Global phase angle mapping method in the generalized two-dimensional correlation infrared spectroscopy for the band shifting and broadening phenomena

We have demonstrated the potential of the pattern recognition method for the band shifting and broadening phenomena using the map drawn by the global phase angle introduced in the generalized two-dimensional correlation spectroscopy (global 2D phase map). The method has been applied to analyze the CH₂ symmetric stretching band in the temperature-dependent infrared (IR) spectra of the Langmuir-Blodgett (LB) film of the dioctadecyldimethyl- Au(dmit)₂ (2C₁₈-Au(dmit)₂) salt. In the temperature range from 30 up to 60°C, the peak position of the CH ₂ symmetric stretching band shifts apparently to the lower wavenumber position in the one-dimensional (1D) spectra. The corresponding global 2D phase map has been calculated, in which the observed pattern is composed of the tilted-structure at 2856 cm^-1 and the saddle-like-structure at 2848 cm^-1, indicating that the peak position of the absorption component at 2856 cm^-1 due to the CH₂ symmetric stretching mode shifts to the lower wavenumber position, and that the component at 2848 cm ^-1 due to the same vibrational mode narrows in the heating process of 30-60°C. In the temperature range from 70 up to 100°C, the peak position shifts apparently to the higher wavenumber position. The global 2D phase mapping analysis has revealed that the observed pattern is composed of the flat-structure at 2860 cm^-1 and the saddle-like-structure at 2848 cm^-1, indicating that the component at 2860 cm^-1 neither shifts nor broadens, and that the component at 2848 cm^-1 broadens in the heating process of 70-100°C. These two absorption components at 2848 and 2856 (2860) cm^-1 are probably related to the trans- and gauche-forms in the hydrocarbon chain, respectively.