The observation of a sharp predissociation threshold in the resonant two-photon ionization spectra of EuO, TmO, and YbO has been used to measure the bond dissociation energies of these species. The resulting values, D0(EuO) = 4.922(3) eV, D0(TmO) = 5.242(6) eV, and D0(YbO) = 4.083(3) eV, are in good agreement with previous values but are much more precise. In addition, the ionization energy of TmO was measured by the observation of a threshold for one-color two-photon ionization of this species, resulting in IE(TmO) = 6.56(2) eV. The observation of a sharp predissociation threshold for EuO was initially surprising because the half-filled 4f7 subshell of Eu in its ground state generates fewer potential energy curves than in the other molecules we have studied by this method. The observation of a sharp predissociation threshold in YbO was even more surprising, given that the ground state of Yb is nondegenerate (4f146s2, 1Sg) and the lowest excited state of Yb is over 2 eV higher in energy. It is suggested that these molecules possess a high density of electronic states at the energy of the ground separated atom limit because ion-pair states drop below the ground limit, providing a sufficient electronic state density to allow predissociation to set in at the thermochemical threshold.
The bond dissociation energies of early transition metal diborides (M–B2, M = Sc, Ti, V, Y, Mo) have been measured by observation of the sharp onset of predissociation in a highly congested spectrum. Density functional and CCSD(T) ab initio calculations, extrapolated to the complete basis set limit, have been used to examine the electronic structure of these species. The computations demonstrate the formation of bonding orbitals between the metal d orbitals and the 1πu bonding orbitals of B2, leading to the transfer of metallic electron density into the bonding 1πu orbitals, strengthening both the M–B and B–B bonds in the molecule. This runs counter to most metal–ligand π interactions, where electron density is generally transferred into π antibonding orbitals of the ligand.
The early transition metal diatomic sulfides, MS, M = Sc, Y, Ti, Zr, Hf, Nb, and Ta, have been investigated using resonant two-photon ionization spectroscopy in the vicinity of their bond dissociation energies (BDEs). Due to the high density of vibronic states in this energy range, the molecular spectra appear quasicontinuous, and when the excitation energy exceeds the ground separated atom limit, excited state decay by dissociation becomes possible. The dissociation process typically occurs so rapidly that the molecule falls apart before a second photon can be absorbed to ionize the species, leading to a sharp drop in ion signal, which is identified as the 0 K BDE. The observed predissociation thresholds yield BDEs of 4.852(10) eV (ScS), 5.391(3) eV (YS), 4.690(4) eV (TiS), 5.660(4) eV (ZrS), 5.780(20) eV (HfS), 5.572(3) eV (NbS), and 5.542(3) eV (TaS). Utilizing thermochemical cycles, the enthalpies of formation, ΔfH0Ko(g), of 182.7(4.3) kJ mol−1 (ScS), 178.3(4.2) kJ mol−1 (YS), 293.1(16.7) kJ mol−1 (TiS), 337.3(8.4) kJ mol−1 (ZrS), 335.0(6.6) kJ mol−1 (HfS), 467.0(8.0) kJ mol−1 (NbS), and 521.5(2.1) kJ mol−1 (TaS) are obtained. Another thermochemical cycle has been used to combine the previously measured M+-S BDEs with the M-S BDEs and atomic ionization energies to obtain the MS ionization energies of 6.44(5) eV (ScS), 6.12(8) eV (YS), 6.78(7) eV (TiS), 6.60(10) eV (ZrS), and 6.88(9) eV (NbS). Using this same cycle, we obtain D0(Hf+-S) = 4.926(20) eV. The bonding trends of the early transition metal sulfides, along with the corresponding selenides, are discussed.
Resonant two-photon ionization spectroscopy has been used to determine the bond dissociation energies (BDEs) and ionization energies (IEs) of rhenium containing small molecules. The ultraviolet spectra of these molecules display a highly congested collection of indeterminate vibronic states. Couplings among these states allow the molecule to find a path to dissociation as soon as the ground separated atom limit is exceeded in energy, allowing a precise measurement of the bond energy from the observation of a sharp predissociation threshold. Measurements provide BDE values of 5.731(3) eV (ReC), 5.359(3) eV (ReC 2 ), 5.635(3) eV (ReN), 5.510(3) eV (ReO), and 3.947(3) eV (ReS). The ionization energy of ReC, 8.425(15) eV, was determined from the observed onset of one-color two-photon ionization. By combining our ReC results with the ionization energy of Re (7.83352(11) eV) in a thermochemical cycle, the BDE of cationic ReC+ was determined as 5.140(15) eV. a This is in excellent agreement with that measured using guided ion beam mass spectrometry, 5.13(12) eV. b
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