Relativistic bond energies

It follows from Table III that relativistic corrections to the actinide-R bond energies are necessary in order to reproduce experimental results.Non-relativistic bond energies are seen to be too small by some 50-60 Kcal mol". The importance of relativistic... 

Figure 4.10 Calculated relativistic bond contractions ARte in A (circles and solid line, axis on the left-hand side) and relativistic change in the dissociation energy contractions (triangles and dashed line, axis on the right-hand side) for various diatomic compounds as a function ofthe electronegativity of the ligand.

Ziegler, T, Baerends, E.J., Snijders, J.G., Ravenek, W. and Tschinke, V. (1989) Calculation of bond energies in compounds of heavy elements by quasi-relativistic approach. The Journal of Physical Chemistry, 93, 3050-3056. 

In the complex xenon functions as a n-donor toward Au2+. This is reflected in the calculated charge distribution within the cation, where the main part of the positive charge resides on the xenon atoms. Relativity plays a large role in stabilizing this and other predicted Au—Xe compounds about half of the Au—Xe bonding energy comes from relativistic effects.1993... 

The growing importance of quantum-chemical calculations is dealt with in a short section, with emphasis on the consideration of relativistic effects, especially in systems containing mercury. These calculations aim at optimization of structures, determination of bond energies, simulation of spectra, and estimation of spectral parameters, independent of but complementary to experiments. 

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]. 

Hence it can be stated that the predicted IPs, bonding energies and the bonding characteristics predicted for ThO using the relativistic and the NRL molecular orbital theories differ considerably and that there are very significant relativistic effects due to the participation of the 6d and 6p DFAO s of the Th atom in the bonding of the ThO diatomic. 

We present in Table I results from calculations on bond energies, bond distances and vibrational frequencies for the simple MH hydrides of the coinage triad M=Cu,Ag,and Au as well as the isoelectronic series MH+ with M=Zn,Cd,and Hg.Table I contains experimental data (lH) as well as results from non-relativistic (JLl) and relativistic (4.) Hartree-Fock-Slater (HFS) calculations. Results from a similar set of calculations on the metal-dimers M2 (M=Cu,Ag,and Au) as well as the dications (M=Zn,Cd,and Hg) are presented in Table II. 

It follows from Tables I and II that relativistic effects have a sizable influence on bond energies,bond distances,as well as vibrational frequencies for the compounds containing the heavier 5d-member (Au or Hg) within the triad. 

In the non-relativistic case bond energies follows the wrong order(compared with experiment) of first row>second row>third-row.Relativistic corrections,which for Au and Hg stabilize the bonds by some 30 Kcal mol", provide on the other hand the correct ordering of third row>first row>second row. 

We shall now turn to a general discusion of the trends in bond energies involving a triad of transition metals,with special emphasis on the role played by relativistic effects. 

Single metal-ligand bonds involving primarily a-type orbitals on both the metal center and the ligand are quite common.Small size examples would be the two hydrides AuH and HgH. However, the two species AuH and HgH are somewhat atypical in that the G-interaction involves the 6s-orbital on the metals rather than 5d. The involvement of 6s is largely responsible for the substantial relativistic contribution to the M-H bond energies in AuH and HgH (Table I). 

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