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Two dimensional correlation spectroscopy applications in vibratioal and optical spectroscopy - Isao N.

Isao N. Two dimensional correlation spectroscopy applications in vibratioal and optical spectroscopy - Wiley publishing , 2004. - 312 p.
ISBN 0-471-62391-1
Download (direct link): twodimensionalcorrela2004.pdf
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The interpretation of the region of 2D correlation spectra showing the asynchronous butterfly pattern can no longer be handled in the manner described in Section 2.3. The presence of multiple correlation peaks in the asynchronous spectrum may give an erroneous impression that there may be multiple hidden bands involved in the formation of such clusters. In reality, only a single band with varying position is involved. Furthermore, the elongated asynchronous cross peak lies in a broad region of the 2D correlation map encompassing both positive and negative synchronous correlation intensity regions, so the simple assignment of sequential order based on the sign rule is not possible for this peak. An alternative form of sequence rule is developed here specifically for a cross peak cluster showing the butterfly pattern. Whenever such a pattern is observed, one may assume that there is a possibility of a single band shifting in position. The direction of the shift is determined by the signs of the asynchronous cross peaks within the butterfly cluster. If the elongated cross peak above the diagonal is negative, and that below the diagonal is positive, the band position is shifting from right to left along the horizontal axis of the 2D spectrum and from top to bottom along the vertical axis. Opposite signs of the cross peaks indicate the opposite direction of band shift. In the simulation example of Figure 4.9(B), the band is moving from the lower wavenumber side to the higher wavenumber side.
It is interesting to point out that the location of peaks found in both the synchronous four-leaf-clover pattern cluster and the asynchronous butterfly pattern cluster are surprisingly insensitive to the extent of the band position shift. Only the correlation intensity level is affected uniformly by the extent of the band shift. The overall graphic appearance of the cluster pattern is dictated only by the cross peak positions and their spread, which are primarily determined by the intrinsic width of the shifting band. Thus, even a slight shift in the band position, as small as 0.5 % of band width, will generate the distinct pattern of butterfly-shaped cluster in 2D correlation spectra, as long as the signal-to-noise ratio is good enough to capture the systematic spectral feature change. In other
62
Generalized 2D Correlation Spectroscopy in Practice
words, if you do not see any butterfly pattern in the asynchronous 2D spectrum, the chance is high that there is no band position shift.
4.3.3.3 Band Position Shift Coupled with Intensity Change
Figure 4.10 shows the 2D spectra corresponding to the band position shift in the higher wavenumber direction coupled with some intensity increase, as depicted in Fig. 4.7(C). The cluster pattern in the synchronous spectrum (Fig. 4.10(A)) is no longer four-way symmetric, as one autopeak becomes disproportionately large compared to the other. Such a pattern is sometimes referred to as the angel pattern with cross peak wings. A similar angel pattern can also be observed in the case of two overlapped bands with intensity changes, so this pattern cannot be used as an unambiguous indicator for a position shift coupled with an intensity change.
The corresponding asynchronous spectrum (Fig. 4.10(B)) is also distorted from the standard butterfly pattern when the intensity change is coupled with the band position shift. The elongated asynchronous cross peaks near the diagonal are now distributed closer to the stronger autopeak side, i.e., the leg side of the angel. The pair of main cross peaks almost looks as if they represent two distinct bands. Indeed, this case is probably the most difficult one to interpret unambiguously, as it can be easily confused with the two overlapped band case. However, one may notice that the spread of the elongated asynchronous cross peaks near the diagonal actually covers 2D spectral regions of both positive and negative synchronous correlation intensities (i.e., the body and wings of the angel). Thus, the simple sign rules cannot be applied unequivocally to the elongated asynchronous cross peaks. This observation provides a clear indication that these asynchronous cross
(A) (B)
Wavenumber, v-, Wavenumber, v-.
Figure 4.10 (A) Synchronous and (B) asynchronous 2D correlation spectrum based on
a single band simultaneously shifting in position from a lower to a higher wavenumber and increasing in intensity
Features Arising from Factors other than Band Intensity Changes
63
peaks are generated from a much more complex process than simple intensity changes of two overlapped bands.
Another possible clue indicating that the origin of the peak cluster is a single shifting band with accompanying change in intensities is the presence of the weak secondary pair of cross peaks in the asynchronous spectrum. Unlike the strong elongated cross peaks near the diagonal, the position of the secondary asynchronous cross peaks are not influenced much by the accompanying increase in the intensity of the moving band. Unfortunately, these weak cross peaks are often overlooked because of their low intensity compared to the strong neighboring cross peaks. Appropriate adjustments of the contour level for 2D spectrum plots often reveal the presence of such weak cross peaks.
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