Inner-shell excitation spectroscopy and fragmentation of small hydrogen-bonded clusters of formic acid after core excitations at the oxygen K edge.

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 H₃O⁺ 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 and/or trimer units. Specifically labeled formic acid-d, HCOOD, was also used to examine the core-excited fragmentation mechanisms. These deuterium-labeled experiments showed that HDO⁺ was formed via site-specific migration of a formyl hydrogen within an individual molecule, and that HD₂O⁺ 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. [ABSTRACT FROM AUTHOR]