1 Pi<--X1 Sigma+ band systems of jet-cooled ScCo and YCo

A Study of NbMo and NbMo- by Anion Photoelectron Spectroscopy

Photoelectron spectra of the niobium-molybdenum diatomic anion, obtained at 488 and 514 nm, display vibrationally resolved transitions from the ground state and one excited electronic state of the anion to the ground state and one excited electronic state of the neutral molecule. The electron affinity of NbMo is measured to be 1.130 ± 0.005 eV. Its ²Δ_3/2 spin-orbit component is observed to lie 870 ± 20 cm^-1 above its previously identified ²Δ_5/2 ground state. For ⁹³Nb⁹⁸Mo, vibrational energies measured for levels up to v = 4 for the ²Δ_5/2 and ²Δ_3/2 states give harmonic frequency and anharmonicity constant values of ω_e = 492 ± 12 cm^-1 and ω_ex_e = 8.0 ± 3.2 cm^-1, the former value corresponding to a force constant of 6.80 ± 0.35 mdyn/Å. These two vibrational parameters suggest a bond dissociation energy that is too low by at least a factor of 3, indicating that the ground state potential energy curve of NbMo deviates markedly from a Morse potential at higher energies. An excited electronic state of NbMo, assigned as a ²Σ⁺ state, is observed at 2900 ± 25 cm^-1 (T₀). Vibrational energies up to v = 8 in this excited state give values of ω_e = 544 ± 8 cm^-1 and ω_ex_e = 1.9 ± 1.2 cm^-1 for ⁹³Nb⁹⁸Mo. The former value corresponds to a high vibrational force constant of 8.30 ± 0.25 mdyn/Å. Both doublet states of the neutral molecule are accessed from the anion ground state, which is assigned as ¹Σ⁺. For the ⁹³Nb⁹⁸Mo^- anion, the fundamental vibrational frequency (ΔG_1/2) is 484 ± 15 cm^-1. Electron affinity data indicate that the bond dissociation energy of NbMo^- is 0.213 ± 0.005 eV greater than that of neutral NbMo, whose previously reported value then gives D₀ = 4.85 ± 0.27 eV for the anion. An excited state of the anion lying 3050 ± 25 cm^-1 (T₀) above its ground state is assigned as ³Δ, and the energies of its spin-orbit components above the ³Δ₃ lowest energy level are measured to be 450 ± 20 cm^-1 (³Δ₂) and 1100 ± 20 cm^-1 (³Δ₁). Their uneven spacing suggests that the energy of the ³Δ₂ level is lowered by interaction with a higher energy Ω = 2 anion state. The vibrational frequency (ΔG_1/2) for the ³Δ₁ and ³Δ₂ states is measured to be 433 ± 20 cm^-1. Bond length differences among the observed states are estimated from Franck-Condon fits to vibrational band intensity profiles. When combined with the previously reported NbMo bond length, these provide bond length estimates for the ground state of the anion (1.940 ± 0.025 Å) and for the observed excited states. These species offer extreme examples of multiple metal-metal bonding, with formal bond orders of 5¹/₂ for the ²Δ ground and ²Σ⁺ excited doublet states of NbMo, 6 for the singlet ground state of the anion, and 5 for its low-lying triplet state. The relationships among their bonding properties and those of related multiply bonded transition metal dimers are discussed.

Thermochemical electronegativities of the elements

Electronegativity is a key property of the elements. Being useful in rationalizing stability, structure and properties of molecules and solids, it has shaped much of the thinking in the fields of structural chemistry and solid state chemistry and physics. There are many definitions of electronegativity, which can be roughly classified as either spectroscopic (these are defined for isolated atoms) or thermochemical (characterizing bond energies and heats of formation of compounds). The most widely used is the thermochemical Pauling's scale, where electronegativities have units of eV-1/2. Here we identify drawbacks in the definition of Pauling's electronegativity scale-and, correcting them, arrive at our thermochemical scale, where electronegativities are dimensionless numbers. Our scale displays intuitively correct trends for the 118 elements and leads to an improved description of chemical bonding (e.g., bond polarity) and thermochemistry.

Bond dissociation energies of diatomic transition metal selenides: ScSe, YSe, RuSe, OsSe, CoSe, RhSe, IrSe, and PtSe

The diatomic transition metal selenides, MSe (M = Sc, Y, Ru, Os, Co, Rh, Ir, and Pt), were studied by resonant two-photon ionization spectroscopy near their respective bond dissociation energies. As these molecules exhibit high densities of vibronic states near their dissociation limits, the spectra typically appear quasicontinuously at these energies. Spin-orbit and nonadiabatic couplings among the multitudes of potential curves allow predissociation to occur on a rapid timescale when the molecule is excited to states lying above the ground separated atom limit. This dissociation process occurs so rapidly that the molecules are dissociated before they can be ionized by the absorption of a second photon. This results in an abrupt drop in the ion signal that is assigned as the 0 K bond dissociation energy for the molecule, giving bond dissociation energies of 4.152(3) eV (ScSe), 4.723(3) eV (YSe), 3.482(3) eV (RuSe), 3.613(3) eV (OsSe), 2.971(6) eV (CoSe), 3.039(9) eV (RhSe), 3.591(3) eV (IrSe), and 3.790(31) eV (PtSe). The enthalpies of formation, Δ_fH_0K° (g), for each diatomic metal selenide were calculated using thermochemical cycles, yielding Δ_fH_0K° (g) values of 210.9(4.5) kJ mol^-1 (ScSe), 203.5(4.5) kJ mol^-1 (YSe), 549.2(4.5) kJ mol^-1 (RuSe), 675.9(6.5) kJ mol^-1 (OsSe), 373.9(2.6) kJ mol^-1 (CoSe), 497.4(2.7) kJ mol^-1 (RhSe), 557.4(6.5) kJ mol^-1 (IrSe), and 433.7(3.6) kJ mol^-1 (PtSe). Utilizing a thermochemical cycle, the ionization energy for ScSe is estimated to be about 7.07 eV. The bonding trends of the transition metal selenides are discussed.

Predicting bond dissociation energy and bond length for bimetallic diatomic molecules: a challenge for electronic structure theory

We test the accuracy of Kohn–Sham density functional theory for strongly correlated metal–metal bonds that occur in catalytically active sites and intermediates and examine the orbitals and configurations involved to analyze the results.

Low-Lying Electronic States of CuAu

Coinage metal diatomic molecules are building blocks for nanostructured materials, electronic devices, and catalytically or photochemically active systems that are currently receiving lively interest in both fundamental and applied research. The theoretical study presented here elucidates the electronic structure in the ground and several low-lying excited states of the diatomic molecule CuAu that result from the combination of the atoms in their ground states nd(10)(n + 1)s(1 2)S and lowest excited d-hole states nd(9)(n + 1)s(2 2)D (n = 3 for Cu, n = 5 for Au). Full and smooth potential energy curves, obtained at the multireference configuration interaction (MRCI) level of theory, are presented for the complete set of the thus resulting 44 Λ-S terms and 86 Ω terms. Our approach is based on a scalar relativistic description using the Douglas-Kroll-Hess (DKH) Hamiltonian, with subsequent perturbative inclusion of spin-orbit (SO) coupling via the spin-orbit terms of the Breit-Pauli (BP) Hamiltonian. The Ω terms span an energy interval of about 7 eV at the ground state's equilibrium distance. Spectroscopic constants, calculated for all terms, are shown to accurately reproduce the observation for those nine terms that are experimentally known.