Inner-shell excitation spectroscopy and fragmentation of small hydrogen-bonded clusters of formic acid after core excitations at the oxygen K edge
JChemPhys_125_194307.pdf 775 KB
Harries, James R.
Suzuki, Isao H.
time of flight mass spectra
spectral line shift
Inner-shell excitation spectra and fragmentation of small clusters of formic acid have been studied in the oxygen K -edge region by time-of-flight fragment mass spectroscopy. In addition to several fragment cations smaller than the parent molecule, we have identified the production of HCOOH H+ and H3 O+ cations characteristic of proton transfer reactions within the clusters. Cluster-specific excitation spectra have been generated by monitoring the partial ion yields of the product cations. Resonance transitions of O1s (C==O/OH) electrons into π CO* orbital in the preedge region were found to shift in energy upon clusterization. A blueshift of the O1s (C==O) → π CO* transition by ∼0.2 eV and a redshift of the O1s (OH) → π CO* by ∼0.6 eV were observed, indicative of strong hydrogen-bond formation within the clusters. The results have been compared with a recent theoretical calculation, which supports the conclusion that the formic-acid clusters consist of the most stable cyclic dimer andor trimer units. Specifically labeled formic acid- d, HCOOD, was also used to examine the core-excited fragmentation mechanisms. These deuterium-labeled experiments showed that HD O+ was formed via site-specific migration of a formyl hydrogen within an individual molecule, and that H D2 O+ was produced via the subsequent transfer of a deuterium atom from the hydroxyl group of a nearest-neighbor molecule within a cationic cluster. Deuteron (proton) transfer from the hydroxyl site of a hydrogen-bond partner was also found to take place, producing deuteronated HCOOD D+ (protonated HCOOH H+) cations within the clusters.
Journal of Chemical Physics
American Institute of Physics
Copyright (c) 2006 American Institute of Physics.