# IR and Raman Dual Modality Markers Differentiate among Three _bis_-Phenols: BPA, BPS, and BPF
## 4 Discussion  
Dual-modality analysis enables addressing exactly the wavenumbers in which IR and Raman spectra are most different. In mono-modality analysis, the presence of specific wavenumbers is used in IR as evidence of identity, except that the absence of a different specific wavenumber in an IR spectrum is not equally taken as evidence against its identification. [PERSON] et al. (2018) calculated discrete vibrational modes, each having a correspondingly discrete and identifying wavenumber for a phenolic ring, not that all of them will occur at equal intensity simultaneously. Our analysis found that the major phenolic ring vibrational mode intensities in BPA, BPS, and BPF are highly significantly different, sufficient to identify one from the other despite their structural similarity.  
A spectral signature for other _bis_-moieties X-A-X in BPA, BPS, and BPF (CH\({}_{3}\)-C-CH\({}_{3}\), O-S-O, and H-C-H) occurs as two peaks (1620 cm\({}^{-1}\) to 1580 cm\({}^{-1}\)), which are assignable to X-A-X symmetrical scissoring and essentially \"asymmetrical\" scissoring. In each of the three, the two peaks appear to have variable intensities in IR versus Raman spectra. Asymmetric in this case refers to the vibrational mode in which X is transiently deformed as different from the other X site. The O-S-O moiety is the most rigid (i.e., also is O=S=O), so the actual \"asymmetrical\" scissoring sites could be at the C-S-C site instead, suggesting that the BPS vibrational modes would be the most symmetrical. BPA scissoring, the easiest to understand, involves methyl groups moving regularly towards and away from each other. Symmetrical scissoring is with H-C stretching modes in each X group _in sync_, and \"asymmetrical\" scissoring is with H-C stretching modes _out of sync_. Spectroscopic evidence is that BPF is the least symmetrical molecule of the three: X-A-X in H-C-H more often bends  
Figure 5: Unique dual-modality fingerprint wavenumbers for distinguishing BPA (**a**), BPS (**b**,**c**), and BPF (**d**).  
or twists than scissors. The difference in scissoring at site A appears relevant in explaining the difference in the _bis_-phenolic portions of the three spectra.  
Dual modality enables interpreting the wavenumbers in which the IR and Raman spectra are most different (in normalized relative intensity). BPA, BPS, and BPF are clearly and cleanly distinguishable despite their otherwise quite similar chemical structure. Dual-modality measurements in which specific wavenumbers are strong in one mode and absent/much weaker in the other in each case were highly statistically significant.  
These plasticizers are commercially present in plastics, resulting in binary matrices of BPA, BPS, and BPF with polymer. Future research is needed to explore the same compounds in more complicated matrices.  
Conceptualization, K.C. and W.S.; methodology, K.C.; data curation, K.C. and F.T.; writing--original draft preparation, W.S. and K.C.; writing--review and editing, J.Q. and M.K. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.  
The authors declare no conflicts of interest.