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

Tabayashi Kiyohiko

Yamamoto Keisuke

Takahashi Osamu

Tamenori Yusuke

Harries James R.

Gejo Tatsuo

Iseda Mitsuhiro

Tamura Takashi

Honma Kenji

Suzuki Isao H.

Nagaoka Shin-ichi

Ibuki Toshio

Journal of Chemical Physics

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.

organic compounds

molecular clusters

hydrogen bonds

dissociation

time of flight mass spectra

spectrochemical analysis

chemical exchanges

spectral line shift

red shift

reaction kinetics

eng

American Institute of Physics

Copyright (c) 2006 American Institute of Physics.

[ISSN] 0021-9606

[DOI] 10.1063/1.2387949

[NCID] AA00694991

[DOI] http://dx.doi.org/10.1063/1.2387949