.5 D-transition Metal Double Atomic Carbides And Nitrides Of The Electronic Structure And Spectral Behavior,

The theoretical study of small molecules containing transition metal atoms is one of the most active areas in quantum chemistry because many of them related to important materials and catalytic applications and also their importance in biology. And another factor of stimulating these interests is that the transition-metal-containing molecules provide ideal and challenging systems for the applications and development of the theoretical methods. Very extensive studies have been done on first and second transition metal hydrides, oxides and transition metal dimers. But theoretical studies on the third row transition metal compounds are rather scarce.In the present work, we describe the nature of the bonding in 5d-transition metal diatomic carbides and nitrides through ab initio calculations by using CASSCF and FOCI techniques with relativistic effective core potentials(RECP) for transition metals; examine their potential energy surfaces while transition metal atoms interacting with C or N atom and calculate their molecular constants; predict and identify electronic configurations of their ground states and some low-lying states; discuss the properties of bonds formed in those compounds and their binding mechanisms and investigate the change trends of the features of bonding in those molecules.The following significant conclusions have been made.1> Although CASSCF method underestimates the dissociation energies of 5d-transition metal diatomic carbides and nitrides, it is a valuable tool to include static electron correlation effects and generally, a qualitatively correct energy surface can be obtained at this level of theory. And a reasonable molecular wave function can be obtained as a reference configuration for the further high-level theoretical calculations. FOCI calculations which including dynamic electron correlation effects are needed for quantitative description of the transition metal-containing compounds. At this level of theory, the calculation molecular constants are quite agreeable with the spectroscopic values.2> The theoretical predictions of the electronic configurations of the ground states of 5d-transition metal diatomic carbides and nitrides exception of ReC, which, we think, is needed further high-level theoretical calculations and spectroscopic experimental to identify, agree with the available spectroscopic experimental.3> Triple-bond (one σ ,two π) is formed in 5d-transition metal diatomic carbides and nitrides. It primarily comes from the 5d σ and 5d π orbital of the transition metal atom and the 2p σ and 2p π orbital of C or N atom. And some hybridization of the 6s and 5d orbital of transition metal atoms and some hybridization of the 2s and 2p orbital of C atom occur while the transition metal atom interact with C or N atom.4> From the view of overlap populations, the bonding strength of the carbides is stronger than that of the corresponding nitrides. This is reasonable because the energy level of the valence orbital of C atom is closer to theenergy level of the valence orbital of transition metal atoms than that of N atom. But there exist obvious ionic bond characteristics in the nitrides. From La to Pt, the character of ionic bond in nitrides getting weaker as the increasing of the nuclear charge of the transition metal atom. The stronger ionic bond in La, Hf, Ta and W diatomic nitrides gives rise to their dissociation energy is larger than that of the corresponding carbides. Following the trend of ironic bond weakening in Os, Ir, Pt diatomic nitrides results in their dissociation energy is smaller than that of the corresponding carbides. Except for PtN (De=2.86eV), the stable diatomic carbides or nitrides can be formed when 5d-transition metal atoms interact with C or N atom (De>4.5eV).5> From the view of atomic orbital population analysis, the promotion of electrons from 6s to 5d occurs while 5d-transition metal atoms interact with C or N atom. Especially for La atom, where almost two electrons of 6s orbital are promoted to 5d orbitals. This results indicate that the low-lying atomic state 5dn+16s1 rather than the ground state 5dn6s2 plays the leading role on the binding between 5d-transition metal atom and C or N atom. But for Re atom, this promotion of electrons is rather small because the low-lying state 5d66s1 of Re atom lies much higher (49.05Kcal/mol) than the ground state(5d56s2) which with a "half-filled" occupancy of 5d orbital.6> The low-lying states of 5d-transiton metal diatomic carbides and nitrides lie higher than their corresponding ground states only by about 0.5-1.8eV. That indicates there exists high density of low-lying molecular electronic states for the 5d-transition metal diatomic carbides and nitrides. And it also...