Probing chemical transformations with mass spectrometry

Tunable synchrotron radiation (VUV and X-rays) provides a universal, yet selective scalpel to decipher molecular information in complex chemical systems when coupled to mass spectrometry and photoelectron spectroscopy. This provides profound insight into molecular growth mechanisms, solvation and electronic structure in clusters, complexes and nanoparticles. Efficient intra-molecular ionization-induced proton transfer for a system with no hydrogen bonds, the 1,3 dimethyuracil dimer, have been demonstrated,1 and upon microhydration,2 this transfer is blocked and a new pathway mediated via water occurs. I will describe recent results on the association and dissociation pathways in acetylene clusters where bonding can change from van der Waals to covalent upon ionization leading to the formation of benzene.
X-ray photoelectron spectroscopy provides a local probe of a sample’s electronic structure with elemental and site-specificity. Velocity Map Imaging X-Ray Photoelectron Spectroscopy (VMI-XPS) coupled to nanoparticle beams3 allows for the visualization of proton transfer in solvation processes. I will describe its’ implementation on aqueous arginine aerosols, where by varying the pH of the constituent solution, evidence is provided that the guanidinium groups are protonated even in a very basic solution (pH 13). A molecular level picture of how charge and proton transport in aqueous solutions of arginine occur emerges by analyzing the energy shifts on the C and N X-ray photoelectron spectra.

[1] A. Golan, K. B. Bravaya, R. Kudirka, O. Kostko, S. R. Leone, A. I. Krylov, and M. Ahmed. Nature Chem. (2012) 4,323
[2] K. Khistyaev, A. Golan, K. B. Bravaya, N. Orms, A. I. Krylov, and M. Ahmed, J. Phys. Chem. A. (2013) 117, 6789
[3] M.I. Jacobs, B. Xu, O. Kostko, N. Heine,  M. Ahmed & K. R. Wilson, J. Phys. Chem. A. (2016) 120, 8645