Transition-metal atoms, molecular

In the present work, we report on a new semi-empirical theoretical approach which allows us to perform spin and symmetry unconstrained total energy calculations for clusters of transition metal atoms in a co .putationally efficient way. Our approach is based on the Tight Binding Molecular Dynamics (TBMD) method. 

There are two basic differences of (sic) free atoms and chemically bound atoms. First, the more diffuse an AO, the stronger it is perturbed in molecular and condensed matter. The (n + )s AOs of the transition metal atoms, especially of the earlier ones, are not of primary importance for chemical bonding. Their relevance is comparable to that of the diffuse orbitals of main group elements ([34], p 653). 

The study of molecular systems containing metal atoms, particularly transition metal atoms, is more challenging than first-row chemistry from both an experimental and theoretical point of view. Therefore, we have systematically studied (3-5) the computational requirements for obtaining accurate spectroscopic constants for diatomic and triatomic systems containing the first- and second-row transition metals. Our goal has been to understand the diversity of mechanisms by which transition metals bond and to aid in the interpretation of experimental observations. 

Hay, P. J. Wadt, W. R. Ah initio effective core potentials for molecular calculations. Potentials for the transition metal atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270-283. 

Some of the earliest experimental studies of neutral transition metal atom reactions in the gas phase focused on reactions with oxidants (OX = O2, NO, N2O, SO2, etc.), using beam-gas,52,53 crossed molecular beam,54,55 and flow-tube techniques.56 A few reactions with halides were also studied. Some of these studies were able to obtain product rovibrational state distributions that could be fairly well simulated using various statistical theories,52,54,55 while others focused on the spectroscopy of the MO products.53 Subsequently, rate constants and activation energies for reactions of nearly all the transition metals and all the lanthanides with various oxidant molecules... 

The experimental techniques that have been used to study transition metal atom reactions (crossed molecular beams, flow tubes, etc.) are powerful ones. However, a complete interpretation of the mechanistic and dynamic aspects of these reactions is greatly facilitated through comparison of experimental results to theoretical predictions.159 The early theoretical work by the group of Siegbahn led to a great number of testable predictions, many of which have been found to be remarkably precise. Our measurements of various thermodynamic quantities have shown these calculations to be generally accurate to within 5-6kcal/mol. Unfortunately, due to the... 

In the present work, correlation consistent basis sets have been developed for the transition metal atoms Y and Hg using small-core quasirelativistic PPs, i.e., the ns and (nA)d valence electrons as well as the outer-core (nA)sp electrons are explicitly included in the calculations. This can greatly reduce the errors due to the PP approximation, and in particular the pseudo-orbitals in the valence region retain some nodal structure. Series of basis sets from double-through quintuple-zeta have been developed and are denoted as cc-pVwZ-PP (correlation consistent polarized valence with pseudopotentials). The methodology used in this work is described in Sec. II, while molecular benchmark calculations on YC, HgH, and Hg2 are given in Sec. III. Lastly, the results are summarized in Sec. IV. 

If the transition metal atom has more than the six d electrons indicated on the diagram, the antibonding 2eg molecular orbitals will also be populated and the metal-ligand bonding will be weakened. An example of this quite-prevalent effect is encountered in the series FeS2, CoS2, NiS2, discussed in section 10.4.2. 

These properties of the d-shell chromophore (group) prove the necessity of the localized description of d-electrons of transition metal atom in TMCs with explicit account for effects of electron correlations in it. Incidentally, during the time of QC development (more than three quarters of century) there was a period when two directions based on two different approximate descriptions of electronic structure of molecular systems coexisted. This reproduced division of chemistry itself to organic and inorganic and took into account specificity of the molecules related to these classical fields. The organic QC was then limited by the Hiickel method, the elementary version of the HFR MO LCAO method. The description of inorganic compounds — mainly TMCs,— within the QC of that time was based on the crystal field... 

In the above examples, a transition-metal atom is invariably present. As such, one might believe that the unpaired d electrons of the transition metal are the main cause of ferro/ferrimagnetic ordering, the organic/molecular species only playing a secondary role. There are a few examples of molecular magnets without the transition-metal atoms, albeit showing low T s. These are p-nitrophenyl nitronyl nitroxide (T = 0.6 K)... 

Small molecular systems containing transition metal atoms are interesting both in themselves and as model systems (see e.g. the recent reviews by Harrison [7] and Noguera [8]). They are also useful as (quite difficult) trial systems for theoretical methods. 

---