Two-Dimensional Correlation Spectroscopy: Effect of Band Position,Width,and Intensity Changes on Correlation Intensities

Simulation studies have demonstrated that the effect of a band shift may be completely removed from two-dimensional (2D) synchronous spectra if a shifting band simultaneously changes its intensity. In contrast, the corresponding asynchronous spectrum develops at least two peaks, even for a small shift coupled with an appreciable intensity variation. The separation between these peaks increases upon an increase in the bandwidth. If the spectral data are changing monotonically, the number and positions of the synchronous features can be readily determined from the difference between the first and the last spectrum in the series. The correlation spectrum calculated without the subtraction of reference spectrum, for a single band that shifts with constant intensity, is similar to that calculated without the subtraction of reference, for a band undergoing shift combined with significant intensity variations. The synchronous peaks resulting from the exponentially decaying intensity changes alone are at least 10 times more intense than the corresponding asynchronous peaks, whereas the analogous intensity ratio due to a moderate band shift is discernibly lower. This result proves that the asynchronous spectra are more sensitive to the band shift than the synchronous spectra. Also, the effect of noise is more apparent in the case of the asynchronous spectrum. The bandwidth variation alone generates noticeably weaker correlation intensity than that due to the band position or intensity changes. It has been shown that the asynchronous intensity strongly depends on the overall extent of the intensity changes at particular wavenumbers. As a result, the bands changing their intensities extensively but at similar rates may develop more intense asynchronicity than the bands with distinct difference in the rates of the intensity changes but smaller magnitude of these changes.