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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 c(_^3)Σ_1^+ state, has been found with T0 = 22442 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. 

The rhenium-containing molecules ReC, ReN, ReO, ReS, and ReC2 have been investigated using a pulsed laser ablation supersonic beam molecular source in resonant two-photon ionization experiments with time-of-flight mass spectrometric detection. Sharp predissociation thresholds have been observed, allowing precise bond dissociation energies (BDEs) to be measured as D0(ReC) = 5.731(3) eV, D0(ReN) = 5.635(3) eV, D0(ReO) = 5.510(3) eV, D0(ReS) = 3.947(3) eV, and D0(Re–C2) = 5.359(3) eV. The threshold for two-photon ionization was also measured for ReC, ReN, and ReO, providing ionization energies (IEs) of IE(ReC) = 8.425(12) eV, IE(ReN) = 8.193(20) eV, and IE(ReO) = 8.561(11) eV. These are the first measurements of these thermochemical quantities to be reported in the literature. The combination of BDEs and IEs allowed the BDEs of the cations ReC+, ReN+, and ReO+ to be determined via a thermochemical cycle as D0(Re+-C) = 5.140(12) eV, D0(Re+-N) = 5.275(20) eV, and D0(Re+-O) = 4.783(11) eV. In addition, computations of these thermochemical values were performed using density functional theory [B3LYP/aug-cc-pVQZ(-PP)] to determine the ground states and their geometric parameters. These were further studied at the CCSD(T) level with extrapolation to the complete basis set limit using aug-cc-pVXZ(-PP) basis sets (X = 3, 4, 5) to obtain computational values of the BDEs and IEs as well. The high-level super correlation consistent composite approach (s-ccCA) was also utilized, providing an additional approach for the prediction of thermochemical values. The electronic structure of the molecules is discussed, along with the periodic trends as the ligand is varied. 

In the present work, the electronic structure and chemical bonding of the MoC X3Σ− ground state and the six lowest excited states, A3Δ, a1Γ, b5Σ−, c1Δ, d1Σ+, and e5Π, have been investigated in detail using multireference configuration interaction methods and basis sets, including relativistic effective core potentials. In addition, scalar relativistic effects have been considered in the second order Douglas–Kroll–Hess approximation, while spin–orbit coupling has also been calculated. Five of the investigated states, X3Σ−, A3Δ, a1Γ, c1Δ, and d1Σ+, present quadruple σ2σ2π2π2 bonds. Experimentally, the predissociation threshold of MoC was measured using resonant two-photon ionization spectroscopy, allowing for a precise measurement of the dissociation energy of the ground state. Theoretically, the complete basis set limit of the calculated dissociation energy with respect to the atomic ground state products, including corrections for scalar relativistic effects, De(D0), is computed as 5.13(5.06) eV, in excellent agreement with our measured value of D0(MoC) of 5.136(5) eV. Furthermore, the calculated dissociation energies of the states having quadruple bonds with respect to their adiabatic atomic products range from 6.22 to 7.23 eV. The excited electronic states A3Δ2 and c1Δ2 are calculated to lie at 3899 and 8057 cm−1, also in excellent agreement with the experimental values of DaBell et al., 4002.5 and 7834 cm−1, respectively.