disordering in the modified NCA determined by the peak intensity ratio I(003)/I(104) in X-ray diffraction patterns decreases. Thanks to the complete and ordered layered structure on the surface of secondary particles, lithium ions can easily intercalate/extract in the discharging-charging process, leading to greatly improved electrochemical properties.