TmS compounds

The m/z 73 ion is prominent in virtually all TMS spectra and is often the base peak. The m/z 147 ion is coaunon in polyhydroxyl TMS compounds containing two. or more TMS groups in close proximity. The TMS group undergoes a prolific number intramolecular... 

Several general reviews describe the state of the art of peroxide epoxidation catalyzed by TM compounds at about a decade ago [2A. Later on, specialized reviews dealt with particular peroxides ofCr, Mo, andW [5], V [6], and with epoxidation reactions catalyzed by methyltrioxorhenium (MTO) [7] that involve Re peroxo complexes as species responsible for the oxygen transfer. 

In the last decade, quantum-chemical investigations have become an integral part of modern chemical research. The appearance of chemistry as a purely experimental discipline has been changed by the development of electronic structure methods that are now widely used. This change became possible because contemporary quantum-chemical programs provide reliable data and important information about structures and reactivities of molecules and solids that complement results of experimental studies. Theoretical methods are now available for compounds of all elements of the periodic table, including heavy metals, as reliable procedures for the calculation of relativistic effects and efficient treatments of many-electron systems have been developed [1, 2] For transition metal (TM) compounds, accurate calculations of thermodynamic properties are of particularly great usefulness due to the sparsity of experimental data. 

For many years, our group has been using quantum-chemical methods to calculate properties of TM compounds. During the last decade, we focused on theoretical analysis of chemical bonds in TM compounds [3-7] and on elucidation of mechanisms of TM-mediated reactions [8],... 

Initially, the level of theory that provides accurate geometries and bond energies of TM compounds, yet allows calculations on medium-sized molecules to be performed with reasonable time and CPU resources, had to be determined. Systematic investigations of effective core potentials (ECPs) with different valence basis sets led us to propose a standard level of theory for calculations on TM elements, namely ECPs with valence basis sets of a DZP quality [9, 10]. The small-core ECPs by Hay and Wadt [11] has been chosen, where the original valence basis sets (55/5/N) were decontracted to (441/2111/N-11) withN = 5,4, and 3, for the first-, second-, and third-row TM elements, respectively. The ECPs of the second and third TM rows include scalar relativistic effects while the first-row ECPs are nonrelativistic [11], For main-group elements, either 6-31G(d) [12-16] all electron basis set or, for the heavier elements, ECPs with equivalent (31/31/1) valence basis sets [17] have been employed. This combination has become our standard basis set II, which is used in a majority of our calculations [18]. 

The presentation of the theoretical BDEs is organized as follows. We list the calculated De values and their zero-point energy corrected counterparts (denoted by Do) for TM compounds that belong to different classes of molecules. This leads in some cases to double presentation, as some species belong to more than one class. However, we believe that the ordering chosen here facilitates comparison between compounds as well as between methods in a balanced way. 

Table 7.11 lists the predicted BDEs of TM compounds with 7r-donor ligands [4, 54, 55, 68-71], The complexes of W(CO)5 with acetylene, ethylene, and formaldehyde belong to the donor-acceptor class. The compounds of WC14 with the same ligands are metallacyclic molecules. 

I outline the main salient points relevant to computations on TM compounds. First, as indicated above, one needs an understanding of the rules behind the... 

In our short survey of the computational techniques available for investigating TM compounds we first mention molecular mechanics (Chapter 3). It may seem humble by the standards of the quantum mechanical ab initio, semiempirical and DFT methods (Chapters 5, 6 and 8, respectively) but MM is useful for obtaining input structures for submission to one of those calculations, may even provide in itself useful information, and it is, of course, extremely fast. Indeed, a recent book on the modelling of inorganic compounds, mainly TM species, is devoted very largely to molecular mechanics and a program specially parameterized for TM compounds, Momec3 [105],... 

Finally, TM compounds have been studied by semiempirical methods. One thinks first of faux-ab initio-type methods like AMI and PM3 (Chapter 6), since these are surrogates for full quantum mechanical ab initio techniques. However, the deepest insights into the nature of these compounds that have been afforded by a semiempirical method have come from the uncomplicated and venerable extended... 

Computational and theoretical chemistry has a very important role to play in helping to predict and rationalize the nature of the electronic ground state of TM compounds. Being able to do so is critical in many respects, if one wishes to predict the structure, properties, and reactivity of such compounds. First of all, the predicted structure and properties (for example the spectroscopic features) will of course be very different for different spin states. Of more interest to our research group is the notion that reactivity often crucially depends on the preferred spin state of reactants, products and intermediates. Thus, we have shown elsewhere (for reviews, see [2-4]) that many reactions of TM compounds involve multiple electronic states, of different spin. In these cases, reactivity is strongly influenced by the energy... 

It seems that cases of strong coupling can be found in e.g. 3 d-TM s and probably also in 3 d-TM compounds and in many adsorbate systems. In such cases, the overlap between the localized orbital and the valence orbitals is quite large. The screening process then rather corresponds to a pronounced shift of the bonding charge towards the central metal ion or towards the adsorbate, and the lowest line picks up most spectral strength and also becomes the main line. 

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