Small Carbon Clusters Doped with Early Transition Metals: A Theoretical Study of ScCn, ScCn+, and ScCn- (n = 1−8) Open-Chain Clusters

Bond dissociation energies of ScSi, YSi, LaSi, ScC, YC, LaC, CoC, and YCH

Predissociation thresholds of the ScSi, YSi, LaSi, ScC, YC, LaC, CoC, and YCH molecules have been measured using resonant two-photon ionization spectroscopy. It is argued that the dense manifold of electronic states present in these molecules causes prompt dissociation when the bond dissociation energy (BDE) is exceeded, allowing their respective predissociation thresholds to provide precise values of their bond energies. The BDEs were measured as 2.015(3) eV (ScSi), 2.450(2) eV (YSi), 2.891(5) eV (LaSi), 3.042(10) eV (ScC), 3.420(3) eV (YC), 4.718(4) eV (LaC), 3.899(13) eV (CoC), and 4.102(3) eV (Y-CH). Using thermochemical cycles, the enthalpies of formation, Δ_fH_0K°(g), were calculated as 627.4(9.0) kJ mol^-1 (ScSi), 633.1(9.0) kJ mol^-1 (YSi), 598.1(9.0) kJ mol^-1 (LaSi), 793.8(4.3) kJ mol^-1 (ScC), 805.0(4.2) kJ mol^-1 (YC), 687.3(4.2) kJ mol^-1 (LaC), 760.1(2.5) kJ mol^-1 (CoC), and 620.8(4.2) kJ mol^-1 (YCH). Using data for the BDEs of the corresponding cations allows ionization energies to be obtained through thermochemical cycles as 6.07(11) eV (ScSi), 6.15(13) eV (YSi), 5.60(10) eV (LaSi), 6.26(6) eV (ScC), 6.73(12) or 5.72(11) eV [YC, depending on the value of D₀(Y⁺-C) employed], and 5.88(35) eV (LaC). Additionally, a new value of D₀(Co⁺-C) = 4.045(13) eV was obtained based on the present work and the previously determined ionization energy of CoC. An ionization onset threshold allowed the measurement of the LaSi ionization energy as 5.607(10) eV, in excellent agreement with a prediction based on a thermochemical cycle. Chemical bonding trends are also discussed.

Small carbides of third-row main group elements: structure and bonding in C3X compounds (X = K-Br)

The molecular structures of third-row main group tricarbides C(3)X (X = K-Br) have been studied by quantum chemical methods. It is found that less electronegative elements (K, Ca, Ga, Ge) favor either fan or rhombic structures (resulting from side interactions with either linear or triangular C(3) units), whereas the more electronegative elements (As, Se, Br) favor linear or three-membered ring structures (resulting from σ-type interactions with either linear or triangular C(3) units). The predicted global minima are of fan type for C(3)K, rhombic for C(3)Ca, C(3)Ga, and C(3)Ge, linear for C(3)As and C(3)Se, and a three-membered ring for C(3)Br. In order to aid in their possible experimental identification the molecular geometries, vibrational frequencies, IR intensities, and dipole moments have been provided. The nature of the interactions has been characterized through an analysis of the electronic charge density. In addition, the relative stability of the different isomers has been also rationalized in terms of an energy decomposition analysis.

Spectroscopic constants and potential energy curves of yttrium carbide (YC)

The potential energy curves of the low-lying electronic states of yttrium carbide (YC) and its cation are calculated at the complete active space self-consistent field and the multireference single and double excitation configuration interaction (MRSDCI) levels of theory. Fifteen low-lying electronic states of YC with different spin and spatial symmetries were identified. The X (4)Sigma- state prevails as the ground state of YC, and a low-lying excited A (4)Pi state is found to be 1661 cm(-1) higher at the MRSDCI level. The computations of the authors support the assignment of the observed spectra to a B (4)Delta(Omega=72)<--A (4)Pi(Omega=52) transition with a reinterpretation that the A (4)Pi state is appreciably populated under the experimental conditions as it is less than 2000 cm(-1) of the X (4)Sigma- ground state, and the previously suggested (4)Pi ground state is reassigned to the first low-lying excited state of YC. The potential energy curves of YC+ confirm a previous prediction by Seivers et al. [J. Chem. Phys. 105, 6322 (1996)] that the ground state of YC+ is formed through a second pathway at higher energies. The calculated ionization energy of YC is 6.00 eV, while the adiabatic electron affinity is 0.95 eV at the MRSDCI level. The computed ionization energy of YC and dissociation energy of YC+ confirm the revised experimental estimates provided by Seivers et al. although direct experimental measurements yielded results with greater errors due to uncertainty in collisional cross sections for YC+ formation.

Small ScCn Cyclic Clusters: A Density Functional Study of Their Structure and Stability

A theoretical study of the ScCn, ScCn+, and ScCn- (n = 1-10) cyclic clusters has been carried out employing the B3LYP density functional method. Predictions for several molecular properties that could help in their possible experimental characterization, such as equilibrium geometries, electronic structures, dipole moments, and vibrational frequencies, are reported. All ScCn cyclic clusters are predicted to have doublet ground states. For cationic clusters the ground state is alternate between singlets (n-even species) and triplets (n-odd members). In the case of anionic clusters the singlet-triplet separation is relatively small, with the singlets being favored in most cases. In general, even-odd parity effects are also observed for different properties, such as incremental binding energies, ionization energies, and electron affinities. For all neutral, cationic, and anionic clusters it is found that cyclic species are more stable than their open-chain counterparts. Therefore, cyclic structures are the most interesting possible targets for an experimental search of scandium-doped carbon clusters.