The last phase of the project has culminated in the completion of the first study of lignin decomposition by using collision-induced dissocation charge-remote fragmentation (CID-CRF).  In this work, lignin model compounds with remote charge tags (G1StySO3-, O1StySO33, M1StySO3-).  Upon dissociation, these compounds decompose first by loss of methyl radical, similar to what is found for pyrolysis of the base molecules.

Further dissociation is dependent on the structure.  In particular, dissociation of the guaiacol and o-dimethoxybenzene deriviatives give o-benzoquinone products, which are not observed in pyrolysis (Figure 1).  On the other hand, dissociation of o-dimethoxybenzene also occurs by formation of hydroxybenzaldehyde, which is observed in pyrolysis.  The similarities and differences in the dissociation of tagged and not-tagged substrates is predicted by densdity functional theory (Figure 2).

Contnuing studies of lignin decomposition is focused on more complicated systems with competing pathways.  In particular, we have been investigating the dissociation of phenylphenethyl ether (PhOCH2CH2PH), which can lead to either formation of phenoxy radical/phenethyl radical or phenol and styrene.  Mass spectrometry using tagged substrates provides a method for detailed characterization of the dissociation process, because the two pathways are clearly and easily distinguished on the basis of the product mass (Figure 3).  We are using energy-resolved mass spectrometry to elucidate the detailed mechanisms by which the competing pathways occur.

A second focus of our work has been oin the chemistry of aromatic azides and the reactivity of nitrenes.  In this reporting period (delayed due to covid), we carried out a fundamental study to assess the electronic properties of an azide substituent.  Azides are popular reagents in organometallic and click chemistry, but are not well-characterized in terms of the electronic effects they have on molecular systems.  By using mass spsectrometry, we have shown that the effect of an azide is "chameleonic" (Figure 4) in that it depends on the electronic needs of the system.  Nominally, the group is pi donating, and can increase electron density.  However, if the system is already electron rich, the azide can be electron-withdrawing.  Whatever is needed to benefit the system the most, the azide can do.

Our interests in azides stems from our interests in p-azidophenoxide.  We have made significant progress on the study of the mechanism of photolysis of p-azidophenoxide to form indophenol.  By using real-time analysis of the reaction with mass spectromtry, we have been able to show that the reaction occurs by a single-electron transfer (SET) mechanism that is enhanced by molecular oxygen.  Although there are still some experiments needed, the basics of the mechanism are shown in Figure 5.

Finally, products that resutled from work carried out in this project that were not foreseen involved the investigation of phenolic content in natural sources.  In the course of our azidophenolide studies, we developed a new analytical methodology for the measurement of phenols.  Although we were not able to patent the process (we tried!), we have still been able to use it in a couple of applications:

1) In our own work, we have carried out an analysis of the phenolic content of Scotch whiskies.  This work has been submitted to a trade journal, The Journal of Distilling Science.

2) Using this methodology, we were able to create a service contract to measure the free phenol content in a sand binder resin for the Bay Area Air Quality Management District in California.  They were unable to find a lab that could measure phenol impurities, and so contacted us.  We were able to use methodology developed in our NSF supported studies to measure the phenol concentrations to guide their environmental policy decisions.

Last Modified: 07/14/2022
Modified by: Paul G Wenthold