Positional Fluctuation of IR Absorption Peaks: Frequency Shift of a Single Band or Relative Intensity Changes of Overlapped Bands?

摘要

There are two schools of thought in in terpreting the so-called positional fluctuation of peaks of IR spectra under the influence of environmental factors, i.e., temperature and concentration. The peak position change may be caused by the actual frequency shift of a single absorption band or alternatively by the relative intensity changes of overlapped bands. The classical view that has been widely accepted for many decades in the field of vibrational spectroscopy is that gradual changes occur in the vibrational frequency associated with a specific chemical bond. Many previous works concerning the peak position shift of IR spectra are based on the notion that the extent of frequency shift can be directly correlated with the level of specific molecular interactions, such as hydrogen bonding and dipole–dipole interactions.' The apparent frequency shift of OH stretching or C=0 stretching band under temperature or concentration change was attributed to the gradual weakening of such interactions. An alternative view is that the apparent peak position shift is caused by the change in the population of dif ferent chemical species. In this case, the position of peak maximum tends to shift due to the variation in the relative intensity contributions of closely overlapped bands with their individual frequencies essentially unchanged.