Bond Dissociation Energies of RuC, RhC, OsC, IrC, and PtC Measured by Resonant Three-Photon Ionization Spectroscopy

Resonant three-photon ionization spectroscopy has been used to study the late 4d and 5d transition metal carbides RuC, RhC, OsC, IrC, and PtC. These species, like most diatomic transition metals with open nd subshells, exhibit an exceptionally high density of states near the ground separated atom limit. Spin–orbit and nonadiabatic interactions provide a means for the molecules to rapidly dissociate as soon as the bond dissociation energy (BDE) is exceeded. The result is a sharp predissociation threshold that is identified as the BDE. The high BDEs of these five molecules required the use of two tunable lasers to reach the BDE. Measured values of D0(RuC) = 6.312(2) eV, D0(RhC) = 6.007(2) eV, D0(OsC) = 6.427(2) eV, D0(IrC) = 6.404(2) eV, and D0(PtC) = 6.260(2) eV were obtained, where the value is parentheses represents the estimated error limit in units of the last quoted digit. A new electronic state of PtC, tentatively assigned as the c3Σ1+ state, has been found with T0 = 22,442 cm–1. These BDEs are combined with recently measured ionization energies to obtain BDEs of the associated cations. Electronic structure calculations are also reported to investigate the chemical bonding in more detail. Trends in the BDEs of the diatomic transition metal carbides are also discussed.