Dissociative photoionization of chromium hexacarbonyl: A round-trip ticket to non-statisticality and a detective story in thermochemistry

Abstract

The fragmentation processes of internal energy selected chromium hexacarbonyl cations, Cr(CO)6+, were investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy at the VUV beamline of the Swiss Light Source. In the 9.3–21.5 eV photon energy range, Cr(CO)6+ dissociates by six sequential carbonyl ligand losses. The fragment ion fractional abundances, plotted in the breakdown diagram, along with the time-of-flight mass spectra for the first three metastable CO-loss channels were modeled using a statistical approach. Between 12 and 16 eV, the statistical model overestimates the degree of fragmentation, which is explained by enhanced kinetic energy release in impulsive CO loss on repulsive electronic states of the parent ion Cr(CO)6+, as confirmed by TD-DFT calculations. This is the first reported example for an embedded non-statistical unimolecular dissociation regime, bracketed by statistical regimes at low and at high energies. The statistical model was employed to derive 0 K appearance energies for Cr(CO)n+ (n = 0–5). The Cr(CO)5+ appearance energy and the literature COCr(CO)5+ bond dissociation energy yield an adiabatic Cr(CO)6+ ionization energy of 8.195 ± 0.120 eV, independent of photoelectron spectroscopy measurements. The 0 K appearance energy of Cr+ is 61 kJ mol–1 higher than predicted based on literature enthalpies of formation, which we suggest is most likely due to an error in the enthalpy of formation of Cr(CO)6, and propose a revised ΔfH[Cr(CO)6, g] = –972.1 ± 4.1 and –968.9 ± 4.1 kJ mol–1 at 0 and 298 K, respectively, and ΔfH298K[Cr(CO)6, c] = –1040.6 ± 4.2 kJ mol–1 for the crystalline state, based on the known enthalpy of sublimation. The measured CrCO bond dissociation energies in [(CO)nCrCO]+ (n = 0–5), and the enthalpies of formation of the chromium carbonyl ion series are also reported.